Random-Access Authenticated Encryption
draft-sullivan-cfrg-raae-03
This document is an Internet-Draft (I-D).
Anyone may submit an I-D to the IETF.
This I-D is not endorsed by the IETF and has no formal standing in the
IETF standards process.
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Nick Sullivan | ||
| Last updated | 2026-07-23 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-sullivan-cfrg-raae-03
Crypto Forum N. Sullivan
Internet-Draft Cryptography Consulting LLC
Intended status: Informational 23 July 2026
Expires: 24 January 2027
Random-Access Authenticated Encryption
draft-sullivan-cfrg-raae-03
Abstract
This document defines random-access authenticated encryption (raAE),
a primitive that partitions a message into an indexed sequence of
segments that can be encrypted and decrypted independently and in any
order. It also specifies SEAL (Segmented Encryption and
Authentication Layer), a parameterized construction that defines a
family of concrete raAE instantiations, one for each valid choice of
an Authenticated Encryption with Associated Data (AEAD) algorithm, a
key derivation function (KDF), and associated parameters.
SEAL supports immutable (write-once), append-only, and mutable (in-
place ciphertext rewrite) operations, each with per-segment
authentication. A separately configured snapshot authenticator can
additionally authenticate the complete, indexed segment set.
The document also defines the security notions of raAE, specifies the
requirements for conforming constructions, and analyzes SEAL against
those requirements. Concrete cipher suites, serialization layouts,
and test vectors for SEAL are specified in a companion document.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Crypto Forum Research
Group mailing list (cfrg@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/cfrg.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Sullivan Expires 24 January 2027 [Page 1]
Internet-Draft raAE July 2026
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 24 January 2027.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Related Work . . . . . . . . . . . . . . . . . . . . . . 6
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 7
2.1. Taxonomy . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . 10
3. The raAE Primitive . . . . . . . . . . . . . . . . . . . . . 11
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2. Interface . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3. Extended Interface . . . . . . . . . . . . . . . . . . . 12
4. The SEAL Construction . . . . . . . . . . . . . . . . . . . . 14
4.1. Construction Overview . . . . . . . . . . . . . . . . . . 16
4.2. Component Suite and Parameters . . . . . . . . . . . . . 16
4.2.1. Suite Choices . . . . . . . . . . . . . . . . . . . . 18
4.2.2. Profile-Level Constants . . . . . . . . . . . . . . . 19
4.2.3. Algorithm-Determined Sizes . . . . . . . . . . . . . 19
4.2.4. Per-Message Inputs . . . . . . . . . . . . . . . . . 19
4.3. The KDF Combiner . . . . . . . . . . . . . . . . . . . . 20
4.3.1. Two-Step KDF . . . . . . . . . . . . . . . . . . . . 21
4.3.2. One-Step KDF . . . . . . . . . . . . . . . . . . . . 22
4.4. SEAL Encodings and Labels . . . . . . . . . . . . . . . . 22
4.4.1. Payload Info Construction . . . . . . . . . . . . . . 22
4.4.2. Segment AAD . . . . . . . . . . . . . . . . . . . . . 23
4.4.3. Labels . . . . . . . . . . . . . . . . . . . . . . . 25
4.5. Key Schedule and Nonce Generation . . . . . . . . . . . . 26
4.5.1. Payload Schedule . . . . . . . . . . . . . . . . . . 26
4.5.2. Epoch Key Derivation . . . . . . . . . . . . . . . . 26
4.5.3. Nonce Generation . . . . . . . . . . . . . . . . . . 27
4.6. Commitment . . . . . . . . . . . . . . . . . . . . . . . 30
Sullivan Expires 24 January 2027 [Page 2]
Internet-Draft raAE July 2026
4.7. Snapshot Authenticator . . . . . . . . . . . . . . . . . 30
4.7.1. Interface and Requirements . . . . . . . . . . . . . 30
4.7.2. None . . . . . . . . . . . . . . . . . . . . . . . . 33
4.8. Segment Algorithms . . . . . . . . . . . . . . . . . . . 33
4.8.1. EncryptSegment . . . . . . . . . . . . . . . . . . . 34
4.8.2. DecryptSegment . . . . . . . . . . . . . . . . . . . 35
4.8.3. RewriteSegment . . . . . . . . . . . . . . . . . . . 35
4.9. Top-level Algorithms . . . . . . . . . . . . . . . . . . 36
4.9.1. Read-Only Operations . . . . . . . . . . . . . . . . 36
4.9.2. Rewritable Operations . . . . . . . . . . . . . . . . 39
4.9.3. Changing Message Length . . . . . . . . . . . . . . . 40
5. Security Analysis . . . . . . . . . . . . . . . . . . . . . . 43
5.1. Security Notions . . . . . . . . . . . . . . . . . . . . 43
5.1.1. ra-ROR . . . . . . . . . . . . . . . . . . . . . . . 44
5.1.2. ra-CMT . . . . . . . . . . . . . . . . . . . . . . . 44
5.1.3. ra-CMT-p . . . . . . . . . . . . . . . . . . . . . . 45
5.1.4. Snapshot Authentication . . . . . . . . . . . . . . . 45
5.2. Relations to Other Notions . . . . . . . . . . . . . . . 45
5.3. Adversary Model and Assumptions . . . . . . . . . . . . . 46
5.4. raAE Construction Requirements . . . . . . . . . . . . . 48
5.5. Segment Confidentiality and Integrity . . . . . . . . . . 50
5.6. Commitment Security . . . . . . . . . . . . . . . . . . . 51
5.7. Snapshot Authenticator Security . . . . . . . . . . . . . 53
5.8. Capacity and Usage Limits . . . . . . . . . . . . . . . . 55
5.8.1. Accounting Model . . . . . . . . . . . . . . . . . . 55
5.8.2. Limit Classes . . . . . . . . . . . . . . . . . . . . 56
5.8.3. Maximum Object Size . . . . . . . . . . . . . . . . . 56
5.8.4. Rewrite Capacity . . . . . . . . . . . . . . . . . . 58
5.8.5. Choosing epoch_length . . . . . . . . . . . . . . . . 59
5.8.6. Derivations . . . . . . . . . . . . . . . . . . . . . 60
6. Security Considerations . . . . . . . . . . . . . . . . . . . 62
6.1. Detection Summary . . . . . . . . . . . . . . . . . . . . 62
6.2. Nonce Misuse . . . . . . . . . . . . . . . . . . . . . . 64
6.3. Parameter Set Mismatch . . . . . . . . . . . . . . . . . 65
6.4. Framing and Label Errors . . . . . . . . . . . . . . . . 65
6.5. Parameter Misuse . . . . . . . . . . . . . . . . . . . . 65
6.6. Snapshot Integrity Limitations . . . . . . . . . . . . . 66
6.7. Salt Reuse . . . . . . . . . . . . . . . . . . . . . . . 67
6.8. Rewrite Hazards . . . . . . . . . . . . . . . . . . . . . 68
6.9. Constant-Time Implementation . . . . . . . . . . . . . . 68
6.10. Properties Not Provided . . . . . . . . . . . . . . . . . 69
6.11. Cross-Application Domain Separation . . . . . . . . . . . 70
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 70
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 70
8.1. Normative References . . . . . . . . . . . . . . . . . . 70
8.2. Informative References . . . . . . . . . . . . . . . . . 71
Appendix A. Optional Plaintext-Bound Nonce Construction . . . . 73
Appendix B. Optional Hedged Randomness . . . . . . . . . . . . . 74
Sullivan Expires 24 January 2027 [Page 3]
Internet-Draft raAE July 2026
Appendix C. Proof Status and Security Claim Provenance . . . . . 75
C.1. Provenance of the Security Claims . . . . . . . . . . . . 75
C.2. Advantage Notation and Adversary Parameters . . . . . . . 77
C.3. ra-ROR Reduction . . . . . . . . . . . . . . . . . . . . 78
C.4. Commitment . . . . . . . . . . . . . . . . . . . . . . . 79
C.5. Nonce Modes . . . . . . . . . . . . . . . . . . . . . . . 79
Appendix D. Design Rationale . . . . . . . . . . . . . . . . . . 80
D.1. Key Schedule Design Rationale . . . . . . . . . . . . . . 80
D.2. Nonce Mode Design Rationale . . . . . . . . . . . . . . . 81
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 81
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 81
1. Introduction
Large encrypted content is often stored as a sequence of fixed-size
segments, so that an application can read or modify any part of it
without processing the whole. Encrypted backups, encrypted file
formats, object stores with partial updates, and full-disk encryption
over fixed-size blocks all work this way. Such a system needs to
encrypt, decrypt, or re-encrypt any individual segment on its own and
in arbitrary order, and to verify that the stored object as a whole
is authentic and complete, without re-encrypting the segments that
did not change.
This document specifies the raAE primitive (Section 3), whose base
interface and security notions come from Fábrega et al. ([FLRR25]),
and defines SEAL (Segmented Encryption and Authentication Layer), a
parameterized construction of raAE. The base algorithms encrypt and
decrypt segments in arbitrary order, so a caller can read or replace
any segment on its own. On top of that base, the extended raAE
interface adds in-place rewrite and a snapshot, a stored value that
authenticates the segment set as the writer last recorded it. A
rewrite re-encrypts only the changed segment and updates the
snapshot, leaving every other segment untouched. Appending to the
end, or truncating from the end, composes the same per-segment
operations, so the snapshot stays consistent without a separate
algorithm.
SEAL builds the primitive from a chosen AEAD algorithm, a key
derivation function (KDF), and configuration parameters. Under a
snapshot authenticator, it binds every segment and the count into a
public snapshot value that an adversary cannot forge without the
content-derived key.
Sullivan Expires 24 January 2027 [Page 4]
Internet-Draft raAE July 2026
.------------------------------.
| raAE primitive |
| abstract interface |
'------------------------------'
|
v
.--------------------------------------------------------------.
| SEAL construction |
| parameterized by a cipher suite (AEAD + KDF), a maximum |
| segment size, and an epoch length |
| |
| .----------------------. .----------------------. |
| | mutability: | | snapshot: | |
| | immutable | | none | present | |
| | append-only | | | |
| | mutable | | | |
| '----------------------' '----------------------' |
'--------------------------------------------------------------'
| provides
+----------------+----------------+
| |
v v
.----------------------------. .------------------------.
| per-segment authenticity | | snapshot integrity |
| (always present) | | (when present) |
'----------------------------' '------------------------'
Figure 1: The raAE primitive and the SEAL construction
raAE separates the two authentication scopes that applications often
conflate:
Per-segment authenticity: One AEAD tag verifies under the segment
index, a finality bit marking the last segment, and any caller-
supplied associated data. It does not establish that the segment
belongs to the current snapshot.
Snapshot integrity: When a snapshot authenticator is configured,
snapshot verification checks that the present segment set and the
count are exactly what the writer last recorded, under a content-
derived key. Verification targets the whole segment set, and,
where the authenticator supports it, can instead confirm that a
designated subset of segments is included at their recorded
positions, without reading the others (Section 4.7.1). It does
not establish freshness against whole-object rollback, which a
consuming protocol must supply.
Sullivan Expires 24 January 2027 [Page 5]
Internet-Draft raAE July 2026
raAE and SEAL are deliberately distinct layers. A consuming protocol
supplies serialization, storage transactions, key management, and
rollback protection, which are out of scope here. The full boundary
of what raAE does not provide is Section 6.10.
The remainder of this document presents raAE first and SEAL second.
Section 1.1 situates raAE against prior segmented authenticated-
encryption (AE) constructions. Section 2 fixes terminology and
notation. Section 3 specifies the extended raAE primitive and its
snapshot operations. Section 4 defines SEAL. Section 5 states the
target security properties an raAE construction must meet, the
assumptions SEAL's components must satisfy, and the operational
limits on its use, and defines the security notions. Cipher suites,
serialization layouts, named instantiations, and test vectors are
specified in the companion document [I-D.sullivan-seal-concrete].
1.1. Related Work
CHAIN ([HRRV15]) is a segmented authenticated-encryption construction
that chains state from one segment to the next. It is sequential by
construction and supports neither independent random-access reads nor
in-place segment rewrites.
STREAM ([HRRV15]) turns a nonce-based AEAD into a segmented
construction using induced nonces that encode a message nonce, the
segment index, and a final-segment bit. STREAM supports random-
access decryption and encryption of individual segments, but defines
neither authenticated snapshots nor authenticated rewrites. SEAL's
derived nonce mode (Section 4.5.3.2) follows STREAM's counter-plus-
final-bit structure and adds those operations.
Tink Streaming AEAD ([Tink]) and OpenPGP v2 Symmetrically Encrypted
and Integrity Protected Data (SEIPD) packets ([RFC9580]) are deployed
STREAM-family formats. Both derive a message key and nonce prefix
from a key and a salt, encrypt each chunk under an AEAD nonce
carrying the chunk index, and detect truncation with a final tag over
the empty string. Tink derives an AES-GCM key with the HMAC-based
Key Derivation Function (HKDF) ([RFC5869]), while v2 SEIPD selects
its cipher and AEAD from packet identifiers. Each chunk is
independently authenticated and can be verified locally at its
boundary. Neither format defines an in-place rewrite operation or a
snapshot authenticator over an updatable set of segment tags.
SEAL's snapshot authenticator addresses exactly that gap. Its per-
segment input is the segment's metadata leaf, which binds the
segment's content when the segment commitment is set. It verifies
the whole segment set against the snapshot, and optionally a single
segment (Section 4.7.1). A consuming protocol that authenticates the
Sullivan Expires 24 January 2027 [Page 6]
Internet-Draft raAE July 2026
one snapshot value obtains integrity over the whole segment set and
its count, and, where the segment commitment is set over a strict
ordering, per-segment origin authentication that holds against
holders of the content-encryption key (CEK).
Fábrega et al. ([FLRR25]) formalized random-access AEAD security
over a raAE primitive syntax in which segment encryption and
decryption may be invoked in arbitrary order, and gave a
construction, FLOE (Fast Lightweight Online Encryption), proved to
meet it. FLOE realizes the base interface: segments are written
once, and there is no snapshot. SEAL realizes that primitive with
the extension of this document, and Section 5.1 defines the security
notions used in its analysis.
Each precedent covers part of the random-access AE design space.
raAE is the abstract interface that generalizes the family with three
capabilities none of the precedents combines: arbitrary-order
encryption, in-place authenticated rewrite, and a snapshot over the
whole segment set. SEAL realizes all three in one construction.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
A message segment is a unit of plaintext that is individually
encrypted and authenticated. Segments need not be uniform in length.
A construction MAY set a maximum segment size, and any segment may be
shorter than that maximum.
A message is the content that one CEK and salt pair protects, the
unit the primitive encrypts. An object is that message in stored
form: the ciphertext segments, their metadata, and any snapshot that
a storage layer holds. Each object carries one message, whose
content may change through rewrite and length-changing operations.
A ciphertext core is the AEAD encryption output corresponding to one
message segment, excluding file-level headers and external segment
metadata. An AEAD-specific split divides this output into ct_i, the
encrypted segment body, and tag_i, the authentication tag.
A ciphertext segment is the stored representation of one encrypted
message segment. Depending on nonce_mode, it contains the ciphertext
core and may also contain a stored nonce or other per-segment
metadata.
Sullivan Expires 24 January 2027 [Page 7]
Internet-Draft raAE July 2026
Segment metadata is per-segment information needed to decrypt or
verify a ciphertext segment, such as a stored nonce, tag, finality
status, or profile-defined values.
This document writes AEAD.Encrypt and AEAD.Decrypt for the
authenticated encryption and decryption operations defined in
[RFC5116].
Symbol form follows the layer a name belongs to. The abstract
interface and the analysis use mathematical notation: a capital
letter is an operand of that interface, either an octet string (K, N,
G, T_g, A_i, M_i, C_i) or the per-message state (S). A lowercase
letter is an integer index or bound (i, e, p, r).
N with a lowercase letter is a size that a chosen component fixes
rather than one a deployment picks: the AEAD (Nk, Nn, Nt, Nb), the
KDF (Nh), the snapshot authenticator (Na), or the nonce mode (Np).
Everything the construction names concretely uses lowercase
snake_case, whatever its type: octet strings (salt, commitment,
segment_key, ct_i), counts (n_seg, n_ep), lengths a deployment
configures (segment_max, commitment_length), and identifiers, which
take an _id suffix (aead_id, kdf_id, snap_id, protocol_id).
Names that take arguments follow the same split. Initialisms are
capitalized (LH, KDF, AEAD, XOF). The raAE tuple is CamelCase
(KeyGen, StartEnc, EncSeg, StartDec, DecSeg, RewriteSeg, SnapVerify),
while the methods of a component it parameterizes over are lowercase
(snapshot, verify, add, remove, set_length), so a name shows which
interface it belongs to. Derivations this document defines are
lowercase (encode, frame, leaf, nonce).
A subscript selects one instance of a value: X_i is X for the segment
at index i, as in A_i, P_i, ct_i, and tag_i, and d_e is the epoch
head for epoch e. A value written f(i), as in segment_key(i) or
nonce(i), is the value that the named derivation defines for index i.
Two suffixes and one prefix recur. A _label suffix is an ASCII
constant that separates one derivation from another (aad_label,
epoch_key_label). A _calc suffix marks a value recomputed for
comparison against a stored one (d_calc(e), leaf_calc(i)). An Adv_
prefix names an adversary's advantage in the analysis (Adv_snap,
Adv_muPRF).
The remaining terms (ikm, label, info, L, salt, CEK, protocol_id,
nonce_mode, epoch, epoch_key, payload_key, segment_key, snap_key, and
the commitment) are defined in Section 4 where they first appear.
Sullivan Expires 24 January 2027 [Page 8]
Internet-Draft raAE July 2026
2.1. Taxonomy
This document uses the following terms to keep the raAE primitive,
the SEAL construction, profiles, and consuming protocols distinct.
raAE primitive: The abstract random-access AE interface and security
target.
Extended raAE interface: The base interface plus snapshot
maintenance and verification.
SEAL construction: The construction specified in Section 4.
Cipher suite: The AEAD and KDF algorithms and their sizes.
Profile: A named set of fixed parameter choices, identified by a
protocol_id, needed for interoperability.
Parameter set: The per-object values that affect derivation and
verification.
Consuming protocol: The protocol or format that stores objects and
manages keys.
Serialization: The consuming protocol's wire or storage encoding.
Segment: A zero-based independently encrypted plaintext unit.
Epoch: A group of 2^epoch_length segment indices sharing one epoch
key.
Snapshot value: The public value that, when a snapshot authenticator
is configured, authenticates one segment set.
Snapshot authenticator: A pluggable SEAL component that produces and
verifies the snapshot value. Its abstract interface is
Section 4.7.1, and its concrete instantiations are in
[I-D.sullivan-seal-concrete].
Per-segment authenticity: AEAD verification for one segment only.
Snapshot integrity: Verification of the complete segment set and
snapshot.
Segment commitment: A hash of one segment's ciphertext, stored in
that segment's metadata. It binds the segment's content, not its
position (Section 4.7.1).
Sullivan Expires 24 January 2027 [Page 9]
Internet-Draft raAE July 2026
External freshness: Rollback protection supplied outside raAE.
2.2. Notation
I2OSP(n, w): The w-octet big-endian encoding of the non-negative
integer n, as defined in [RFC8017].
uint8(n), uint16(n), uint32(n), uint64(n): Shorthand for I2OSP(n,
1), I2OSP(n, 2), I2OSP(n, 4), and I2OSP(n, 8) respectively.
||: Octet string concatenation.
XOR: Bitwise exclusive-or of two octet strings of equal length.
X[a:b]: Octets a through b-1 inclusive of X (zero-based, half-open
interval).
