Internet-Draft SCITT Receipts March 2023
Birkholz, et al. Expires 21 September 2023 [Page]
Workgroup:
TBD
Internet-Draft:
draft-birkholz-scitt-receipts-latest
Published:
Intended Status:
Standards Track
Expires:
Authors:
H. Birkholz
Fraunhofer SIT
M. Riechert
Microsoft
A. Delignat-Lavaud
Microsoft
C. Fournet
Microsoft

Countersigning COSE Envelopes in Transparency Services

Abstract

A transparent and authentic Transparent Registry service in support of a supply chain's integrity, transparency, and trust requires all peers that contribute to the Registry operations to be trustworthy and authentic. In this document, a countersigning variant is specified that enables trust assertions on Merkle-tree based operations for global supply chain registries. A generic procedure for producing payloads to be signed and validated is defined and leverages solutions and principles from the Concise Signing and Encryption (COSE) space.

Discussion Venues

This note is to be removed before publishing as an RFC.

Source for this draft and an issue tracker can be found at https://github.com/ietf-scitt/draft-birkholz-scitt-receipts.

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/.

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This Internet-Draft will expire on 21 September 2023.

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Table of Contents

1. Introduction

This document defines a method for issuing and verifying countersignatures on COSE_Sign1 messages included in an authenticated data structure such as a Merkle Tree.

We adopt the terminology of the Supply Chain Integrity, Transparency, and Trust (SCITT) architecture document (An Architecture for Trustworthy and Transparent Digital Supply Chains, see [I-D.birkholz-scitt-architecture]): Claim, Envelope, Transparency Service, Registry, Receipt, and Verifier.

From the Verifier's viewpoint, a Receipt is similar to a countersignature V2 on a single signed message: it is a universally-verifiable cryptographic proof of endorsement of the signed envelope by the countersigner.

Compared with countersignatures on single COSE envelopes,

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. Common Parameters

Verifiers are configured by a collection of parameters to identify a Transparency Service and verify its Receipts. These parameters MUST be fixed for the lifetime of the Transparency Service and securely communicated to all Verifiers.

At minimum, these parameters include:

3. Generic Receipt Structure

A Receipt represents a countersignature issued by a Transparency Service.

The Receipt structure is a CBOR array with two items, in order:

Receipt = [
  protected: bstr .cbor {
    * label => value
  },
  contents: any
]
label = tstr / int
value = any

Each tree algorithm MUST define its contents type and procedures for issuing and verifying a receipt.

4. COSE_Sign1 Countersigning

While the tree algorithms may differ in the way they aggregate multiple envelopes to compute a digest to be signed by the TS, they all share the same representation of the individual envelopes to be countersigned (intuitively, their leaves).

This document uses the principles and structure definitions of COSE_Sign1 countersigning V2 ([I-D.ietf-cose-countersign]). Each envelope is authenticated using a Countersign_structure array, recalled below.

Countersign_structure = [
    context: "CounterSignatureV2",
    body_protected: empty_or_serialized_map,
    sign_protected: empty_or_serialized_map,
    external_aad: bstr,
    payload: bstr,
    other_fields: [
        signature: bstr
    ]
]

The body_protected, payload, and signature fields are copied from the COSE_Sign1 message being countersigned.

The sign_protected field is provided by the TS, see Section 4.1 below. This field is included in the Receipt contents to enable the Verifier to re-construct Countersign_structure, as specified by the tree algorithm.

By convention, the TS always provides an empty external_aad: a zero-length bytestring.

Procedure for reconstruction of Countersign_structure:

  1. Let Target be the COSE_Sign1 message that corresponds to the countersignature. Different environments will have different mechanisms to achieve this. One obvious mechanism is to embed the Receipt in the unprotected header of Target. Another mechanism may be to store both artifacts separately and use a naming convention, database, or other method to link both together.
  2. Extract body_protected, payload, and signature from Target.
  3. Create a Countersign_structure using the extracted fields from Target, and sign_protected from the Receipt contents.

