Description of changes

• Added MPC context ID to signcrypted elements
• Moved things around to minimize unauthenticated or redundant data
• Added a new backwards compatibility for 0.14.0
• Removed custodian backup tests from the 0.13.20 backwards compatibility test (since they are not in use yet)
• Added a check and tests on IV's for encryption and decryption

Note that almost all line additions are from the new backwards compatibility test.

Issue ticket number and link

This closes [https://github.com/zama-ai/kms-internal/issues/3025]

PR Checklist

I attest that all checked items are satisfied. Any deviation is clearly justified above.

• Title follows conventional commits (e.g. chore: ...).
• Tests added for every new pub item and test coverage has not decreased.
• Public APIs and non-obvious logic documented; unfinished work marked as TODO(#issue).
• unwrap/expect/panic only in tests or for invariant bugs (documented if present).
• No dependency version changes OR (if changed) only minimal required fixes.
• No architectural protocol changes OR linked spec PR/issue provided.
• No breaking deployment config changes OR devops label + infra notified + infra-team reviewer assigned.
• No breaking gRPC / serialized data changes OR commit marked with ! and affected teams notified.
• No modifications to existing versionized structs OR backward compatibility tests updated.
• No critical business logic / crypto changes OR ≥2 reviewers assigned.
• No new sensitive data fields added OR Zeroize + ZeroizeOnDrop implemented.
• No new public storage data OR data is verifiable (signature / digest).
• No unsafe; if unavoidable: minimal, justified, documented, and test/fuzz covered.
• Strongly typed boundaries: typed inputs validated at the edge; no untyped values or errors cross modules.
• Self-review completed.

Dependency Update Questionnaire (only if deps changed or added)

Answer in the Cargo.toml next to the dependency (or here if updating):

1. Ownership changes or suspicious concentration?
2. Low popularity?
3. Unusual version jump?
4. Lacking documentation?
5. Missing CI?
6. No security / disclosure policy?
7. Significant size increase?

More details and explanations for the checklist and dependency updates can be found in CONTRIBUTING.md