crypto: allow to restrict valid GCM tag length

This change allows users to restrict accepted GCM authentication tag
lengths to a single value.

Fixes: nodejs#17523

doc/api/crypto.md

@@ -1457,6 +1457,10 @@ to create the `Decipher` object.
 <!-- 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 restrict accepted
+                 GCM authentication tag lengths.
   - 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
@@ -1474,7 +1478,9 @@ and 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 restrict accepted authentication tags
+to those with the specified length.
 
 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

@@ -2806,6 +2806,10 @@ void CipherBase::InitIv(const FunctionCallbackInfo<Value>& args) {
 }
 
 
+static bool IsValidGCMTagLength(int tag_len) {
+  return tag_len == 4 || tag_len == 8 || (tag_len >= 12 && tag_len <= 16);
+}
+
 bool CipherBase::InitAuthenticated(const char *cipher_type, int iv_len,
                                    int auth_tag_len) {
   CHECK(IsAuthenticatedMode());
@@ -2818,7 +2822,8 @@ bool CipherBase::InitAuthenticated(const char *cipher_type, int iv_len,
     return false;
   }
 
-  if (EVP_CIPHER_CTX_mode(ctx_) == EVP_CIPH_CCM_MODE) {
+  const int mode = EVP_CIPHER_CTX_mode(ctx_);
+  if (mode == EVP_CIPH_CCM_MODE) {
     if (auth_tag_len < 0) {
       char msg[128];
       snprintf(msg, sizeof(msg), "authTagLength required for %s", cipher_type);
@@ -2851,6 +2856,19 @@ bool CipherBase::InitAuthenticated(const char *cipher_type, int iv_len,
     } else {
       max_message_size_ = INT_MAX;
     }
+  } else {
+    CHECK_EQ(mode, EVP_CIPH_GCM_MODE);
+
+    if (auth_tag_len >= 0 && !IsValidGCMTagLength(auth_tag_len)) {
+      char msg[50];
+      snprintf(msg, sizeof(msg),
+          "Invalid GCM authentication tag length: %u", auth_tag_len);
+      env()->ThrowError(msg);
+      return false;
+    }
+
+    // Remember the given authentication tag length for later.
+    auth_tag_len_ = auth_tag_len;
   }
 
   return true;
@@ -2886,10 +2904,11 @@ void CipherBase::GetAuthTag(const FunctionCallbackInfo<Value>& args) {
   // Only callable after Final and if encrypting.
   if (cipher->ctx_ != nullptr ||
       cipher->kind_ != kCipher ||
-      cipher->auth_tag_len_ == 0) {
+      cipher->auth_tag_len_ <= 0) {
     return args.GetReturnValue().SetUndefined();
   }
 
+  CHECK_GE(cipher->auth_tag_len_, 0);
   Local<Object> buf =
       Buffer::Copy(env, cipher->auth_tag_, cipher->auth_tag_len_)
       .ToLocalChecked();
@@ -2911,7 +2930,8 @@ void CipherBase::SetAuthTag(const FunctionCallbackInfo<Value>& args) {
   unsigned int tag_len = Buffer::Length(args[0]);
   const int mode = EVP_CIPHER_CTX_mode(cipher->ctx_);
   if (mode == EVP_CIPH_GCM_MODE) {
-    if (tag_len > 16 || (tag_len < 12 && tag_len != 8 && tag_len != 4)) {
+    if ((cipher->auth_tag_len_ >= 0 && cipher->auth_tag_len_ != tag_len) ||
+        !IsValidGCMTagLength(tag_len)) {
       char msg[50];
       snprintf(msg, sizeof(msg),
           "Invalid GCM authentication tag length: %u", tag_len);

src/node_crypto.h

@@ -398,7 +398,7 @@ class CipherBase : public BaseObject {
         ctx_(nullptr),
         kind_(kind),
         auth_tag_set_(false),
-        auth_tag_len_(0),
+        auth_tag_len_(-1),
         pending_auth_failed_(false) {
     MakeWeak<CipherBase>(this);
   }
@@ -407,7 +407,7 @@ class CipherBase : public BaseObject {
   EVP_CIPHER_CTX* ctx_;
   const CipherKind kind_;
   bool auth_tag_set_;
-  unsigned int auth_tag_len_;
+  int auth_tag_len_;
   char auth_tag_[EVP_GCM_TLS_TAG_LEN];
   bool pending_auth_failed_;
   int max_message_size_;

test/parallel/test-crypto-authenticated.js

@@ -726,9 +726,46 @@ for (const test of TEST_CASES) {
       type: Error,
       message: `Invalid GCM authentication tag length: ${length}`
     });
+
+    common.expectsError(() => {
+      crypto.createDecipheriv('aes-256-gcm',
+                              'FxLKsqdmv0E9xrQhp0b1ZgI0K7JFZJM8',
+                              'qkuZpJWCewa6Szih',
+                              {
+                                authTagLength: length
+                              });
+    }, {
+      type: Error,
+      message: `Invalid GCM authentication tag length: ${length}`
+    });
   }
 }
 
+// Test that users can manually restrict the GCM tag length to a single value.
+{
+  const decipher = crypto.createDecipheriv('aes-256-gcm',
+                                           'FxLKsqdmv0E9xrQhp0b1ZgI0K7JFZJM8',
+                                           'qkuZpJWCewa6Szih', {
+                                             authTagLength: 8
+                                           });
+
+  common.expectsError(() => {
+    // This tag would normally be allowed.
+    decipher.setAuthTag(Buffer.from('1'.repeat(12)));
+  }, {
+    type: Error,
+    message: 'Invalid GCM authentication tag length: 12'
+  });
+
+  // The Decipher object should be left intact.
+  decipher.setAuthTag(Buffer.from('445352d3ff85cf94', 'hex'));
+  const text = Buffer.concat([
+    decipher.update('3a2a3647', 'hex'),
+    decipher.final()
+  ]);
+  assert.strictEqual(text.toString('utf8'), 'node');
+}
+
 // Test that create(De|C)ipher(iv)? throws if the mode is CCM and an invalid
 // authentication tag length has been specified.
 {