Encrypted Attachments for MLS
draft-sullivan-mls-attachments-01
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Nick Sullivan , Raphael Robert | ||
| Last updated | 2026-07-21 | ||
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draft-sullivan-mls-attachments-01
Messaging Layer Security N. Sullivan
Internet-Draft Cryptography Consulting LLC
Intended status: Standards Track R. Robert
Expires: 22 January 2027 Phoenix R&D GmbH
21 July 2026
Encrypted Attachments for MLS
draft-sullivan-mls-attachments-01
Abstract
This document defines random-access authenticated encryption of large
write-once files for Messaging Layer Security (MLS) groups. A file
is encrypted so that a receiver can decrypt and authenticate any byte
range without processing the whole file. The encryption is SEAL-
attachment, SEAL's named write-once attachment scheme (raAE),
parameterized by the AEAD and key derivation function of the group's
MLS cipher suite and keyed from the MLS exporter. The encrypted
bytes are carried by any means, and a recipient needs only a small
reference (the object's identifier, length, and snapshot value, and
an optional locator) to fetch, key, and verify the object. Carrying
that reference in an MLS message attributes the object to the member
that sent the message. MLS application messages cannot carry large
files, and existing attachment encryption produces an opaque,
immutable blob with no partial access. This extension supplies the
random-access layer those uses need.
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/.
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 22 January 2027.
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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. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Key Derivation . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Deriving the CEK . . . . . . . . . . . . . . . . . . . . 5
4.2. Choosing object_id . . . . . . . . . . . . . . . . . . . 5
5. Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Object Layout . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Encrypting and Reading . . . . . . . . . . . . . . . . . 7
6. Referencing an Object . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7.1. Confidentiality Lifetime and Membership . . . . . . . . . 10
7.2. Capacity and Limits . . . . . . . . . . . . . . . . . . . 10
7.3. Salt Uniqueness . . . . . . . . . . . . . . . . . . . . . 11
7.4. Writer Attribution . . . . . . . . . . . . . . . . . . . 11
7.5. Position Commitment under Adversarial Keys . . . . . . . 12
7.6. Operational Considerations . . . . . . . . . . . . . . . 12
8. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 12
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . 14
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
This document defines an MLS extension for random-access
authenticated encryption of large files. A file is encrypted with a
random-access authenticated encryption (raAE) scheme
([I-D.sullivan-cfrg-raae]): it is split into fixed-size segments,
each independently decryptable and authenticated, so a receiver can
read any byte range at a cost proportional to the range rather than
to the whole file. The scheme is SEAL-attachment, SEAL's named
write-once attachment scheme, taking the AEAD and KDF of the group's
MLS cipher suite (Section 5) and its key from the MLS exporter
(Section 4).
This fills a gap in MLS. An MLS application message protects its
content with a single AEAD operation under a forward-secret ratchet
key, so the content is all-or-nothing to decrypt and bounded by the
AEAD's per-invocation input limit. Large files therefore cannot be
sent as messages, and the existing attachment mechanism
[I-D.robert-mimi-attachments] encrypts each file as a single opaque
blob that is immutable and cannot be partially decrypted.
The file bytes are an opaque object distributed by any means, and the
members exchange only a small object reference (Section 6) that
refers to it. This document specifies the object's encryption and
keying. Upload, download, storage, and the in-band reference are
left to the consuming application, for example the MIMI attachments
mechanism. It keys SEAL from the MLS exporter, following the pattern
SFrame [RFC9605] established for real-time media. Deriving the CEK
deterministically from (epoch, object_id) is a new keying discipline
for SEAL, not a new primitive, and carries its own salt-uniqueness
obligation (Section 7.3).
Its confidentiality is at the granularity of the creating epoch's
membership, under a persistent key rather than the per-message
forward secrecy of MLS application messages (Section 7).
2. Conventions and Definitions
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.
This document uses the following terms.
