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User prompt 1 of 1 - 10/1/2025, 11:34:14 AM:
When making a user account like this:

```rust
        // Allocate user ID
        let user_id = next_user_id().context("Unable to allocate user id")?;

        let (kek, kek_params) = auth::derive_kek(password);

        // Generate DEK (stub 256-bit)
        let dek = auth::generate_dek();

        // Compute PHC password hash (stub)
        let phc = hash(password)?;

        // Wrap DEK under KEK with aad=user_id (stub: no-op wrap)
        // AAD = "additional authenticated data" in AEAD
        let aad = user_id.to_be_bytes();
        let wrapped_dek = auth::wrap_key(&kek, &dek, &aad);

```

Am I correct in thinking that the use of user_id as the AAD will break the AEAD's authentication check and introduce a vulnerability if the wrapped KEK is copied to a different user database with different user IDs? I'm very new to trying to understand this stuff. If I'm correct about that, what would be best practice for an app that has a set of local user IDs (for local accounts) and a set of remote IDs (for remote accounts, which may or may not exist for a given local user - and at least as of right now, aren't allocated until *after* the local user has been created)? Thanks!

GPT-5 mini:
Short answer: Yes — using a mutable or environment-specific value (like a local user_id that can differ across installations/databases) as AEAD AAD will cause verification to fail when ciphertext is moved between contexts, and it can be a correctness/vulnerability problem if you intended the wrapped key to be portable. Use AAD only for stable, canonical context you really want authenticated.

Recommended approach (practical, prescriptive):

1. Decide portability semantics
   - If wrapped DEKs must be tied to a local-account identity only usable in this database, authenticating with the local user_id is fine.
   - If you want wrapped DEKs to be usable in other systems/databases (or to survive user-id remapping), do NOT use a local, mutable ID as AAD.

2. Use a stable canonical identifier for AAD when you need cross-system binding
   - Option: use a stable, globally unique user identifier (UUID, user's opaque account GUID, or remote-account ID) that will be the same across systems.
   - Option: use the user's public key or an immutable principal identifier (e.g., "user:urn:service:1234") if available.
   - Ensure that this canonical id is assigned/known before wrapping, or defer wrapping until it is.

3. If you must wrap before a canonical remote id exists
   - Generate and store a stable local wrapping identifier (wrap_id) at account creation that you will keep the same across migrations. Use that as AAD.
   - Or wrap with no AAD (or only constant AAD) and store an additional HMAC/metadata binding separately; then re-wrap or rebind when the remote id appears. (Avoid unauthenticated binding if you need authenticity guarantees.)

4. Practical patterns
   - Local-only: AAD = local_user_id (clearly document non-portability).
   - Cross-system: AAD = canonical_user_guid (must be persistent across systems).
   - Hybrid: AAD = concat(version||principal_type||canonical_id) so you can change format safely (include a version tag).
   - Migration: if moving an account to a new database that assigns new local IDs, either rewrap under the new AAD or keep and migrate the canonical id too.

5. Implementation notes
   - AAD is authenticated but not encrypted — do not include secrets in it.
   - Keep AAD encoding unambiguous (length-prefix or fixed fields) to avoid subtle collisions.
   - When using AEAD libraries, pass the exact same AAD bytes to unwrap that were used to wrap.
   - Consider key rotation: re-wrap DEKs when KEKs rotate and ensure canonical id survives rotation/migration.

Minimal concrete example for your flow
- If remote ID may not exist at local creation:
  - At local create: generate user_guid (random UUID), persist it with the account.
  - Use user_guid.to_be_bytes() as AAD for wrap_key.
  - When/if a remote account is created, store its remote_id mapped to user_guid; no rewrap needed.
- If you cannot add a stable id: wrap without user-specific AAD and then rewrap (unwrap+wrap) once a stable id exists.

Bottom line: AAD should be a stable, authoritative context identifier for the object you want authenticated. Do not rely on transient/local-only numeric DB IDs if you expect portability.