stupidgcm: use "dst" as the output buffer it is big enough

This saves an allocation of the ciphertext block.

internal/stupidgcm/stupidgcm.go

@@ -55,7 +55,17 @@ func (g stupidGCM) Seal(dst, iv, in, authData []byte) []byte {
 	if len(in) == 0 {
 		log.Panic("Zero-length input data is not supported")
 	}
-	buf := make([]byte, len(in)+tagLen)
+
+	// If the "dst" slice is large enough we can use it as our output buffer
+	outLen := len(in) + tagLen
+	var buf []byte
+	inplace := false
+	if cap(dst)-len(dst) >= outLen {
+		inplace = true
+		buf = dst[len(dst) : len(dst)+outLen]
+	} else {
+		buf = make([]byte, outLen)
+	}
 
 	// https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Encryption_using_GCM_mode
 
@@ -115,6 +125,9 @@ func (g stupidGCM) Seal(dst, iv, in, authData []byte) []byte {
 	// Free scratch space
 	C.EVP_CIPHER_CTX_free(ctx)
 
+	if inplace {
+		return dst[:len(dst)+outLen]
+	}
 	return append(dst, buf...)
 }
 

internal/stupidgcm/stupidgcm_test.go

@@ -73,6 +73,45 @@ func TestEncryptDecrypt(t *testing.T) {
 	}
 }
 
+// Seal re-uses the "dst" buffer it is large enough.
+// Check that this works correctly by testing different "dst" capacities from
+// 5000 to 16 and "in" lengths from 1 to 5000.
+func TestInplaceSeal(t *testing.T) {
+	key := randBytes(32)
+	sGCM := New(key, false)
+	authData := randBytes(24)
+	iv := randBytes(16)
+
+	gAES, err := aes.NewCipher(key)
+	if err != nil {
+		t.Fatal(err)
+	}
+	gGCM, err := cipher.NewGCMWithNonceSize(gAES, 16)
+	if err != nil {
+		t.Fatal(err)
+	}
+	max := 5016
+	// Check all block sizes from 1 to 5000
+	for i := 1; i < max-16; i++ {
+		in := make([]byte, i)
+		dst := make([]byte, max-i)
+		dst = dst[:16]
+
+		sOut := sGCM.Seal(dst, iv, in, authData)
+		dst2 := make([]byte, 16)
+		gOut := gGCM.Seal(dst2, iv, in, authData)
+
+		// Ciphertext must be identical to Go GCM
+		if !bytes.Equal(sOut, gOut) {
+			t.Fatalf("Compare failed for encryption, size %d", i)
+			t.Log("sOut:")
+			t.Log("\n" + hex.Dump(sOut))
+			t.Log("gOut:")
+			t.Log("\n" + hex.Dump(gOut))
+		}
+	}
+}
+
 // TestCorruption verifies that changes in the ciphertext result in a decryption
 // error
 func TestCorruption(t *testing.T) {