Encryption and Descyption issue in Flutter and C#

Question

I am using encryption in Flutter using the link: https://pub.dev/packages/encrypt

Code is

    final plainText = 'some plain text here';
    final key = encrypt.Key.fromUtf8('16 characters key');
    final iv = IV.fromLength(16);
    final encrypter = Encrypter(AES(key,mode: AESMode.cbc,padding: 'PKCS7'));
    final encrypted = encrypter.encrypt(plainText, iv: iv);
    final decrypted = encrypter.decrypt(encrypted, iv: iv);
    print(decrypted); 
    print(encrypted.base64);

C# code (for decryption)

public static String TestDecrypt(String encryptedText)
{
    var encryptedBytes = Convert.FromBase64String(encryptedText);
    return Encoding.UTF8.GetString(Decrypt(encryptedBytes, GetRijndaelManaged("16 characters key")));
}

public static RijndaelManaged GetRijndaelManaged(String secretKey)
{
     var keyBytes = new byte[16];
     var secretKeyBytes = Encoding.UTF8.GetBytes(secretKey);

     Array.Copy(secretKeyBytes, keyBytes, Math.Min(keyBytes.Length, secretKeyBytes.Length));

     return new RijndaelManaged
        {
            Mode = CipherMode.CBC,
            Padding = PaddingMode.PKCS7,
            KeySize = 128,
            BlockSize = 128,
            Key = keyBytes,
            IV = keyBytes
        };
}

I encrypt some plaintext in flutter dart code, But I didn't get proper plain text after decrypting the encrypted text using c# code.

I'm getting a message

Padding is invalid and cannot be removed

Solution

This works for me: Keys:

String keySTR = "16 characters"; //16 byte
String ivSTR = "16 characters"; //16 byte

Flutter

final plainText = 'Works!';

final key = encrypt.Key.fromUtf8(keySTR);
final iv = encrypt.IV.fromUtf8(ivSTR);
final encrypter = encrypt.Encrypter(encrypt.AES(key,mode: encrypt.AESMode.cbc,padding: 'PKCS7'));
final encrypted = encrypter.encrypt(plainText, iv: iv);
final decrypted = encrypter.decrypt(encrypted, iv: iv);
print('decrypted:'+decrypted);
print('encrypted.base64:'+encrypted.base64);

        

C#

using (System.Security.Cryptography.RijndaelManaged rjm = 
                        new System.Security.Cryptography.RijndaelManaged 
                            { 
                                KeySize = 128, 
                                BlockSize = 128, 
                                Key = ASCIIEncoding.ASCII.GetBytes(keySTR),
                                IV = ASCIIEncoding.ASCII.GetBytes(ivSTR) 
                            }
            )
        {           
                Byte[] input = Encoding.UTF8.GetBytes(stringToEncrypt);
                Byte[] output = rjm.CreateEncryptor().TransformFinalBlock(input, 0, input.Length);
                return Convert.ToBase64String(output);                
        }
    }

Comments

[500 - Internal Server Error]

A key is not 16 bytes long just because it says it is :)

[Maarten Bodewes]

IV = keyBytes is suspicious and ` final iv = IV.fromLength(16);` is as well. Whatever you're doing, it is not secure. To find out what is happening, print all the input / output to AES out in hexadecimals - mainly key and IV - and check that they are identical on both sides. The key is the most likely culprit if your plaintext is indeed over 16 characters / bytes in size.

[bartonjs]

Don’t set both Key and KeySize. Each updates/resets the other.