[x, y, ...]: An ordered list of octet strings. A single-element
list is written [x]. The brackets denote a list, not an optional
parameter.
frame, encode: The length-prefixed injective encoding used by SEAL,
defined in Section 4.3. frame is total over all field lengths.
The construction requires an injective framing (Section 5.4) but
does not mandate this encoding. Another profile MAY use any
injective encoding that meets Section 5.4.
Nk: Key size in octets for the chosen AEAD algorithm.
Nn: Nonce size in octets for the chosen AEAD algorithm.
Np: Presented nonce size in octets: the per-segment nonce stored in
metadata. Np = Nn in random nonce mode and Np = 0 in derived
nonce mode, where the nonce is recomputed from the key schedule.
Nh: Hash or pseudorandom function (PRF) output size in octets for
the chosen KDF.
Nt: Authentication tag size in octets for the chosen AEAD algorithm.
Nb: Cipher block size in octets for the chosen AEAD algorithm,
defined only for an AEAD built on a block cipher. The block-size
birthday limit of Section 5.8 is expressed in Nb and does not
apply to an AEAD with no block-cipher structure.
Na: The snapshot value length in octets, the output size of the
configured snapshot authenticator. It MAY depend on the segment
count n_seg, and is 0 when none (Section 4.7).
Sullivan Expires 24 January 2027 [Page 10]
Internet-Draft raAE July 2026
n_seg: The number of ciphertext segments in a stored message.
Written n_seg (lowercase) to distinguish it from the per-message
nonce N in the raAE interface (Section 3.2).
3. The raAE Primitive
3.1. Overview
Random-access authenticated encryption (raAE) partitions a message
into segments, each encrypted and decrypted on its own and in any
order. A reader can open one segment without the rest of the
message, and a writer can encrypt one without holding the others. In
the base interface [FLRR25], each segment is written once and
thereafter only read.
The extended raAE interface keeps that base and adds one capability:
a segment already written may be replaced in place, without
disturbing the rest. Rewriting changes the threat model. Per-
segment authentication binds each segment in isolation, so it cannot
tie the segments a reader assembles to a single recorded version of
the object: a storage layer holding the ciphertext can roll a segment
back to an older valid copy, substitute one valid same-index segment
for another, or drop one, and each segment it presents still
authenticates. An optional snapshot authenticates the segment set as
the writer last recorded it, so a reader can detect these changes.
The extension (Section 3.3) defines the snapshot and the operations
that maintain it.
The security notions follow the interface. raAE and its rewrites
target ra-ROR (random-access real-or-random) and ra-CMT (random-
access context commitment) (Section 5.1). Rewriting leaves both
untouched, since replacing a segment is an encryption the read-only
adversary could already ask for. Snapshot authentication adds the
one notion of its own, snapshot integrity (Section 5.1.4).
3.2. Interface
The base raAE interface, from [FLRR25], is a tuple of five
algorithms: KeyGen, StartEnc, EncSeg, StartDec, and DecSeg.
KeyGen: Randomized key generation. Takes no input. Outputs a
secret key K.
StartEnc(K, N, G): Deterministic. Takes a secret key K, nonce N,
and global associated data G. Outputs a ciphertext header T_g and
an initial per-message encryption state S.
EncSeg(S, p, A_i, M_i): Randomized. Takes the encryption state S, a
Sullivan Expires 24 January 2027 [Page 11]
Internet-Draft raAE July 2026
position identifier p, per-segment associated data A_i, and
message segment M_i. Outputs a ciphertext segment C_i. S is not
modified. EncSeg may be called with the same S at any position,
in any order, or concurrently.
StartDec(K, N, G, T_g): Deterministic. Takes key K, nonce N, global
associated data G, and ciphertext header T_g. Outputs a per-
message decryption state S or an error.
DecSeg(S, p, A_i, C_i): Deterministic. Takes the decryption state
S, position identifier p, per-segment associated data A_i, and
ciphertext segment C_i. Outputs message segment M_i or an error.
The position identifier p = (i, b) consists of a segment number i and
a terminal bit b in {0, 1}. They impose a total ordering on segments
and carry a truncation defense: a ciphertext lacking a segment with b
= 1 is incomplete. The encryption state S is immutable. That
immutability is what makes arbitrary-order and fully parallel
operation possible. The ciphertext header T_g commits to K, the
nonce N, and global associated data G. A headerless scheme emits T_g
= empty.
3.3. Extended Interface
Replacing a segment already written, without re-encrypting the
others, requires no new operation: re-encrypting a position is an
ordinary EncSeg call over the immutable state, and it leaves ra-ROR
and ra-CMT untouched.
A scheme that supports rewriting MAY carry a snapshot, a value over
the current set of segments that a reader checks to detect tampering
that per-segment authentication does not catch. A write-once scheme
MAY carry one too: a single value that a consuming protocol
authenticates once for the whole segment set (Section 4.7.1).
Two operations, added by this document, maintain and check the
snapshot. RewriteSeg replaces a segment and updates the snapshot in
the same step. SnapVerify decides whether a given set of segments is
the one the writer last recorded.
RewriteSeg(S, p, A_i, M'_i, C_i, snapshot): Randomized. Takes the
encryption state S and position identifier p, per-segment
associated data A_i, the new message segment M'_i, the existing
ciphertext C_i at position p, and the current snapshot. Outputs
replacement ciphertext C'_i and an updated snapshot value,
snapshot'.
SnapVerify(S, segments, snapshot): Deterministic. Takes the
Sullivan Expires 24 January 2027 [Page 12]
Internet-Draft raAE July 2026
encryption state S, the segments to verify as (position,
ciphertext segment) pairs, and the snapshot. These are either the
whole current object state or, where the authenticator supports
it, a designated subset of it. Outputs accept or reject.
The extension also changes two base signatures. In the extended
interface:
StartEnc(K, N, G): Deterministic. As in Section 3.2, and
additionally outputs the initial snapshot value, the snapshot over
the empty segment set.
EncSeg(S, p, A_i, M_i): Randomized. As in Section 3.2, and
additionally outputs the snapshot value over the segment set that
results from adding C_i at position p.
The snapshot is an authenticator over the set of segments belonging
to the current object state. The base primitive has no snapshot
value.
These additions define the extended raAE interface of this document:
the tuple (KeyGen, StartEnc, EncSeg, StartDec, DecSeg, RewriteSeg,
SnapVerify) over the snapshot value. The first five algorithms are
those of Section 3.2. The snapshot value, RewriteSeg, and SnapVerify
are the extension. A scheme can realize the base interface alone, as
FLOE ([FLRR25]) does. SEAL (Section 4) realizes the full extended
interface.
Changing a message's length is a composition of these operations, not
a separate algorithm in either direction. Position-addressed
encryption over immutable state already permits re-encrypting a
position or encrypting at a higher index (see Section 5.1.1).
A holder appends by encrypting the new segments and re-marking the
old final segment with RewriteSeg to clear its terminal bit. It
truncates by re-marking the new final segment with RewriteSeg,
dropping the trailing segments, and updating the snapshot over the
surviving set. Appending to an empty object skips the re-mark, since
there is no old final segment, and truncating away every segment
yields the empty object, with no new final segment to re-mark.
A length change appends segments or drops a suffix. Each step
updates the snapshot, and Section 4.9.3 gives the construction for
both directions.
Sullivan Expires 24 January 2027 [Page 13]
Internet-Draft raAE July 2026
4. The SEAL Construction
SEAL is a construction for the raAE primitive of Section 3, realizing
its extended interface from a KDF, an AEAD, the labels, a nonce-
generation method, and an optional snapshot authenticator. Its KDF
combiner (Section 4.3) frames every derived key and input over any
KDF with a two-step or one-step form. SEAL provides commitment
around the AEAD, which need not itself commit.
A SEAL construction MAY select any AEAD, KDF, nonce mode, commitment
configuration, or snapshot authenticator that satisfies the
requirements of this section. Section 5 derives SEAL's bounds from
the abstract properties of these components. Concrete parameter
choices and interoperable suites are in [I-D.sullivan-seal-concrete].
In the key hierarchy below, a CEK and per-content salt feed the
payload schedule, which derives four per-message values
(Section 4.5).
Sullivan Expires 24 January 2027 [Page 14]
Internet-Draft raAE July 2026
CEK + salt
|
v
.-----------.
| Payload |
| Schedule |
'-----------'
/ | | \
v v v v
commit payload nonce snap
key base key
| | | |
v v v |
.---. .------. .-----. |
|cmt| |seg | |nonce| |
| | |key(i)| | (i) | |
'---' '------' '-----' |
\ / |
\ / |
v v |
.----------. |
| AEAD | |
| Seal(i) | |
'----------' |
/ \ |
v v |
ct_i tag_i |
\ |
v v
.-----------------.
| Snapshot |
| Authenticator |
'-----------------'
|
v
snapshot
Figure 2: SEAL Key Hierarchy
The figure shows each segment's tag feeding the authenticator. Its
actual per-segment input is the segment's metadata leaf
(Section 4.7.1).
Sullivan Expires 24 January 2027 [Page 15]
Internet-Draft raAE July 2026
4.1. Construction Overview
The commitment lets a reader reject a wrong key or parameter set
before decrypting. The snapshot, when present, authenticates the
current segment set. SEAL realizes the raAE operations of
Section 3.2 as follows:
+============+==========================================+
| raAE | SEAL realization |
| operation | |
+============+==========================================+
| StartEnc | Derive the payload schedule from the CEK |
| | and salt, committing G (Section 4.5) |
+------------+------------------------------------------+
| StartDec | Re-derive the schedule and verify the |
| | stored commitment (Section 4.6) |
+------------+------------------------------------------+
| EncSeg, | EncryptSegment and DecryptSegment |
| DecSeg | (Section 4.8) |
+------------+------------------------------------------+
| RewriteSeg | RewriteSegment, then update the snapshot |
| | authenticator for the changed segment |
| | (Section 4.9.2.1) |
+------------+------------------------------------------+
| SnapVerify | Check the present indices and finality, |
| | then verify the snapshot via the |
| | authenticator (Section 4.7) |
+------------+------------------------------------------+
Table 1: raAE operations and their SEAL realizations
SEAL's salt, payload_info, and G carry the raAE per-message inputs,
and the commitment stands in for the header (Section 4.2).
4.2. Component Suite and Parameters
Table 2 lists the parameters, inputs, and derived values, with what
sets each and where it is defined.
+===================+===================+=============+==========+
| Parameter | Type or range | Set by | Defined |
| | | | in |
+===================+===================+=============+==========+
| AEAD | RFC 5116 | suite | Section |
| | algorithm | | 4.2.1 |
+-------------------+-------------------+-------------+----------+
| KDF | KDF algorithm | suite | Section |
| | | | 4.3 |
Sullivan Expires 24 January 2027 [Page 16]
Internet-Draft raAE July 2026
+-------------------+-------------------+-------------+----------+
| segment_max | power of two, at | suite | Section |
| | least 4096 | | 4.2.1 |
+-------------------+-------------------+-------------+----------+
| epoch_length | integer r in 0 to | suite | Section |
| | 63 | | 4.5.2 |
+-------------------+-------------------+-------------+----------+
| protocol_id | octet string | application | Section |
| | | | 6.11 |
+-------------------+-------------------+-------------+----------+
| nonce_mode | "random" or | message, | Section |
| | "derived" | within the | 4.5.3 |
| | | profile | |
+-------------------+-------------------+-------------+----------+
| aad_label | ASCII string | profile | Section |
| | | | 4.4.2 |
+-------------------+-------------------+-------------+----------+
| commitment_length | integer, at least | profile | Appendix |
| | 16, default Nh | | C.4 |
+-------------------+-------------------+-------------+----------+
| Nk, Nn, Nt, Nb | sizes in octets | AEAD | Section |
| | | | 2.2 |
+-------------------+-------------------+-------------+----------+
| Nh | size in octets | KDF | Section |
| | | | 2.2 |
+-------------------+-------------------+-------------+----------+
| CEK | 32 uniform random | message | Section |
| | octets | input | 4.5 |
+-------------------+-------------------+-------------+----------+
| salt | 32 uniform random | message | Section |
| | octets | input | 4.5 |
+-------------------+-------------------+-------------+----------+
| payload_info | ordered element | message | Section |
| | list | input | 4.4 |
+-------------------+-------------------+-------------+----------+
| G | octet string, | message | Section |
| | empty by default | input | 4.6 |
+-------------------+-------------------+-------------+----------+
| commitment | commitment_length | derived per | Section |
| | octets | message | 4.5 |
+-------------------+-------------------+-------------+----------+
| payload_key | Nk octets | derived per | Section |
| | | message | 4.5 |
+-------------------+-------------------+-------------+----------+
| snap_key | Nh octets | derived per | Section |
| | | message | 4.5 |
+-------------------+-------------------+-------------+----------+
| nonce_base | Nn octets, | derived per | Section |
Sullivan Expires 24 January 2027 [Page 17]
Internet-Draft raAE July 2026
| | derived mode only | message | 4.5 |
+-------------------+-------------------+-------------+----------+
| epoch_key(i) | Nk octets | derived per | Section |
| | | segment | 4.5.2 |
+-------------------+-------------------+-------------+----------+
| segment_key(i) | Nk octets | derived per | Section |
| | | segment | 4.5.2 |
+-------------------+-------------------+-------------+----------+
| nonce(i) | Nn octets | derived per | Section |
| | | segment | 4.5.3 |
+-------------------+-------------------+-------------+----------+
| segment_aad(i, | octet string | derived per | Section |
| is_final, A_i) | | segment | 4.4.2 |
+-------------------+-------------------+-------------+----------+
| tag_i | Nt octets | derived per | Section |
| | | segment | 4.8 |
+-------------------+-------------------+-------------+----------+
Table 2: Parameters, inputs, and derived values
SEAL fixes the segment additional authenticated data (AAD) encoding
(Section 4.4.2), the commitment derivation (Section 4.6), and the
snapshot value (Section 4.7) identically across all suites. Their
octet lengths still vary by suite, because Nh and Nt are suite-
dependent.
4.2.1. Suite Choices
A suite is assembled from the following choices.
AEAD: An authenticated encryption with associated data algorithm
satisfying the interface of [RFC5116], with Nk-octet keys, Nn-
octet nonces, and Nt-octet authentication tags.
KDF: A key derivation function supporting the abstract interface
defined in Section 4.3.
segment_max: A positive integer, a power of two and at least 4096
octets. Each segment carries at most this many plaintext octets.
The length-dependent limits of Section 5.8 are computed at this
size.
epoch_length: A non-negative integer r in 0 to 63. Each epoch
covers 2^r consecutive segments aligned to a multiple of 2^r, so r
sets the key-rotation granularity: r = 0 gives a fresh epoch key
per segment and r = 63 one key for all segments (Section 4.5.2).
Profiles MAY restrict r further per AEAD.
Sullivan Expires 24 January 2027 [Page 18]
Internet-Draft raAE July 2026
protocol_id: An application-chosen octet string that binds all
derived values to a specific protocol version. Selection guidance
is in Section 6.11.
nonce_mode: A payload_info element selecting one of two nonce
constructions, "random" or "derived". The constructions are
defined in Section 4.5.3, and the valid (nonce_mode, snap_id)
tuples depend on the profile.
4.2.2. Profile-Level Constants
One constant is fixed by the profile rather than carried per message.
aad_label: A profile-level ASCII string bound as the first element
of the segment AAD encoding (Section 4.4.2). Consuming protocols
MAY define their own profiles with a distinct aad_label.
4.2.3. Algorithm-Determined Sizes
The sizes Nk, Nn, Nt, and Nh are fixed by the chosen AEAD and KDF
rather than by the parameter set directly, as is Nb where it is
defined. They are defined in Section 2.2. One configurable length
defaults from Nh.
commitment_length: A positive integer of at least 16, defaulting to
Nh, the length in octets of the key commitment prefix. The
collision bounds for each length are given in Appendix C.4.
4.2.4. Per-Message Inputs
An encrypted message is built from four input values.
CEK: A 32-octet uniform random Content Encryption Key generated
fresh for each message. The CEK is input keying material for the
key schedule, not an AEAD key.
salt: A per-content value of 32 uniformly random octets. The salt
separates payload schedule outputs across messages even when the
CEK is reused. Section 4.9.1.1 and Section 4.9.3 give the rules
for generating and reusing it, and Section 6.7 analyzes the
consequences of reuse.
payload_info: The committed record of the object's configuration: an
ordered list of parameter set context elements covering key
derivation, AEAD operations, AAD construction, and nonce
construction, which the commitment (Section 4.6) binds. The
profile fixes the concrete element set and their encodings.
SEAL's payload_info is specified in Section 4.4.
Sullivan Expires 24 January 2027 [Page 19]
Internet-Draft raAE July 2026
G: An octet string of global associated data, the G input of
StartEnc in the raAE primitive (Section 3.2), binding whole-
message application context such as a name, version, or policy. G
defaults to the empty octet string and is never stored: a
decryptor supplies it from application context, and the commitment
binds it (Section 4.6).
The remaining symbols in Table 2 are derived values. The payload
schedule computes the per-message values once from the CEK and
payload_info, fixing them for the message's lifetime (Section 4.5).
The commitment additionally binds G (Section 4.6). Per-segment
values are defined where they are computed, in the sections Table 2
cites.
4.3. The KDF Combiner
The KDF combiner uses a length-prefixed encoding for all of its
inputs.
frame(x):
if len(x) <= 0xFFFE: return uint16(len(x)) || x ;; literal
else: return uint16(0xFFFF) || LH(x) ;; digest
encode(x1, ..., xn) = frame(x1) || ... || frame(xn)
frame is total over every field length. A field too large to emit
literally is instead prefixed by the reserved length 0xFFFF and an
Nh-octet digest LH(x).
LH(x), the over-large-field digest, runs the cipher suite's native
KDF primitive directly on x, invoking neither encode nor frame, and
returns Nh octets. The two-step and one-step classes are HPKE's: a
two-step KDF ([RFC9180]) uses its extract step, and a one-step KDF
([I-D.ietf-hpke-pq]) its XOF, with x carried under the label "raAE-
LP-v1":
two-step: LH(x) = Extract("raAE-LP-v1", x)
one-step: LH(x) = XOF("raAE-LP-v1" || x, Nh)
Running LH through the full KDF would re-enter frame on the over-
large x and recurse. The direct native call avoids that.
Sullivan Expires 24 January 2027 [Page 20]
Internet-Draft raAE July 2026
Section 5.4 requires the construction to justify the collision
resistance of the over-large-field digest. LH's is that of the
cipher suite's KDF primitive at the suite's Nh. In the two-step
class LH(x) is HMAC under the fixed public key "raAE-LP-v1", so a
collision in LH yields a collision in the underlying hash, by the
reduction Section 5.3 gives for the commit derivation. In the one-
step class LH(x) is the XOF itself over a domain-separated input, so
a collision in LH is a collision in the XOF at that output length.
The injectivity of encode, and LH's domain separation from the
literal encoding, are established in Section 5.4.
Key material is derived through a KDF with the interface
KDF(protocol_id, label, ikm, info, L)
* protocol_id: binds the output to a specific protocol version and
application context.
* label: a unique ASCII string identifying the derivation role.
* ikm: input keying material, provided as a single octet string or
an ordered list of octet strings.
* info: context information, provided as a single octet string or an
ordered list of octet strings.
* L: the requested output length in octets.
This KDF binds protocol_id, a label, and inputs of any size into a
derived value. frame reduces any over-large input to a fixed-size
digest before the KDF sees it. Any protocol instantiates the
combiner by supplying its own protocol_id, which domain-separates its
derivations from those of every other protocol.
The KDF is built from the cipher suite's hash in one of two forms.
The cipher suite's kdf_id selects which one.
