crypto: support authTagLength in GCM encryption

doc/api/crypto.md

@@ -1321,6 +1321,11 @@ This property is deprecated. Please use `crypto.setFips()` and
 <!-- YAML
 added: v0.1.94
 deprecated: v10.0.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/???
+    description: The `authTagLength` option can now be used to produce shorter
+                 authentication tags in GCM mode and defaults to 16 bytes.
 -->
 
 > Stability: 0 - Deprecated: Use [`crypto.createCipheriv()`][] instead.
@@ -1336,7 +1341,9 @@ Creates and returns a `Cipher` object that uses the given `algorithm` and
 The `options` argument controls stream behavior and is optional except when a
 cipher in CCM mode is used (e.g. `'aes-128-ccm'`). In that case, the
 `authTagLength` option is required and specifies the length of the
-authentication tag in bytes, see [CCM mode][].
+authentication tag in bytes, see [CCM mode][]. In GCM mode, the `authTagLength`
+option is not required but can be used to set the length of the authentication
+tag that will be returned by `getAuthTag()` and defaults to 16 bytes.
 
 The `algorithm` is dependent on OpenSSL, examples are `'aes192'`, etc. On
 recent OpenSSL releases, `openssl list-cipher-algorithms` will display the
@@ -1366,6 +1373,10 @@ Adversaries][] for details.
 <!-- YAML
 added: v0.1.94
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/???
+    description: The `authTagLength` option can now be used to produce shorter
+                 authentication tags in GCM mode and defaults to 16 bytes.
   - version: v9.9.0
     pr-url: https://github.com/nodejs/node/pull/18644
     description: The `iv` parameter may now be `null` for ciphers which do not
@@ -1383,7 +1394,9 @@ initialization vector (`iv`).
 The `options` argument controls stream behavior and is optional except when a
 cipher in CCM mode is used (e.g. `'aes-128-ccm'`). In that case, the
 `authTagLength` option is required and specifies the length of the
-authentication tag in bytes, see [CCM mode][].
+authentication tag in bytes, see [CCM mode][]. In GCM mode, the `authTagLength`
+option is not required but can be used to set the length of the authentication
+tag that will be returned by `getAuthTag()` and defaults to 16 bytes.
 
 The `algorithm` is dependent on OpenSSL, examples are `'aes192'`, etc. On
 recent OpenSSL releases, `openssl list-cipher-algorithms` will display the

src/node_crypto.cc

@@ -3113,9 +3113,10 @@ bool CipherBase::Final(unsigned char** out, int *out_len) {
     ok = EVP_CipherFinal_ex(ctx_, *out, out_len) == 1;
 
     if (ok && kind_ == kCipher && IsAuthenticatedMode()) {
-      // For GCM, the tag length is static (16 bytes), while the CCM tag length
-      // must be specified in advance.
-      if (mode == EVP_CIPH_GCM_MODE)
+      // In GCM mode, the authentication tag length can be specified in advance,
+      // but defaults to 16 bytes when encrypting. In CCM mode, it must always
+      // be given by the user.
+      if (mode == EVP_CIPH_GCM_MODE && auth_tag_len_ == kNoAuthTagLength)
         auth_tag_len_ = sizeof(auth_tag_);
       // TOOD(tniessen) Use EVP_CTRL_AEAP_GET_TAG in OpenSSL 1.1.0
       static_assert(EVP_CTRL_CCM_GET_TAG == EVP_CTRL_GCM_GET_TAG,

test/parallel/test-crypto-authenticated.js

@@ -727,6 +727,18 @@ for (const test of TEST_CASES) {
       message: `Invalid GCM authentication tag length: ${length}`
     });
 
+    common.expectsError(() => {
+      crypto.createCipheriv('aes-256-gcm',
+                            'FxLKsqdmv0E9xrQhp0b1ZgI0K7JFZJM8',
+                            'qkuZpJWCewa6Szih',
+                            {
+                              authTagLength: length
+                            });
+    }, {
+      type: Error,
+      message: `Invalid GCM authentication tag length: ${length}`
+    });
+
     common.expectsError(() => {
       crypto.createDecipheriv('aes-256-gcm',
                               'FxLKsqdmv0E9xrQhp0b1ZgI0K7JFZJM8',
@@ -741,6 +753,23 @@ for (const test of TEST_CASES) {
   }
 }
 
+// Test that GCM can produce shorter authentication tags than 16 bytes.
+{
+  const fullTag = '1debb47b2c91ba2cea16fad021703070';
+  for (const [authTagLength, e] of [[undefined, 16], [12, 12], [4, 4]]) {
+    const cipher = crypto.createCipheriv('aes-256-gcm',
+                                         'FxLKsqdmv0E9xrQhp0b1ZgI0K7JFZJM8',
+                                         'qkuZpJWCewa6Szih', {
+                                           authTagLength
+                                         });
+    cipher.setAAD(Buffer.from('abcd'));
+    cipher.update('01234567', 'hex');
+    cipher.final();
+    const tag = cipher.getAuthTag();
+    assert.strictEqual(tag.toString('hex'), fullTag.substr(0, 2 * e));
+  }
+}
+
 // Test that users can manually restrict the GCM tag length to a single value.
 {
   const decipher = crypto.createDecipheriv('aes-256-gcm',