Attachment object: The encrypted representation of one attachment: a
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SEAL-attachment object (SEAL Named Instantiations of
[I-D.sullivan-seal-concrete]), whose byte layout is Section 5.1.
Segment: A fixed-size unit of attachment plaintext. All segments
except possibly the last contain exactly segment_size octets.
object_id: An octet string of at most 255 octets (Section 4.2),
unique within an MLS epoch, that names one attachment object and
feeds its key derivation (Section 4).
CEK: The content-encryption key (CEK) of an attachment object,
derived from the MLS exporter and used as the input keying
material of SEAL.
Object reference: The application's in-band reference to an
attachment object, carrying at least the object_id, the object's
plaintext length, and the object's snapshot value (Section 6).
Terms from MLS ([RFC9420]), the MLS Safe Extension framework
([I-D.ietf-mls-extensions]), and SEAL ([I-D.sullivan-cfrg-raae]) are
used as defined there.
3. Architecture
An attachment is handled on three independent planes: a key plane
that derives the content key from the MLS exporter, an opaque bytes
plane that holds the encrypted object, and a control plane that
carries a small object reference among the members.
key plane (this document) control plane (the application)
MLS exporter object reference
| (object_id, length, locator, ...)
| derive (never on wire) |
v |
CEK ------- keys ------. | refers to
| |
v v
.---------------------------------.
bytes plane | encrypted SEAL object |
| (opaque; see Object Layout) |
'---------------------------------'
stored and moved by any transport
Figure 1: The three planes of an attachment
The CEK never appears on the wire; each side derives it from the
exporter (Section 4).
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4. Key Derivation
This extension is a Safe Application component
([I-D.ietf-mls-extensions]) and is assigned a Component ID, a four-
octet value, registered in Section 10. All of its uses of the MLS
key schedule are domain-separated by that Component ID.
4.1. Deriving the CEK
For each attachment object, the encryptor chooses an object_id and
derives the content-encryption key:
component_secret = SafeExportSecret(ComponentID)
CEK = ExpandWithLabel(component_secret,
ComponentOperationLabel(ComponentID,
"attachment"),
object_id, 32)
where:
component_secret: the component's exported secret. SafeExportSecret
is defined in Exported Secrets of [I-D.ietf-mls-extensions], and
ComponentOperationLabel in Component IDs of
[I-D.ietf-mls-extensions]. SafeExportSecret derives from the MLS
exporter, so the CEK is rooted in it.
object_id: the per-object identifier (Section 4.2), passed to
ExpandWithLabel as raw octets; the KDFLabel structure of [RFC9420]
already length-prefixes it, so this document adds no framing of
its own.
CEK: the 32-octet content-encryption key, SEAL's input keying
material ([I-D.sullivan-cfrg-raae]).
The group's MLS cipher suite is fixed for the group's lifetime
([RFC9420]), so an attachment uses the same AEAD and KDF throughout.
4.2. Choosing object_id
An honest encryptor:
* MUST choose a non-empty object_id of at most 255 octets, unique
within the epoch;
* SHOULD draw it from a cryptographically secure pseudorandom number
generator (CSPRNG) with no embedded semantics, for example a
version 4 UUID ([RFC9562]), which is virtually collision-free; and
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* MUST NOT reuse an object_id across epochs.
These uniqueness requirements bind the encryptor alone: a receiver
cannot detect reuse of an object_id, so the unlinkability they
provide rests on encryptor discipline, not on any receiver check
(Section 9).
An application MAY instead derive the object_id deterministically,
provided the derivation still meets the requirements above. Deriving
it from the epoch, the sender's leaf index, and a per-object counter,
for example object_id = H(epoch, leaf_index, counter), makes non-
collision between honest senders structural rather than
probabilistic, since distinct leaf indices separate different
senders' objects and a counter separates a sender's own. Such an
object_id embeds semantics by design: a party that sees it can link a
sender's objects within an epoch, a linkability the CSPRNG option
avoids (Section 9). It does not attribute an object to its writer,
which only the signature does (Section 7.4).
