docs(a2a): reconcile IBCT wire-format drift between specs and implementation - #6198
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July 13, 2026 01:10
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specs/010-security/spec.md and specs/014-a2a/spec.md described two
different IBCT wire formats, and neither fully matched
crates/zeph-a2a/src/ibct.rs. Reconcile both against the actual
implementation: single base64-JSON blob (not a dot-separated triplet),
signed over key_id|task_id|endpoint|issued_at|expires_at, default TTL
300s (not 60s), and ibct_keys as an array of {key_id, key_hex} entries
(not a key_id -> vault_ref map). Also correct both specs' claim that
IBCT prevents replay attacks — Ibct::verify has no invocation_id/nonce
dedup, so a captured valid token is replayable until it expires; this
is now documented as a known limitation instead of silently
contradicted between the two spec files.
Closes #6197
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Summary
specs/010-security/spec.mdandspecs/014-a2a/spec.mddescribed two different IBCT (Invocation-Bound Capability Token) wire formats. Investigation showed neither spec fully matched the actual implementation incrates/zeph-a2a/src/ibct.rs— the issue's premise that014-a2awas already accurate turned out to be only half right.ibct.rsandcrates/zeph-config/src/channels.rsground truth:<mac>.<key_id>.<timestamp>triplet), signed over{key_id}|{task_id}|{endpoint}|{issued_at}|{expires_at}.ibct_keysis an array of{key_id, key_hex}entries (inline hex signing key), not akey_id -> vault_refmap;ibct_signing_key_vault_refis the separate vault-resolved primary-key path that takes precedence overibct_keys[0].Ibct::verifyperforms noinvocation_id/nonce dedup, so a captured, still-valid token is replayable against the sametask_id+endpointuntil it expires — this is now documented as a known implementation limitation in both specs' Key Invariants sections instead of being silently misdescribed in opposite directions.crates/was modified.Test plan
gitleaks protect --staged— no leaks foundcrates/zeph-a2a/src/ibct.rsandcrates/zeph-config/src/channels.rs(signed-field format, base64-JSON wire encoding,default_ibct_ttl()= 300,CLOCK_SKEW_GRACE_SECS= 30,IbctKeyConfig { key_id, key_hex }struct shape,Mac::verify_slice, absence of anyinvocation_iddedup)git diff --statscoped tospecs/010-security/spec.md,specs/014-a2a/spec.md,CHANGELOG.mdonlyconfig/default.toml, docker compose files, ordocs/src/— no config keys or user-facing behavior changed, only spec text corrected to match existing behaviorCloses #6197