change expiresAt to be nullable and do default check with at for backward compatibility with mobile apps

[kf7mxe]

made expiresAt nullable and do default check with at for backward compatibility with mobile apps

[UnknownJoe796]

Automated Review (commit c2e4045)## PR #127 Review: Make expiresAt nullable for backwards compatibility

Overall this is a reasonable compatibility shim, but there's one significant concern and a couple of minor ones.

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1. Fragile fallback — 1.hours hardcoded in two places must match proofExpiration

Files: Signer.ext.kt:35, Proof.ext.kt:10

Issue: The signingInfo function falls back to at + 1.hours when expiresAt is null, and the expired check does the same. This only produces correct signature verification if the original proof was signed with a proofExpiration of exactly 1 hour.

All current proof method implementations default proofExpiration to 1.hours, so this works today. However, PinBasedProofEndpoints takes proofExpiration as a required parameter with no default — any consumer of this library can set it to something other than 1 hour. If they do, and a mobile client strips expiresAt during the round-trip, the signingInfo fallback will compute a different payload than what was originally signed, and signature verification will silently fail.

Why it matters: This is an auth-critical code path. The backwards compatibility guarantee is actually conditional on a specific configuration, which isn't documented anywhere. Future developers (or current library consumers) won't know this.

Suggested fix: Either:

• Extract the fallback duration into a shared constant with a clear name like DEFAULT_PROOF_EXPIRATION_FALLBACK and document that it must match the signing-time expiration for null-expiresAt proofs to verify, or
• Remove the fallback from signingInfo entirely and instead have Signer.verify / verifyBlocking try verification with the fallback only if the first attempt fails (try original payload, then try with at + 1.hours).

Confidence: High

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2. Stale doc comment on Proof.expiresAt

File: proofModels.kt:113

The doc comment still says @property expiresAt The timestamp when this proof is no longer valid. without mentioning it's nullable or what null means. The inline comment // Nullable for now for backwards compatibility with mobile apps is only visible in source, not in generated docs.

Suggested fix: Update the KDoc:

* @property expiresAt The timestamp when this proof is no longer valid.
*           Null for backwards compatibility with older clients; when null, defaults to [at] + 1 hour.

Confidence: High

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3. No test coverage for the null expiresAt path

Files: No test changes in the diff.

The backwards compatibility scenario (proof deserialized without expiresAt → signature still verifies → expiry check still works) has no test. Given this is security-critical code with a subtle correctness requirement (the fallback must match the signing duration), a test would catch regressions.

Confidence: Medium