5. Parameters
An attachment is a SEAL-attachment(aead_id, kdf_id) object (SEAL
Named Instantiations of [I-D.sullivan-seal-concrete]), where aead_id
and kdf_id are the IANA code points ([RFC5116], [RFC9180]) of the
group's MLS cipher suite's AEAD and KDF. The name fixes every other
parameter of the scheme, so the cipher suite is the only choice this
document makes; the name fixes a segment size, written segment_size,
of 65536 octets. A receiver learns the cipher suite from the group
state, so it is not carried in the reference; a holder that keeps the
object past its creating epoch retains the CEK and the cipher suite's
AEAD and KDF identifiers, since the group state may no longer supply
them. The object is keyed by the CEK (Section 4). Its capacity and
security bounds are in Section 7.2.
Every such object carries a snapshot value, and so does the reference
to it (Section 6). A receiver checks one segment against the
snapshot without reading the others (Section 5.2).
5.1. Object Layout
The attachment object uses SEAL-attachment's aligned layout, which
fixes the object's byte layout and its offset arithmetic (Aligned
Layout of [I-D.sullivan-seal-concrete]). That layout leaves two
details to the consuming protocol, and a receiver cannot compute a
segment's offset until both are settled. This document settles them:
* There is no prefix. An object begins with SEAL's header at offset
0.
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* The slot between the header and the first segment boundary is
zero-padded, so every ciphertext segment except the last is a full
segment_size octets.
A receiver then derives every offset from the object's length and the
group's cipher suite, with no further input: the length gives the
segment count, the segment count and the cipher suite's hash length
give the header's size, and the first segment begins at the next
segment_size boundary after it.
Checking one segment takes three reads: the per-epoch digests, which
a receiver reads once and caches; the metadata entries of that
segment's own epoch; and the segment itself.
object each read
+--------------+
| 96 B |
+--------------+
| digests | 64 KiB ----> +----------+
+--------------+ | 64 KiB | all
| | +----------+
| metadata |
| 2048 epochs | 96 MiB ----> +----------+
| | | 48 KiB | 1 epoch
| | +----------+
+--------------+
| |
| segments |
| 2M segments | 128 GiB ----> +----------+
| | | 64 KiB | 1 segment
| | +----------+
| |
+~~~~~~~~~~~~~~+ 176 KiB
Figure 2: Three reads check any segment of a 128 GiB object (SHA-256)
The entries are grouped by epoch, which is what bounds the second
read: an epoch's 1024 entries are adjacent, so one read returns all
of them. An epoch's digest is 32 octets, so 2048 of them fill one
read, which is the 128 GiB above.
5.2. Encrypting and Reading
To encrypt, the encryptor derives the CEK, generates a fresh salt per
the freshness requirement of Section 7.3, and runs SEAL encryption
with the parameters of Section 5, writing the salt, commitment, and
segments in the layout of Section 5.1.
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On StartEnc and StartDec, both sides supply the object_id as G,
SEAL's global associated data (Per-Message Inputs of
[I-D.sullivan-cfrg-raae]). SEAL's commitment binds G, so a wrong or
missing object_id fails the commitment check as a wrong CEK does.
The object_id thus serves twice: it derives the CEK (Section 4) and
is the committed G.
A receiver opens an object in two phases: it initializes once, then
reads ranges, and it needs the header only for the first phase.
To initialize, the receiver derives the CEK, fetches the header (the
salt and commitment), and calls StartDec to obtain the decryption
state. StartDec checks the commitment, rejecting a wrong key before
any segment is read. The receiver retains the state and does not
need the header again.
To read octet range [a, b), the receiver fetches the segments that
cover it, at offsets the layout fixes (Section 5.1), and opens each
with DecSeg, which runs the key schedule and AEAD inside.
Opening a segment authenticates it only to the group, since every
member holds the CEK (Section 7.4); whole-object integrity comes from
the snapshot the reference carries (Section 6).