4.3.1. Two-Step KDF
KDF(protocol_id, label, ikm, info, L):
extract_input = encode(protocol_id, label,
...ikm)
prk = Extract(salt=protocol_id,
ikm=extract_input)
expand_info = encode(protocol_id, label,
...info, uint16(L))
return Expand(prk, expand_info, L)
Sullivan Expires 24 January 2027 [Page 21]
Internet-Draft raAE July 2026
The notation ...x means each element of the sequence x, whether ikm
or info, is a separate argument to encode. A single octet string is
the one-element sequence containing it. An absent value is the empty
sequence and contributes no arguments. The empty sequence and the
one-element sequence whose element is the empty octet string are
distinct: the first adds nothing to the encode, the second adds one
zero-length field. A derivation with no info MUST use the empty
sequence, not the empty octet string, so that every implementation
encodes a given (protocol_id, label, ikm, info, L) tuple to the same
octets. The protocol_id appears in both the Extract salt and the
extract_input as a defensive measure, and binding in either position
suffices for domain separation.
The label appears in both Extract and Expand so that label binding is
preserved even if one phase is weak in isolation. Implementations
MAY amortize the Extract computation internally when deriving
multiple outputs from the same protocol_id, label, and ikm.
4.3.2. One-Step KDF
KDF(protocol_id, label, ikm, info, L):
M = encode(protocol_id, label,
encode(...ikm), encode(...info),
uint16(L))
return XOF(M, L)
The XOF-based form frames ikm and info each as a single element of
one length-prefixed encode and squeezes L octets of output.
protocol_id binding is via the message-side encode prefix only. In
both forms the encoded input ends with uint16(L): the two-step form
places it last in expand_info and the one-step form places it last in
M. Within a fixed kdf_id, distinct (protocol_id, label, ikm, info,
L) tuples yield distinct outputs, by the injectivity of encode
(established in Section 5.4). The two forms are not mutually
injective: a cipher suite's kdf_id selects one form for that suite,
and payload_info commits to the kdf_id (Section 4.4), so no
derivation ever compares outputs across forms. The two-step form
binds ikm in Extract and info in Expand. A multi-value ikm or info
is the encode of its elements, so any number of elements is admitted.
4.4. SEAL Encodings and Labels
4.4.1. Payload Info Construction
payload_info = [aead_id, segment_max_be, kdf_id, snap_id,
segment_commitment, nonce_mode, epoch_length_u8, salt]
Element encodings for SEAL:
Sullivan Expires 24 January 2027 [Page 22]
Internet-Draft raAE July 2026
* aead_id: 2 octets, uint16(id), the unsigned 16-bit AEAD
identifier.
* segment_max_be: 4 octets, uint32(segment_max).
* kdf_id: 2 octets, uint16(id), the unsigned 16-bit KDF identifier.
* snap_id: 2 octets, uint16(id), the snapshot authenticator
identifier.
* segment_commitment: 1 octet, 0x00 when the leaf does not bind the
segment's content and 0x01 when it does (Section 4.7.1). It is a
choice independent of snap_id, but not every (snap_id,
segment_commitment) pair is sound, so a profile selects a valid
one.
* nonce_mode: 1 octet, the nonce construction: 0x00 for random, 0x01
for derived. A profile MAY select any valid (nonce_mode, snap_id,
segment_commitment) tuple.
* epoch_length_u8: 1 octet, uint8(epoch_length). Implementations
MUST reject epoch_length >= 64 on both encode and decode.
* salt: 32 raw octets, the per-message salt.
The aad_label is a profile-level constant for SEAL (see
Section 4.4.2) and is not carried in payload_info. The KDF
(Section 4.3) applies frame to each element of the list.
4.4.2. Segment AAD
The segment AAD binds each segment to its context. What it must
carry depends on the nonce mode: a random nonce binds neither the
segment index nor the finality bit, so the AAD must, whereas a
derived nonce binds both (Section 4.5.3.2).
segment_aad(i, is_final, A_i):
if nonce_mode is "random":
if A_i is empty:
return encode(aad_label, uint64(i), uint8(is_final))
return encode(aad_label, uint64(i), uint8(is_final), A_i)
;; derived nonce mode
if A_i is empty:
return ""
return encode(aad_label, A_i)
Sullivan Expires 24 January 2027 [Page 23]
Internet-Draft raAE July 2026
+=============+======================+============================+
| | Random nonce mode | Derived nonce mode |
+=============+======================+============================+
| Index i and | bound in the AAD | bound in the nonce |
| is_final | | |
+-------------+----------------------+----------------------------+
| A_i, when | appended as a fourth | the sole variable element |
| present | encode element | |
+-------------+----------------------+----------------------------+
| A_i, when | omitted, not a zero- | segment_aad is empty, with |
| empty | length element | no associated-data pass |
+-------------+----------------------+----------------------------+
Table 3: Segment AAD by nonce mode
aad_label is the profile-chosen label from the parameter set, the
index i is a big-endian 8-octet integer, and is_final is 1 for the
last segment and 0 for all others. Because encode length-prefixes
every element, omitting an empty A_i stays injective, so the index
and finality bit remain authenticated. Any profile that defines a
random-mode segment_aad MUST preserve injectivity and MUST
authenticate the segment index and finality bit. aad_label is bound
transitively through protocol_id (Section 6.3), so segment_aad need
not repeat it for cross-profile separation.
More generally, the binding of the triple (i, is_final, A_i) MUST be
unambiguous under one segment key, so that no two distinct triples
present the same inputs to the AEAD. In random nonce mode
segment_aad alone MUST injectively bind the triple, because the nonce
is an independent random draw. In derived nonce mode i and is_final
are bound in the nonce and A_i, when present, in segment_aad, and the
(nonce, segment_aad) pair MUST jointly determine the triple
(Section 4.5.3.2).
A_i is the caller-supplied per-segment associated data of EncSeg and
DecSeg (Section 3.2), passed to the segment algorithms (Section 4.8).
It MAY be empty and MAY differ per segment. Unlike a global
associated data value G, which the commitment fixes at creation
(Section 4.6), A_i rides the per-segment AEAD associated data and is
rewritable: a rewrite can change a segment's A_i.
The salt is not included in the AAD because it is already bound into
segment_key via the payload schedule. Different messages produce
different segment keys even at the same index. Transposing two
segments within a message fails because the AAD at each position
encodes the expected index.
Sullivan Expires 24 January 2027 [Page 24]
Internet-Draft raAE July 2026
Because each segment binds its index and finality bit (through the
AAD or the nonce, per Table 3), these truncation and extension
attacks are caught by per-segment authentication alone, not by the
snapshot, and a reader checks them locally before decrypting any one
segment. Unauthorized truncation is detectable and Section 4.9.1.2
requires a decryptor to reject it: a highest-indexed present segment
carrying is_final = 0 signals missing data. Authorized truncation by
the CEK holder instead re-marks the new final segment and updates the
snapshot (Section 4.9.3). Appending after a segment marked is_final
= 1 either invalidates that segment on re-read or produces a
ciphertext that cannot verify.
The per-segment AEAD binds a segment's index and finality, not its
length, by the same means. A segment's ciphertext length is
authenticated implicitly by the AEAD tag over the ciphertext:
altering, truncating, or splicing a segment ciphertext is an AEAD
forgery, not a parsing ambiguity. Because segments may be shorter
than segment_max, the consuming format MUST convey each segment's
ciphertext length to the decoder. A wrong length yields an AEAD
rejection, never a silent accept.
4.4.3. Labels
+============================+===================+===============+
| Derivation role | Label variable | Value |
+============================+===================+===============+
| Commitment | commit_label | "commit" |
+----------------------------+-------------------+---------------+
| Payload key | payload_key_label | "payload_key" |
+----------------------------+-------------------+---------------+
| Snapshot authenticator key | snap_key_label | "snap_key" |
+----------------------------+-------------------+---------------+
| Nonce base | nonce_base_label | "nonce_base" |
+----------------------------+-------------------+---------------+
| Epoch key | epoch_key_label | "epoch_key" |
+----------------------------+-------------------+---------------+
Table 4: SEAL core KDF labels by role
SEAL fixes each label to the value above. The construction formulas
reference labels by variable name, so the labels are SEAL parameters
rather than constants of the raAE construction. Each value is an
ASCII string and is distinct from all others. These are the always-
present key-schedule labels. The configured snapshot authenticator
(Section 4.7.1), the plaintext-bound nonce construction (Appendix A),
and hedged randomness (Appendix B) each define their own labels.
Sullivan Expires 24 January 2027 [Page 25]
Internet-Draft raAE July 2026
4.5. Key Schedule and Nonce Generation
A message's entire key hierarchy grows from two random values. All
other keys are derived deterministically through the KDF of
Section 4.3.
4.5.1. Payload Schedule
Four values are derived from the CEK and per-content salt.
payload_info = [aead_id,
uint32(segment_max),
kdf_id,
snap_id,
segment_commitment,
nonce_mode,
uint8(epoch_length),
salt]
commitment = KDF(protocol_id, commit_label,
[CEK], [...payload_info, G], commitment_length)
payload_key = KDF(protocol_id, payload_key_label,
[CEK], payload_info, Nk)
snap_key = KDF(protocol_id, snap_key_label,
[CEK], payload_info, Nh)
;; For derived nonce mode only:
nonce_base = KDF(protocol_id, nonce_base_label,
[CEK], payload_info, Nn)
G is the global associated data of Section 4.2.4, always the last
element of the commitment info. It defaults to the empty octet
string, one zero-length element, so every commitment derivation
includes it. The other three derivations never take G.
4.5.2. Epoch Key Derivation
An AEAD with a 96-bit nonce (Nn = 12) has, at a 2^(-32) collision-
probability target, a random-nonce budget of about 2^32 encryptions
per key. For rewritable content, this limit can be reached through
repeated modifications to the same segments. Epoch keys partition
the nonce space: each epoch key covers at most 2^r segment positions
(initial writes plus rewrites), so the per-key invocation count is
bounded regardless of the content's total size.
When epoch_length = r is specified, the segment key for segment i is
an epoch key derived from the payload key:
Sullivan Expires 24 January 2027 [Page 26]
Internet-Draft raAE July 2026
segment_key(i):
epoch_index = i >> epoch_length
return KDF(protocol_id, epoch_key_label,
[payload_key],
[uint64(epoch_index)], Nk)
At epoch_length = 0 the shift is the identity, so each segment has
its own epoch key. At a large epoch_length every segment shares one
epoch key. Implementations MUST apply this derivation at every
epoch_length, including 0, and MUST NOT use payload_key directly as a
segment key. Epoch keys implement the parallel external rekeying
pattern of [RFC8645], adapted for random-access patterns.
4.5.3. Nonce Generation
Two nonce modes, random and derived, trade off AEAD flexibility,
storage cost, and trust in a cryptographically secure pseudorandom
number generator (CSPRNG). Derived mode needs misuse-resistant
authenticated encryption (MRAE) unless the profile's writer
discipline keeps every derived nonce unique. The chosen mode is part
of the parameter set and MUST be consistent across all segments of a
message.
Random: Derived:
nonce_base
Random(Nn) |
| XOR ((i<<1)|is_final)
v |
nonce(i) nonce(i)
Figure 3: The Two Nonce Modes
The two modes differ as follows:
Sullivan Expires 24 January 2027 [Page 27]
Internet-Draft raAE July 2026
+===============+===================+=============================+
| Property | Random nonce mode | Derived nonce mode |
+===============+===================+=============================+
| Per-segment | yes, in segment | no |
| nonce stored | metadata | |
+---------------+-------------------+-----------------------------+
| CSPRNG | yes | no |
| required | | |
+---------------+-------------------+-----------------------------+
| AEAD | any AEAD | MRAE, or any under a nonce- |
| requirement | | uniqueness discipline |
+---------------+-------------------+-----------------------------+
| Rewrite | fresh nonce each | nonce fixed by index, |
| | write | reused on rewrite |
+---------------+-------------------+-----------------------------+
| Per-segment | binds index and | empty when no A_i (index/ |
| AAD | finality | finality in nonce) |
+---------------+-------------------+-----------------------------+
| Minimum nonce | no added | at least 8 octets |
| size Nn | constraint | |
+---------------+-------------------+-----------------------------+
Table 5: Random and derived nonce modes
4.5.3.1. Random Nonce Mode
In random nonce mode (nonce_mode = "random"), each segment gets an
independent random nonce:
nonce(i) = Random(Nn)
Each segment's nonce is stored in per-segment metadata and MUST be
accessible to any reader that decrypts individual segments. This
mode requires a functioning CSPRNG. The nonce collision probability
across independently generated per-segment nonces follows the
standard birthday bound. See Section 5.8.
Two optional encryptor-side hedges harden this mode against different
threats. Hedged randomness (Appendix B) mixes in a long-term key and
defends against a weak or predictable CSPRNG. It does not defend
against CSPRNG state duplication, since identical CSPRNG output still
yields identical hedged output. Only the plaintext-bound
construction (Appendix A), which mixes in plaintext content, defends
against state duplication. Both are implementation-local, leave the
wire format unchanged, and are not separate nonce modes.
Sullivan Expires 24 January 2027 [Page 28]
Internet-Draft raAE July 2026
4.5.3.2. Derived Nonce Mode
Derived nonce mode is intended for profiles that do not store a per-
segment nonce. Nonces are derived deterministically from the payload
schedule. Derived nonce mode is defined only for AEADs whose nonce
size Nn is at least 8 octets. The segment index occupies the high
bits of an 8-octet counter and the finality bit occupies the low bit,
so the per-segment AAD need not carry either one. The XOR is applied
to the last 8 octets of nonce_base, with the remaining octets
unchanged:
nonce(i) = nonce_base[0:Nn-8]
|| (nonce_base[Nn-8:Nn]
XOR uint64((i << 1) | is_final))
The encoded value (i << 1) | is_final MUST fit in the low 8 octets of
nonce_base, so the segment index i MUST be less than 2^63. This caps
an object at 2^63 segments, far beyond any reachable object. See
Section 5.8.3 for the resulting size limit.
A derived nonce is fixed by the segment's index and finality bit, so
re-encrypting a segment at the same index and finality repeats its
nonce. The repetition is harmless when the plaintext and the
associated data are unchanged, because AEAD.Encrypt is a function of
its inputs: the second invocation reproduces the first one's
ciphertext and tells an adversary nothing it did not already hold.
With changed inputs it is not harmless. An MRAE AEAD tolerates it,
degrading to equality leakage rather than plaintext recovery, within
the per-segment rewrite limit given in Section 5.8. A non-MRAE AEAD
does not: it leaks plaintext. So with a non-MRAE AEAD, derived nonce
mode MUST be confined to a profile whose discipline never re-encrypts
a segment with a different plaintext or associated data.
Two such disciplines matter here. A write-once profile rewrites
nothing and performs no length change, so no segment is re-encrypted
at all. An append-only profile re-encrypts one segment per append,
the re-mark that supersedes the old terminal segment, and that re-
mark carries the segment's existing plaintext and changes only the
finality bit, which changes the nonce (Section 4.9.3.1).
Under either discipline the encryptor MUST draw a fresh salt for
every object, so the nonce base differs, and MUST NOT reuse any (i,
is_final) pair under that salt, including after a crash
(Section 6.2).
Sullivan Expires 24 January 2027 [Page 29]
Internet-Draft raAE July 2026
Because the nonce binds index and finality, an immutable, write-once
profile in derived nonce mode coincides with STREAM (Section 1.1) at
the nonce-and-AEAD layer: a key-derived counter-plus-final-bit nonce
over a per-segment AEAD with no associated data.
4.6. Commitment
The commitment is a commitment_length-octet value the payload
schedule derives from the CEK, payload_info, and the global
associated data G (Section 4.5).
A reader MUST verify the commitment before decrypting any segment,
deriving the expected value from the CEK, payload_info, and G and
comparing it octet-for-octet with the stored value, and MUST treat a
mismatch as an authentication failure for the object. A mismatch
indicates a wrong key, parameter set, or G, or a corrupted header.
This is the ra-CMT context commitment (Section 5.1.2). The reduction
outline and bound are in Section 5.6.
Because the commitment is fixed at creation, G is immutable: changing
it invalidates the stored commitment, while segment contents stay
rewritable through the snapshot authenticator (Section 4.7).
G defaults to the empty octet string, so an application that supplies
no context commits over an empty final element. A wrong G fails the
commitment check (Section 4.9.1.2) the same way a wrong CEK does
(Section 6.3).
4.7. Snapshot Authenticator
The snapshot authenticator binds the current segment set into the
public snapshot value. A SEAL profile selects one with snap_id.
4.7.1. Interface and Requirements
A snapshot authenticator defines a per-segment input, one value per
segment. It produces and checks a snapshot value over the (index,
input) set and the count with two operations:
snapshot(): Produce the snapshot value over the current (index,
input) set and the count n_seg.
verify(snapshot): Recompute the snapshot value over the current
(index, input) set and the count, and accept only if it equals the
stored snapshot under a constant-time comparison.
Sullivan Expires 24 January 2027 [Page 30]
Internet-Draft raAE July 2026
The per-segment input is the segment's metadata leaf, binding the
segment's index and presented nonce. A CEK holder can reproduce that
binding for altered content. Setting the segment commitment binds
the segment's ciphertext into the leaf, so changing the ciphertext
changes the snapshot value. The commitment is therefore needed where
the object is shared among CEK holders, and MAY be omitted where the
CEK has a single holder.
The segment commitment is necessary for that binding but not
sufficient. An ordering is linear when a holder of the snapshot key
can solve for a different segment set giving the same snapshot value,
and strict when finding one is a collision problem. Under a linear
ordering a snap_key holder can find such a set even when every leaf
commits to its content, so content binding against a CEK holder
requires the segment commitment, a strict ordering, and a snapshot
authenticated outside the construction (Section 5.3).
Every authenticator supports the whole-object target: verify the full
(index, input) set and the count. An authenticator MAY additionally
support a subset target: verify that a designated subset of (index,
input) pairs is included at their positions in the recorded set,
without the rest. The snapshot value stays a single target-
independent value of length Na, with no per-segment proof stored.
To change a stored object, an authenticator updates its snapshot
value. It MAY provide add(i, input), remove(i, input), and
set_length(n) to update the value in place from the changed segment
alone, or it MAY recompute the value over the changed (index, input)
set.
A snapshot update is not atomic against a second concurrent update,
so a consuming protocol MUST serialize concurrent updates to one
object. This document requires that a serialization exist and does
not say how to obtain it, which remains a storage-transaction concern
of the consuming protocol. Serialization is needed whether the
authenticator recomputes the value or updates it in place, and only
the failure differs. An authenticator that recomputes over the full
set binds whatever set it read, which is neither the state before the
interleaved update nor the state after. One that updates in place
loses an update, when two read-modify-write cycles on the stored
value interleave and the later write never observed the earlier
delta. This is the concurrent case, distinct from the crash case of
Section 6.8, where one update is interrupted rather than two
interleaved.
The scheme's RewriteSeg and SnapVerify (Section 3.3) are realized
from these operations. SnapVerify(S, segments, snapshot) checks a
stored snapshot:
Sullivan Expires 24 January 2027 [Page 31]
Internet-Draft raAE July 2026
1. Let n_seg be the number of present segments.
2. Verify that the present segments occupy exactly the indices
0..n_seg-1, each once.
3. Call verify(snapshot) over the present (index, input) set and the
count n_seg.
Finality is not checked in SnapVerify: is_final is authenticated by
the segment AEAD at decryption, and Section 4.9.1.2 rejects a present
set whose highest-indexed segment does not carry is_final = 1.
A duplicate or missing index is caught only by step 2, because an
authenticator's verify MAY accept a malformed index multiset. An
implementation MUST NOT skip the index-set check.
A conforming authenticator's snapshot() MUST bind both the (index,
input) set and the count n_seg, so a verifier detects any added,
dropped, reordered, re-marked, or otherwise altered segment, and the
snapshot value MUST be unforgeable by a party without the
authenticator's key, under that authenticator's security assumption.
