ERS Long-Term Archival

Status

Draft. Reference implementation: crates/confium-transparency/src/ers.rs.

Motivation

Cryptographic algorithms weaken over time. A signature valid today may be forgeable in 20 years. Evidence Record Syntax (ERS, RFC 4998) provides a structure for renewing archival evidence without re-signing the original artifact.

Scope

• ERS envelope format
• Renewal cadence: every algorithm degradation epoch (default: 5 years)
• Hash and signature renewal procedures
• Verification of an ERS archive

Out of scope

• Storage media longevity (out of scope for crypto layer)
• Archive format for artifacts themselves (ERS only covers evidence)

Specification

Envelope

An ERS envelope contains:

. Original artifact hash . Sequence of (timestamp, algorithm_suite, evidence) renewal records . Each renewal record is timestamped by a TSA and signed by an archive authority

Renewal

On each epoch:

. Compute current strongest available hash of artifact (e.g., SHA-512 then later SHA-3). . Get a timestamp from a TSA (RFC 3161). . Sign the new hash + timestamp + previous envelope. . Append as a new renewal record.

Verification

Verify each renewal record in sequence, confirming the chain of custody and that each algorithm was unbroken at its timestamp.

Security considerations

• TSA must be trusted at the time of timestamping; revocation of TSA trust retroactively invalidates the chain.
• Archive authority key compromise breaks the chain forward (but not backward).

References

• RFC 4998: Evidence Record Syntax (ERS)
• Confium source: crates/confium-transparency/src/ers.rs