A receiver checks a segment against that snapshot by SEAL's random-
access verification, which is not restated here (Epoch Digest Tree of
[I-D.sullivan-seal-concrete]).
One input to that check differs here. The value a receiver compares
against MUST be the snapshot the reference carries, and a receiver
MUST NOT substitute the copy stored in the object. SEAL compares
against the stored copy, which suffices where each object has a fresh
random CEK. Here every member derives the CEK and can compute a
stored copy that matches any object it writes, so the stored copy
attributes nothing. Only the value the group authenticated does.
This check is only as strong as the snapshot it starts from, so it
gives whole-object completeness and attribution only when the
reference is authenticated to the group (Section 6).
A valid attachment object has at least one segment. A receiver MUST
reject the object when:
* StartDec rejects the header (a wrong key or parameter set);
* SEAL's verification fails against the snapshot the reference
carries;
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* DecSeg rejects a segment it opens (the contents, index, or
finality fail to authenticate);
* a segment it needs is absent (the object is shorter than the
reference's length implies);
* a requested byte range lies beyond the length carried in the
reference (Section 6); or
* the object_id is empty.
6. Referencing an Object
A consuming application refers to an attachment object with a small
reference. This document does not define the reference's structure
or its transport. Those are the application's, for example the MIMI
attachments mechanism ([I-D.robert-mimi-attachments]). This section
states only what the reference MUST convey for this extension's
guarantees to hold.
The reference MUST carry the object_id, because a receiver derives
the CEK from it (Section 4) and cannot otherwise key the object. The
reference MUST also carry the object's plaintext length, which the
receiver needs to locate segments in the layout (Section 5.1) and to
validate a requested range. The reference MUST also carry the
object's snapshot value, against which the receiver checks every
segment it reads (Section 5.2).
Writer attribution and whole-object completeness need the reference
authenticated to the group. For that, it MUST be bound into an MLS
message's authenticated data, which the sender's signature covers:
the signature authenticates the reference and attributes the object
to the sending member (Section 7.4). This document defines no
signature of its own.
The length needs that authentication because the snapshot binds only
the segment count: an adversary can understate the length within the
final segment's span without changing the count, and a receiver that
never fetches the final segment cannot detect the truncation.
MLS does not give a total order across different senders' application
messages within an epoch, so delivery order alone does not establish
which object is current. An application that needs a current-object
notion MUST scope the ordering to a single sender or carry an
explicit application-level sequence number in the authenticated
reference.
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Any locator or other fields the reference carries are the
application's.
7. Security Considerations
7.1. Confidentiality Lifetime and Membership
The CEK is derived from one MLS epoch's exporter and, unlike an MLS
message key, is not deleted after use: it is a persistent key for the
life of the attachment. The MLS exporter itself is forward-secure
across epochs, so once that epoch's secrets are deleted the CEK
cannot be re-derived from MLS. Holders that retain the CEK keep
access and others do not. Attachment confidentiality is therefore at
the granularity of the creating epoch's membership and lasts as long
as any holder retains the CEK, not at the per-message granularity of
MLS application messages. A re-readable, seekable object needs a
persistent key, so it cannot have the per-message forward secrecy of
MLS application messages.
Removing a member does not by itself revoke access to attachments
created while they were present, since they may have retained the
CEK. An application that must revoke access to an existing
attachment MUST distribute a new object encrypted under a CEK derived
in a later epoch. This costs the size of the attachment, because the
removed member knows the old CEK.
Because the AEAD and KDF are inherited from the MLS cipher suite, an
attachment inherits the cipher suite's AEAD and KDF strength. The
CEK is exported from the MLS key schedule, so an attachment's long-
term confidentiality rests on the group's key agreement, not on the
AEAD key size alone. A quantum adversary that records a group's
handshake can, once a quantum computer is available, recover the
exported secrets of past epochs, and thus the CEK, regardless of the
AEAD key size. Deployments that need long-lived confidentiality for
attachments SHOULD therefore run a post-quantum MLS cipher suite. A
larger symmetric key does not address this threat.
