Ensure Cipher.Decrypt doesn't mangle input ciphertext []byte

pkg/encryption/cipher.go

@@ -50,7 +50,7 @@ func NewBase64Cipher(initCipher func([]byte) (Cipher, error), secret []byte) (Ci
 	return &Base64Cipher{Cipher: c}, nil
 }
 
-// Encrypt encrypts a value with AES CFB & Base64 encodes it
+// Encrypt encrypts a value with the embedded Cipher & Base64 encodes it
 func (c *Base64Cipher) Encrypt(value []byte) ([]byte, error) {
 	encrypted, err := c.Cipher.Encrypt([]byte(value))
 	if err != nil {
@@ -60,7 +60,7 @@ func (c *Base64Cipher) Encrypt(value []byte) ([]byte, error) {
 	return []byte(base64.StdEncoding.EncodeToString(encrypted)), nil
 }
 
-// Decrypt Base64 decodes a value & decrypts it with AES CFB
+// Decrypt Base64 decodes a value & decrypts it with the embedded Cipher
 func (c *Base64Cipher) Decrypt(ciphertext []byte) ([]byte, error) {
 	encrypted, err := base64.StdEncoding.DecodeString(string(ciphertext))
 	if err != nil {
@@ -103,12 +103,12 @@ func (c *CFBCipher) Decrypt(ciphertext []byte) ([]byte, error) {
 		return nil, fmt.Errorf("encrypted value should be at least %d bytes, but is only %d bytes", aes.BlockSize, len(ciphertext))
 	}
 
-	iv := ciphertext[:aes.BlockSize]
-	ciphertext = ciphertext[aes.BlockSize:]
+	iv, ciphertext := ciphertext[:aes.BlockSize], ciphertext[aes.BlockSize:]
+	plaintext := make([]byte, len(ciphertext))
 	stream := cipher.NewCFBDecrypter(c.Block, iv)
-	stream.XORKeyStream(ciphertext, ciphertext)
+	stream.XORKeyStream(plaintext, ciphertext)
 
-	return ciphertext, nil
+	return plaintext, nil
 }
 
 type GCMCipher struct {
@@ -135,6 +135,8 @@ func (c *GCMCipher) Encrypt(value []byte) ([]byte, error) {
 	if _, err = io.ReadFull(rand.Reader, nonce); err != nil {
 		return nil, err
 	}
+	// Using nonce as Seal's dst argument results in it being the first
+	// chunk of bytes in the ciphertext. Decrypt retrieves the nonce/IV from this.
 	ciphertext := gcm.Seal(nonce, nonce, value, nil)
 	return ciphertext, nil
 }

pkg/encryption/cipher_test.go

@@ -84,9 +84,17 @@ func TestEncryptAndDecrypt(t *testing.T) {
 									assert.Equal(t, nil, err)
 									assert.NotEqual(t, encrypted, data)
 
+									// Ensure our Decrypt function doesn't decrypt in place
+									immutable := make([]byte, len(encrypted))
+									copy(immutable, encrypted)
+
 									decrypted, err := c.Decrypt(encrypted)
 									assert.Equal(t, nil, err)
+									// Original data back
 									assert.Equal(t, data, decrypted)
+									// Decrypt didn't operate in-place on []byte
+									assert.Equal(t, encrypted, immutable)
+									// Encrypt/Decrypt actually did something
 									assert.NotEqual(t, encrypted, decrypted)
 								})
 							}