Server-side crypto invariants

[codebestia]

Write apps/backend/docs/concepts-crypto-invariants.md covering lib/ciphertextInvariant.ts and lib/signalInvariants.ts — the guards that keep the server from ever accepting or storing material it must not hold, and the security-ci job that regression-tests them.

Acceptance criteria:

• States the invariants plainly: the server stores ciphertext only, and never accepts session state, ratchet state, or private keys on any inbound payload
• Documents findForbiddenSessionStateField and the rejected field names, and that rejection happens before any database lookup
• Explains why the relevant Zod schemas are .strict() — a non-strict schema silently strips a forbidden field instead of rejecting the request
• Documents the security-ci regression job that keeps these enforced

[attyolu]

@attyolu has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello please assign this issue to me I will definitely do a great job solving

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[BigManly4]

@BigManly4 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I'd like to contribute to this issue. As a full-stack and backend developer with open-source experience, I'll provide a high-quality implementation and keep the discussion updated throughout the process.

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[drips-wave]

Congratulations, @BigManly4! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

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[github-actions]

Closed by #599, merged into dev.

[drips-wave]

This issue has been completed by @BigManly4 as part of the Stellar Wave Program's 8th Wave 🥳

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