7.2. Capacity and Limits
An attachment inherits SEAL-attachment's capacity and security bounds
([I-D.sullivan-cfrg-raae]), which rest on the group's key agreement
as well as the AEAD (Section 7.1). An application MUST stop writing
an object before a per-key budget is exceeded. The CEK is fixed per
object, so continued storage requires a new object under a fresh CEK.
The per-segment metadata adds one hash per segment, so both an
attachment's storage overhead and its per-segment read cost scale
with the cipher suite's hash length ([I-D.sullivan-cfrg-raae]).
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SEAL-attachment pins the epoch length for every suite, so the SHA-512
suites, whose entries are largest, cost one extra metadata read per
segment checked.
7.3. Salt Uniqueness
Unlike base SEAL, which keys each object with a fresh random CEK,
this extension derives the CEK deterministically from (epoch,
object_id) (Section 4). SEAL-attachment uses a derived nonce, so the
per-object salt is the only value that separates two encryptions
under the same CEK. Reusing an (object_id, salt) pair within an
epoch reuses the derived nonces, which for an AEAD that is not
misuse-resistant, such as AES-GCM or ChaCha20-Poly1305, is a two-time
pad and breaks confidentiality. The encryptor therefore MUST
generate a fresh, uniformly random 32-octet salt from a CSPRNG for
each object, and MUST NOT re-encrypt an object under a previously
used (object_id, salt) pair, including after a crash or retry.
7.4. Writer Attribution
All epoch members derive the CEK, so per-segment authentication binds
an attachment to the group, not to a writer: any member can produce a
segment that others accept. Attribution comes from the sender's
signature. The reference carries the object's snapshot value, and
MLS covers a message's authenticated data by the sender's signature,
so binding the reference into an MLS message attributes the object to
the sending member (Section 6). A receiver checks one segment
against the snapshot and attributes it without reading the rest
(Section 5.2).
The snapshot binds each segment's ciphertext by a collision-resistant
digest, so a signature over it attributes content even against a
member that holds the CEK (Epoch Digest Tree of
[I-D.sullivan-seal-concrete]).
Attribution is to the member that sent the reference, in the epoch it
was sent, and nothing re-attests the object afterward. MLS gives no
total order across senders within an epoch, so an application that
needs the original writer MUST make that writer the object's only
sender, and one that needs a current-object notion MUST carry the
ordering itself (Section 6).
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7.5. Position Commitment under Adversarial Keys
No AEAD an MLS cipher suite uses is key-committing, so SEAL's
commitment does not bind a segment's position: without the snapshot,
a member that controls the CEK could open one ciphertext segment at
two positions (Commitment Security of [I-D.sullivan-cfrg-raae]). An
authenticated snapshot pins each segment's ciphertext to its index by
collision resistance, so no CEK holder can move a segment, which
restores ra-CMT-p against adversarially chosen keys (Epoch Digest
Tree of [I-D.sullivan-seal-concrete]). It holds only for a snapshot
the group authenticates.
7.6. Operational Considerations
Superseded objects are not deleted automatically: the profile is
write-once and revocation re-encrypts to a new object (Section 7.1),
so the application owns garbage collection and any link from a
replacement to its predecessor. Revocation cost scales with the
total volume of attachments the removed member could read, not with
one object. Recovering from corruption means fetching the object
again, since there is no in-place repair.
8. Test Vectors
An attachment object is a SEAL-attachment object and uses a derived
nonce, so it stores no per-segment nonce (Section 5). SEAL's
published example vectors use a random nonce and a different layout,
so they do not apply byte for byte. Its derived-nonce example vector
is the closest format reference but is computed for AES-256-GCM-SIV.
Test vectors specific to SEAL-attachment under each MLS cipher suite
are TBD, and this document defines none of its own.
