Exposing EVP_CIPHER_CTX_ctrl and ECDH_compute_key for AEAD ciphers

[russ]

Problem

Crystal's OpenSSL::Cipher doesn't expose methods needed for authenticated encryption modes (AEAD) like AES-GCM. Specifically:

• No way to set Additional Authenticated Data (AAD)
• No way to retrieve/set authentication tags
• No ECDH key agreement support

Use Cases

I've encountered this limitation in two separate production projects:

1. JWE (JSON Web Encryption) with AES-128-GCM

• Implements RFC 7516 for secure token generation
• Requires AAD support to authenticate the JWT header alongside the encrypted payload
• Currently binding LibCrypto directly to call EVP_EncryptUpdate with AAD and retrieve tags via EVP_CIPHER_CTX_ctrl

2. Web Push (RFC 8291) - see github.com/russ/vapid

• Implements Web Push message encryption
• Requires ECDH key agreement (ECDH_compute_key) and GCM authentication tags
• Also binding LibCrypto directly

Current Workaround

Both projects duplicate these LibCrypto bindings:

From internal JWE project:

@[Link("crypto")]
lib LibCrypto
  fun evp_aes_128_gcm = EVP_aes_128_gcm : Void*
  fun evp_cipher_ctx_new = EVP_CIPHER_CTX_new : Void*
  fun evp_cipher_ctx_free = EVP_CIPHER_CTX_free(ctx : Void*)
  fun evp_encryptinit_ex = EVP_EncryptInit_ex(ctx : Void*, type : Void*, impl : Void*, key : UInt8*, iv : UInt8*) : Int32
  fun evp_encryptupdate = EVP_EncryptUpdate(ctx : Void*, out : UInt8*, outl : Int32*, in : UInt8*, inl : Int32) : Int32
  fun evp_encryptfinal_ex = EVP_EncryptFinal_ex(ctx : Void*, out : UInt8*, outl : Int32*) : Int32
  fun evp_cipher_ctx_ctrl = EVP_CIPHER_CTX_ctrl(ctx : Void*, type : Int32, arg : Int32, ptr : Void*) : Int32
  EVP_CTRL_GCM_SET_IVLEN = 0x9
  EVP_CTRL_GCM_GET_TAG   = 0x10
end

From github.com/russ/vapid:

lib LibCrypto
  fun ecdh_compute_key = ECDH_compute_key(out : UInt8*, outlen : LibC::SizeT, pub_key : Void*, ecdh : Void*, kdf : Void*) : LibC::Int
  fun ec_group_get_degree = EC_GROUP_get_degree(group : Void*) : LibC::Int

  # EVP cipher functions
  fun evp_cipher_ctx_new = EVP_CIPHER_CTX_new : Void*
  fun evp_cipher_ctx_free = EVP_CIPHER_CTX_free(ctx : Void*)
  fun evp_get_cipherbyname = EVP_get_cipherbyname(name : LibC::Char*) : Void*
  fun evp_encryptinit_ex = EVP_EncryptInit_ex(ctx : Void*, type : Void*, impl : Void*, key : UInt8*, iv : UInt8*) : LibC::Int
  fun evp_encryptupdate = EVP_EncryptUpdate(ctx : Void*, out : UInt8*, outl : LibC::Int*, in : UInt8*, inl : LibC::Int) : LibC::Int
  fun evp_encryptfinal_ex = EVP_EncryptFinal_ex(ctx : Void*, out : UInt8*, outl : LibC::Int*) : LibC::Int
  fun evp_cipher_ctx_ctrl = EVP_CIPHER_CTX_ctrl(ctx : Void*, type : LibC::Int, arg : LibC::Int, ptr : Void*) : LibC::Int

  # GCM control constants
  EVP_CTRL_GCM_SET_IVLEN = 0x9
  EVP_CTRL_GCM_GET_TAG = 0x10
end

Notes

• This code is admittedly over my head, but it does work in production environments

Would this be something that makes sense to add to openssl_ext?