API design: wolfCrypt sign/verify functions take non-const key pointers

[MarkAtwood]

wolfCrypt's C API takes non-const (mutable) key pointers for operations that are logically read-only, including signature verification and public-key access. Affected functions include:

• wc_Ed25519Sign, wc_Ed25519Verify
• wc_Ed448Sign, wc_Ed448Verify
• wc_ecc_sign_hash, wc_ecc_verify_hash
• wc_RsaSSL_Sign, wc_RsaSSL_Verify
• wc_MlDsaKey_Sign, wc_MlDsaKey_Verify

Impact: For Rust FFI wrappers, the standard Verifier trait takes &self (shared/immutable reference). There is no safe way to obtain *mut from &self without interior mutability, so every key type that backs sign or verify must be wrapped in UnsafeCell, propagating !Sync throughout the key hierarchy. This is a significant ergonomic burden.

The same pattern affects other language FFI wrappers (Python, Go, C++) that distinguish const/non-const or immutable/mutable references.

Suggested approach: Extend const-correctness to the sign/verify function set as an ongoing effort. The Ed448 DER functions were already fixed in PR #10372 as a contained starting point.

Migrated from internal tracking (ZD-21739).

[MarkAtwood]

Where this surfaces

This was discovered in wolfssl-rs, the Rust FFI wrapper for wolfCrypt. Every key type that backs sign or verify must wrap the key in UnsafeCell, propagating !Sync through the entire key hierarchy.

Affected files in wolfssl-rs:

• wolfcrypt/src/ed25519.rs (line 29):

  /// Interior mutability: wolfCrypt sign requires `*mut` even though
  /// the `Signer` trait provides only `&self`.
  key: UnsafeCell<wc_ed25519_key>,

• wolfcrypt/src/lms.rs (lines 91–97):

  /// wolfCrypt's `wc_LmsKey_Verify` requires a mutable pointer even though
  /// verification does not logically modify the key. We use `UnsafeCell` to
  /// satisfy this requirement while presenting an `&self` API.
  key: UnsafeCell<WcLmsKey>,

• Same pattern in ed448.rs (line 86), mldsa.rs (line 155), ecdsa_native.rs (line 307), ecdh.rs (line 717)

Every key type in the crate needs this workaround. It's not one function — it's a crate-wide architectural constraint imposed by the missing const.