CCF Profile for COSE Receipts
draft-ietf-scitt-receipts-ccf-profile-01
| Document | Type | Active Internet-Draft (scitt WG) | |
|---|---|---|---|
| Authors | Henk Birkholz , Antoine Delignat-Lavaud , Cedric Fournet , Amaury Chamayou | ||
| Last updated | 2026-04-02 | ||
| Replaces | draft-birkholz-cose-receipts-ccf-profile | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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draft-ietf-scitt-receipts-ccf-profile-01
SCITT H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track A. Delignat-Lavaud
Expires: 4 October 2026 C. Fournet
A. Chamayou
Microsoft Research
2 April 2026
CCF Profile for COSE Receipts
draft-ietf-scitt-receipts-ccf-profile-01
Abstract
This document defines a new verifiable data structure (VDS) type for
COSE Receipts and inclusion proof specifically designed for append-
only logs produced by the Confidential Consortium Framework (CCF) to
provide stronger tamper-evidence guarantees.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 4 October 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. Description of the Confidential Consortium Framework Ledger
Verifiable Data Structure . . . . . . . . . . . . . . . . 3
2.1. Merkle Tree Shape . . . . . . . . . . . . . . . . . . . . 3
2.2. Transaction Components . . . . . . . . . . . . . . . . . 4
3. CCF Inclusion Proofs . . . . . . . . . . . . . . . . . . . . 5
3.1. CCF Inclusion Proof Signature . . . . . . . . . . . . . . 6
3.2. Inclusion Proof Verification Algorithm . . . . . . . . . 6
4. Usage in COSE Receipts . . . . . . . . . . . . . . . . . . . 7
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. Trusted Execution Environments . . . . . . . . . . . . . 8
6.2. Operators . . . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7.1. Additions to Existing Registries . . . . . . . . . . . . 9
7.1.1. Tree Algorithms . . . . . . . . . . . . . . . . . . . 9
8. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The COSE Receipts document [I-D.ietf-cose-merkle-tree-proofs] defines
a common framework for expressing different types of proofs about
verifiable data structures (VDS), providing a standardized way to
convey trust relevant evidence. For instance, inclusion proofs
guarantee to a verifier that a given serializable element is recorded
at a given state of the VDS, while consistency proofs are used to
establish that an inclusion proof is still consistent with the new
state of the VDS at a later time.
In this document, we define a new type of VDS and inclusion proof
associated with an application of the Confidential Consortium
Framework (CCF) ledger that implements the SCITT Architecture defined
in [I-D.ietf-wg-scitt-architecture]. This VDS carries indexed
transaction information in a binary Merkle Tree, where new
transactions are appended to the right, so that the binary
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decomposition of the index of a transaction can be interpreted as the
position in the tree if 0 represents the left branch and 1 the right
branch. Compared to [RFC9162], the leaves of CCF trees carry
additional internal information for the following purposes:
1. To bind the full details of the transaction executed, which is a
super-set of what is exposed in the proof and captures internal
information details useful for detailed system audit, but not for
application purposes.
2. To allow the distributed system executing the application logic
in Trusted Execution Environments (TEE) to persist signatures to
storage early. Receipt production in only enabled once
transactions are fully committed by the consensus protocol.
1.1. Requirements Notation
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.
2. Description of the Confidential Consortium Framework Ledger
Verifiable Data Structure
This documents extends the VDS registry of
[I-D.ietf-cose-merkle-tree-proofs] with the following value:
+===================+===============+==================+===========+
| Name | Value | Description | Reference |
+===================+===============+==================+===========+
| CCF_LEDGER_SHA256 | TBD_1 | Historical | RFCthis |
| | (requested | transaction | |
| | assignment 2) | ledgers, such as | |
| | | the CCF ledger | |
+-------------------+---------------+------------------+-----------+
Table 1: Verifiable Data Structure Algorithms
2.1. Merkle Tree Shape
A CCF ledger is a binary Merkle Tree constructed from a hash function
H, which is defined from the log type. For instance, the hash
function for CCF_LEDGER_SHA256 is SHA256, whose HASH_SIZE is 32
bytes.
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The Merkle tree encodes an ordered list of n transactions T_n =
{T[0], T[1], ..., T[n-1]}. We define the Merkle Tree Hash (MTH)
function, which takes as input a list of serialized transactions (as
byte strings), and outputs a single HASH_SIZE byte string called the
Merkle root hash, by induction on the list.
