BCR-2026-XXX: Anchor Predicates

papers/bcr-2026-004-signing-event-assertions.md

@@ -0,0 +1,288 @@
+# Signing Event Assertions
+
+## BCR-2026-004
+
+**© 2026 Blockchain Commons**
+
+Authors: Christopher Allen<br/>
+Date: February 4, 2026
+
+---
+
+## Abstract
+
+A cryptographic signature proves a key signed content. It does not prove who holds the key, when signing occurred, or what the signature means. This BCR defines predicates for signers to attest these facts, bound to their signatures using the pattern from BCR-2024-009.
+
+## Status: Draft
+
+## Introduction
+
+### What Signatures Prove
+
+A signature proves exactly one thing:
+
+> **The private key for a specific public key produced this signature over this content.**
+
+A signature does not prove who holds the key, whether the key was compromised or delegated, when signing occurred, why it happened, or that the signer agreed to anything.
+
+```
+{
+    Digest(contract)
+} 'signed': Signature
+```
+
+This proves the key signed. It does not prove Alice controls that key, that she is CEO, or that she signed for Acme Corp.
+
+### The Problem
+
+Alice signing personally and Alice signing as CEO for Acme Corp produce identical signatures. The difference is what Alice attests about her signing act. To express this in Gordian Envelope:
+
+1. Predicates linking signatures to identity (XIDs)
+2. Predicates expressing representation
+3. A pattern binding assertions to signatures
+
+### Solution
+
+This BCR defines:
+
+- `signer` (300) — links signature to signer's identity
+- `signedOnBehalfOf` (301) — identifies who the signer represents
+- References `xades:ClaimedRole` and `xades:CommitmentType` from XAdES
+
+Two patterns bind assertions to signatures:
+- **Signature-with-assertions** — signer's own assertions (primary)
+- **Wrapped signing** — third-party assertions
+
+## Binding Assertions to Signatures
+
+### The Problem
+
+Assertions alongside a signature aren't cryptographically bound:
+
+```
+// WRONG: Anyone could add these assertions
+{
+    Digest(contract)
+} [
+    'signed': Signature
+    'signer': XID(alice)
+]
+```
+
+### Pattern 1: Signature-with-Assertions
+
+For a signer's own assertions, use the pattern from BCR-2024-009: make the Signature the subject, add assertions, wrap, sign.
+
+```
+{
+    Digest(contract)
+} [
+    'signed': {
+        Signature [
+            'signer': XID(alice)
+            'signedOnBehalfOf': XID(acme-corp)
+            'xades:ClaimedRole': "CEO"
+            'xades:CommitmentType': "approval"
+        ]
+    } ['signed': Signature]
+]
+```
+
+The inner signature covers content. The outer signature binds assertions to the inner signature. Both must verify against the same key.
+
+### Pattern 2: Wrapped Signing
+
+For third-party assertions (timestamps, notarization), wrap the signed content, add assertions, sign:
+
+```
+{
+    {
+        Digest(contract)
+    } [
+        'signed': {
+            Signature ['signer': XID(alice)]
+        } ['signed': Signature]
+    ]
+    [
+        'anchoredAt': 2026-02-04T12:00:00Z
+        'anchoredBy': XID(timestamp-authority)
+    ]
+} [
+    'signed': {
+        Signature [
+            'signer': XID(timestamp-authority)
+            'xades:CommitmentType': "timestamp"
+        ]
+    } ['signed': Signature]
+]
+```
+
+The third party's signature binds their assertions about the already-signed content.
+
+### When to Use Each
+
+| Pattern | Who Asserts | About What |
+|---------|-------------|------------|
+| Signature-with-assertions | Signer | Their signing act |
+| Wrapped signing | Third party | Signed content |
+
+Examples of wrapped signing: timestamp authorities, notaries, witnesses, endorsers.
+
+### Dates vs Timestamps
+
+A date in signing assertions is a claim, not proof. For provable timestamps, a third party must counter-sign — they couldn't sign content that didn't exist.
