crypto/cipher: replace NewGCMWithNonceAndTagSize with NewGCMWithTagSize

This function was added during the Go 1.11 dev cycle and isn't part of
the API compatibility promise yet.

In the previous implementation, NewGCMWithNonceAndTagSize was being used
as a helper function for NewGCM, NewGCMWithTagSize, and NewGCMWithNonceSize.
With the removal of Nonce size from the name and parameters, we needed to
add an unexported helper function newGCMWithNonceAndTagSize.

Fixes #24977

Change-Id: Ie70f2a192d0556c4f890deb62e68cff6bbbccd33
Reviewed-on: https://go-review.googlesource.com/116435Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/cipher/gcm.go

@@ -80,7 +80,7 @@ type gcm struct {
 // An exception is when the underlying Block was created by aes.NewCipher
 // on systems with hardware support for AES. See the crypto/aes package documentation for details.
 func NewGCM(cipher Block) (AEAD, error) {
-	return NewGCMWithNonceAndTagSize(cipher, gcmStandardNonceSize, gcmTagSize)
+	return newGCMWithNonceAndTagSize(cipher, gcmStandardNonceSize, gcmTagSize)
 }
 
 // NewGCMWithNonceSize returns the given 128-bit, block cipher wrapped in Galois
@@ -90,18 +90,22 @@ func NewGCM(cipher Block) (AEAD, error) {
 // cryptosystem that uses non-standard nonce lengths. All other users should use
 // NewGCM, which is faster and more resistant to misuse.
 func NewGCMWithNonceSize(cipher Block, size int) (AEAD, error) {
-	return NewGCMWithNonceAndTagSize(cipher, size, gcmTagSize)
+	return newGCMWithNonceAndTagSize(cipher, size, gcmTagSize)
 }
 
-// NewGCMWithNonceAndTagSize returns the given 128-bit, block cipher wrapped in Galois
-// Counter Mode, which accepts nonces of the given length and generates tags with the given length.
+// NewGCMWithTagSize returns the given 128-bit, block cipher wrapped in Galois
+// Counter Mode, which generates tags with the given length.
 //
 // Tag sizes between 12 and 16 bytes are allowed.
 //
 // Only use this function if you require compatibility with an existing
 // cryptosystem that uses non-standard tag lengths. All other users should use
 // NewGCM, which is more resistant to misuse.
-func NewGCMWithNonceAndTagSize(cipher Block, nonceSize, tagSize int) (AEAD, error) {
+func NewGCMWithTagSize(cipher Block, tagSize int) (AEAD, error) {
+	return newGCMWithNonceAndTagSize(cipher, gcmStandardNonceSize, tagSize)
+}
+
+func newGCMWithNonceAndTagSize(cipher Block, nonceSize, tagSize int) (AEAD, error) {
 	if tagSize < gcmMinimumTagSize || tagSize > gcmBlockSize {
 		return nil, errors.New("cipher: incorrect tag size given to GCM")
 	}

src/crypto/cipher/gcm_test.go

@@ -231,9 +231,28 @@ func TestAESGCM(t *testing.T) {
 		plaintext, _ := hex.DecodeString(test.plaintext)
 		ad, _ := hex.DecodeString(test.ad)
 		tagSize := (len(test.result) - len(test.plaintext)) / 2
-		aesgcm, err := cipher.NewGCMWithNonceAndTagSize(aes, len(nonce), tagSize)
-		if err != nil {
-			t.Fatal(err)
+
+		var aesgcm cipher.AEAD
+		switch {
+		// Handle non-standard nonce sizes
+		case tagSize != 16:
+			aesgcm, err = cipher.NewGCMWithTagSize(aes, tagSize)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+		// Handle non-standard tag sizes
+		case len(nonce) != 12:
+			aesgcm, err = cipher.NewGCMWithNonceSize(aes, len(nonce))
+			if err != nil {
+				t.Fatal(err)
+			}
+
+		default:
+			aesgcm, err = cipher.NewGCM(aes)
+			if err != nil {
+				t.Fatal(err)
+			}
 		}
 
 		ct := aesgcm.Seal(nil, nonce, plaintext, ad)
@@ -277,12 +296,11 @@ func TestAESGCM(t *testing.T) {
 
 func TestGCMInvalidTagSize(t *testing.T) {
 	key, _ := hex.DecodeString("ab72c77b97cb5fe9a382d9fe81ffdbed")
-	nonce, _ := hex.DecodeString("54cc7dc2c37ec006bcc6d1db")
 
 	aes, _ := aes.NewCipher(key)
 
 	for _, tagSize := range []int{0, 1, aes.BlockSize() + 1} {
-		aesgcm, err := cipher.NewGCMWithNonceAndTagSize(aes, len(nonce), tagSize)
+		aesgcm, err := cipher.NewGCMWithTagSize(aes, tagSize)
 		if aesgcm != nil || err == nil {
 			t.Fatalf("NewGCMWithNonceAndTagSize was successful with an invalid %d-byte tag size", tagSize)
 		}