Witness Gossip Protocol

Status

Draft. Reference implementation: crates/confium-log-server (gossip module).

Motivation

A single transparency log can silently rewrite history by re-issuing a tree head. Witness gossip distributes the latest tree head to multiple independent witnesses, so any rewrite becomes detectable.

Scope

• Tree head cross-confirmation across witnesses
• Gossip transport (HTTP, WebSocket, or manual)
• Witness signature on observed tree heads
• Fork detection: log presenting different heads to different witnesses

Out of scope

• Certificate Transparency’s specific gossip (RFC 6962 §5; we cover the abstract pattern)
• Specific witness operators (anyone can run a witness)

Specification

Tree head shape

{
  "log_id": "sha256:...",
  "tree_size": 4823,
  "timestamp": 1722988800,
  "root_hash": "sha256:abc..."
}

Witness signature

Witness signs the tree head canonical JSON with their Ed25519 private key. The signed head is republishable to other witnesses (2nd-hand gossip).

Fork detection

A monitor observes two witnesses reporting different root_hash for the same (log_id, tree_size, timestamp). This is a provable fork — the log lied.

Security considerations

• Witnesses MUST sign canonical-encoded tree heads (any whitespace variance breaks verifiability).
• 2nd-hand gossip (republishing) requires that witnesses trust each other’s signatures; otherwise, every witness must directly query the log.
• Witnesses MUST publish their public keys out-of-band (DNS, transparency log of their own, etc.).

References

• Confium source: crates/confium-log-server/src/gossip.rs
• Related: 42-transparency-log