how to implement decrypt in flutter like java?

Question

I have to change java code for decrypt to flutter code. I use library "encrypt" for flutter. my flutter code show

Key length not 128/192/256 bits.

my java spring boot code.

public static String decryptAES256(String str, String secretKey) throws java.io.UnsupportedEncodingException,
                NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException,
                InvalidAlgorithmParameterException, IllegalBlockSizeException, BadPaddingException {
            String AES = "AES";
            String AES_PADDING = "AES/CBC/PKCS5Padding";
            byte[] keyData = java.util.Base64.getDecoder().decode(secretKey);
            String iv = secretKey.substring(0, 16);
            SecretKey secureKey = new SecretKeySpec(keyData, AES);
            Cipher c = Cipher.getInstance(AES_PADDING);
            c.init(Cipher.DECRYPT_MODE, secureKey, new IvParameterSpec(iv.getBytes(StandardCharsets.UTF_8)));
            byte[] byteStr = Base64.getDecoder().decode(str.getBytes());
            return new String(c.doFinal(byteStr), StandardCharsets.UTF_8);
        }

my flutter code.

final encData = codefModel.data!.encData!;
    final key = en.Key.fromUtf8(decryptKey);
    final b64key = en.Key.fromUtf8(base64Url.encode(key.bytes));
    final iv = en.IV.fromLength(16);
    final encrypter = en.Encrypter(en.AES(b64key, mode: en.AESMode.cbc));
    logger.d(encrypter.decrypt64(encData, iv: iv));

Topaco · Accepted Answer · 2022-11-01 12:16:23Z

1

From the Java code it can be deduced that the AES key secretKey and the ciphertext str are Base64 encoded. The first 16 bytes of the Base64 encoded key (not of the raw key!) are used as IV.
In the Dart code the key is incorrectly converted and a wrong IV is used. A possible fix is (together with sample data):

const decryptKey = "MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2Nzg5MDE=";
const encData = "lo0dOoJrHNRuefPXgkEa6jtDUhV5CguF9MHWTJ4Y8eGP8zHaKJEQIuOTZxstyk3X";
final key = en.Key.fromBase64(decryptKey);
final iv = en.IV(Uint8List.fromList(utf8.encode(decryptKey).sublist(0, 16)));
final encrypter = en.Encrypter(en.AES(key, mode: en.AESMode.cbc));
final decrypted = encrypter.decrypt64(encData, iv: iv); // The quick brown fox jumps over the lazy dog

As test, key and ciphertext can be used in the Java code that returns the same plaintext for them.

Be aware that deriving the IV from the key is generally insecure, since encryptions with the same key automatically lead to the reuse of key/IV pairs. Instead, a random IV should be used for each encryption.

answered Nov 1, 2022 at 12:16

Topaco

50.5k4 gold badges47 silver badges85 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

broheat Over a year ago

Thank you!!.. it's problem is iv!!! it's very helpful.

Collectives™ on Stack Overflow

how to implement decrypt in flutter like java?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related