4.1. Countersigner Header Parameters

The following parameters MUST be included in the protected header of the countersigner (sign_protected in Section 4):

  • Service ID (label: TBD): The Service identifier, as defined in the Transparency Service parameters.
  • Tree Algorithm (label: TBD): The Tree Algorithm used for issuing the receipt, as defined in the Transparency Service parameters.
  • Issued At (label: TBD): The time at which the countersignature was issued as the number of seconds from 1970-01-01T00:00:00Z UTC, ignoring leap seconds.

5. Receipt Verification

Given a signed envelope and a Receipt for it, the following steps must be followed to verify this Receipt.

  1. Decode the protected header of the Receipt and look-up the TS parameters using the Service ID header parameter.
  2. Verify that the Tree Algorithm parameter value in the receipt protected header matches the one in the TS parameters.
  3. Construct a Countersign_structure as described in Section 4, using the protected header of the Receipt as sign_protected.
  4. CBOR-encode Countersign_structure as To-Be-Included, using the CBOR encoding described in Section 7.
  5. Invoke the Tree Algorithm receipt verification procedure with the TS parameters and To-Be-Included as inputs.

The Verifier SHOULD apply additional checks before accepting the countersigned envelope as valid, based on its protected headers and payload.

6. CCF Tree Algorithm

The CCF tree algorithm specifies an algorithm based on a binary Merkle tree over the sequence of all ledger entries, as implemented in the CCF framework (see [CCF_Merkle_Tree]).

6.1. Additional Parameters

The algorithm requires that the TS define additional parameters:

  • Signature Algorithm: The ECDSA signature algorithm used to sign the Merkle tree root (see Section 10.2.2).
  • Service Certificate: The self-signed X.509 certificate used as trust anchor to verify signatures generated by the transparency service using the Signature Algorithm.

All definitions in this section use the hash algorithm required by the signature algorithm set in the TS parameters (see Section Section 6.1). We write HASH to refer to this algorithm, and HASH_SIZE for the fixed length of its output in bytes.

6.2. Cryptographic Components

Note: This section is adapted from Section 2.1 of [RFC9162], which provides additional discussion of Merkle trees.

6.2.1. Binary Merkle Trees

The input of the Merkle Tree Hash (MTH) function is a list of n bytestrings, written D_n = {d[0], d[1], ..., d[n-1]}. The output is a single HASH_SIZE bytestring, also called the tree root hash.

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).

6.2.2. Merkle Inclusion Proofs

A Merkle inclusion proof for a leaf in a Merkle Tree is the shortest list of intermediate hash values required to re-compute the tree root hash from the digest of the leaf bytestring. Each node in the tree is either a leaf node or is computed from the two nodes immediately below it (i.e., towards the leaves). At each step up the tree (towards the root), a node from the inclusion proof is combined with the node computed so far. In other words, the inclusion proof consists of the list of missing nodes required to compute the nodes leading from a leaf to the root of the tree. If the root computed from the inclusion proof matches the true root, then the inclusion proof proves that the leaf exists in the tree.

6.2.2.1. Verifying an Inclusion Proof

When a client has received an inclusion proof and wishes to verify inclusion of a leaf_hash for a given root_hash, the following algorithm may be used to prove the hash was included in the root_hash:

recompute_root(leaf_hash, proof):
  h := leaf_hash
  for [left, hash] in proof:
    if left
      h := HASH(hash || h)
    else
      h := HASH(h || hash)
  return h
6.2.2.2. Generating an Inclusion Proof

Given the MTH input D_n = {d[0], d[1], ..., d[n-1]} and an index i < n in this list, run the MTH algorithm and record the position and value of every intermediate hash concatenated and hashed first with the digest of the leaf, then with the resulting intermediate hash value. (Most implementations instead record all intermediate hash computations, so that they can produce all inclusion proofs for a given tree by table lookups.)