This function is defined as follows:
The hash of an empty list is the hash of an empty string:
MTH({}) = HASH().
The hash of a list with one entry (also known as a leaf hash) is:
MTH({d[0]}) = HASH(d[0]).
For n > 1, let k be the largest power of two smaller than n (i.e., k
< n <= 2k). The Merkle Tree Hash of an n-element list D_n is then
defined recursively as:
MTH(D_n) = HASH(MTH(D[0:k]) || MTH(D[k:n])),
where:
* || denotes concatenation
* : denotes concatenation of lists
* D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] =
d[k1+1], ..., d'[k2-k1-1] = d[k2-1]} of length (k2 - k1).
2.2. Transaction Components
Each leaf in a CCF ledger carries the following components:
ccf-leaf = [
; Byte string of size HASH_SIZE(32)
internal-transaction-hash: bstr .size 32
; Text string of at most 1024 bytes
internal-evidence: tstr .size (1..1024)
; Byte string of size HASH_SIZE(32)
data-hash: bstr .size 32
]
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The internal-transaction-hash and internal-evidence byte strings are
internal to the CCF implementation. They can be safely ignored by
receipt Verifiers, but they commit the transparency service (TS) to
the whole tree contents and may be used for additional, CCF-specific
auditing.
internal-transaction-hash is a hash over the complete entry in the
[CCF-Ledger-Format], and internal-evidence is a revealable
[CCF-Commit-Evidence] value that allows early persistence of ledger
entries before distributed consensus can be established. This
mechanism is useful to implement high-throughput transparency
applications in Trusted Execution Environments (TEEs) that only
provide a limited amount of memory, while maintaining high
availability afforded by distributed consensus.
data-hash summarizes the application data included in the ledger at
this transaction, which is a Signed Statement as defined by
[I-D.ietf-wg-scitt-architecture].
3. CCF Inclusion Proofs
CCF inclusion proofs consist of a list of digests tagged with a
single left-or-right bit.
ccf-proof-element = [
; Position of the element
left: bool
; Hash of the proof element: byte string of size HASH_SIZE(32)
hash: bstr .size 32
]
ccf-inclusion-proof = bstr .cbor {
&(leaf: 1) => ccf-leaf
&(path: 2) => [+ ccf-proof-element]
}
Unlike some other tree algorithms, the index of the element in the
tree is not explicit in the inclusion proof, but the list of left-or-
right bits can be treated as the binary decomposition of the index,
from the least significant (leaf) to the most significant (root).
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3.1. CCF Inclusion Proof Signature
The proof signature for a CCF inclusion proof is a COSE signature
(encoded with the COSE_Sign1 CBOR type) which includes the following
additional requirements for protected and unprotected headers.
Please note that there may be additional header parameters defined by
the application.
The protected header parameters for the CCF inclusion proof signature
MUST include the following:
* verifiable-data-structure: int/tstr. This header MUST be set to
the verifiable data structure algorithm identifier for ccf-ledger
(TBD_1).
* label: int. This header MUST be set to the value of the inclusion
proof type in the IANA registry of Verifiable Data Structure Proof
Type (-1).
The unprotected header for a CCF inclusion proof signature MUST
include the following:
* inclusion-proof: bstr .cbor ccf-inclusion-proof. This contains
the serialized CCF inclusion proof, as defined above.
The payload of the signature is the CCF ledger Merkle root digest,
and MUST be detached in order to force verifiers to recompute the
root from the inclusion proof in the unprotected header. This
provides a safeguard against implementation errors that use the
payload of the signature but do not recompute the root from the
inclusion proof.
3.2. Inclusion Proof Verification Algorithm
CCF uses the following algorithm to verify an inclusion receipt:
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compute_root(proof):
h := proof.leaf.internal-transaction-hash
|| HASH(proof.leaf.internal-evidence)
|| proof.leaf.data-hash
for [left, hash] in proof:
h := HASH(hash + h) if left
HASH(h + hash) else
return h
verify_inclusion_receipt(inclusion_receipt):
let label = INCLUSION_PROOF_LABEL
assert(label in inclusion_receipt.unprotected_header)
let proof = inclusion_receipt.unprotected_header[label]
assert(inclusion_receipt.payload == nil)
let payload = compute_root(proof)
# Use the Merkle Root as the detached payload
return verify_cose(inclusion_receipt, payload)
A description can also be found at [CCF-Receipt-Verification].