The snapshot value is a fixed-length value of Na octets, with Na = 0
meaning no value.
A segment's finality is a function of its index and the count:
is_final(i) = 1 if i = n_seg - 1, else 0
It is carried by that segment's input. Because finality follows the
count, changing n_seg re-marks the boundary segment on its own, so
rewrite, extend, and truncate are one update over the segments whose
input changes:
1. Under the current count, remove each changed segment's current
input (remove(i, input)). The current n_seg fixes which segment
is terminal, so each removal carries that segment's current
finality.
2. Set the length to the new count (set_length(n)).
3. Under the new count, re-encrypt each surviving changed segment
under its new finality and add its new input (add(i, input)).
New segments are added here. Discarded segments are not added
back.
4. Call snapshot(), which recomputes the snapshot value over the new
count and segment set.
Sullivan Expires 24 January 2027 [Page 32]
Internet-Draft raAE July 2026
Each operation is an instance of that shape over different segments:
+===========+===========+=============+==============+==============+
| Operation | Count | Inputs | Inputs | Terminal re- |
| | | removed | added | mark |
+===========+===========+=============+==============+==============+
| Rewrite | unchanged | the changed | the changed | none |
| | | segment | segment | |
+-----------+-----------+-------------+--------------+--------------+
| Extend by | n_seg to | old | old | old final, 1 |
| k | n_seg+k | terminal | terminal | to 0 (none |
| | | | and k new | if empty) |
+-----------+-----------+-------------+--------------+--------------+
| Truncate | n_seg to | the tail | new | new final, 0 |
| to m | m | and old | terminal | to 1 (none |
| | | terminal | | if m=0) |
+-----------+-----------+-------------+--------------+--------------+
Table 6: Rewrite, extend, and truncate
This sequence uses remove, the mutable-authenticator path. One that
omits remove instead resets and re-adds the surviving segments to
rebuild the value.
4.7.2. None
This selection configures no snapshot authenticator: no snapshot
value is produced (Na = 0), snapshot() and verify() are no-ops, and a
mutable object rests on per-segment authentication alone. Such a
profile MUST NOT produce the empty object (n_seg = 0). With no
snapshot value to bind the zero count, a truncation that removed
every segment would be indistinguishable from a legitimate empty
object, so a valid object always has at least one segment
(Section 6.6).
4.8. Segment Algorithms
The construction defines the per-segment subroutines EncryptSegment,
DecryptSegment, and RewriteSegment. In this random-access
construction, is_final is an explicit input to each subroutine.
Full-message encryption derives it from the total segment count
n_seg, while random-access decryption and rewrite callers supply it
directly. The full-message procedures (Section 4.9.1.1,
Section 4.9.1.2, Section 4.9.3) and the rewrite procedure
(Section 4.9.2.1) compose them.
Sullivan Expires 24 January 2027 [Page 33]
Internet-Draft raAE July 2026
Each subroutine is the cryptographic core of one operation of the
base or extended raAE interface (Section 3.2, Section 3.3), refining
Table 1 to the signature level. The primitive's position becomes the
explicit arguments i and is_final, and its state is the payload
schedule, which the segment_key(i), nonce(i), and segment_aad calls
below read from context rather than take as an argument. A segment's
plaintext is P_i, and its stored record is stored_i =
(nonce_metadata_i, ct_i, tag_i). leaf(i) is the segment's leaf
(Section 4.7.1), and new_leaf(i) the leaf of a rewritten segment.
+================+================+==========================+
| Subroutine | Realizes | Composed by |
+================+================+==========================+
| EncryptSegment | the encryption | Section 4.9.1.1 and |
| | core of EncSeg | Section 4.9.3, which add |
| | | the snapshot bookkeeping |
+----------------+----------------+--------------------------+
| DecryptSegment | DecSeg | Section 4.9.1.2 |
+----------------+----------------+--------------------------+
| RewriteSegment | the encryption | Section 4.9.2.1 and |
| | core of | Section 4.9.3, which add |
| | RewriteSeg | the snapshot bookkeeping |
+----------------+----------------+--------------------------+
Table 7: Segment subroutines and the primitive operations
they realize
These subroutines emit ct_i and tag_i as separate fields, with ct_i
of length len(P_i), at most segment_max. DecryptSegment splits
stored_i using the ciphertext length the consuming format conveys
(Section 4.4.2).
In every subroutine, i is the segment index (a non-negative integer),
is_final is 1 for the last segment and 0 for all others, A_i is the
caller-supplied associated data for segment i, and P_i is the segment
plaintext. The subroutines call segment_key(i) and nonce(i) as
defined in Section 4.5, with nonce(i) following the chosen
nonce_mode, and segment_aad(i, is_final, A_i) as defined in
Section 4.4.2.
nonce_metadata_i denotes the stored nonce metadata for segment i:
nonce(i) when nonce_mode is "random", and empty when it is "derived".
In random mode DecryptSegment reads its nonce from nonce_metadata_i,
and in derived mode it recomputes nonce(i).
4.8.1. EncryptSegment
Sullivan Expires 24 January 2027 [Page 34]
Internet-Draft raAE July 2026
EncryptSegment(i, is_final, A_i, P_i):
key = segment_key(i)
nonce = nonce(i)
aad = segment_aad(i, is_final, A_i)
C_i = AEAD.Encrypt(key, nonce, aad, P_i)
split C_i into ct_i and tag_i
return (nonce_metadata_i, ct_i, tag_i)
AEAD.Encrypt and AEAD.Decrypt are the interfaces of [RFC5116]
Sections 2.1 and 2.2. For an AEAD whose ciphertext places the tag in
its final Nt octets, ct_i is the first len(P_i) octets and C_i =
ct_i || tag_i.
The caller supplies the segment's leaf to the snapshot authenticator
(Section 4.7.1). EncryptSegment itself does no snapshot work.
4.8.2. DecryptSegment
DecryptSegment returns the segment plaintext, or a decryption error:
DecryptSegment(i, is_final, A_i, stored_i):
nonce_metadata_i, ct_i, tag_i = stored_i
key = segment_key(i)
nonce = nonce_metadata_i or nonce(i)
aad = segment_aad(i, is_final, A_i)
C_i = ct_i || tag_i
P_i = AEAD.Decrypt(key, nonce, aad, C_i)
(on verification failure, return a decryption error)
return P_i
4.8.3. RewriteSegment
RewriteSegment assumes the caller has already verified the commitment
or otherwise confirmed it holds the correct CEK. A rewrite under the
wrong key produces unreadable ciphertext and corrupts the snapshot
authenticator's state. The error is detectable only on a subsequent
read.
RewriteSegment(i, is_final, A_i, new_P_i) is defined as
EncryptSegment(i, is_final, A_i, new_P_i): a rewrite is a fresh
encryption of the new plaintext new_P_i. The output record
new_stored_i replaces the stored record for segment i.
RewriteSegment produces the replacement record new_stored_i only.
The caller updates the snapshot authenticator with the old and new
leaves, calling remove(i, leaf(i)) then add(i, new_leaf(i))
(Section 4.9.2.1).
Sullivan Expires 24 January 2027 [Page 35]
Internet-Draft raAE July 2026
4.9. Top-level Algorithms
The top-level algorithms compose the segment subroutines
(Section 4.8) over complete messages. They call the snapshot-
authenticator operations of Section 4.7 (snapshot() and verify() in
every profile, and add(i, input), remove(i, input), and set_length(n)
in a mutable or append-only profile) without referring to how any is
computed. The profile's selected authenticator realizes those
operations (Section 4.7.1).
These procedures do not expose per-segment associated data, so they
pass an empty A_i to each subroutine. An application that needs per-
segment A_i calls the subroutines directly.
4.9.1. Read-Only Operations
Encryption and decryption are available in every SEAL profile. They
write a message once and read individual segments. Neither rewrites
a stored segment. When the profile configures no snapshot
authenticator, the snapshot steps below are omitted, and a reader
relies on per-segment authentication alone.
4.9.1.1. Encryption
Encryption runs in two phases. The first derives the per-message
schedule and writes the commitment. The second encrypts each segment
independently, feeds its input to the snapshot authenticator, and
then calls snapshot() to bind the whole (index, input) set and the
count. Because the segments are independent, the second phase may
encrypt them in any order or in parallel. The snapshot calls mutate
one value under a single writer, so encryption is never the
concurrent case that Section 4.7.1 requires a protocol to order.
CEK, salt
|
v
payload schedule
|
+----+----------+----------+
v v v
commitment payload_key snap_key
|
v
EncryptSegment(i, P_i) using payload_key (one per segment)
|
v
ct_i, tag_i
Sullivan Expires 24 January 2027 [Page 36]
Internet-Draft raAE July 2026
Figure 4: Key Schedule and Segment Encryption
An application that writes a new message MUST generate a fresh,
uniformly random salt. Given a CEK, that salt, a parameter set, G,
and a plaintext P split into segments P_0 through P_{n_seg-1}:
derive commitment, payload_key, snap_key from the payload schedule
(and nonce_base, in derived mode)
store commitment
for i in 0 .. n_seg-1:
is_final = 1 if i = n_seg-1, else 0
stored_i = EncryptSegment(i, is_final, empty, P_i)
store stored_i
if snapshot authentication is enabled:
snapshot = snapshot()
store snapshot
A zero-length plaintext produces a single segment of length 0 with
is_final = 1.
4.9.1.2. Decryption of Segment i
Opening a segment is a local operation: check the commitment to
reject a wrong key early, then AEAD-decrypt the one segment. This
yields per-segment authenticity over the ciphertext core, the segment
index, and the finality bit. Under a snapshot authenticator, whole-
set authenticity is a separate and stronger check that requires the
full present segment set. It detects any altered segment set,
including same-index rollback to an older sibling (Section 4.7.1). A
reader holding only one segment gets per-segment authenticity but
MUST NOT claim snapshot integrity until it runs SnapVerify
(Section 4.7). A profile with no snapshot authenticator defines no
snapshot, so per-segment authenticity is the whole integrity
guarantee.
Given the CEK, salt, parameter set, G, segment index i, finality
status, and the stored ciphertext segment for index i:
derive commitment from CEK, salt, and G
derive payload_key, snap_key from CEK and salt
if stored commitment != derived commitment: return error
P_i = DecryptSegment(i, is_final, empty, stored_i)
(return its error on failure)
if snapshot authentication is enabled and the full set is available:
if SnapVerify(snapshot) fails: return error (do not deliver P_i)
return P_i
Sullivan Expires 24 January 2027 [Page 37]
Internet-Draft raAE July 2026
Here SnapVerify is the three-argument SnapVerify(S, segments,
snapshot) of Section 3.3, with S the derived schedule and segments
the present set, each carrying the authenticator's per-segment input
(Section 4.7.1).
Whole-object SnapVerify needs the leaf of every segment
(Section 4.7.1), so a reader that streams segments in order cannot
gate their release on whole-object snapshot integrity without first
buffering those leaves or using a layout that carries them in a
header. Where the authenticator supports a subset target, a reader
can instead verify a subset it holds against the snapshot without the
leaves of the other segments. Delivering on per-segment authenticity
alone forgoes snapshot freshness for that segment (Section 6.6). A
streaming reader likewise cannot rule out truncation before the end
of the stream: plaintext delivered before the highest-indexed segment
verifies under is_final = 1, and before SnapVerify passes where a
snapshot authenticator is configured, MUST be treated as unverified
with respect to completeness.
A decryptor MUST reject a truncated object: one whose highest-indexed
present segment carries is_final = 0 is incomplete, and the decryptor
MUST fail rather than return the partial plaintext.
The check works because the application supplies the segment count
n_seg, from which the decryptor derives the finality each segment
must carry (Section 4.8). It then verifies the final segment under
is_final = 1. A truncated object, whose last surviving segment was
written as non-final, fails that verification. The finality bit is
bound per segment and unforgeable without the CEK, through the nonce
or the segment AAD per the nonce mode (Section 4.4.2).
Under a write-once profile no segment ever carried an earlier
terminal form, so an adversary cannot present a valid is_final = 1
segment at a truncated boundary: it cannot re-mark a non-final
segment, and this check detects every dropped trailing segment. A
profile that changes length re-marks a terminal segment
(Section 4.9.3), so one segment may have carried is_final = 1 under
an earlier nonce. An adversary can replay that retained terminal
ciphertext at the shorter count, which this check alone accepts.
SnapVerify over the complete segment set then rejects the replay.
The mutable profile requires a snapshot authenticator, so SnapVerify
always runs there (Section 6.6).
Authorized truncation by the CEK holder re-encrypts the new final
segment under is_final = 1 with a fresh nonce (Section 4.9.3), so a
legitimately truncated object still ends with is_final = 1 and
verifies. This check needs no snapshot authenticator.
Sullivan Expires 24 January 2027 [Page 38]
Internet-Draft raAE July 2026
Truncation to an empty object is covered too. Without a snapshot
authenticator a valid object has at least one segment, so a decryptor
MUST reject one with none. With a snapshot authenticator the
snapshot binds the count, making the empty object (n_seg = 0) a
distinct authenticated state (Section 6.6).
On a commitment mismatch the implementation MUST NOT proceed with
decryption, SHOULD zeroize derived key material, and MUST return an
error. The SnapVerify gate in the procedure above runs whenever a
snapshot authenticator is configured and the full present set is
available, for example a full read or an integrity audit.
Decryption can fail in up to three distinguishable ways:
+===========================+===============================+
| Failure | Cause |
+===========================+===============================+
| Commitment mismatch | wrong key or parameters |
+---------------------------+-------------------------------+
| AEAD verification failure | corrupted or tampered segment |
+---------------------------+-------------------------------+
| Snapshot mismatch | segment set or count modified |
| | (snapshot authenticator only) |
+---------------------------+-------------------------------+
Table 8: Decryption failure conditions
Implementations SHOULD report these as distinct error conditions for
local diagnosis, but SHOULD return a single opaque error to untrusted
callers to avoid leaking an oracle (for example, distinguishing
"wrong key" from "tampered segment" over a network).
4.9.2. Rewritable Operations
Rewriting is available only in a mutable profile, which requires a
snapshot authenticator. Length change is a separate operation
(Section 4.9.3), available in both directions in a mutable profile
and in the append direction alone in an append-only profile
(Section 4.9.3.1). The procedure below is written against the in-
place update path (add, remove, set_length; Section 4.7.1). Under an
authenticator that recomputes instead, the remove, add, and
set_length calls in this procedure collapse into a single snapshot()
over the updated (index, input) set, and the operations are otherwise
identical. A caller that cannot recover a re-encrypted segment's
plaintext cannot perform it.
Sullivan Expires 24 January 2027 [Page 39]
Internet-Draft raAE July 2026
4.9.2.1. Rewriting Segment i
A rewrite touches exactly one segment's ciphertext, and no other
segment is read or decrypted (Section 4.7). In-place update trusts,
rather than re-verifies, the stored snapshot state, whereas an
authenticator that recomputes rebuilds the value from the stored per-
segment leaves and trusts none.
The procedure is:
read leaf(i)
new_stored_i = RewriteSegment(i, is_final, empty, new_P_i)
remove(i, leaf(i))
add(i, new_leaf(i))
snapshot = snapshot()
store new_stored_i, snapshot
In derived nonce mode the nonce for segment i is deterministic, so a
rewrite reuses the same nonce under a key whose derivation context is
identical. This is safe because a mutable profile that selects
derived nonce mode requires an MRAE AEAD, which tolerates the reuse
(Section 4.5.3.2) within the per-segment rewrite limit
(Section 5.8.4).
The stored snapshot cannot be validated during an in-place rewrite:
SnapVerify recomputes the authenticator from every segment's tag and
so requires reading all tags (Section 4.7). Because the rewrite
trusts the stored snapshot state, the writer MUST take it from
locally trusted storage or run SnapVerify first. Otherwise a storage
adversary can roll the stored snapshot back and have the writer
rebuild on it, laundering the rollback into a snapshot that verifies
going forward (Section 6.6). Full validation is a separate operation
performed with access to all tags, for example on a full read or an
integrity audit.
4.9.3. Changing Message Length
An application that holds the CEK MAY change a stored message's
length in either direction: appending new segments, or truncating to
a prefix. The two directions share one shape. Exactly one retained
segment, the old or the new terminal one, is re-encrypted to move the
finality bit, with none re-encrypted when appending from the empty
object or truncating to it. Every other changed segment is removed
or added by tag without re-encryption, and the snapshot is updated
over the new count (Table 6). Both reuse the existing salt: an
application extending an existing message MUST reuse the salt from
the object it extends. A fresh salt would change every payload
schedule output and force re-encryption of the retained segments.
Sullivan Expires 24 January 2027 [Page 40]
Internet-Draft raAE July 2026
def Append(P_new):
k = len(P_new)
t = n_seg - 1
if n_seg > 0:
P_t = DecryptSegment(t, 1, empty, stored_t)
new_stored_t = RewriteSegment(t, 0, empty, P_t)
remove(t, leaf(t))
set_length(n_seg + k)
if n_seg > 0:
store new_stored_t
add(t, new_leaf(t))
for j in n_seg .. n_seg+k-1:
is_final = 1 if j == n_seg+k-1 else 0
stored_j = EncryptSegment(j, is_final, empty, P_new[j-n_seg])
store stored_j
add(j, leaf(j))
store snapshot()
Figure 5: Append
def Truncate(m):
t = m - 1
if m > 0:
P_t = DecryptSegment(t, 0, empty, stored_t)
new_stored_t = RewriteSegment(t, 1, empty, P_t)
remove(t, leaf(t))
for j in m .. n_seg-1:
remove(j, leaf(j))
set_length(m)
if m > 0:
store new_stored_t
add(t, new_leaf(t))
discard segments m .. n_seg-1
store snapshot()
Figure 6: Truncate
Sullivan Expires 24 January 2027 [Page 41]
Internet-Draft raAE July 2026
With in-place update each add and remove takes the segment's stored
tag, which already encodes that segment's is_final bit, so an
implementation never tracks finality separately.
A botched in-place update corrupts the snapshot authenticator's state
silently, surfacing only at a later SnapVerify. An implementation
MAY run SnapVerify over the new segment set immediately after a
length change to catch this. A botched update yields a value that
the from-scratch recomputation will not match, turning a silent
corruption into a caught error. An authenticator that recomputes
rebuilds the snapshot from the stored per-segment leaves on every
update, so it has no analogous silent-corruption failure mode.
Both operations count against the same per-CEK AEAD usage budget as
initial encryption (Section 5.8). Each re-encrypts one terminal
segment, except a truncate to the empty object (m = 0), which re-
encrypts none. In derived nonce mode the nonce for the re-marked
segment is fixed by its index and finality bit. Re-marking flips the
finality bit, so this nonce differs from the segment's previous one,
but repeated length changes in both directions can make the same
(index, finality bit) pair recur, and only an MRAE AEAD tolerates the
resulting nonce reuse (Section 4.5). An append-only profile admits
no truncation, so no pair recurs and any AEAD is safe
(Section 4.9.3.1).
Applications that do not need length changes MAY forbid them. The
salt-reuse requirement applies only when a length change is
performed. An application that re-encrypts the entire content can
generate a fresh salt and is not bound by the constraints in this
section.
4.9.3.1. Appending Under an Append-Only Profile
An append-only profile admits the append direction of this section
(Figure 5) and forbids truncation.
An encryptor under an append-only profile MUST NOT rewrite a segment
with a different plaintext or associated data, and MUST NOT reduce
n_seg.
The first rule is what lets the profile pair a derived nonce with any
AEAD, including a non-MRAE one. At a fixed index and finality the
key and the nonce are already fixed by the payload schedule, so the
plaintext and the associated data are the only inputs that can
differ, and the rule forbids exactly those. No nonce ever carries
two different plaintexts, which is the condition Section 4.5.3.2 sets
for a non-MRAE AEAD.
