Improve key store passphrase crypto

* Change MAC-then-Encrypt to Encrypt-then-MAC
* Change AES256 to AES128
* Use first 16 bytes of KDF derived key for AES and
  remaining 16 for MAC

crypto/crypto.go

@@ -252,7 +252,7 @@ func aesCBCDecrypt(key []byte, cipherText []byte, iv []byte) (plainText []byte,
 	decrypter.CryptBlocks(paddedPlainText, cipherText)
 	plainText = PKCS7Unpad(paddedPlainText)
 	if plainText == nil {
-		err = errors.New("Decryption failed: PKCS7Unpad failed after decryption")
+		err = errors.New("Decryption failed: PKCS7Unpad failed after AES decryption")
 	}
 	return plainText, err
 }