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

This saves an allocation of the ciphertext block.

1 files changed, 14 insertions, 1 deletions

diff --git a/internal/stupidgcm/stupidgcm.go b/internal/stupidgcm/stupidgcm.go
index 5cc6c3c..54915df 100644
--- a/internal/stupidgcm/stupidgcm.go
+++ b/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...)
 }