Sullivan Expires 24 January 2027 [Page 42]
Internet-Draft raAE July 2026
The re-mark satisfies the first rule without an exception: it changes
only the finality bit, not the plaintext or associated data
(Section 4.5.3.2).
The second rule makes the profile append-only rather than non-
rewriting. It forbids the truncate direction, so a stored object
only grows, and an index the object has spent is never freed for a
later append to fill with different content.
An append re-encrypts one retained segment, so each segment is
encrypted at most twice, and an epoch key covers at most
2^(epoch_length + 1) segment encryptions (Section 5.8.3).
Because the re-mark preserves the segment's length, an append to an
object whose terminal segment is short leaves a short interior
segment, which a layout with uniformly sized interior segments cannot
represent. A consuming protocol using this profile supplies a
serialization that conveys each segment's ciphertext length
(Section 4.4.2).
5. Security Analysis
The formal proofs are not reproduced here. The ra-ROR and ra-CMT
notions are taken unchanged from [FLRR25], which proved them for a
construction that instantiates the same component contracts other
than the snapshot authenticator (Section 5.4). SEAL's analysis of
those notions follows the same structure. Snapshot integrity is the
one notion this document defines. The configured authenticator's
definition argues its snapshot-integrity bound (Section 5.7), and
integrating that argument into a combined proof is deferred to the
companion proof [SEALPROOFS] (in preparation). The limits of
Section 5.8 are operational deployment bounds, not proof obligations.
5.1. Security Notions
raAE has two security tiers. The base tier comprises the notions
inherited from [FLRR25]: ra-ROR, ra-CMT, and its position-choosing
variant ra-CMT-p, summarized here. The extension tier adds one
notion, snapshot integrity, defined by this document. A construction
MAY provide the base tier only. Advantage definitions and proofs for
ra-ROR, ra-CMT, and ra-CMT-p are in [FLRR25].
Sullivan Expires 24 January 2027 [Page 43]
Internet-Draft raAE July 2026
5.1.1. ra-ROR
ra-ROR (random-access real-or-random) is the joint confidentiality
and authenticity notion for raAE in a multi-user, nonce-respecting
setting. An adversary with adaptive encryption and decryption access
across many key instances, sessions, and arbitrary positions cannot
distinguish real ciphertexts and headers from random, and cannot make
an out-of-context ciphertext segment decrypt. The game is defined in
[FLRR25].
The encryption core of the extension operations is ordinary EncSeg.
A RewriteSeg produces its replacement ciphertext as a repeated EncSeg
at the same position, and appending is EncSeg at fresh positions plus
one RewriteSeg of the old final segment.
In random nonce mode these draw fresh nonces and stay within the
nonce-respecting ra-ROR game. In derived nonce mode the analysis
turns on whether the profile permits a rewrite in place.
Where it does, a rewrite repeats the position's derived nonce with
new content. That is a deliberate nonce-repeating query, and the
underlying MRAE AEAD handles it under the equality-leakage relaxation
documented for derived nonce mode (Appendix C.5), not the unmodified
nonce-respecting game.
Where it does not, no query repeats a position with new content. The
write-once profile re-encrypts nothing beyond an identical replay of
an interrupted write. The append-only profile re-encrypts only at
the append's RewriteSeg, which flips the finality bit and so writes a
position the initial write did not use, and at an identical replay.
Both profiles bind every position to one set of inputs
(Section 4.9.3.1), so their repeated queries carry the inputs of the
query they repeat. A reduction answers such a query from the first
one's output, and both profiles stay in the nonce-respecting game and
need no MRAE AEAD (Section 4.5.3.2).
These operations also maintain a snapshot value. That value is
outside the [FLRR25] syntax. It is governed not by ra-ROR but by the
snapshot integrity notion of Section 5.1.4.
5.1.2. ra-CMT
ra-CMT (random-access context commitment) is the segment-level
commitment notion: no single ciphertext segment decrypts successfully
under two different decryption contexts (key, nonce, global
associated data, and per-segment associated data). In this position-
respecting notion the two contexts share a position. Because the
scheme is random access, committing one segment's context commits the
Sullivan Expires 24 January 2027 [Page 44]
Internet-Draft raAE July 2026
full ciphertext's context. ra-CMT is defined in [FLRR25].
ra-CMT is distinct from the per-AEAD notions CMT-1 and CMT-4 of
[RFC9771]: a scheme can achieve ra-CMT through an external commitment
mechanism even when its underlying AEAD commits to nothing. The
Invisible Salamanders attacks ([DGRW18]) show what a non-committing
AEAD permits, and [ADG22] frames it as a key-commitment failure with
fixes.
5.1.3. ra-CMT-p
ra-CMT-p is the position-choosing variant of ra-CMT: the two
decryption contexts need not share a position, so the adversary binds
a segment's content to a position of its choice.
Under honest keys a construction inherits ra-CMT-p from the
underlying AEAD's commitment. Against a holder of the CEK it does
not follow from per-segment AEAD alone: a snapshot authenticator
restores it by committing each segment's content and ordering
segments strictly, with the snapshot value authenticated outside the
construction (Section 5.3, Appendix C.4).
5.1.4. Snapshot Authentication
Per-segment AEAD authenticates a segment at its position, but it does
not authenticate the current object snapshot: which set of segments
belongs to the current stored object state. An adversary with write
access to stored segments can silently substitute an old valid same-
index segment or delete a segment, and per-segment AEAD verification
at each presented segment still passes. (A segment moved to a
different index fails its AEAD check through the index binding of
Section 4.4.2.)
Snapshot authentication covers that set. The resulting notion is
snapshot integrity. Its interface and requirements are in
Section 4.7.1, and its general bound is in Section 5.7. A combined
formal treatment with the ra-ROR framework is in preparation
([SEALPROOFS]).
A construction MAY omit snapshot integrity, providing only the per-
segment guarantees and ra-CMT. SEAL provides it under a snapshot
authenticator and omits it otherwise.
5.2. Relations to Other Notions
AEAD ([RFC5116]): raAE generalizes AEAD to multi-segment content
with arbitrary-order encryption. A single-segment raAE is an
AEAD.
Sullivan Expires 24 January 2027 [Page 45]
Internet-Draft raAE July 2026
nOAE2 ([Tink]): nOAE2 is the nonce-based online authenticated
encryption notion for schemes that encrypt in order and support
random-access decryption. Hoang and Shen introduced nOAE2 in
their analysis of Tink Streaming AEAD ([Tink]), where they also
show STREAM satisfies it. ra-ROR is strictly stronger: every ra-
ROR-secure scheme is nOAE2-secure when used in order, but
nOAE2-secure schemes can fail when encryption order is arbitrary
([FLRR25]).
Commitment ([RFC9771]): CMT-1 and CMT-4 ([RFC9771]) are per-AEAD
commitment notions. ra-CMT is the corresponding notion for
segmented random-access schemes. See Section 5.1.2.
Rewrite safety and extension safety are corollaries of the notions
above, not additional notions. Replacing a segment in place is an
EncSeg at an already-used position together with a snapshot update.
In random nonce mode it adds no adversary capability beyond ra-ROR
(Section 5.1.1) and snapshot integrity (Section 5.1.4). In derived
nonce mode the rewrite repeats the segment nonce, so it MUST use an
MRAE AEAD (Section 4.5.3.2), and it adds the equality-leakage
relaxation that derived nonce mode already documents (Appendix C.5).
Each rewrite also counts against the per-key AEAD usage budget
(Section 5.8).
Appending and truncating likewise add no algorithm (Section 4.9.3)
and no notion. An adversary's ability to add, drop, reorder, or re-
mark a terminal segment is exactly the snapshot integrity adversary
of Section 5.1.4, and detection follows from the index and finality
binding of Section 4.4.2 together with snapshot verification.
Rollback of the whole object to an earlier honest (state, snapshot)
after truncation is the freshness case that snapshot integrity
excludes (Section 6.6). The application supplies it.
5.3. Adversary Model and Assumptions
The adversary model is that of the ra-ROR game (Section 5.1.1,
[FLRR25]). In the construction's terms: the adversary can observe
all ciphertexts and content metadata, tamper with individual segments
or their ordering, replace the snapshot value, and attempt decryption
with chosen keys. The adversary does not know the CEK, any derived
key, or the per-content salt. The confidentiality and integrity
bounds assume the CEK is secret and uniform, the single-owner regime:
an application MUST supply a CEK that is either generated uniformly
at random or derived so that it is computationally indistinguishable
from uniform, such as the output of a KDF keyed by a secret. When
multiple messages share a CEK, the per-content salt separates payload
schedule outputs across messages under the PRF assumption. The
multi-message advantage grows linearly in the number of messages.
Sullivan Expires 24 January 2027 [Page 46]
Internet-Draft raAE July 2026
The commitment notions ra-CMT and ra-CMT-p (Section 5.6) also cover a
shared-CEK regime, where the adversary holds the CEK. A CEK holder
defeats the per-segment AEAD: it holds the key, so it can decrypt and
forge tags at will, and confidentiality and per-segment integrity do
not bind it.
One binding can still hold against a CEK holder, under three
conditions that each close a distinct gap. The snapshot value MUST
be authenticated outside the construction, because a CEK holder
derives snap_key and can otherwise forge any snapshot value directly.
The segment commitment MUST be set: without it a segment's leaf binds
its ciphertext only through the AEAD tag, which a CEK holder forges
for altered content. Setting it adds a collision-resistant
commitment to the ciphertext that holding the CEK does not defeat.
The authenticator's ordering MUST be strict: under a linear ordering
a snap_key holder can present a different segment set with the same
snapshot value even when every leaf commits to its content, and only
a strict ordering makes finding one a collision problem. A snapshot
meeting all three binds each segment's content to its position
against a CEK holder.
The persistent state of an raAE object, and the party that owns each
item, is:
CEK: Secret key-management state. It is the root secret for the
object.
salt: Stored object metadata. It separates objects under one CEK.
parameter set: Profile or object metadata. It fixes suite, maximum
segment size, and epoch policy.
commitment: Stored object metadata. It rejects wrong key or
parameter context before decryption.
segment metadata: Storage-format state. It holds AEAD tags, the
per-segment nonces in random nonce mode, and, when the segment
commitment is set, each segment's content commitment
(Section 4.7.1).
snapshot: Stored object metadata. It authenticates the segment set
and segment count.
external freshness state: Consuming-protocol state. It detects
whole-object rollback.
Sullivan Expires 24 January 2027 [Page 47]
Internet-Draft raAE July 2026
The security argument relies on three distinct assumptions about the
KDF. For derivations whose input keying material is secret and
uniformly random, including payload_key, snap_key, nonce_base,
epoch_key, and the snapshot authenticator's keyed derivations, the
KDF is assumed to be a multi-user PRF with domain separation by
protocol_id and label.
For commitment, the function
Commit(CEK, payload_info, G, L) =
KDF(protocol_id, "commit", [CEK], [...payload_info, G], L)
is additionally assumed to be collision resistant over adversarially
chosen tuples (CEK, payload_info, G), with G empty by default. PRF
security alone is not sufficient for this property, because the
commitment adversary may choose the CEK and context values.
For the plaintext-bound nonce construction, the plaintext digest step
targets collision resistance, not PRF security.
5.4. raAE Construction Requirements
The bounds below rely on properties of the construction's components,
namely the KDF framing and the snapshot authenticator. The
requirements those components must satisfy are obligations on the
raAE construction, not part of the raAE primitive of Section 3.
A profile selects the concrete framing for every derivation surface:
the KDF input assembly (Section 4.5), the payload_info construction
(Section 4.2), the segment AAD (Section 4.4.2), and the snapshot
authenticator's keyed inputs (Section 4.7). The requirements below
are the contract that any such framing MUST meet. SEAL is one such
framing (Section 4.3, Section 4.4).
Pseudorandomness: KDF outputs MUST be computationally
indistinguishable from random to an adversary that does not know
the ikm, assuming the underlying hash or PRF is a secure
pseudorandom function.
Injectivity: A construction encodes the tuple (protocol_id, label,
ikm, info, L) into the underlying primitive input, and that
encoding MUST be injective: distinct tuples map to distinct
primitive inputs. This is a requirement on the encoding, not on
the KDF, which need not itself be injective. ikm and info MUST be
unambiguously separable in the encoded input, whatever the number
of elements each contains. The two-step form binds ikm and info
in its separate Extract and Expand inputs. The one-step form
frames ikm and info each as a single encode element. Where the
Sullivan Expires 24 January 2027 [Page 48]
Internet-Draft raAE July 2026
encoding admits inputs too large to encode literally, it MAY
substitute a fixed-length digest of the over-large field. The
encoding is then injective only up to the collision resistance of
that digest, which the construction MUST justify and domain-
separate from the literal encoding (Section 4.3).
Label uniqueness: Each derivation role, including commitment,
payload key, snapshot authenticator key, nonce base, epoch key,
segment nonce, hedged key, and the authenticator's internal roles,
MUST use a distinct label string within each protocol version.
Protocol binding: The protocol_id MUST appear as a distinct
component of the KDF primitive input. Different application
protocols using the same AEAD and KDF MUST use different
protocol_id values to ensure that derived values from one protocol
cannot be confused with those from another.
Output length commitment: The requested output length L MUST be part
of the KDF primitive input. This prevents attacks in which an
adversary attempts to use a truncated version of a longer derived
output as a valid shorter one.
Cross-role isolation: Cross-role isolation follows from label
uniqueness and PRF security of the underlying primitive: a KDF
output for one derivation role, such as payload_key, is
computationally independent of the output for any other role, such
as snap_key, even when derived from the same CEK and payload_info.
Snapshot authenticator: The construction MUST bind the current
segment set and the segment count into a public snapshot value
that an adversary without the authenticator's secret key cannot
forge for a modified segment set or count, even after observing
snapshot values and their deltas across rewrites. How the value
is computed, and whether a single-segment change updates it in
place or rebuilds it, is the authenticator's choice. SEAL's
snapshot authenticators are in Section 4.7.
Sullivan Expires 24 January 2027 [Page 49]
Internet-Draft raAE July 2026
SEAL's construction (Section 4) satisfies these requirements. Its
KDF (Section 4.3), keyed by the CEK, is modeled as a pseudorandom
function. Its encode framing (Section 4.3) is injective and places
protocol_id and the output length L in the primitive input, so
injectivity, protocol binding, and output-length commitment hold.
SEAL's labels (Table 4, each configured snapshot authenticator's own
labels, the plaintext-bound construction's labels in Appendix A, and
the hedged-randomness label in Appendix B) are distinct, giving label
uniqueness and, with PRF security, cross-role isolation. Each
configured snapshot authenticator satisfies the snapshot
authenticator requirement, with its forgery argument accounted in
Section 5.7.
SEAL's encode framing meets the injectivity requirement. Its frame/
encode construction, the LH digest for over-large fields, and LH's
domain separation from the literal encoding are defined and justified
in Section 4.3. SEAL's own derived and fixed fields (the
payload_info elements, the commitment, the leaf, and the uint-encoded
scalars) are all at most 0xFFFE octets, so for them frame is byte-
identical to a plain 2-octet length prefix. The two caller-supplied
octet strings, G (Section 4.2.4) and A_i, are unbounded, and a field
longer than 0xFFFE octets is bound as 0xFFFF || LH of it. Cross-
protocol separation comes from protocol_id in the KDF inputs and from
aad_label in the segment AAD, not from LH (Section 4.3).
SEAL's additional components, the epoch keys and the snapshot
maintained across rewrite and length change, are analyzed in
Section 5.5 and Section 5.7.
5.5. Segment Confidentiality and Integrity
The construction targets ra-ROR security (Section 5.1.1). The ra-ROR
advantage bound, its adversary parameters, and the per-term analysis
are in Appendix C, with the full proof in preparation ([SEALPROOFS]).
Ciphertext cores are indistinguishable from random under the ra-ROR
definition, up to the public length leakage, the equality leakage
allowed by derived nonce mode, and public metadata exposed by the
consuming format (file-level headers, stored nonces in random mode,
the salt, and the commitment). Each segment AEAD ciphertext binds
the segment to its own index and finality bit (through segment_aad in
random nonce mode and through the nonce in derived nonce mode) so an
adversary cannot modify or substitute a segment under a given index,
or flip its finality bit, without causing an AEAD verification
failure on that segment. Same-index rollback to an older valid
segment within the same message is not detected by per-segment AEAD
alone. This requires the snapshot verification of Section 4.7.
Sullivan Expires 24 January 2027 [Page 50]
Internet-Draft raAE July 2026
Three quadratic floors bound the multi-message advantage:
salt collision q_m^2 / 2^256 all suites
epoch-key collision n_ep^2 / 2^128 128-bit-key suite
nonce collision q^2 / 2^(8*Nn + 1) random nonce mode
q^2 / 2^97 at Nn = 12
In derived nonce mode the salt-collision floor is the only one for
the 256-bit-key suites. The 128-bit-key suite also has the epoch-key
collision floor, which dominates (Appendix C.2). In random nonce
mode, with or without the plaintext-bound hedge, the nonce-collision
floor applies. Here q is the number of segment encryptions under one
key, and Nn is the nonce length in octets. The block-size birthday
term is derived in Section 5.8.
Segment size enters the analysis only through the forgery and block-
size terms, both bounded in Section 5.8. It does not affect the
confidentiality or commitment terms. A segment's length is
authenticated implicitly by its AEAD tag, not through segment_aad or
the nonce, so the analysis assumes the consuming format conveys each
segment's true ciphertext length. Segments shorter than segment_max
consume no more than their share of the per-key budget and need no
separate accounting.
5.6. Commitment Security
raAE's commitment target is ra-CMT (Section 5.1.2), not the per-AEAD
CMT-1 ([RFC9771]): the commitment binds the CEK and payload_info,
which together carry the full parameter context affecting key
derivation, AEAD operation, and AAD and nonce construction.
ra-CMT relies on the collision resistance of the commitment
derivation map over the tuple (protocol_id, "commit", CEK,
payload_info, G, commitment_length), with G empty by default. PRF
security alone does not suffice, because the commitment adversary may
choose the CEK and the context values (Section 4.5). SEAL sets
commitment_length = Nh, so the commitment offers about 2^128
collision-search work at Nh = 32 and about 2^256 at Nh = 64. The
reduction outline for this assumption and the collision quantities
for a general commitment_length are in Appendix C.4.
Sullivan Expires 24 January 2027 [Page 51]
Internet-Draft raAE July 2026
The commitment bounds the ra-CMT variant in which the positions match
(Section 5.1.2). It does not bind the segment position. Position
binding for the ra-CMT-p variant is inherited from the underlying
AEAD's commitment level. Because the SEAL AEADs are not relied upon
to provide it, position confusion under an adversarially chosen key
is out of scope, consistent with [FLRR25]. A malicious encryptor
that controls the CEK can craft a single ciphertext segment that
opens at two positions, and the per-segment AEAD tag does not prevent
it.
A snapshot authenticator that sets the segment commitment over a
strict ordering, with the snapshot authenticated outside the
construction, restores ra-CMT-p against adversarially chosen keys:
the collision-resistant leaf commitment and the strict ordering
together pin each segment's ciphertext to its index even against a
CEK holder (Section 4.7.1, Section 5.3). It applies only where a
profile selects that authenticator and a consuming protocol
authenticates the one snapshot value. The base construction pins
position no more strongly than the AEAD commits under an adversarial
key.
The per-segment associated data A_i is likewise outside the
commitment: SEAL binds A_i through segment_aad and the AEAD tag for
that segment. Contexts that differ only in A_i are therefore not
commitment collisions. Their separation relies on AEAD commitment
for the segment AAD. A_i is also rewritable across rewrites and is
not bound at the snapshot level. An application that needs it bound
there MUST bind it externally. The exact accounting of this term in
the ra-CMT reduction is deferred to Appendix C.
