在C＃中使用AesCryptoServiceProvider时出现无效的填充错误

Question

I've written a simple encryp/decrypt method in c# which uses the AES alg. When I try to encrypt and then decrypt a string with certain lengths like 4 or 7 characters, it works fine, with other lengths however It says that the padding is invalid and cannot be removed.

    public static string Decrypt(string text)
    {
        Aes a = System.Security.Cryptography.AesCryptoServiceProvider.Create();
        a.Padding = PaddingMode.PKCS7;
        a.Key = Convert.FromBase64String("UDlArN63HCk15fHBski/zvaWiMZJi+jR1BADvVgenCU=");
        a.IV = Convert.FromBase64String("xZG/eLY8eq0mQhUXvKbUDQ==");
        var dc = a.CreateDecryptor();

        byte[] encryptedBytes = Encoding.Unicode.GetBytes(text);
        byte[] decryptedBytes = dc.TransformFinalBlock(encryptedBytes, 0, encryptedBytes.Length);

        return Encoding.Unicode.GetString(decryptedBytes);
    }

    public static string Encrypt(string text)
    {
        Aes a = System.Security.Cryptography.AesCryptoServiceProvider.Create();
        a.Padding = PaddingMode.PKCS7;
        a.Key = Convert.FromBase64String("UDlArN63HCk15fHBski/zvaWiMZJi+jR1BADvVgenCU=");
        a.IV = Convert.FromBase64String("xZG/eLY8eq0mQhUXvKbUDQ==");
        var dc = a.CreateEncryptor();

        byte[] decryptedBytes = Encoding.Unicode.GetBytes(text);
        byte[] encryptedBytes = dc.TransformFinalBlock(decryptedBytes, 0, decryptedBytes.Length);

        return Encoding.Unicode.GetString(encryptedBytes);
    }

Answer 1

Ciphertexts are binary data which might contain bytes that are not printable. If try to encode the byte array as a Unicode string, you will lose some bytes. It will be impossible to recover them during decryption.

If you actually want to handle the ciphertext as a string, you need to convert it into a textual representation like Base 64 or Hex.

// encryption
return Convert.ToBase64String(decryptedBytes);

// decryption
byte[] decryptedBytes = Convert.FromBase64String(text);