add:SealWithRandomNonce method (#279)

* add:SealWithRandomNonce method

* address pr comments

Author: mertakman

cipher.go

@@ -325,6 +325,7 @@ type cipherGCM struct {
 }
 
 const (
+	aesBlockSize         = 16
 	gcmTagSize           = 16
 	gcmStandardNonceSize = 12
 	// TLS 1.2 additional data is constructed as:
@@ -480,6 +481,56 @@ func (g *cipherGCM) Seal(dst, nonce, plaintext, aad []byte) []byte {
 	return ret
 }
 
+func (g *cipherGCM) SealWithRandomNonce(out, nonce, plaintext, aad []byte) {
+	if uint64(len(plaintext)) > uint64((1<<32)-2)*aesBlockSize {
+		panic("crypto/cipher: message too large for GCM")
+	}
+	if len(nonce) != gcmStandardNonceSize {
+		panic("crypto/cipher: incorrect nonce length given to GCMWithRandomNonce")
+	}
+	if len(out) != len(plaintext)+gcmTagSize {
+		panic("crypto/cipher: incorrect output length given to GCMWithRandomNonce")
+	}
+	if inexactOverlap(out, plaintext) {
+		panic("crypto/cipher: invalid buffer overlap of output and input")
+	}
+	if anyOverlap(out, aad) {
+		panic("crypto/cipher: invalid buffer overlap of output and additional data")
+	}
+
+	if g.tls != cipherGCMTLSNone {
+		panic("cipher: encryption failed")
+	}
+
+	RandReader.Read(nonce)
+	ctx, err := newCipherCtx(g.c.kind, cipherModeGCM, cipherOpNone, g.c.key, nil)
+	if err != nil {
+		panic(err)
+	}
+	defer ossl.EVP_CIPHER_CTX_free(ctx)
+
+	if _, err := ossl.EVP_EncryptInit_ex(ctx, nil, nil, nil, base(nonce)); err != nil {
+		panic(err)
+	}
+	var outl, discard int32
+	if _, err := ossl.EVP_EncryptUpdate(ctx, nil, &discard, baseNeverEmpty(aad), int32(len(aad))); err != nil {
+		panic(err)
+	}
+	if _, err := ossl.EVP_EncryptUpdate(ctx, base(out), &outl, baseNeverEmpty(plaintext), int32(len(plaintext))); err != nil {
+		panic(err)
+	}
+	if len(plaintext) != int(outl) {
+		panic("cipher: incorrect length returned from GCM EncryptUpdate")
+	}
+	if _, err := ossl.EVP_EncryptFinal_ex(ctx, base(out[outl:]), &discard); err != nil {
+		panic(err)
+	}
+	if _, err := ossl.EVP_CIPHER_CTX_ctrl(ctx, ossl.EVP_CTRL_GCM_GET_TAG, 16, unsafe.Pointer(base(out[outl:]))); err != nil {
+		panic(err)
+	}
+	runtime.KeepAlive(g)
+}
+
 var errOpen = errors.New("cipher: message authentication failed")
 
 func (g *cipherGCM) Open(dst, nonce, ciphertext, aad []byte) (_ []byte, err error) {