6.3. Encoding Signed Envelopes into Tree Leaves

This section describes the encoding of signed envelopes and auxiliary ledger entries into the leaf bytestrings passed as input to the Merkle Tree function.

Each bytestring is computed from three inputs:

  • internal_hash: a string of HASH_SIZE bytes;
  • internal_data: a string of at most 1024 bytes; and
  • data_hash: either the HASH of the CBOR-encoded Countersign_structure of the signed envelope, using the CBOR encoding described in Section 7, or a bytestring of size HASH_SIZE filled with zeroes for auxiliary ledger entries.

as the concatenation of three hashes:

LeafBytes = internal_hash || HASH(internal_data) || data_hash

This ensures that leaf bytestrings are always distinct from the inputs of the intermediate computations in MTH, which always consist of two hashes, and also that leaf bytestrings for signed envelopes and for auxiliary ledger entries are always distinct.

The internal_hash and internal_data bytestrings are internal to the CCF implementation. Similarly, the auxiliary ledger entries are internal to CCF. They are opaque to receipt Verifiers, but they commit the TS to the whole ledger contents and may be used for additional, CCF-specific auditing.

6.4. Receipt Contents Structure

The Receipt contents structure is a CBOR array. The items of the array in order are:

  • signature: the ECDSA signature over the Merkle tree root as bstr. Note that the Merkle tree root hash is the prehashed input to ECDSA and is not hashed twice.
  • node_certificate: a DER-encoded X.509 certificate for the public key for signature verification. This certificate MUST be a valid CCF node certificate for the service; in particular, it MUST form a valid X.509 certificate chain with the service certificate.

  • inclusion_proof: the intermediate hashes to recompute the signed root of the Merkle tree from the leaf digest of the envelope.

    • The array MUST have at most 64 items.
    • The inclusion proof structure is an array of [left, hash] pairs where left indicates the ordering of digests for the intermediate hash compution. The hash MUST be a bytestring of length HASH_SIZE.

  • leaf_info: auxiliary inputs to recompute the leaf digest included in the Merkle tree: the internal hash and the internal data.

    • internal_hash MUST be a bytestring of length HASH_SIZE;
    • internal_data MUST be a bytestring of length less than 1024.

The inclusion of an additional, short-lived certificate endorsed by the TS enables flexibility in its distributed implementation, and may support additional CCF-specific auditing.

The CDDL fragment that represents the above text follows.

ReceiptContents = [
    signature: bstr,
    node_certificate: bstr,
    inclusion_proof: [+ ProofElement],
    leaf_info: LeafInfo
]

ProofElement = [
    left: bool
    hash: bstr
]

LeafInfo = [
    internal_hash: bstr,
    internal_data: bstr
]

6.5. Receipt Contents Verification

Given the To-Be-Included bytes (see Section 5) and the TS parameters, the following steps must be followed to verify the Receipt contents.

  1. Verify that the Receipt Content structure is well-formed, as described in Section 6.4.

  2. Compute LeafBytes as the bytestring concatenation of the internal hash, the hash of internal data, and the hash of the To-Be-Included bytes.

     LeafBytes := internal_hash || HASH(internal_data) || HASH(To-Be-Included)

  3. Compute the leaf digest.

     LeafHash := HASH(LeafBytes)

  4. Compute the root hash from the leaf hash and the Merkle proof using the Merkle Tree Hash Algorithm found in the service's parameters (see Section 6.1):

     root := recompute_root(LeafHash, inclusion_proof)
  5. Verify the certificate chain established by the node certificate embedded in the receipt and the fixed service certificate in the TS parameters (see Section 6.1). TBD needs more details
  6. Verify that signature is a valid signature value of the root hash, using the public key of the node certificate and the Signature Algorithm of the TS parameters.

6.6. Receipt Generation

This document provides a reference algorithm for producing valid receipts, but it omits any discussion of TS registration policy and any CCF-specific implementation details.

