# ADR 0018: Linked SSHSIGCHAIN v2

## Status

Accepted for the new generic core and geth keychain profile. Legacy static
sigchain publication is deprecated pending explicit v2 command migration.

## Context

The prior static JSONL keychain flow replayed records after sorting mutable
timestamps and used a downloaded allowed-signers projection to verify its own
checkpoint. That permits self-bootstrapping trust and makes revocation,
rollback, and fork semantics inadequate for a trust foundation.

## Decision

Geth adopts SSHSIGCHAIN v2 as the replacement design:

- root trust is an explicit `(chain ID, profile, namespace, root key)` tuple;
- records are linearly ordered by sequence and linked by a digest over their
  signed content and SSHSIG signature;
- the initial root is only a genesis requirement, not a permanent bypass;
- profile authorization runs against the causally preceding state;
- key lifecycle is represented by causal add/revoke records, not timestamp
  validity fields; and
- payloads use deterministic binary codecs with a strict round-trip check.

The generic core is transport-independent. Geth continues using Iroh for all
node-to-node communication; SSH remains an identity/signature integration and
not a geth transport.

## Consequences

The v2 core can be published and implemented by applications without importing
geth's resource model. Geth's keychain profile is intentionally narrow and
tested against self-bootstrap, fork, and backdated-revocation attacks.

Existing static v1 bundles cannot safely migrate in place. Operators must pin a
new v2 trust tuple and create a new genesis chain. Head persistence and later
witness/transparency support remain follow-up work; neither is implied by a
single signed checkpoint.