Post-Quantum Evidence Records with Algorithm Agility (Wathīqa Profile)

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Table of Contents

1. Introduction

A signature proves authenticity now. Preserving that proof for decades raises two problems RFC 4998 (ERS) [RFC4998] already addresses — the signing key or algorithm may weaken, and the timestamp authority may change — and one it does not: the signature algorithm itself may be catastrophically broken by a quantum computer. NIST's FIPS 203/204/205 [FIPS203] [FIPS204] [FIPS205] (and draft 206) standardise post-quantum primitives, but a record signed today only with a classical or single post-quantum algorithm has no path forward if that algorithm falls.¶

This document profiles ERS for that setting. The core idea, following RFC 4998's Archive Timestamp chain, is to keep extending a record under fresh primitives before the old one is broken, with each extension cryptographically bound to its predecessor, so a verifier can establish an unbroken sequence in which every link was sound at the time it was made.¶

This document describes a format with a complete reference implementation, but the security argument has not undergone independent cryptographic review.¶

2. Terminology

• Content address. A hash-based identifier of the protected data: a SHA-3-256 Merkle root over fixed-size chunks together with the total byte length.¶

• Chain entry / link. One renewal layer: a signature over the content address under a stated primitive, an anchor, the hash of the prior link, and an optional witness receipt.¶

• Anchor. Per-link not-before metadata (beacon id, epoch, root) self-bound to the link by witnessed_hash == HASH(signature.bytes). In this profile it is carried unauthenticated — the verifier checks no beacon proof (Section 8).¶

• Witness receipt. A signed transparency-log tree head plus an inclusion proof showing the link's signature was logged — the authenticated not-after temporal evidence, enforced when a witness-trust policy is supplied.¶

3. Architecture

data ──► content address ──► chain entry v1
                                 (primitive A, anchor, witness)
                                  │ prior_chain_hash
                                  ▼
                              chain entry v2
                                 (primitive B, anchor, witness)
                                  │
                                  ▼      … extend before A breaks

A record is { content_address, chain[] }. Verification (Section 5) walks the chain from v1 outward, requiring each link's signature to verify over the content address under its stated primitive, each non-initial link to commit to its predecessor's chain hash, and (when a trust policy is supplied) each link to carry a valid witness receipt from a trusted witness with monotonic time.¶

4. Data Structures

EvidenceRecord = {
  content_address: { root: 32 octets, byte_length: uint },
  chain: [ ChainEntry, ... ]
}

ChainEntry = {
  version:          uint,            ; 1 = initial, >1 = extension
  signature: {
    algorithm:         tstr, ; e.g. "SLH-DSA-SHAKE-256s", "ML-DSA-87"
    algorithm_version: uint,
    bytes:             bstr,
    public_key:        bstr,
    signed_at_unix:    uint
  },
  anchor: {
    beacon_id:         tstr,
    epoch:             uint,
    witnessed_at_unix: uint,
    beacon_root:       32 octets,
    witnessed_hash:    32 octets     ; = SHA3-256(signature.bytes)
  },
  prior_chain_hash:    32 octets / null,   ; null iff version == 1
  inclusion:           WitnessReceipt / null
}

{
 "anchor":{"beacon_id":<tstr>,"beacon_root_hex":<hex>,
           "epoch":<uint>,"witnessed_at_unix":<uint>,
           "witnessed_hash_hex":<hex>},
 "prior_chain_hash_hex":<hex|null>,
 "signature":{"algorithm":<tstr>,"algorithm_version":<uint>,
              "bytes_hex":<hex>,"public_key_hex":<hex>,
              "signed_at_unix":<uint>},
 "version":<uint>
}

WathiqaEvidenceRecord ::= SEQUENCE {
  contentType  OBJECT IDENTIFIER,   -- 1.3.6.1.4.1.66038.1.1
  content      EvidenceRecord }     -- SEQUENCE { version,
                                    --   contentAddress…, chain }

WitnessReceipt = {
  witnessed_hash:  32 octets,   ; = SHA3-256(link signature.bytes)
  leaf_index:      uint,
  sth:             SignedTreeHead,
  inclusion_proof: [ 32 octets, ... ]
}
SignedTreeHead = { log_id, tree_size, root, timestamp_unix,
                   signature }

5. Verification

A verifier is given the protected data D, the record, and OPTIONALLY a set of trusted witness public keys W. It MUST:¶

1. Recompute the content address of D and check it equals record.content_address.¶

2. For each link, in chain order, with prev = the previous link (none for v1): a. Verify signature over the content address under signature.algorithm and the evidence signing context (Section 6). Reject on failure. b. If version == 1: require prior_chain_hash == null. Else: require prior_chain_hash == SHA3-256(canonicalJSON(prev)). c. If W is supplied (temporal-binding enforcement): require inclusion to be present and valid; require inclusion.witnessed_hash == SHA3-256(signature.bytes); require inclusion.sth.signature to be by a key in W; and require STH timestamps to be non-decreasing along the chain.¶

3. Accept iff every link passed and the chain is contiguous from v1.¶

Acceptance means: there exists an unbroken sequence of attestations in which each primitive was applied while sound, the data was unchanged throughout, and (under a trust policy) each step was publicly logged by a trusted witness in monotonic time. Which primitives a relying party still trusts at verification time is the relying party's policy input, not a property of the record.¶