9. Privacy Considerations
An untrusted observer of the object's storage or transport does not
learn the plaintext, but it does learn metadata about the object. It
learns the exact plaintext length, which is recoverable from the
object byte count regardless of the reference. It learns which byte
ranges each reader fetches, an access-pattern channel. It can link
fetches by object_id or locator, including across epochs if an
object_id were to repeat (Section 4 forbids that repetition). This
unlinkability across objects and epochs rests on honest-encryptor
discipline in choosing object_id (Section 4.2). It is not a property
a receiver or observer can verify, since neither can detect a
repeated or predictable object_id on its own.
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The reference also carries the object's snapshot value (Section 6),
which discloses nothing further: it is a keyed value, pseudorandom
without the CEK.
These metadata exposures are by design and out of scope for the
encryption layer. Length-hiding by padding and access-pattern hiding
by full-object fetch or oblivious access are left to the application
and the transport. Transport-level sender metadata, such as the
connection and timing of a fetch, is likewise out of scope for this
document.
10. IANA Considerations
This document requests one registration in the "MLS Component Types"
registry established by [I-D.ietf-mls-extensions].
The component domain-separates this extension's uses of the MLS key
schedule (Section 4), which is its only use of an MLS facility.
* Value: (assigned by IANA)
* Name: attachment_encryption
* Where: ES (an exported secret is used)
* Recommended: Y
* Reference: This document
This document defines no new registries.
11. References
11.1. Normative References
[I-D.ietf-mls-extensions]
Robert, R., "The Messaging Layer Security (MLS)
Extensions", Work in Progress, Internet-Draft, draft-ietf-
mls-extensions-10, 6 July 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
extensions-10>.
[I-D.sullivan-cfrg-raae]
Sullivan, N., "Random-Access Authenticated Encryption",
Work in Progress, Internet-Draft, draft-sullivan-cfrg-
raae-02, 13 July 2026,
<https://datatracker.ietf.org/doc/html/draft-sullivan-
cfrg-raae-02>.
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[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>.
[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>.
[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>.
[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>.
[RFC9420] Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
July 2023, <https://www.rfc-editor.org/rfc/rfc9420>.
11.2. Informative References
[FLOE] Fabrega, A., Len, J., Ristenpart, T., and G. Rubin,
"Random-Access AEAD for Fast Lightweight Online
Encryption", IACR ePrint 2025/2275, 2025,
<https://eprint.iacr.org/2025/2275>.
[I-D.robert-mimi-attachments]
Robert, R. and K. Kohbrok, "MIMI Attachments", Work in
Progress, Internet-Draft, draft-robert-mimi-attachments-
06, 29 June 2026, <https://datatracker.ietf.org/doc/html/
draft-robert-mimi-attachments-06>.
[RFC9562] Davis, K., Peabody, B., and P. Leach, "Universally Unique
IDentifiers (UUIDs)", RFC 9562, DOI 10.17487/RFC9562, May
2024, <https://www.rfc-editor.org/rfc/rfc9562>.
Sullivan & Robert Expires 22 January 2027 [Page 14]
Internet-Draft MLS Attachments July 2026
[RFC9605] Omara, E., Uberti, J., Murillo, S. G., Barnes, R., Ed.,
and Y. Fablet, "Secure Frame (SFrame): Lightweight
Authenticated Encryption for Real-Time Media", RFC 9605,
DOI 10.17487/RFC9605, August 2024,
<https://www.rfc-editor.org/rfc/rfc9605>.
Acknowledgments
This extension builds on the SEAL construction
[I-D.sullivan-cfrg-raae], the MLS Safe Extension framework
[I-D.ietf-mls-extensions], and the attachments transport
[I-D.robert-mimi-attachments]. The random-access goals follow
[FLOE].
Authors' Addresses
Nick Sullivan
Cryptography Consulting LLC
Email: nicholas.sullivan+ietf@gmail.com
Raphael Robert
Phoenix R&D GmbH
Email: ietf@raphaelrobert.com
Sullivan & Robert Expires 22 January 2027 [Page 15]