Migrate HMAC_* to EVP_MAC interface

[VladGud]

Migrate HMAC_* to EVP_MAC interface

Issue Summary

Migrate the HMAC implementation in gost_keyexpimp.c from deprecated HMAC_* APIs to the modern EVP_MAC interface to ensure compatibility with OpenSSL builds that disable deprecated functionality.

Problem Description

The current HMAC usage in key derivation functions relies on deprecated HMAC APIs (HMAC_CTX_new, HMAC_CTX_free, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_reset), which are marked for removal in future OpenSSL versions. This prevents the codebase from building or running with OPENSSL_NO_DEPRECATED_3_0 enabled. The migration to EVP_MAC provides a stable, provider-based alternative that aligns with OpenSSL's modern architecture.

Current Implementation

• gost_kdftree2012_256() uses HMAC_CTX *ctx with HMAC_CTX_new(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final(), HMAC_CTX_reset(), and HMAC_CTX_free()
• Context is created, initialized with key and digest, updated with data, finalized, and reset for multiple iterations

Required Changes

1. Replace HMAC_CTX with EVP_MAC context

• Modify gost_kdftree2012_256() to use EVP_MAC *mac and EVP_MAC_CTX *mac_ctx
• Replace HMAC_CTX_new() with EVP_MAC_fetch("HMAC") and EVP_MAC_CTX_new()

2. Update MAC initialization

• Replace HMAC_Init_ex() with EVP_MAC_init() using OSSL_MAC_PARAM_DIGEST parameter (set to NID_id_GostR3411_2012_256)

3. Update MAC update and final operations

• Replace HMAC_Update() with EVP_MAC_update()
• Replace HMAC_Final() with EVP_MAC_final()

4. Update context reset and cleanup

• Replace HMAC_CTX_reset() with re-running EVP_MAC_init() with the same key and parameters
• Replace HMAC_CTX_free() with EVP_MAC_CTX_free() and EVP_MAC_free()

5. Handle EVP_MAC availability

• Add checks for EVP_MAC support; provide fallback or error if not available

Files to Modify

• gost_keyexpimp.c: Update gost_kdftree2012_256() function to use EVP_MAC APIs

Dependencies

This task has no dependencies on other issues but should be completed before full provider decoupling (#116, #117).

Acceptance Criteria

• HMAC implementation uses only EVP_MAC APIs, no HMAC_* calls remain
• Key derivation function gost_kdftree2012_256() works correctly with EVP_MAC

Testing

• Unit tests for key derivation (gost_kdftree2012_256) pass with new implementation
• Integration tests with TLS tree key derivation (gost_tlstree) succeed