Implement SEP-10 web authentication

[blockchain-maxis]

lib/auth.ts implements a custom Sign-In-With-Stellar challenge. SEP-10 is the
ecosystem standard and unlocks interoperability with existing Stellar tooling.

Acceptance: compliant challenge/verify endpoints (correct home domain, web auth
domain, timebounds, signature checks), tests against the spec's vectors, SIWS kept
working during migration.

[collinsezedike]

@collinsezedike has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

The domain fields are the part most likely to get subtly wrong in a SEP-10 implementation, since home_domain and web_auth_domain have precise semantics (which service is verifying versus which domain the client claims to be authenticating for), and transposing them silently breaks the replay protection SEP-10 is designed to provide.

Here's my plan:

• Implement GET /auth (challenge) building the transaction per spec: Manage Data operation with the correct home_domain and web_auth_domain values, a random 64-byte nonce, and timebounds set to the spec's recommended window (typically 15 minutes)
• Implement POST /auth (verify), validating: the returned transaction's signature against the client's public key, that the transaction matches the original challenge exactly, that timebounds haven't expired, and that the domain fields match this service's actual configuration rather than accepting arbitrary values
• Test against the spec's own reference vectors directly, not just hand-written fixtures, to catch any subtle encoding mismatch
• Keep the existing custom SIWS flow fully functional in parallel during migration, likely behind a feature flag or a version-negotiated endpoint, so existing clients aren't broken mid-transition
• Add tests for expired timebounds, tampered transaction, and mismatched domain fields, confirming each is rejected

Happy to start as soon as I'm assigned.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @collinsezedike to this issue.

[drips-wave]

Congratulations, @collinsezedike! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @collinsezedike: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[drips-wave]

This issue has been marked as completed by a Drips Wave moderator for @collinsezedike as part of the Stellar Wave Program's 7th Wave 🥳

Moderator's reason: [Auto-assessed] The maintainer @blockchain-maxis explicitly approved the implementation, stating, 'This is careful work and the security reasoning is sound' and confirming there is 'nothing for you to change here.' The contributor also successfully addressed the non-blocking request to include fixed SEP-10 spec vectors in the test suite.

😎 @collinsezedike: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.

ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.