The algorithm takes as input a list of entries to be jointly countersigned, each entry consisting of internal_hash, internal_data, and an optional signed envelope. (This optional item reflects that a CCF ledger records both signed envelopes and auxiliary entries.)

  1. For each signed envelope, create the countersigner protected header and compute the Countersign_structure as described in Section 4.
  2. For each item in the list, compute LeafBytes as the bytestring concatenation of the internal hash, the hash of internal data and, if the envelope is present, the hash of the CBOR-encoding of Countersign_structure, using the CBOR encoding described in Section 7, otherwise a HASH_SIZE bytestring of zeroes.
  3. Compute the tree root hash by applying MTH to the resulting list of leaf bytestrings, keeping the results for all intermediate HASH values.
  4. Select a valid node_certificate and compute a signature of the root of the tree with the corresponding signing key.

  5. For each signed envelope provided in the input,

    • Collect an inclusion_proof by selecting intermediate hash values, as described above.
    • Produce the receipt contents using this inclusion_proof, the fixed node_certificate and signature, and the bytestrings internal_hash and internal_data provided with the envelope.
    • Produce the receipt using the countersigner protected header and this receipt's contents.

7. CBOR Encoding Restrictions

In order to always regenerate the same byte string for the "to be included" and "to be hashed" values, the core deterministic encoding rules defined in Section 4.2.1 of [RFC8949] MUST be used for all their CBOR structures.

8. Privacy Considerations

TBD

9. Security Considerations

TBD

10. IANA Considerations

10.1. Additions to Existing Registries

10.1.1. New Entries to the COSE Header Parameters Registry

IANA is requested to register the new COSE Header parameters defined below in the "COSE Header Parameters" registry.

10.1.1.1. COSE_Sign1 Countersign receipt

Name: COSE_Sign1 Countersign receipt

Label: TBD (temporary: 394, see also [I-D.birkholz-scitt-architecture])

Value Type: [+ Receipt]

Description: One or more COSE_Sign1 Countersign Receipts to be embedded in the unprotected header of the countersigned COSE_Sign1 message.

10.1.1.2. Issued At

Name: Issued At

Label: TBD

Value Type: uint

Description: The time at which the signature was issued as the number of seconds from 1970-01-01T00:00:00Z UTC, ignoring leap seconds.

11. References

11.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC6234]
Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, , <https://www.rfc-editor.org/rfc/rfc6234>.
[RFC8032]
Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, , <https://www.rfc-editor.org/rfc/rfc8032>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8949]
Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <https://www.rfc-editor.org/rfc/rfc8949>.
[RFC9053]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, , <https://www.rfc-editor.org/rfc/rfc9053>.
[RFC9162]
Laurie, B., Messeri, E., and R. Stradling, "Certificate Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162, , <https://www.rfc-editor.org/rfc/rfc9162>.

11.2. Informative References

[CCF_Merkle_Tree]
Microsoft Research, "CCF - Merkle Tree", n.d., <https://microsoft.github.io/CCF/main/architecture/merkle_tree.html>.
[I-D.birkholz-scitt-architecture]
Birkholz, H., Delignat-Lavaud, A., Fournet, C., and Y. Deshpande, "An Architecture for Trustworthy and Transparent Digital Supply Chains", Work in Progress, Internet-Draft, draft-birkholz-scitt-architecture-02, , <https://datatracker.ietf.org/doc/html/draft-birkholz-scitt-architecture-02>.
[I-D.ietf-cose-countersign]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Countersignatures", Work in Progress, Internet-Draft, draft-ietf-cose-countersign-10, , <https://datatracker.ietf.org/doc/html/draft-ietf-cose-countersign-10>.

Authors' Addresses

Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Maik Riechert
Microsoft
United Kingdom
Antoine Delignat-Lavaud
Microsoft
United Kingdom
Cedric Fournet
Microsoft
United Kingdom