A deployment that must defend against a malicious encryptor who
controls the CEK can close the adversarial-key position gap by one of
two changes. These are non-normative. SEAL adds none of them to the
base construction.
* A context-committing AEAD, so that each segment ciphertext commits
to its full decryption context rather than relying on the external
commitment over (CEK, payload_info, G).
* A per-segment collision-resistant commitment over the segment's
position and ciphertext, on the order of 16 to 64 octets per
segment, bound alongside the segment so that no segment opens at
two positions.
Sullivan Expires 24 January 2027 [Page 52]
Internet-Draft raAE July 2026
Both changes act in the base construction. A snapshot authenticator
with the segment commitment set over a strict ordering
(Section 4.7.1) is a separate, conditional route: it leaves the base
construction unchanged and closes the gap only where a profile
selects it and a consuming protocol authenticates the one snapshot
value.
None of the SEAL AEADs is relied upon to provide CMT-1 or CMT-4
([RFC9771]). In particular AES-256-GCM-SIV is not key-committing,
with known attacks such as partitioning oracles. raAE obtains ra-CMT
context commitment from the external commitment over (CEK,
payload_info, G) regardless of the AEAD, provided the commitment
check is not bypassed and payload_info carries the full parameter
context (Section 5.1.2).
With commitment_length = Nh and Nt = 16, ra-CMT is bounded by the
commitment collision resistance, and ra-CMT-p adds the per-segment
AEAD commitment term. The ra-CMT collision bound is derived in
Appendix C.4, and the ra-CMT-p bound is stated there as a sum. The
two key regimes differ. Under honestly generated keys the idealized
per-pair tag-forgery floor is 2^(-128) because Nt = 16, and concrete
per-AEAD authenticity bounds degrade with segment length
(Section 5.8). Under an adversarially chosen CEK the position term
is the AEAD's commitment level, which the SEAL AEADs are not relied
upon to provide.
The global associated data G (Section 4.6) enters the committed map
as an additional input, empty by default, and the collision-
resistance argument above applies unchanged with G in the committed
context. This matches the ra-CMT treatment of global associated data
in [FLRR25].
5.7. Snapshot Authenticator Security
The snapshot authenticator adds two operations (RewriteSeg,
SnapVerify) over a snapshot value and one new security property,
snapshot integrity (Section 5.1.4). Integrating its reduction into a
single combined proof with the ra-ROR framework is in preparation and
will be published separately ([SEALPROOFS]).
The keyed snapshot is not read as sender attribution.
Sullivan Expires 24 January 2027 [Page 53]
Internet-Draft raAE July 2026
+=========================+================+=========+==============+
| Guarantee | Established | Scope | Provided by |
| | by | | |
+=========================+================+=========+==============+
| Segment confidentiality | AEAD | segment | this |
| and integrity, index | decrypt, | | construction |
| and finality bound | per segment | | |
+-------------------------+----------------+---------+--------------+
| Segment-set integrity: | SnapVerify | whole | this |
| content, position, | over the | object | construction |
| count | snapshot | | |
| | value | | |
+-------------------------+----------------+---------+--------------+
| Sender attribution, | signature | whole | consuming |
| third-party | over the | object | protocol, |
| verifiability | snapshot | | not provided |
| | context | | |
+-------------------------+----------------+---------+--------------+
| Freshness, whole-object | external | whole | consuming |
| rollback protection | version or | object | protocol, |
| | freshness | | not provided |
| | state | | |
+-------------------------+----------------+---------+--------------+
Table 9: What each layer guarantees
Snapshot integrity is keyed by snap_key and protects the writer's
current segment set against a storage or write adversary that does
not hold snap_key. A verifier who holds snap_key, hence the CEK from
which it is derived, detects any alteration of the segment set
(Section 4.7.1). Forgery of a fresh snapshot value gives no
protection among parties that share the CEK, because any CEK-holder
can recompute snap_key, and the value is not a third-party-verifiable
commitment to a segment set. Whether a fixed, externally
authenticated snapshot value binds segment content against key
holders depends on the authenticator (Section 4.7.1). An application
that needs sender attribution or third-party verifiability MUST add a
signing layer over the snapshot context, and the configured
authenticator determines whether its value can carry that layer.
The snapshot authenticator adds authentication of the full segment
set (over the authenticator's per-segment input) and the count n_seg
as a single unit, which detects same-index rollback, segment-set
modification, and count changes. Its mandatory index-set check
catches a duplicated or dropped index that an authenticator's verify
might otherwise accept (Section 4.7.1).
Sullivan Expires 24 January 2027 [Page 54]
Internet-Draft raAE July 2026
A profile MUST keep each of an authenticator's KDF labels inside the
injective encode frame (Section 4.3) so distinct roles cannot collide
on the primitive input.
Whole-object rollback to a previously valid snapshot is out of scope
and is treated in Section 6.6. snap_key is not exposed through any
public API. It is derived internally per message from the CEK and
salt, so snap_key exposure is not a threat surface in the
construction itself.
5.8. Capacity and Usage Limits
These usage limits are organized around independent limit classes,
not around named suites. The accounting unit is one segment
encryption under one segment-encryption key (an epoch key). For
random-nonce AEADs that budget is a single per-epoch-key pool. For a
derived-nonce MRAE AEAD there are two separate budgets: a per-epoch-
key budget for distinct derived nonces, and a per-derived-nonce
budget for repeated encryption of one segment. Such an AEAD
therefore does not follow a simple "divide the per-key budget by
2^epoch_length" model.
The budgets depend on the AEAD's nonce, key, tag, and cipher block
sizes (Nn, Nk, Nt, Nb), on the AEAD's own published confidentiality
and integrity analysis, on the KDF's output size Nh and its mu-PRF
security (Appendix C.2), and on the suite's segment_max. A suite
that fixes those works out every figure below. The mu-PRF security
is what makes each epoch key's budget independent of every other's,
which the per-epoch-key accounting assumes.
The length-dependent limits in this section are computed at the
suite's segment_max, the largest plaintext one segment encryption
carries. Independent of these per-key budgets, a segment-encryption
procedure MUST respect each underlying AEAD's per-invocation input
limits.
5.8.1. Accounting Model
One segment encryption consumes one write from a segment-encryption
key's budget. Initial writes and rewrites are both segment
encryptions. The budget belongs to the epoch key, not the CEK.
epoch-key budget: the segment encryptions allowed under one epoch
key.
write (initial write): the first encryption of a segment.
rewrite: a later encryption of the same segment.
Sullivan Expires 24 January 2027 [Page 55]
Internet-Draft raAE July 2026
epoch_length (r): the base-2 log of the number of segments that
share one epoch key.
n_blk: the blocks per segment, n_blk = segment_max / Nb, for a
block-cipher AEAD.
advantage target: the bound a budget is solved against. Every
confidentiality budget in this section is computed at 2^-32. A
budget quoted from an AEAD's own analysis holds at the target that
analysis states.
5.8.2. Limit Classes
Six phenomena bound how much may be encrypted. Each is set by one
property of the AEAD or the segment size, and each bounds
confidentiality or integrity.
+================+===============+===============+=================+
| Limit class | Applies to | Depends on | Failure mode |
+================+===============+===============+=================+
| Random-nonce | random nonce | nonce size | confidentiality |
| collision | modes | | |
+----------------+---------------+---------------+-----------------+
| Forgery bound | all AEADs | MAC strength, | integrity |
| | | segment size | |
+----------------+---------------+---------------+-----------------+
| Block-size | block-cipher | total blocks | confidentiality |
| birthday | AEADs | | |
+----------------+---------------+---------------+-----------------+
| MRAE distinct- | derived-nonce | derived-nonce | confidentiality |
| nonce | MRAE across | count | and integrity |
| | segments | | |
+----------------+---------------+---------------+-----------------+
| Fixed-nonce | derived-nonce | segment size, | confidentiality |
| data volume | hot rewrites | rewrites | |
+----------------+---------------+---------------+-----------------+
| Epoch-key | 128-bit-key | key size | confidentiality |
| collision | AEADs | | |
+----------------+---------------+---------------+-----------------+
Table 10: Usage-limit classes
5.8.3. Maximum Object Size
Two limits bound a write-once object: each epoch key's AEAD write
budget, and the 2^63 segment index ceiling. The same two bound an
append-only object, at the write counts given at the end of this
section.
Sullivan Expires 24 January 2027 [Page 56]
Internet-Draft raAE July 2026
Under a flat key (one epoch key for the whole object), the AEAD
budget sets the size. Which class binds follows from the AEAD:
+=====================+===========================================+
| Binding class | Flat-key write-once limit |
+=====================+===========================================+
| Block-size birthday | 2^((8*Nb + 1 - 32)/2) / n_blk segments |
+---------------------+-------------------------------------------+
| MRAE distinct-nonce | the AEAD's distinct-derived-nonce ceiling |
+---------------------+-------------------------------------------+
| No block-cipher | 2^63 segments, the index ceiling |
| structure | |
+---------------------+-------------------------------------------+
| No published block- | 2^63 segments, the index ceiling |
| size analysis | |
+---------------------+-------------------------------------------+
Table 11: Flat-key write-once size limit by binding class
A block-cipher AEAD binds on the block-size birthday, a
confidentiality bound of about 2^36 segments at Nb = 16 and n_blk =
4096 (Section 5.8.6.2). A derived-nonce MRAE AEAD binds instead on
the distinct-derived-nonce ceiling its own analysis gives
(Section 5.8.6.4).
The last two classes carry the same limit for different reasons. An
AEAD with no block-cipher structure, such as one built on a stream
cipher, has no block-size term at all: that is a property of the
AEAD. An AEAD whose analysis publishes no such term may still have
one: that is a statement about the literature, not about the
algorithm, and a suite that places an AEAD in this class says which
of the two reasons applies.
These figures assume derived nonce mode, which a non-MRAE AEAD may
use only under a nonce-uniqueness discipline, the write-once or
append-only profiles (Section 4.5.3.2). In random nonce mode such an
AEAD binds earlier, at the random-nonce-collision budget of 2^((8*Nn
+ 1 - 32)/2), about 2^32 per epoch key at Nn = 12 (Section 5.8.6.1).
Sullivan Expires 24 January 2027 [Page 57]
Internet-Draft raAE July 2026
An append-only object re-encrypts one retained segment per append
(Section 4.9.3.1), so a segment appended one at a time is written
twice rather than once, and an identical replay after a crash adds a
further write without reusing the pair (Section 4.5.3.2). Every
figure above that binds on a write count therefore halves, before any
replays: the block-size birthday figure, which counts blocks, and the
distinct-derived-nonce figure, which counts distinct nonces. The
2^63 entry is the index ceiling and is unchanged. An epoch key
correspondingly spans at most 2^(epoch_length + 1) pairs, and the
budget it needs is that many writes plus any replays
(Section 4.9.3.1).
With epoch rotation each epoch key carries a fresh budget, so any
suite can reach the 2^63-segment ceiling, about 2^79 octets at
segment_max 65536. An AEAD subject to the epoch-key collision class
reaches it only with a sufficiently large epoch_length. At
epoch_length 0 that floor caps the object near 2^(4*Nk - 16)
segments, about 2^48 at Nk = 16 (Section 5.8.5). Reaching 2^63 then
needs epoch_length at least 15, so the distinct epoch keys stay near
2^48.
A rotating profile MUST keep each epoch key's 2^epoch_length initial
writes within its per-key budget. For a derived-nonce MRAE AEAD that
means 2^epoch_length below the AEAD's distinct-derived-nonce budget.
An append-only profile adds one re-mark per superseded segment, so
each epoch key spans up to 2^(epoch_length + 1) pairs, and an
encryptor MUST size the per-key budget to cover those plus the
replays an interrupted append costs (Section 4.9.3.1). Rewrite
capacity is a separate condition (Section 5.8.4).
5.8.4. Rewrite Capacity
A rewrite consumes the same budget as an initial write. How that
budget is shared depends on the nonce mode.
For a random-nonce AEAD, epoch_length divides one per-epoch-key pool
among the 2^epoch_length segments that share the key. If rewrites
are spread evenly, the count each segment can take falls as
epoch_length rises:
rewrites per segment = 2^((8*Nn + 1 - 32)/2) / 2^epoch_length
At Nn = 12 that is about 2^(32 - epoch_length): about 2^32 rewrites
per segment at epoch_length 0, about 2^26 at 6, and about 2^22 at 10.
For a derived-nonce MRAE AEAD the hot-segment rewrite cap is not
divided by 2^epoch_length, because a rewrite reuses the segment's one
derived nonce. The cap is length-dependent instead. Where the
Sullivan Expires 24 January 2027 [Page 58]
Internet-Draft raAE July 2026
AEAD's per-record encryption is a block cipher in counter mode, the
cap is the keystream block-size birthday over the rewritten blocks,
quantified in Section 5.8.6.4, and applies at every epoch_length. An
MRAE AEAD built another way has whatever cap its own analysis gives.
The distinct-derived-nonce budget is separate and per epoch key. A
derived-nonce MRAE AEAD thus has two separate constraints, the
distinct derived nonces per epoch key and the repeated encryptions at
one derived nonce. An AEAD whose nonce size makes a flat key safe
uses one, and a forgery budget that exceeds any reachable rewrite
workload leaves no rewrite constraint at all.
Applications MUST track segment encryptions per key and freeze the
object before a budget is exceeded. The CEK is fixed per object and
cannot be rotated in place, so continued writing requires a new
object under a fresh CEK.
5.8.5. Choosing epoch_length
epoch_length means different things by mode. For the random-nonce
AEADs, a smaller epoch_length puts fewer segments under each key, so
each segment keeps a larger share of the random-nonce collision pool,
at the cost of more epoch keys to derive and hold. For a derived-
nonce MRAE AEAD, a smaller epoch_length reduces the distinct derived
nonces under an epoch key but does not raise the hot-segment rewrite
cap, which is per derived nonce and length-dependent. An AEAD with a
256-bit nonce (Nn = 32) makes collision negligible and has a high
forgery floor, so a flat key (epoch_length 63) is the natural choice.
The cipher suite's default epoch_length, used when no epoch_length is
chosen, is 0 for the suites with an epoch_length range and 63 for the
suites whose nonce size makes a flat key safe. A referencing
protocol may choose directly, or by citing a profile or named
instantiation that fixes a value. Either way a value is chosen, and
the default does not apply.
A referencing protocol that chooses an epoch_length tuned to its
expected segment count, rather than taking the default, trades some
rotation for fewer epoch keys while staying within budget. Because a
segment may be shorter than segment_max, size epoch_length by the
number of segments rather than the object's octet length. At a fixed
octet size, smaller segments mean more segments and more epoch keys.
The epoch keys are independent under the PRF security of the KDF
([RFC8645]), so the epoch_length parameter controls budget
distribution, not key strength.
Sullivan Expires 24 January 2027 [Page 59]
Internet-Draft raAE July 2026
An AEAD with a 128-bit key (Nk = 16) adds one further constraint.
The epoch-key collision term (Appendix C.2) is about n_ep^2/2^(8*Nk)
over the n_ep distinct epoch keys, so a profile whose AEAD has Nk =
16 SHOULD keep distinct epoch keys per payload_key below about
2^(4*Nk - 16), which is about 2^48 at Nk = 16 and holds that term
within the 2^-32 target. At epoch_length 0 this caps such an object
at about 2^48 segments, far below the 2^63 index ceiling, so it
constrains only extreme object sizes.
5.8.6. Derivations
5.8.6.1. Confidentiality (Nonce Collision)
For random nonce mode with Nn-octet nonces and q segment encryptions
under one key, the collision probability follows the birthday bound:
P(collision) <= q^2 / 2^(8*Nn + 1)
When epoch_length = r is specified, q counts encryptions per epoch
key (initial writes plus rewrites within that epoch), not across the
whole content. Each epoch key has an independent budget.
For derived nonce mode, nonces are deterministic and distinct across
segment indices, so this collision term does not apply. The limits
on reusing a derived nonce are in Section 5.8.6.4.
5.8.6.2. Confidentiality (Block-Size Birthday Bound)
Nonce collisions alone do not exhaust the confidentiality bound.
Following the AEAD usage-limits analysis ([I-D.irtf-cfrg-aead-limits]
Section 5), a block-cipher AEAD has a distinguishing bound that grows
with the total number of cipher blocks processed under a key. If s
is the total number of AAD-plus-plaintext blocks, each Nb octets, and
q is the number of encryption queries under a key, the
confidentiality advantage is at most:
CA <= ((s + q + 1)^2) / 2^(8*Nb + 1)
A segment holds n_blk blocks and s = n_blk * q in the worst case, so
CA scales as q^2 * n_blk^2 / 2^(8*Nb + 1). Solved against the 2^-32
target, the per-key budget is:
q <= 2^((8*Nb + 1 - 32)/2) / n_blk
about 2^36 at Nb = 16 and n_blk = 4096. In random nonce mode the
nonce-collision bound q^2 / 2^(8*Nn + 1) dominates for typical
deployments because n_blk^2 / 2^(8*(Nb - Nn)) is small: the block-
size term overtakes it only once n_blk^2 exceeds 2^(8*(Nb - Nn)),
Sullivan Expires 24 January 2027 [Page 60]
Internet-Draft raAE July 2026
which is 2^32 at Nb = 16 and Nn = 12. In derived nonce mode the
nonce-collision term vanishes and the block-size term is the binding
confidentiality constraint.
An AEAD with no block-cipher structure, such as one built on a stream
cipher, has no comparable block-size bound, leaving nonce collision
and forgery as the only relevant terms. Nonce collision is
negligible at a large Nn, and the per-query forgery bound governs
integrity. Section 5.8.6.4 gives the derived-nonce block-size
limits.
SEAL implementations MUST compute usage budgets from the AEAD-
specific confidentiality and integrity bounds, not from nonce-
collision probability alone.
5.8.6.3. Integrity (Forgery)
Each AEAD decryption query gives the adversary a chance to forge a
valid ciphertext. The forgery advantage per query depends on the
AEAD's own analysis and on segment_max:
IA <= v * F(segment_max)
where v is the number of forgery attempts and F(segment_max) is the
per-query forgery advantage the AEAD's analysis gives at a segment of
segment_max octets. F grows with segment_max, so a larger segment
lowers the attempt budget for a fixed target. The tag length bounds
F from below, at about 2^(-8*Nt), so no AEAD offers more than about
8*Nt bits of forgery resistance per query. An AEAD's authenticator
may achieve less than its tag length suggests, so F is not determined
by Nt and a suite takes F from the analysis of its own AEAD.
[I-D.irtf-cfrg-aead-limits] gives F for several AEADs.
5.8.6.4. Derived-Nonce Bounds
A derived-nonce MRAE AEAD has two limits the random-nonce analysis
above does not capture. Both come from the AEAD's own analysis
rather than from its interface.
Across the distinct segment nonces under one epoch key, an MRAE AEAD
that derives a per-record key from the key and the nonce has a
derived-key collision ceiling on the number of distinct derived
nonces. Its value is the AEAD's analysis to state.
For a single hot segment, every rewrite reuses that segment's one
derived nonce, so n_rw rewrites of an n_blk-block segment encrypt
n_rw * n_blk blocks under one derived per-record key. Where that
per-record encryption is a block cipher in counter mode, the binding
Sullivan Expires 24 January 2027 [Page 61]
Internet-Draft raAE July 2026
term is the keystream block-size birthday over the total blocks, in
the form of [I-D.irtf-cfrg-aead-limits] Section 5 with s = n_rw *
n_blk blocks and q = n_rw queries:
((s + q + 1)^2) / 2^(8*Nb + 1) <= 2^-32
The n_rw * n_blk term dominates, so this holds while n_rw * n_blk
stays below about 2^((8*Nb + 1 - 32)/2), giving:
n_rw <= 2^((8*Nb + 1 - 32)/2) / n_blk
the same expression as the flat-key block-size budget, counting
rewrites of one segment at one nonce instead of writes under one key.
