[SECURITY] HMAC signature verification should use constant-time comparison

[shunkica]

Summary

HMAC signature verification in src/signature-algorithms.ts uses JavaScript's === operator which is not constant-time. This violates cryptographic best practices and should be fixed to use crypto.timingSafeEqual().

Issue

The === operator performs short-circuit evaluation, potentially creating timing side-channels:

• Stops comparing as soon as first mismatch is found
• Different error positions = different comparison times

While timing variance is measurable, I could not demonstrate practical exploitation in a test environment. This appears to be more of a theoretical vulnerability and code quality issue.

Security Best Practice

Per cryptographic best practices and CWE-208 (Observable Timing Discrepancy), secret comparisons should use constant-time algorithms to prevent timing attacks.

Node.js provides crypto.timingSafeEqual() specifically for this purpose.

Impact

Who is affected:

• Applications using HMAC signatures (via sig.enableHMAC())

References

• CWE-208: Observable Timing Discrepancy - https://cwe.mitre.org/data/definitions/208.html
• Node.js crypto.timingSafeEqual() - https://nodejs.org/api/crypto.html#cryptotimingsafeequala-b