geth/docs/adr/0018-sshsigchain.md

ADR 0018: Sequence-free SSHSIGCHAIN authority protocol

Status

Accepted for the generic core. The protocol is still pre-deployment and has no legacy compatibility requirement.

Context

The first linked draft improved on a timestamp-sorted static bundle, but still made application profiles responsible for all key lifecycle semantics. It also included both a sequence counter and a parent hash, fixed one application profile for the whole chain, exposed every payload, and left rollback anchors outside the protocol model.

That design would let applications fragment device/key behavior, made multi-profile least privilege awkward, and offered no standard way to disclose only selected profile data or compare independently anchored heads.

Decision

SSHSIGCHAIN v1 is redesigned before deployment:

• the trust tuple is (chain ID, SSHSIG namespace, root public key);
• parent hashes alone define order; there is no sequence number;
• link identity hashes the signed outer bytes, not the signature encoding;
• every link carries a mandatory public Authority v1 transition;
• authority owns devices, keys, proof-of-possession, causal revocation, permission ceilings, delegation, and anchor policy;
• applications use profile-ID-scoped, salted payload commitments and cannot mutate authority;
• disclosures are transport additions which do not change a link or its hash;
• authorized keys can sign head claims;
• distinct head attesters and backend-neutral receipts are evaluated by separate weighted/required thresholds and required backend classes;
• cached ancestors are never replaced by older heads and incomparable verified histories fail as forks; and
• a policy-change head is witnessed under the preceding policy before the new policy governs descendants.

The generic core defines backend interfaces and deterministic policy behavior, not Nostr, HTTP, blockchain, or transparency-log clients. Those adapters belong outside geth-keychain. Backends provide evidence and discovery, not consensus.

The generic protocol remains independent of geth transports. All geth node-to-node communication remains Iroh-only; SSH remains a trust and signature integration.

Consequences

Device and key behavior is interoperable rather than reinvented in each profile. A verifier can validate current authority while withholding application data, but must report affected profile state as incomplete. Proofs of possession prevent an administrator from silently enrolling a key it does not control. Device ceilings and delegable permission sets constrain later amplification.

The wire format is intentionally incompatible with every pre-deployment test draft. No migration parser or version alias is retained.

Rollback protection still depends on persistent local accepted heads and the operator's anchor policy. A Nostr relay, mutable HTTP URL, blockchain, or transparency service has only the guarantees its receipt verifier and deployment actually establish. Concrete publication, durable accepted-head storage, and cross-implementation vectors remain follow-up work.