6. Signing contexts and the v2 evidence message

Every signature commits to a context tag so a signature for one protocol cannot be replayed in another. The base message layout is:¶

context-utf8 || 0x00 || algorithm-utf8 || 0x00 || root(32) ||
    byte_length(8, big-endian) || signed_at_unix(8, big-endian) ||
    extra

extra is empty for every v1 context. Evidence-record links produced from wire profile v0.3 onward sign under wathiqa.evidence.v2 with a fixed-width extra tail:¶

signer_id(32)
    -- SHA3-256 of the signer's public key
prior_flag(1) || prior(32)
    -- 0x00 + zeros = genesis; 0x01 + hash = link
nb_flag(1) || nb_chain(8, BE) || nb_pulse(8, BE) || nb_output(64)
    -- 0x00 + zeros = no not-before binding;
    -- 0x01 = NIST Beacon 2.0 pulse claim

A chain entry declares its rule on the wire via wire_v (absent = 1, the legacy rule, which verifiers MUST support indefinitely; 2 = the rule above; any other value MUST be rejected at decode). Mixed chains are legal: a v1 record extended under v0.3 gains v2 entries. The not-before pulse claim is a signed CLAIM; its authentication is the (optional) pulse-signature verification of Section 8. Other contexts are unchanged: wathiqa.evidence.v1 (legacy record links), wathiqa.sth.v1 (transparency-log tree heads), wathiqa.witness_roster.v1 (witness-key succession), and others per protocol.¶

7. Algorithm Identifiers

Signature primitives are identified by the NIST CSOR OIDs for ML-DSA ([FIPS204]) and SLH-DSA ([FIPS205]); FN-DSA/Falcon awaits FIPS 206. The Wathīqa-specific structures are identified under the IANA Private Enterprise Number arc 1.3.6.1.4.1.66038 (PEN 66038, granted to Wathīqa). The FN-DSA algorithm OID remains the one outstanding value, pending FIPS 206. An implementation MUST treat the algorithm string and the OID as equivalent identifiers of the same primitive and MUST reject a record whose algorithm it does not recognise (rather than accepting it unverified).¶

8. Security Considerations

• Agility window. The scheme protects data only if each link is added before the prior primitive is broken; an operational renewal policy (out of scope for this document) is REQUIRED. A record left un-renewed past a primitive's break is no stronger than that primitive.¶

• Temporal trust over decades. The witness keys in W must themselves survive rotation; this profile recommends a quorum-authorised, append-only witness-key succession pinned to a single long-lived anchor. Without it, W is unmaintainable on a century scale.¶

• Anchor trust (current limitation). This profile's verifier does NOT verify any beacon proof. The anchor carries not-before metadata (beacon id, epoch, root) but is unauthenticated: the only checked binding is witnessed_hash == HASH(signature.bytes), which ties the anchor record to the link's signature but does NOT prove the beacon ever emitted that value. An adversary can therefore populate the anchor with arbitrary not-before metadata; it MUST NOT be relied on as authenticated time. The authenticated temporal evidence is the transparency-log inclusion receipt (not-after), enforced when a witness-trust policy is supplied, and trustworthy to the extent the log is monitored for consistency ([RFC6962] gossip). Authenticating the beacon anchor (e.g. verifying a signed NIST Randomness Beacon [NIST-BEACON] pulse and binding the pulse value into the signed message) is future work; until then, do not treat the beacon anchor as a security guarantee.¶

• Fail-closed verification. A verifier MUST treat any malformed field, unrecognised algorithm, or unverifiable proof as verification failure, never as success.¶

• Canonical-form attacks. Because the chain hash is over a canonical form, implementations MUST reject inputs that do not round-trip to that exact form (e.g. non-minimal numbers, duplicate keys), to deny an attacker a second pre-image via encoding ambiguity.¶

9. IANA Considerations

The Wathīqa structures of Section 4 are identified under IANA Private Enterprise Number 66038 (1.3.6.1.4.1.66038), already granted — no further OID registration is requested for them.¶

Still outstanding (a separate IANA process, not yet filed): registration of the two media types application/wathiqa-evidence+cbor and application/wathiqa-evidence+der.¶

10. References

Appendix A. Implementation status

A complete reference implementation (Python, authoritative for wire formats), plus JavaScript and Rust verifiers that reproduce the canonical chain hash byte-for-byte, exists and is cross-validated by a shared conformance-vector suite. This draft documents that implementation's formats; divergences are implementation bugs, not intended profile variation.¶

Appendix B. Document History

This is the first public revision. The following changes were made in the pre-submission revisions that preceded it:¶

• Wire profile v0.3: chain entries carry wire_v (version gate); the v2 signed message binds the signer identity, the prior chain hash, and an optional NIST-pulse not-before claim (Section 6). v1 records remain verifiable indefinitely.¶

• Anchor structure gains chain_index and certificate_id (beacon chain and pulse-certificate pinning for authenticated re-verification).¶

• ASN.1 DER profile v4 (per-entry wire version, optional NotBefore SEQUENCE, extended anchor); v1–v3 remain decodable, only v4 is emitted.¶

• The machine-checked model now proves the strengthened per-link property: acceptance implies an honest attestation of (document, algorithm, POSITION), or a prior primitive break.¶

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