4. Usage in COSE Receipts
A COSE Receipt with a CCF inclusion proof is described by the
following CDDL definition:
protected-header-map = {
&(alg: 1) => int
&(vds: 395) => 2
* cose-label => cose-value
}
* alg (label: 1): REQUIRED. Signature algorithm identifier. Value
type: int.
* vds (label: 395): REQUIRED. verifiable data structure algorithm
identifier. Value type: int.
The unprotected header for an inclusion proof signature is described
by the following CDDL definition:
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inclusion-proof = ccf-inclusion-proof
inclusion-proofs = [ + inclusion-proof ]
verifiable-proofs = {
&(inclusion-proof: -1) => inclusion-proofs
}
unprotected-header-map = {
&(vdp: 396) => verifiable-proofs
* cose-label => cose-value
}
5. Privacy Considerations
See the privacy considerations section of:
* [I-D.ietf-cose-merkle-tree-proofs]
6. Security Considerations
The security considerations of [I-D.ietf-cose-merkle-tree-proofs]
apply.
6.1. Trusted Execution Environments
CCF networks of nodes rely on executing in TEEs to secure their
function, in particular:
1. The evaluation of registration policies
2. The creation and usage of receipt signing keys
A compromise in the TEE platform used to execute the network may
allow an attacker to produce invalid and divergent ledger branches.
Clients can mitigate this risk in two ways: by regularly auditing the
consistency of the CCF ledger; and by regularly fetching attestation
information about the TEE instances, available in the ledger and from
the network itself, and confirming that the nodes composing the
network are running up-to-date, trusted platform components.
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6.2. Operators
An operator has the ability to start successor networks with a
distinct identity. The operator of a CCF network can recover the
service by starting a successor network, for example a new CCF
network with its own service identity, that endorses the ledger state
of the previous instance. This provides service continuity after a
catastrophic failure of a majority of the nodes. However, a
malicious operator could exploit this mechanism and truncate the
ledger’s history by initializing the successor network from an
earlier ledger prefix, thereby omitting some later entries. Clients
can mitigate this risk by auditing the successor ledger and verifying
that their latest known receipts from the prior service are included
in the successor’s ledger.
7. IANA Considerations
7.1. Additions to Existing Registries
7.1.1. Tree Algorithms
This document requests IANA to add the following new value to the
'COSE Verifiable Data Structures' registry:
* Name: CCF_LEDGER_SHA256
* Value: 2 (requested assignment)
* Description: Append-only logs that are integrity-protected by a
Merkle Tree and signatures produced via Trusted Execution
Environments containing a mix of public and confidential
information, as specified by the Confidential Consortium
Framework.
* Reference: This document
8. Normative References
[CCF] "Confidential Consortium Framework", n.d.,
<https://github.com/microsoft/ccf>.
[CCF-Commit-Evidence]
"CCF Commit Evidence", n.d.,
<https://microsoft.github.io/CCF/main/use_apps/
verify_tx.html#commit-evidence>.
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[CCF-Ledger-Format]
"CCF Ledger Format", n.d.,
<https://microsoft.github.io/CCF/main/architecture/
ledger.html>.
[CCF-Receipt-Verification]
"CCF Receipt Verification", n.d.,
<https://microsoft.github.io/CCF/main/use_apps/
verify_tx.html#receipt-verification>.
[I-D.ietf-cose-merkle-tree-proofs]
Steele, O., Birkholz, H., Delignat-Lavaud, A., and C.
Fournet, "COSE (CBOR Object Signing and Encryption)
Receipts", Work in Progress, Internet-Draft, draft-ietf-
cose-merkle-tree-proofs-18, 2 December 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
merkle-tree-proofs-18>.
[I-D.ietf-wg-scitt-architecture]
"*** BROKEN REFERENCE ***".
[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>.
[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>.
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, <https://www.rfc-editor.org/rfc/rfc9162>.
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@ietf.contact
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Antoine Delignat-Lavaud
Microsoft Research
21 Station Road
Cambridge
CB1 2FB
United Kingdom
Email: antdl@microsoft.com
Cedric Fournet
Microsoft Research
21 Station Road
Cambridge
CB1 2FB
United Kingdom
Email: fournet@microsoft.com
Amaury Chamayou
Microsoft Research
21 Station Road
Cambridge
CB1 2FB
United Kingdom
Email: amaury.chamayou@microsoft.com
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