That is about 2^36 rewrites at Nb = 16 and n_blk = 4096, and about
2^38 at n_blk = 1024.
An MRAE AEAD whose per-record encryption gives distinct plaintexts
distinct keystreams degrades under the fixed nonce only by this
block-birthday term and not by nonce reuse. Two rewrites with
identical plaintext and associated data produce identical ciphertext
(deterministic-MRAE equality leakage), independent of this count.
6. Security Considerations
raAE's ra-ROR security target (Section 5.1.1) rests on three
assumptions: the AEAD is multi-user real-or-random (mu-ROR) secure,
the KDF is a secure multi-user pseudorandom function (mu-PRF), and
nonces do not collide.
Integrity is bounded by the AEAD tag: no AEAD offers more than
approximately 8*Nt bits of forgery resistance per query. A larger
AEAD key raises confidentiality margins, not the tag-length forgery
floor.
6.1. Detection Summary
The rows detected by SnapVerify assume a snapshot authenticator is
configured. Without one, those modes go undetected.
The one exception is a dropped trailing segment under a write-once
profile, which the per-segment finality requirement of
Section 4.9.1.2 detects with no snapshot authenticator (the
truncation row below).
Sullivan Expires 24 January 2027 [Page 62]
Internet-Draft raAE July 2026
+==============+===================================================+
| Failure mode | Detected by |
+==============+===================================================+
| Wrong CEK, | The commitment, before any segment decryption |
| salt, or | (Section 4.6), when reader and writer share the |
| parameter | payload_info construction. |
| set | |
+--------------+---------------------------------------------------+
| Modified | AEAD.Decrypt for the affected segment. |
| ciphertext | |
| core or AEAD | |
| tag | |
+--------------+---------------------------------------------------+
| Segment | AEAD.Decrypt: index and finality are bound by |
| moved to a | segment_aad (random nonce mode) or the nonce |
| different | (derived nonce mode). |
| index | |
+--------------+---------------------------------------------------+
| Segment | AEAD.Decrypt, when the source used a different |
| copied from | CEK or salt; a reused salt under the same CEK |
| another | breaks this separation (Section 6.7). |
| object at | |
| the same | |
| index | |
+--------------+---------------------------------------------------+
| Segment | SnapVerify, not per-segment AEAD alone, unless |
| copied from | the whole object including its snapshot is rolled |
| another | back to that earlier valid version. |
| version at | |
| the same | |
| index | |
+--------------+---------------------------------------------------+
| Dropped | Write-once: the finality requirement of |
| trailing | Section 4.9.1.2 (top present segment carries |
| segment(s) | is_final = 0). Length-changing: SnapVerify, |
| (truncation) | since a re-marked terminal (Section 4.9.3) can be |
| | replayed; the mutable profile always configures |
| | an authenticator. Truncation to zero: the zero- |
| | segment prohibition, or SnapVerify (Section 6.6). |
+--------------+---------------------------------------------------+
| Missing | SnapVerify (segment count, finality, recomputed |
| interior, | snapshot value). |
| duplicated, | |
| reordered, | |
| or inserted | |
| segment | |
+--------------+---------------------------------------------------+
| Stale | SnapVerify, unless the stale value is part of a |
Sullivan Expires 24 January 2027 [Page 63]
Internet-Draft raAE July 2026
| snapshot | whole-object rollback. |
| value | |
+--------------+---------------------------------------------------+
| Whole-object | Not detected by raAE alone; the consuming |
| rollback | protocol needs authenticated external freshness |
| | state. |
+--------------+---------------------------------------------------+
| Equality | Not an authentication failure; rewriting equal |
| leakage in | plaintext at the same index can reveal equality |
| derived | (Section 4.5.3, Section 6.2). |
| nonce mode | |
+--------------+---------------------------------------------------+
Table 12: Failure modes and the checks that detect them
6.2. Nonce Misuse
Nonce reuse under a non-MRAE AEAD leaks plaintext: an adversary who
observes two ciphertexts under the same key and nonce recovers the
XOR of the plaintexts (for CTR-based AEADs) and can forge new
ciphertexts. For an AEAD whose authenticator is a polynomial
universal hash, nonce reuse also recovers the authentication key,
enabling forgery of arbitrary messages.
Random nonce mode depends entirely on the CSPRNG. If the CSPRNG
returns duplicated state, segments collide. Derived nonce mode
removes that dependence, since nonces are deterministic, but the
determinism is itself a hazard: re-encrypting a segment with
different content under its repeated derived nonce is a two-time pad,
catastrophic for a non-MRAE AEAD.
A non-MRAE AEAD therefore uses derived nonces only under a discipline
that keeps every (index, finality bit) pair unique, the write-once or
append-only profiles, drawing a fresh salt per object and never
reusing a pair under that salt, including after a crash
(Section 4.5.3.2). Write-once meets this by never re-encrypting with
changed inputs, and by drawing a fresh salt rather than retrying an
object it did not complete. Append-only meets it by permitting at
most one re-encryption per segment, the finality re-mark, under the
three rules of Section 4.9.3.1. An MRAE AEAD instead degrades only
to equality leakage, not plaintext recovery.
The plaintext-bound construction (Appendix A) partially defends
against CSPRNG duplication: different plaintexts at the same index
produce different nonces because the plaintext digest differs, but
equal plaintexts still collide. Implementations that need a full
defense against random number generator (RNG) state duplication MUST
use derived nonce mode with an MRAE AEAD.
Sullivan Expires 24 January 2027 [Page 64]
Internet-Draft raAE July 2026
6.3. Parameter Set Mismatch
The full parameter set that affects encryption, decryption, AAD
construction, nonce construction, and key derivation is bound into
payload_info (see Section 4.2) and therefore into the commitment. A
reader using a different parameter set than the writer triggers a
commitment mismatch before any AEAD operation is attempted. A reader
supplying a different G than the encryptor likewise triggers a
commitment mismatch (Section 4.6). Consuming protocols MUST still
reject unrecognized or unsupported parameter values before
decryption, since the commitment check detects mismatch but does not
by itself indicate which parameter value the recipient is unable to
support.
One profile-level constant is bound transitively rather than via
payload_info: aad_label is bound through protocol_id (each profile
fixes its own aad_label). nonce_mode, by contrast, is carried in
payload_info, so the commitment binds it directly. Profiles MUST NOT
share a protocol_id across distinct aad_label values. Reusing a
protocol_id with a changed aad_label produces objects whose
commitment matches the wrong reader but whose per-segment AEAD
verification fails with no clear error attribution.
6.4. Framing and Label Errors
Two classes of implementation error break cross-role isolation. A
non-injective framing function maps distinct KDF input tuples to the
same primitive input, correlating outputs that should be independent.
Implementations MUST verify injectivity per Section 5.4. Reusing a
label across roles (for example, "commit" for both commitment and
payload key) has the same effect. Labels MUST be distinct within a
protocol version, and a new version that changes any derivation MUST
change the protocol_id.
6.5. Parameter Misuse
The nonce mode and AEAD choice are coupled, and the table gives the
rule for each pairing:
Sullivan Expires 24 January 2027 [Page 65]
Internet-Draft raAE July 2026
+======================+==============+===============+
| AEAD class / profile | random nonce | derived nonce |
+======================+==============+===============+
| non-MRAE, mutable | valid | unsafe |
+----------------------+--------------+---------------+
| non-MRAE, write-once | valid | valid |
| or append-only | | |
+----------------------+--------------+---------------+
| MRAE, any profile | wasteful | valid |
+----------------------+--------------+---------------+
Table 13: Nonce mode by AEAD class and profile
Rewrite is where the coupling matters: a derived nonce is reused
where a random nonce is redrawn, and the consequences of that reuse
by AEAD class are in Section 6.2 and Section 4.5.3.2.
6.6. Snapshot Integrity Limitations
First, a reader that does not run snapshot verification (Section 4.7)
does not verify that a segment belongs to the current authenticated
snapshot. Such a reader gets per-segment AEAD authenticity only,
without the snapshot integrity guarantee of Section 5.1.4: the
ciphertext core is authenticated under the segment index and finality
bit, but the reader cannot detect that the segment was substituted
from a previous valid version of the same message at the same index,
nor that other segments were dropped or rolled back. Such readers
MUST still verify the commitment per Section 4.6. Applications that
support random-access single-segment reads MUST either run snapshot
verification on every read or explicitly document that they accept
per-segment authenticity without snapshot freshness.
Conversely, snapshot verification authenticates the segment set, its
positions, and the count. Without the segment commitment it does not
confirm that a tag is a valid AEAD tag for the ciphertext beside it.
With the segment commitment set it binds the ciphertext bytes and the
tag, but not that the tag opens the ciphertext under the AEAD
(Section 4.7.1). Decrypting the segment is what establishes that, so
snapshot verification layers set, position, and count binding on top
of per-segment AEAD without replacing it.
Second, snapshot integrity does not provide freshness against whole-
object rollback. A storage adversary that rolls back the entire
encrypted object, including the snapshot, to a previously valid
version is not detected by raAE alone, because the rolled-back
snapshot is itself a valid snapshot for that prior state. This is
replay of an intact prior snapshot, not a forgery: it stays out of
scope unless the consuming protocol authenticates external freshness
Sullivan Expires 24 January 2027 [Page 66]
Internet-Draft raAE July 2026
into the snapshot context. Applications that require whole-object
rollback resistance MUST bind an authenticated version field,
timestamp, monotonic counter, or authenticated storage layer into
that context. Relatedly, Section 4.9.2.1 requires a writer to update
only trusted snapshot state, so a rewrite cannot launder a rolled-
back snapshot into a valid ongoing history.
Third, same-index rollback within a snapshot is detected when
snapshot verification runs. If an adversary replaces one segment
with a previously valid same-index segment but leaves the current
snapshot value in place, the recomputed snapshot value no longer
matches the stored one, so verification fails except with negligible
forgery probability (see Section 5.7).
Truncation, including removal of the final segment marked is_final =
1, is detected by snapshot verification. The removed segment is
absent from the recomputed snapshot value and the count n_seg no
longer matches the segments present, so verification fails.
Truncation is a special case of the first limitation above, for
readers that skip snapshot verification.
6.7. Salt Reuse
Reusing a salt with the same CEK across two files produces identical
payload schedule outputs: the same payload_key, the same snap_key,
and, in derived nonce mode, the same nonce_base. The damage depends
on the nonce mode.
In every nonce mode, an adversary can silently swap same-index
segments between the two files: both per-segment AEAD checks and the
snapshot authenticator accept the swap. Salt reuse is therefore an
integrity break in every configuration.
In derived nonce mode the nonces also repeat across the two files, so
an MRAE AEAD degrades to deterministic encryption and leaks plaintext
equality between same-index segments. A non-MRAE AEAD then permits
plaintext recovery. Such an AEAD is safe under a derived nonce only
within a single object, where the write-once or append-only
discipline keeps each derived nonce unique (Section 4.5.3.2).
Reusing the salt repeats nonce_base across two objects, hence every
derived nonce, and breaks that scoping.
Sullivan Expires 24 January 2027 [Page 67]
Internet-Draft raAE July 2026
In random nonce mode (and with the plaintext-bound hedge) fresh
nonces keep confidentiality intact, although the per-key nonce-
collision budget of Section 5.8.6.1 then counts encryptions across
both files under the single CEK. The integrity break above remains.
Applications MUST therefore ensure salt uniqueness per CEK when
creating a new message. The freshness requirement in Section 4.9.1.1
exists for this reason.
6.8. Rewrite Hazards
Applications MUST track total segment encryptions per key and freeze
the object before exceeding the per-epoch-key budget of
Section 5.8.4. For a 96-bit-nonce AEAD (Nn = 12) that budget is
roughly 2^32 encryptions per epoch key, and exceeding it risks nonce
collisions and plaintext recovery (Section 5.8.5).
A mutable profile with derived nonces and an MRAE AEAD reuses a
segment's deterministic nonce across non-terminal rewrites, and a
crash that replays a write reuses it again. Confidentiality there
rests on the per-segment rewrite limit (Section 5.8.4), not on nonce
uniqueness: the AEAD degrades to equality leakage between identical
rewrites rather than to plaintext recovery, as long as that budget is
respected.
The append-only profile has no such margin. It admits a non-MRAE
AEAD, so its safety rests on never re-encrypting a segment with
different content (Section 4.9.3.1), and a writer that breaks that
rule gets a two-time pad rather than a bounded leak.
Per-segment associated data A_i is rewritable, so a reader cannot
treat it as context fixed at creation (Section 4.4.2). An
application that needs creation-fixed context places it in G, which
the commitment binds (Section 4.6), not in A_i.
raAE also does not guarantee atomic rewrites. A segment rewrite
touches the nonce metadata, ciphertext core, AEAD tag, and the
snapshot value. A crash between any two of these leaves the content
inconsistent. Applications MUST use write-ahead logging, copy-on-
write, or an equivalent mechanism to make rewrites recoverable.
6.9. Constant-Time Implementation
Several raAE operations handle secret data and MUST be implemented in
constant time to prevent timing side-channels.
Sullivan Expires 24 January 2027 [Page 68]
Internet-Draft raAE July 2026
The KDF calls in the payload schedule and epoch key derivation take
the CEK or payload_key as input keying material. Implementations
MUST ensure that the KDF's keyed operations execute in constant time
with respect to their key inputs. For an HMAC-based KDF this is
satisfied by implementations that do not branch on key octets.
Both the commitment comparison (Section 4.9.1.2) and the snapshot
comparison in SnapVerify MUST use a constant-time octet comparison.
A variable-time comparison reveals the position of the first
differing octet. An adversary who can retry tampered inputs learns
the expected value one octet at a time and can then present a
matching stored value. This reduces the forgery cost of either check
to a linear number of trials (for the snapshot, the bound of
Section 5.7).
AEAD.Encrypt and AEAD.Decrypt operations inherit the constant-time
requirements of the underlying AEAD. Implementations SHOULD use AEAD
libraries that document constant-time guarantees.
6.10. Properties Not Provided
raAE protects segment content and binds segments together.
Everything a complete storage or messaging system needs beyond that
is the consuming protocol's responsibility. This section is the
authoritative boundary of what raAE does not provide.
Freshness and whole-object rollback: Snapshot integrity binds the
segment set of one recorded object state, not its recency. A
party holding the ciphertext can present an earlier honest state
in full, and both per-segment and snapshot verification still
pass. Telling the current state from a stale one requires
external freshness (Section 6.6).
Forward secrecy: The CEK must remain available as long as any reader
needs access, so raAE provides none.
Sender identity: Ciphertexts are not bound to any sender. A signing
or MAC layer is needed for sender authentication.
Unlinkability: Key identifiers and structural features of the
encrypted format are visible, so unlinkability requires
application-layer measures.
Whole-message replacement: Snapshot verification covers segments
within a message but cannot detect replacement of the message
itself. An adversary who swaps one encrypted message for another
goes undetected unless the application binds message identity
externally.
Sullivan Expires 24 January 2027 [Page 69]
Internet-Draft raAE July 2026
Serialization, storage, and key management: raAE emits and consumes
component values but defines no wire container, storage
transactions, or key lifecycle. A consuming protocol supplies
these.
6.11. Cross-Application Domain Separation
Two applications that both reuse the same SEAL profile label with the
same input keying material, the same payload_info, and the same G
derive identical commitments, keys, nonces, and AADs. The
protocol_id provides cross-application domain separation only when it
is distinct per application. Applications whose keying material may
be shared across systems SHOULD use an application-specific
protocol_id (for example, "myapp-backup-v1") rather than a SEAL
profile label. An application adopting a SEAL profile unchanged MAY
use that profile's label.
7. IANA Considerations
This document has no IANA actions. It defines an abstract primitive
and a parameterized construction and requests no code points or
registries of its own. The cipher-suite code points and the
registries of named instantiations and snapshot authenticators are
specified in the companion ([I-D.sullivan-seal-concrete]).
The normative reference to [I-D.ietf-hpke-pq], which defines the one-
step KDF class this document's combiner admits, is a reference to a
work in progress, so publication of this document waits on it.
8. References
8.1. Normative References
[I-D.ietf-hpke-pq]
Barnes, R. and D. Connolly, "Post-Quantum and Post-
Quantum/Traditional Hybrid Algorithms for HPKE", Work in
Progress, Internet-Draft, draft-ietf-hpke-pq-05, 6 July
2026, <https://datatracker.ietf.org/doc/html/draft-ietf-
hpke-pq-05>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/rfc/rfc5116>.
Sullivan Expires 24 January 2027 [Page 70]
Internet-Draft raAE July 2026
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/rfc/rfc5869>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<https://www.rfc-editor.org/rfc/rfc8017>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8452] Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV:
Nonce Misuse-Resistant Authenticated Encryption",
RFC 8452, DOI 10.17487/RFC8452, April 2019,
<https://www.rfc-editor.org/rfc/rfc8452>.
[RFC9180] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.
[RFC9861] Viguier, B., Wong, D., Ed., Van Assche, G., Ed., Dang, Q.,
Ed., and J. Daemen, Ed., "KangarooTwelve and TurboSHAKE",
RFC 9861, DOI 10.17487/RFC9861, October 2025,
<https://www.rfc-editor.org/rfc/rfc9861>.
8.2. Informative References
[ADG22] Albertini, A., Duong, T., Gueron, S., Kölbl, S., Luykx,
A., and S. Schmieg, "How to Abuse and Fix Authenticated
Encryption Without Key Commitment", USENIX Security
2022 pp. 3291-3308, 2022,
<https://www.usenix.org/conference/usenixsecurity22/
presentation/albertini>.
[BHT18] Bose, P., Hoang, V. T., and S. Tessaro, "Revisiting AES-
GCM-SIV: Multi-user Security, Faster Key Derivation, and
Better Bounds", IACR ePrint 2018/136, 2018,
<https://eprint.iacr.org/2018/136>.
[DGRW18] Dodis, Y., Grubbs, P., Ristenpart, T., and J. Woodage,
"Fast Message Franking: From Invisible Salamanders to
Encryptment", CRYPTO 2018, LNCS vol. 10991, pp. 155-186,
DOI 10.1007/978-3-319-96884-1_6, 2018,
<https://doi.org/10.1007/978-3-319-96884-1_6>.
Sullivan Expires 24 January 2027 [Page 71]
Internet-Draft raAE July 2026
[FLRR25] Fábrega, A., Len, J., Ristenpart, T., and G. Rubin,
"Random-Access AEAD for Fast Lightweight Online
Encryption", EUROCRYPT 2026,
DOI 10.1007/978-3-032-25333-0_10, 2026,
<https://eprint.iacr.org/2025/2275>.
[HRRV15] Hoang, V. T., Reyhanitabar, R., Rogaway, P., and D. Vizár,
"Online Authenticated-Encryption and its Nonce-Reuse
Misuse-Resistance", IACR ePrint 2015/189, 2015,
<https://eprint.iacr.org/2015/189>.
[I-D.irtf-cfrg-aead-limits]
Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on
AEAD Algorithms", Work in Progress, Internet-Draft, draft-
irtf-cfrg-aead-limits-11, 4 December 2025,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
aead-limits-11>.
[I-D.sullivan-seal-concrete]
Sullivan, N., "SEAL Cipher Suites and Instantiations",
Work in Progress, Internet-Draft, draft-sullivan-seal-
concrete-00, 2026, <https://datatracker.ietf.org/doc/html/
draft-sullivan-seal-concrete-00>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
[RFC8645] Smyshlyaev, S., Ed., "Re-keying Mechanisms for Symmetric
Keys", RFC 8645, DOI 10.17487/RFC8645, August 2019,
<https://www.rfc-editor.org/rfc/rfc8645>.
[RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
and C. Wood, "Randomness Improvements for Security
Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
<https://www.rfc-editor.org/rfc/rfc8937>.
[RFC9580] Wouters, P., Ed., Huigens, D., Winter, J., and Y. Niibe,
"OpenPGP", RFC 9580, DOI 10.17487/RFC9580, July 2024,
<https://www.rfc-editor.org/rfc/rfc9580>.
[RFC9771] Bozhko, A., Ed., "Properties of Authenticated Encryption
with Associated Data (AEAD) Algorithms", RFC 9771,
DOI 10.17487/RFC9771, May 2025,
<https://www.rfc-editor.org/rfc/rfc9771>.
Sullivan Expires 24 January 2027 [Page 72]
Internet-Draft raAE July 2026
[SEALPROOFS]
Sullivan, N., "Security Analysis of the SEAL Construction:
raAE, Snapshot Integrity, and Commitment", Work in
Progress manuscript in preparation.
[Tink] Hoang, V. T. and Y. Shen, "Security of Streaming
Encryption in Google's Tink Library", IACR ePrint
2020/1019, 2020, <https://eprint.iacr.org/2020/1019>.
Appendix A. Optional Plaintext-Bound Nonce Construction
This appendix is informative. It describes an optional encryptor-
side construction that mixes plaintext content into the per-segment
nonce derivation to defend against RNG state duplication. The
construction is encryptor-only: the decryptor reads the resulting
nonce from the wire (in the same slot used by random mode) and never
invokes any of the machinery below. Implementations MAY use this
construction in place of a fresh CSPRNG call when generating nonces
under nonce_mode "random". The wire format is indistinguishable from
random mode and decryption is unaffected.
One such construction:
pt_digest(i) = LH(P_i)
encryption_params = [aead_id, segment_max_be, kdf_id]
pt_hash(i) = KDF(protocol_id, pt_hash_label,
[pt_digest(i)],
encryption_params, Nh)
nonce_ctx = encode(protocol_id, uint64(i), pt_hash(i))
nonce(i) = KDF(protocol_id, pt_nonce_label,
[Random(Nn), payload_key],
[...payload_info, nonce_ctx], Nn)
This construction fixes two labels:
+==========================+================+============+
| Derivation role | Label variable | Value |
+==========================+================+============+
| Plaintext digest binding | pt_hash_label | "pt-nonce" |
+--------------------------+----------------+------------+
| Final nonce derivation | pt_nonce_label | "nonce" |
+--------------------------+----------------+------------+
Table 14: Plaintext-bound construction labels
Sullivan Expires 24 January 2027 [Page 73]
Internet-Draft raAE July 2026
Both labels are distinct from all other SEAL labels. Neither
derivation uses nonce_base_label (Table 4): the "nonce_base" label
belongs to derived nonce mode and does not appear in this
construction. In the final derivation nonce_ctx enters the info list
as one element after the payload_info elements: its encode output is
framed behind a length prefix like any other element, not spliced
flat into the list.
LH is the over-large-field digest of Section 4.3. Its collision
resistance is justified in Section 4.3.
When the encryptor's CSPRNG produces duplicated state (for example,
from a virtual machine snapshot or a fork without reseed), two
encryptions of distinct plaintexts at the same segment index still
produce distinct nonces because pt_hash(i) differs. Two encryptions
of identical plaintexts at the same index produce identical nonces,
resulting in deterministic encryption: the ciphertexts are identical,
revealing only that the plaintexts are equal.
For rewrites under this construction, the encryptor recomputes
pt_hash(i) with the new plaintext, derives a fresh nonce(i) using the
new pt_hash(i) and a new Random(Nn), seals the new plaintext under
the new nonce, and updates the snapshot value as in Section 4.9.2.1.
Appendix B. Optional Hedged Randomness
When a long-term symmetric key sk of at least Nh octets is available
to the encryptor, implementations SHOULD mix it into random
generation using the hedging pattern of [RFC8937]. If only an
asymmetric private key is available, it MUST first be processed
through a KDF to produce a uniform symmetric key.
hedge_key = KDF(protocol_id, hedge_label, sk, [], Nh)
HedgedRandom(n, label):
return KDF(protocol_id, label,
[hedge_key, Random(n)], [], n)
This construction fixes one label, hedge_label = "hedge", distinct
from all other SEAL labels.
HedgedRandom output depends on both the CSPRNG and sk, so a weak
CSPRNG alone cannot predict it. Hedging does not help when the
CSPRNG state itself is duplicated (VM snapshots, fork without
reseed). Identical CSPRNG output still produces identical
HedgedRandom output. Appendix A describes an optional encryptor-side
construction that defends against state duplication. It is
orthogonal to hedging and the two MAY be combined.
Sullivan Expires 24 January 2027 [Page 74]
Internet-Draft raAE July 2026
Appendix C. Proof Status and Security Claim Provenance
This appendix is informative. Table 15 gives each security claim's
source, argument, and assumptions. Each is inherited from [FLRR25],
argued in this document, referenced from external work, or deferred
to the proof paper in preparation ([SEALPROOFS]). The one claim
proved in full here is the KDF combiner's injectivity.
The combiner's input encoding is injective and canonical
(Section 4.3, Section 5.4): distinct (protocol_id, label, ikm, info,
L) tuples map to distinct primitive inputs, from the length-prefixed
encoding rather than the KDF. Collision resistance is assumed only
of LH, and only for over-large fields. A snapshot authenticator with
the segment commitment additionally relies on LH collision resistance
to bind each segment's ciphertext of any length (Section 4.7.1), and
the commitment derivation carries its own separate collision-
resistance assumption (Appendix C.4).
C.1. Provenance of the Security Claims
+============+==============+========+==============================+
|Property |Source |Argument| Rests on |
+============+==============+========+==============================+
|ra-ROR |[FLRR25] |Appendix| mu-PRF KDF (two keying |
| |notion and |C.2, | levels), mu-ROR AEAD, no |
| |base proof; |Appendix| nonce collision, fresh per- |
| |SEAL's |C.3 | message salt |
| |arbitrary- | | |
| |position ra- | | |
| |ROR syntax is | | |
| |a SEAL- | | |
| |specific | | |
| |obligation | | |
+------------+--------------+--------+------------------------------+
|ra-CMT |[FLRR25] |Appendix| collision resistance of the |
|(commitment,| |C.4 | commitment derivation map |
|position- | | | over (protocol_id, "commit", |
|respecting) | | | CEK, payload_info, G, |
| | | | commitment_length) |
+------------+--------------+--------+------------------------------+
|ra-CMT-p |[FLRR25] |Appendix| ra-CMT collision bound plus |
|(per-segment|notion and |C.4 | the underlying AEAD's |
|associated- |base proof; | | commitment bound for the |
|data and |SEAL | | forged position, restored |
|position |position- | | conditionally by the segment |
|commitment) |binding | | commitment over a strict |
| |realization | | ordering where the snapshot |
| |and per- | | is authenticated outside the |
Sullivan Expires 24 January 2027 [Page 75]
Internet-Draft raAE July 2026
| |segment | | construction (Section 5.6) |
| |associated- | | |
| |data and | | |
| |position | | |
| |accounting | | |
| |argued here, | | |
| |formal write- | | |
| |up in | | |
| |[SEALPROOFS] | | |
+------------+--------------+--------+------------------------------+
|KDF combiner|this document |Section | injectivity of the length- |
|injectivity |(structural, |4.3, | prefixed encoding (KDF need |
|and domain |proof- |Section | not be injective); collision |
|separation |complete) |5.4 | resistance of LH for over- |
| | | | large fields only |
+------------+--------------+--------+------------------------------+
|Snapshot |this document |Section | unforgeability of the |
|integrity |(notion, |5.7 | snapshot value by a party |
| |Section | | without snap_key; per-object |
| |5.1.4); | | separation via salt-bound |
| |configured | | snap_key |
| |authenticator | | |
| |(bound) | | |
+------------+--------------+--------+------------------------------+
|AEAD usage, |external work |Section | [I-D.irtf-cfrg-aead-limits], |
|rewrite, and|(operational, |5.8, | [RFC8452] Section 9 with |
|max-object |not proven |Section | [BHT18], 2^63 segment-index |
|bounds |here) |5.8.3, | ceiling |
| | |Section | |
| | |5.8.6.4 | |
+------------+--------------+--------+------------------------------+
|Derived- |precedent, |Section | write-once or append-only |
|nonce |not proof |4.5.3.2,| uniqueness discipline |
|unique-nonce|(TLS 1.3 |Appendix| reducing to nonce-respecting |
|transform |[RFC8446], |C.5 | AEAD security |
| |STREAM | | |
| |[HRRV15]) | | |
+------------+--------------+--------+------------------------------+
|Derived- |this |Appendix| MRAE AEAD ([RFC8452]); per- |
|nonce |document; |C.5 | segment fixed-nonce data- |
|equality |full proof in | | volume cap |
|leakage |[SEALPROOFS] | | |
|(MRAE) | | | |
+------------+--------------+--------+------------------------------+
Table 15: Provenance of the security claims
Sullivan Expires 24 January 2027 [Page 76]
Internet-Draft raAE July 2026
C.2. Advantage Notation and Adversary Parameters
For a ra-ROR adversary (Section 5.3) using at most u distinct CEKs
and q_m messages, each encrypted with an independent fresh 32-octet
per-content salt (Section 4.9.1.1), across n_ep total epochs and
making q_e encryption and q_d decryption segment queries with sigma =
q_e + q_d, the advantage is bounded by:
Adv_raROR(A) <= Adv_salt_coll
+ Adv_muPRF(KDF, B_KDF)
+ Adv_muROR(AEAD, B_AEAD)
+ Adv_nonce_coll
+ Adv_snap
Adv_salt_coll The probability that two messages encrypted under one
CEK draw the same salt. Because the salt is a 32-octet value
drawn uniformly at random, this term is at most q_m^2 / 2^256. A
salt collision is not a PRF-distinguishing event: the KDF
deterministically produces identical payload_key, snap_key, and
nonce_base outputs whenever (CEK, payload_info) repeats, so the
event must be charged separately. Conditioned on no salt
collision, distinct messages under one CEK have distinct
payload_info values and therefore independent KDF outputs under
the mu-PRF assumption. This term holds only under the fresh per-
message uniform salt requirement of Section 4.9.1.1. A profile
that reuses salts or draws them non-uniformly forfeits it and must
argue payload-schedule separation by other means (Section 6.7).
Adv_muPRF(KDF, B_KDF) The multi-user PRF advantage of the KDF,
covering two keying levels. Level A is keyed by the CEK and
produces payload_key, snap_key, and nonce_base. It has u users,
queried once per message. Level B is keyed by payload_key
(producing epoch_key per Section 4.5.2 and, in the plaintext-bound
construction, per-segment nonces per Appendix A) and by snap_key
(producing the snapshot authenticator's keyed derivations). It
has q_m users. The snap_key subset of Level B feeds the
configured snapshot authenticator and is counted under Adv_snap
below. It is not counted again here.
Adv_muROR(AEAD, B_AEAD) The multi-user real-or-random advantage of
Sullivan Expires 24 January 2027 [Page 77]
Internet-Draft raAE July 2026
the AEAD over sigma total segment queries. There is one mu-ROR
user per distinct segment key, for n_ep epoch keys total (n_ep =
q_m when each message uses a single flat epoch key). This
advantage absorbs a key-collision birthday term of about
n_ep^2/2^(8*Nk) over the n_ep distinct keys. For the 256-bit-key
suites (Nk = 32) that term is about n_ep^2/2^256 and is
negligible. For the 128-bit-key suite (Nk = 16) it is about
n_ep^2/2^128 and is the binding epoch-key collision floor.
Section 5.8.5 bounds n_ep for that suite.
Adv_nonce_coll The probability of a nonce collision under any
segment key. In derived nonce mode it is zero, conditional on no
salt collision (already charged via Adv_salt_coll). In random
nonce mode (with or without the plaintext-bound hedge) it is
bounded per Section 5.8.6.1. The derived-mode statement relies on
the deterministic-MRAE analysis in Appendix C.5.
Adv_snap The snapshot-forgery advantage of the configured snapshot
authenticator, present only under an authenticator that produces a
snapshot value and zero under None (Section 4.7.2). It covers the
mu-PRF cost of the authenticator's snap_key derivations (the
snap_key subset of Level B, counted here and not again in
Adv_muPRF(KDF, B_KDF)) with the forgery and any further terms the
authenticator's own construction requires. A key-holder content-
binding property, where an authenticator provides one, is a
separate notion from this without-key advantage and is not counted
here.
C.3. ra-ROR Reduction
The reduction follows the [FLRR25] hybrid over the two-level key
schedule (Section 4.5). Condition on no salt collision
(Adv_salt_coll). Replace the CEK-keyed KDF outputs and the
payload_key-keyed outputs with uniformly random values, charged to
Adv_muPRF; these are Level A and the payload_key subset of Level B.
The snap_key-keyed outputs are idealized likewise, but their mu-PRF
cost is the snap_key subset of Level B, charged to Adv_snap and not
to Adv_muPRF (Section 5.7). Bound the resulting segment AEAD outputs
(Adv_muROR). A flat epoch key is the single-epoch case of the same
argument. The segment AEAD calls are nonce-respecting only under no
nonce collision, the event charged to Adv_nonce_coll (Appendix C.2).
In derived nonce mode this term is zero conditional on the same no-
salt-collision event (Appendix C.5). The snapshot authenticator's
forgery is the remaining part of Adv_snap (Section 5.7). The
reduction constructs adversaries B_KDF against multi-user PRF
security of the KDF and B_AEAD against multi-user real-or-random
security of the AEAD, each running in time approximately that of A.
Sullivan Expires 24 January 2027 [Page 78]
Internet-Draft raAE July 2026
C.4. Commitment
ra-CMT security reduces to collision resistance of the commitment
derivation map over the tuple (protocol_id, "commit", CEK,
payload_info, G, commitment_length), with G empty by default. PRF
security alone is not sufficient for this reduction, because the
commitment adversary may choose the CEK and the context values
(Section 4.5).
For commitment_length = L octets, the relevant quantities are:
fixed-pair collision probability: 2^(-8*L)
q-query birthday probability: q^2 / 2^(8*L + 1)
collision-search work factor: about 2^(4*L)
Thus 16 octets gives about 2^64 collision-search work, 32 octets
about 2^128, 48 octets about 2^192, and 64 octets about 2^256, each
capped by the collision resistance of the underlying hash. The
fixed-pair collision probabilities at 32, 48, and 64 octets are
2^(-256), 2^(-384), and 2^(-512), respectively.
The ra-CMT-p advantage is bounded by the sum of two terms: the
commitment collision bound above and the underlying AEAD's commitment
bound for the forged position.
Position binding is inherited from the underlying AEAD, restored by
the segment commitment over a strict ordering where the snapshot is
authenticated outside the construction (Section 5.6). The notion
derives from [FLRR25], and the per-segment associated-data and
position accounting is deferred to [SEALPROOFS] (Appendix C.1).
C.5. Nonce Modes
The unique-nonce transform of Section 4.5.3.2, which XORs the segment
index and finality bit into nonce_base, follows TLS 1.3 ([RFC8446])
static-IV-XOR-identifier and STREAM ([HRRV15]) counter-plus-final-bit
precedent. The transform is injective in the pair (i, is_final).
The leading Nn - 8 octets of nonce_base are unchanged; XOR by its
fixed low 8-octet value is a bijection on 8-octet strings; and (i,
is_final) maps to the integer (i << 1) | is_final, which is a
bijection from the index range i < 2^63 onto the 64-bit values.
Distinct pairs therefore yield distinct nonces, and a derived nonce
repeats only when an object encrypts twice at one pair.
Both disciplines of Section 4.5.3.2 keep every derived nonce carrying
one plaintext, so each reduces unique-nonce record protection to
nonce-respecting AEAD security. That reduction is precedent, not a
SEAL-specific proof.
Sullivan Expires 24 January 2027 [Page 79]
Internet-Draft raAE July 2026
Under both disciplines no segment is ever re-encrypted with a
different plaintext or associated data (Section 6.2,
Section 4.9.3.1), so where a nonce is used twice both uses carry the
same inputs, AEAD.Encrypt reproduces the same ciphertext, and a
reduction answers the second query from the first one's output. The
bound is therefore the same in both cases: the underlying AEAD's
bound under distinct nonces.
In derived nonce mode the nonce-collision term is zero conditional on
no salt collision, because each segment index maps to one derived
nonce and a fresh per-message salt makes the segment keys distinct
across messages. A rewrite reuses a segment's derived nonce, so the
construction relies on the underlying MRAE AEAD: re-encrypting the
same plaintext and associated data under the same key and nonce
reproduces the same ciphertext, which leaks only equality of those
inputs, while distinct inputs remain real-or-random secure
([RFC8452]). The formal real-or-random treatment of this equality
leakage for derived nonce mode is in preparation ([SEALPROOFS]).
That treatment builds on the [FLRR25] ra-ROR analysis and changes its
nonce-respecting hypothesis: a rewrite repeats a segment's derived
nonce, so the proof replaces the nonce-respecting AEAD assumption
with deterministic-MRAE security, under which nonce reuse leaks only
input equality.
Appendix D. Design Rationale
This appendix is informative.
D.1. Key Schedule Design Rationale
The per-content salt makes the payload schedule unique even when a
CEK is reused across messages, which matters for applications that
derive CEKs from group keys. The encryptor chooses the salt locally
at write time. A per-message counter would instead require
synchronized state.
The CEK is 32 octets regardless of the AEAD key size Nk, so a 128-
bit-key AEAD still derives its keys from a 256-bit CEK.
Epoch keys bound the number of AEAD invocations under any one segment
encryption key.
AEAD per-key bounds: The per-key bounds of the underlying AEAD
(birthday for 96-bit-nonce schemes, block-size for AES, integrity
forgery for all schemes) become the limiting factor on security
long before the nonce space is fully exhausted.
Write-once content: Large content with many segments can place many
Sullivan Expires 24 January 2027 [Page 80]
Internet-Draft raAE July 2026
AEAD invocations under one key, even when the content is written
once.
Rewrites: Rewrites consume additional invocations and can exhaust
the same budget faster than write-once use, but they are not the
only reason for epoching. Write-once large content benefits from
the same partitioning.
Random access: Rekeying the entire content would defeat the random-
access property, so epoch keys bound the per-key invocation count
without requiring a full re-encryption.
Labels separate derivation roles: commitment and payload_key share
the same inputs but different labels, making them independent under
the PRF assumption. Once CEK and salt are chosen the hierarchy is
fixed, with no mutable state to synchronize across writers.
D.2. Nonce Mode Design Rationale
Random mode is simplest but trusts the CSPRNG completely. Derived
mode removes that trust. In a mutable profile it requires an MRAE
AEAD, because an in-place rewrite reuses the segment nonce
(Section 6.5). In the write-once profile the write-once rule keeps
every derived nonce unique, and in the append-only profile the
append-only rule does, so under either, derived mode pairs with any
AEAD. A third rule was available and rejected: permitting a length
change in either direction would let a truncation free indices the
object had already spent, for a later append to encrypt new content
at (Section 4.9.3.1).
The nonce mode is a profile parameter: a profile selects a valid
(nonce_mode, snap_id) tuple, subject to the MRAE requirement above.
Acknowledgments
The author thanks Andrés Fábrega, Thomas Ristenpart, Gregory Rubin,
Richard Barnes, Brendan McMillion, Thibault Meunier, Kenny Paterson,
Christopher Patton, Martin Thomson, Stanislav V. Smyshlyaev, and
Christopher A. Wood for their reviews, comments, and discussions.
Author's Address
Nick Sullivan
Cryptography Consulting LLC
Email: nicholas.sullivan+ietf@gmail.com
Sullivan Expires 24 January 2027 [Page 81]