RFC 3161 Timestamp Authority Support for Certificate Signing

[jzeng4]

Description

What needs to be improved?

Current Gap in Certificate-Based Signing

The model-transparency project currently supports certificate-based signing as an alternative to Sigstore, but lacks support for trusted timestamps, creating several limitations:

1. No Long-Term Signature Validity

• Signatures become invalid when signing certificates expire
• Cannot prove signing occurred while certificate was valid
• No mechanism to extend signature lifetime beyond certificate expiration
• Problematic for archival requirements (5-10+ year model retention)

2. Regulatory Compliance Blockers

• eIDAS (EU): Requires qualified timestamps from approved Timestamp Authorities
• FDA (US): Electronic records regulations (21 CFR Part 11) require trusted timestamps
• Financial Services: Regulations mandate timestamped audit trails
• HIPAA (Healthcare): Requires timestamps for electronic signatures
• Organizations in these sectors cannot adopt model-signing due to lack of timestamp support

3. No Independent Time Attestation

• No third-party proof of when signature was created
• Vulnerable to clock tampering claims
• Cannot provide non-repudiation of signing time
• Missing independent witness required by many compliance frameworks

4. Limited Offline Verification

• Cannot verify signature without trusting local system time
• No cryptographic proof of signing time
• Insufficient for air-gapped or disconnected environments

5. No Alternative to Transparency Logs

For organizations that:

• Cannot use public transparency logs (Rekor) due to privacy requirements
• Need qualified timestamps for compliance (not just transparency)
• Operate in regulated industries with specific timestamp requirements
• Require both certificate-based signing AND trusted timestamps

Result: These gaps prevent enterprise adoption in regulated industries, limiting the project's reach and impact.

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What should be done?

Proposed Solution: Add RFC 3161 Timestamp Authority Support

Implement RFC 3161 (Time-Stamp Protocol) support for certificate-based signing, enabling:

Signing Enhancement:

• Add optional tsa_url parameter to certificate signer
• Obtain RFC 3161 timestamp token from TSA after signing
• Store timestamp in Sigstore bundle's timestamp_verification_data field
• Support configurable timeout and error handling
• Maintain backward compatibility (timestamps are optional)

Verification Enhancement:

• Validate RFC 3161 timestamp tokens when present
• Verify message imprint matches signature hash
• Check timestamp against certificate validity period
• Optional enforcement of timestamp presence
• Optional timestamp age validation

Benefits

Enables New Use Cases:

• Regulated industries (finance, healthcare, government)
• Long-term model archival (decades)
• Compliance-driven organizations
• Private model signing with qualified timestamps

Expands Project Reach:

• EU market (eIDAS compliance)
• Financial services sector
• Healthcare organizations
• Government contracts

Improves Security Model:

• Independent time attestation
• Long-term signature validity
• Stronger non-repudiation
• Offline verification capability

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Has there been any related work before?

Prior Art and Related Work

1. Sigstore Bundle Specification Already Supports This

The Sigstore bundle protobuf specification already defines timestamp_verification_data field:

message VerificationMaterial {
  oneof content {
    PublicKeyIdentifier public_key = 1;
    X509CertificateChain x509_certificate_chain = 2;
  }
  repeated dev.sigstore.rekor.v1.TransparencyLogEntry tlog_entries = 3;
  TimestampVerificationData timestamp_verification_data = 4;  // ← Already defined!
}

message TimestampVerificationData {
  repeated RFC3161SignedTimestamp rfc3161_timestamps = 1;
}

This means:

• ✅ The specification anticipates this feature
• ✅ The bundle format already supports it
• ✅ This is not a spec deviation
• ✅ Implementation aligns with Sigstore's design

Reference: https://github.com/sigstore/protobuf-specs/blob/main/protos/sigstore_bundle.proto#L125-L145

2. Industry Standards and Precedents

Adobe Document Cloud:

• Uses both Adobe timestamps AND RFC 3161 TSA timestamps
• Demonstrates value of dual timestamp approach
• Industry-standard for document signing

Microsoft Authenticode:

• Supports RFC 3161 timestamping since 2005
• Required for driver signing
• Proven model for code signing

PDF Signatures (ISO 32000):

• Standardized RFC 3161 timestamp integration
• Used by Adobe, DocuSign, etc.
• Well-established pattern

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Any relevant links or material?

Technical Standards

RFC 3161 - Time-Stamp Protocol (TSP)

• URL: https://www.ietf.org/rfc/rfc3161.txt
• Description: Official specification for RFC 3161 timestamp protocol
• Status: IETF Proposed Standard (widely implemented)

Sigstore Bundle Specification

• URL: https://docs.sigstore.dev/about/bundle/
• Protobuf Specs: https://github.com/sigstore/protobuf-specs/blob/main/protos/sigstore_bundle.proto
• Timestamp Field: Lines 125-145 define timestamp_verification_data

ISO/IEC 18014 - Time-Stamping Services

• Description: International standard for time-stamping
• Relationship: RFC 3161 implements this ISO standard

Regulatory Requirements

eIDAS Regulation (EU)

• URL: https://digital-strategy.ec.europa.eu/en/policies/eidas-regulation
• Article 41: Qualified electronic time stamps
• Impact: Mandatory for legal electronic signatures in EU
• Market Size: 450+ million people in EU

FDA 21 CFR Part 11 (US Healthcare)

• URL: https://www.fda.gov/regulatory-information/search-fda-guidance-documents/part-11-electronic-records-electronic-signatures-scope-and-application
• Requirement: Electronic signatures must be timestamped
• Impact: Healthcare and pharmaceutical ML models

Financial Services Regulations

• SOX (Sarbanes-Oxley): Requires timestamped audit trails
• MiFID II (EU): Timestamp requirements for financial transactions
• Impact: Financial ML models need compliant signatures

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