+
+## Multi-Party Signatures
+
+### Parallel Signatures
+
+Multiple parties independently signing the same content:
+
+```
+{
+    Digest(contract)
+} [
+    'signed': {
+        Signature [
+            'signer': XID(alice)
+            'xades:CommitmentType': "approval"
+        ]
+    } ['signed': Signature]
+    'signed': {
+        Signature [
+            'signer': XID(bob)
+            'xades:CommitmentType': "approval"
+        ]
+    } ['signed': Signature]
+]
+```
+
+### Counter-Signatures
+
+A party signing over another's signed content:
+
+```
+{
+    {
+        Digest(contract)
+    } [
+        'signed': {
+            Signature ['signer': XID(alice)]
+        } ['signed': Signature]
+    ]
+} [
+    'signed': {
+        Signature [
+            'signer': XID(bob)
+            'xades:CommitmentType': "witness"
+        ]
+    } ['signed': Signature]
+]
+```
+
+Bob's signature covers Alice's complete signed envelope.
+
+## Terminology
+
+**Assertion**: Envelope term for a predicate-object pair. **Attestation**: The act of declaring facts. Signers *attest*; attestations are expressed as *assertions*.
+
+**Signing Event**: A key producing a signature over content.
+
+**Signing Event Assertions**: Assertions on a Signature subject — who signed, representing whom, in what capacity, for what purpose.
+
+## Known Value Assignments
+
+Proposed for the Reserved range (256-999) per BCR-2023-002.
+
+### 300: `signer`
+
+Links a signature to a document identifying the signer.
+
+```
+Signature [
+    'signer': XID(alice)
+]
+```
+
+A signature proves a key signed; `signer` links to identity. Required because some schemes (EdDSA, BBS+, Longfellow) don't embed or allow recovery of the public key.
+
+Within the Gordian ecosystem (XIDs, Clubs, GSTP), references an XID or Club. Gordian Envelope also supports URIs and DIDs.
+
+### 301: `signedOnBehalfOf`
+
+Optional. Identifies who the signer represents.
+
+```
+Signature [
+    'signer': XID(alice)
+    'signedOnBehalfOf': XID(acme-corp)
+]
+```
+
+Only include when acting for another party. This is a claim — verification requires checking delegation authority (see BCR-2026-006).
+
+## Referenced Standards
+
+### xades:ClaimedRole
+
+From ETSI TS 101 903. The capacity in which the signer acts. Self-asserted.
+
+```
+Signature [
+    'signer': XID(alice)
+    'xades:ClaimedRole': "CEO"
+]
+```
+
+### xades:CommitmentType
+
+From ETSI TS 101 903. The purpose of the signature.
+
+```
+Signature [
+    'signer': XID(alice)
+    'xades:CommitmentType': "approval"
+]
+```
+
+Common values: approval, acknowledgment, witness, receipt, origin.
+
+## Security Considerations
+
+Signing event assertions are claims by the signer. Relying parties must verify both signatures, confirm they use the same key, resolve the XID, and evaluate plausibility.
+
+For signature-with-assertions, different keys indicate tampering. For wrapped signing, different keys are expected.
+
+When `signer` is elided, signatures remain valid but identity is hidden.
+
+## References
+
+- [BCR-2023-002: Known Value Registry](https://github.com/BlockchainCommons/Research/blob/master/papers/bcr-2023-002-known-value.md)
+- [BCR-2024-009: Signatures with Metadata](https://github.com/BlockchainCommons/Research/blob/master/papers/bcr-2024-009-signature-metadata.md)
+- [ETSI TS 101 903: XAdES](https://www.etsi.org/deliver/etsi_ts/101900_101999/101903/01.04.02_60/ts_101903v010402p.pdf)
+- [Gordian Envelope Specification](https://github.com/BlockchainCommons/Research/blob/master/papers/bcr-2024-003-envelope.md)
+- [BBS Signature Scheme](https://identity.foundation/bbs-signature/draft-irtf-cfrg-bbs-signatures.html)
+- [Longfellow ZK Scheme](https://datatracker.ietf.org/doc/html/draft-google-cfrg-libzk-01)
+
+## Related BCRs
+
+- BCR-2026-005: General Assertion Predicates
+- BCR-2026-006: Principal Authority Predicates
+
+---
+
+## Appendix A: XAdES Commitment Type OIDs
+
+| Commitment Type | OID |
+|-----------------|-----|
+| Proof of origin | 1.2.840.113549.1.9.16.6.1 |
+| Proof of receipt | 1.2.840.113549.1.9.16.6.2 |
+| Proof of delivery | 1.2.840.113549.1.9.16.6.3 |
+| Proof of sender | 1.2.840.113549.1.9.16.6.4 |
+| Proof of approval | 1.2.840.113549.1.9.16.6.5 |
+| Proof of creation | 1.2.840.113549.1.9.16.6.6 |
+
+---
+
+*BCR-2026-004: Signing Event Assertions*
+*Draft - February 4, 2026*