Add OpenSSL::Cipher#gcm_tag and #gcm_tag= for AEAD ciphers

spec/std/openssl/cipher_spec.cr

@@ -58,4 +58,27 @@ describe OpenSSL::Cipher do
     cipher = OpenSSL::Cipher.new("aes-128-cbc")
     cipher.authenticated?.should be_false
   end
+
+  it "encrypts/decrypts with GCM authentication tag" do
+    key = Random::Secure.random_bytes(32)
+    iv = Random::Secure.random_bytes(12)
+    plaintext = "Hello, GCM!"
+
+    enc = OpenSSL::Cipher.new("aes-256-gcm")
+    enc.encrypt
+    enc.key = key
+    enc.iv = iv
+    ciphertext = enc.update(plaintext)
+    ciphertext += enc.final
+    tag = enc.gcm_tag
+
+    dec = OpenSSL::Cipher.new("aes-256-gcm")
+    dec.decrypt
+    dec.key = key
+    dec.iv = iv
+    dec.gcm_tag = tag
+    result = dec.update(ciphertext)
+    result += dec.final
+    String.new(result).should eq(plaintext)
+  end
 end

src/openssl/cipher.cr

@@ -161,6 +161,29 @@ class OpenSSL::Cipher
     LibCrypto.evp_cipher_flags(cipher).includes?(LibCrypto::CipherFlags::EVP_CIPH_FLAG_AEAD_CIPHER)
   end
 
+  # Returns the authentication tag for an AEAD cipher (e.g. AES-GCM).
+  # Must be called after `#final` during encryption.
+  # Raises `Error` if the cipher is not authenticated.
+  def gcm_tag : Bytes
+    raise Error.new("Cipher is not authenticated") unless authenticated?
+    tag = Bytes.new(LibCrypto::EVP_GCM_TLS_TAG_LEN)
+    if LibCrypto.evp_cipher_ctx_ctrl(@ctx, LibCrypto::EVP_CTRL_GCM_GET_TAG, tag.size, tag.to_unsafe.as(Void*)) != 1
+      raise Error.new "EVP_CIPHER_CTX_ctrl GET_TAG"
+    end
+    tag
+  end
+
+  # Sets the authentication tag for an AEAD cipher (e.g. AES-GCM).
+  # Must be called before `#final` during decryption.
+  # Raises `Error` if the cipher is not authenticated.
+  def gcm_tag=(tag : Bytes) : Bytes
+    raise Error.new("Cipher is not authenticated") unless authenticated?
+    if LibCrypto.evp_cipher_ctx_ctrl(@ctx, LibCrypto::EVP_CTRL_GCM_SET_TAG, tag.size, tag.to_unsafe.as(Void*)) != 1
+      raise Error.new "EVP_CIPHER_CTX_ctrl SET_TAG"
+    end
+    tag
+  end
+
   private def cipherinit(cipher = nil, engine = nil, key = nil, iv = nil, enc = -1)
     if LibCrypto.evp_cipherinit_ex(@ctx, cipher, engine, key, iv, enc) != 1
       raise Error.new "EVP_CipherInit_ex"

src/openssl/lib_crypto.cr

@@ -99,6 +99,9 @@ lib LibCrypto
   EVP_MAX_IV_LENGTH   = 16
   EVP_GCM_TLS_TAG_LEN = 16
 
+  EVP_CTRL_GCM_GET_TAG = 0x10
+  EVP_CTRL_GCM_SET_TAG = 0x11
+
   SSL3_RT_HEADER_LENGTH = 5
 
   TLS1_2_VERSION_MAJOR = 0x03
@@ -258,6 +261,7 @@ lib LibCrypto
   fun evp_cipherfinal_ex = EVP_CipherFinal_ex(ctx : EVP_CIPHER_CTX, out : UInt8*, outl : Int32*) : Int32
   fun evp_cipher_ctx_set_padding = EVP_CIPHER_CTX_set_padding(ctx : EVP_CIPHER_CTX, padding : Int32) : Int32
   fun evp_cipher_ctx_cipher = EVP_CIPHER_CTX_cipher(ctx : EVP_CIPHER_CTX) : EVP_CIPHER
+  fun evp_cipher_ctx_ctrl = EVP_CIPHER_CTX_ctrl(ctx : EVP_CIPHER_CTX, type : Int32, arg : Int32, ptr : Void*) : Int32
 
   @[Flags]
   enum CipherFlags : ULong