AES/GCM encryption in Flutter (Dart)

Question

I am trying to implement AES/GCM/NoPadding encryption in flutter. I have successfully implemented it in JAVA but when I tried to decrypt it in flutter, I got no success.

I have tried writing decryption code in dart but I got Unhandled Exception: SecretBox has wrong message authentication code (MAC)

My Java Code

public class GCMEncryption {
    public static final int AES_KEY_SIZE = 128;
    public static final int GCM_IV_LENGTH = 12;
    public static final int GCM_TAG_LENGTH = 16;

    public String getEncryptedText(String plainText) {
        try {
            byte[] IV = new byte[GCM_IV_LENGTH];
            SecureRandom random = new SecureRandom();
            random.nextBytes(IV);
            String iv = Base64.getEncoder().encodeToString(IV);
            System.out.println("IV : "+iv);
            
            byte[] cipherText = encrypt(plainText.getBytes(), getKeySpec(), IV);
            String text = Base64.getEncoder().encodeToString(cipherText);
            text = iv+text; // Concating iv and encrypted text together 
            return text;
        } catch (Exception e) {
            e.printStackTrace();
            return "";
        }
    }

    public String getDecryptedText(String cipherText) {
        try {

            // Splitting IV and Encrypted text
            String iv = cipherText.substring(0,16);
            System.out.println("IV : "+iv);
            byte[] IV = Base64.getDecoder().decode(iv);
            cipherText = cipherText.substring(16);
            
            
            byte[] data = Base64.getDecoder().decode(cipherText);
            return decrypt(data, getKeySpec(), IV);
        } catch (Exception e) {
            e.printStackTrace();
            return "";
        }
    }

    private SecretKeySpec getKeySpec() {
        SecretKeySpec spec = null;

        try {
            byte[] bytes = new byte[32];
            String pwd = "Test!ng012345678"; //Temporary
            bytes = pwd.getBytes();
            spec = new SecretKeySpec(bytes, "AES");
            return spec;

        } catch (Exception e) {
            e.printStackTrace();
        }

        return spec;
    }

    private byte[] encrypt(byte[] plaintext, SecretKey key, byte[] IV) throws Exception {
        // Get Cipher Instance
        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");

        // Create SecretKeySpec
        SecretKeySpec keySpec = new SecretKeySpec(key.getEncoded(), "AES");

        // Create GCMParameterSpec
        GCMParameterSpec gcmParameterSpec = new GCMParameterSpec(GCM_TAG_LENGTH * 8, IV);

        // Initialize Cipher for ENCRYPT_MODE
        cipher.init(Cipher.ENCRYPT_MODE, keySpec, gcmParameterSpec);

        // Perform Encryption
        byte[] cipherText = cipher.doFinal(plaintext);

        return cipherText;
    }

    private String decrypt(byte[] cipherText, SecretKey key, byte[] IV) throws Exception {
        // Get Cipher Instance
        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");

        // Create SecretKeySpec
        SecretKeySpec keySpec = new SecretKeySpec(key.getEncoded(), "AES");

        // Create GCMParameterSpec
        GCMParameterSpec gcmParameterSpec = new GCMParameterSpec(GCM_TAG_LENGTH * 8, IV);

        // Initialize Cipher for DECRYPT_MODE
        cipher.init(Cipher.DECRYPT_MODE, keySpec, gcmParameterSpec);

        // Perform Decryption
        byte[] decryptedText = cipher.doFinal(cipherText);

        return new String(decryptedText);
    }
}

My Dart Code

 Future<String> decrypt(String textBlock) async {
    Uint8List data  = base64.decode("4NA09I5VpmdD0o1k3VP8eIfyZRKDtzOwgJv5nh0nmEPZ/Q==");
    Uint8List passphrase = utf8.encode('Test!ng012345678');
    SecretKey secretKey = new SecretKey(passphrase);

    Uint8List iv = utf8.encode('/U0OI/AdHDM4QFVC');
    SecretBox secretBox = new SecretBox(data, nonce: iv);
    List<int> decrypted = await AesGcm.with128bits().decrypt(secretBox, secretKey: secretKey);
    String dec = utf8.decode(decrypted);
    print("DATA : "+dec);
    return dec;
  }

For this I got below error in Flutter

Unhandled Exception: SecretBox has wrong message authentication code (MAC) #0 DartAesGcm.decryptSync (package:cryptography/src/dart/aes_gcm.dart:112:7) #1 DartAesGcm.decrypt (package:cryptography/src/dart/aes_gcm.dart:58:12) #2 EncryptionHandler.decrypt (package:investment_app/encryption/encryption_handler.dart:45:27)

Can you please help me to solve this. Sample code will be helpful.

Answer 1

The SunJCE provider in Java concatenates the ciphertext and the MAC: ciphertext|MAC. In the Dart code, both must be specified separately, which does not happen in the posted code. Also, the IV is not Base64 decoded.

The following code is a possible implementation that fixes the bugs:

Uint8List ivCiphertextMac = base64.decode("/U0OI/AdHDM4QFVC4NA09I5VpmdD0o1k3VP8eIfyZRKDtzOwgJv5nh0nmEPZ/Q=="); // from the Java code
Uint8List iv = ivCiphertextMac.sublist(0, 12);
Uint8List ciphertext  = ivCiphertextMac.sublist(12, ivCiphertextMac.length - 16);
Uint8List mac = ivCiphertextMac.sublist(ivCiphertextMac.length - 16);

Uint8List passphrase = utf8.encode('Test!ng012345678');
SecretKey secretKey = new SecretKey(passphrase);

SecretBox secretBox = new SecretBox(ciphertext, nonce: iv, mac: new Mac(mac));

List<int> decrypted = await AesGcm.with128bits().decrypt(secretBox, secretKey: secretKey);
String dec = utf8.decode(decrypted);
print("Decrypted text : " + dec); // Decrypted text : Mayur, You got it!

The decryption gives: Mayur, You got it!

Actually SecretBox provides the method fromConcatenation() which should separate a concatenation of IV, ciphertext and MAC. But this implementation seems to return a corrupted ciphertext, which is probably a bug.

EDIT:

Regarding your question in the comment: The MAC is automatically generated during encryption. The following code implements the encryption, where ivCiphertextMacB64 contains the Base64 encoding of IV | ciphertext | MAC:

Uint8List plaintext  = utf8.encode("Mayur, You got it!");
Uint8List iv = AesGcm.with128bits().newNonce();
Uint8List passphrase = utf8.encode('Test!ng012345678');
SecretKey secretKey = new SecretKey(passphrase);

SecretBox secretBox = await AesGcm.with128bits().encrypt(plaintext, nonce: iv, secretKey: secretKey);
String ivCiphertextMacB64 = base64.encode(secretBox.concatenation()); // Base64 encoding of: IV | ciphertext | MAC
print("ivCiphertextMacB64 : " + ivCiphertextMacB64);