AES to encrypt a single block shows error

Question

when run below program it shows error.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Security.Cryptography;
using System.Web.UI.WebControls;
using System.Configuration;
using System.Text;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Your code goes here
            Console.WriteLine("Hello, world!");

            byte[] plainText = UTF8Encoding.UTF8.GetBytes("textt");
            byte[] key = UTF8Encoding.UTF8.GetBytes("1234567890123456");
            AES_encrypt_block(plainText,key);
        }
        private static byte[] AES_encrypt_block(byte[] plainText, byte[] Key)
        {
        byte[] output_buffer = new byte[plainText.Length];

        using (AesManaged aesAlg = new AesManaged())
        {
        //If CBC, must initialize IV = O_{128}
        //aesAlg.Mode = CipherMode.CBC;
        //aesAlg.IV = new byte[] { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
        aesAlg.Mode = CipherMode.ECB;

        aesAlg.BlockSize = 128;
        aesAlg.KeySize = 128;
        aesAlg.Padding = PaddingMode.None;
        aesAlg.Key = Key;

        // Create a decrytor to perform the stream transform.
        ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);
        encryptor.TransformBlock(plainText, 0, plainText.Length, output_buffer, 0);
        }

        return output_buffer;
    }
    }

}

ERROR

Exception in user code:

System.ArgumentException: Value was invalid. at >System.Security.Cryptography.RijndaelManagedTransform.TransformBlock(Byte[] >inputBuffer, Int32 inputOffset, Int32 inputCount, Byte[] outputBuffer, Int32 >outputOffset) at Rextester.Program.AES_encrypt_block(Byte[] plainText, Byte[] Key) at Rextester.Program.Main(String[] args)"

Answer 1

You can do it like this (example). But I recommend to use CBC instead of ECB. And, because of the padding, the decrypted block will always have the size of the block, even if the original content (5-byte-string) is shorter. So when creating the string, insert the string length before the real content (2nd example).

encrypting/decrypting ECB:

private static AesManaged Create(byte[] key)
{
    var aes = new AesManaged();

    aes.Mode = CipherMode.ECB;
    aes.BlockSize = 128;
    aes.KeySize = 128;
    aes.Padding = PaddingMode.Zeros;
    aes.Key = key;

    return aes;
}
private static byte[] AES_encrypt_block(byte[] plainText, byte[] key)
{
    using (AesManaged aes = Create(key))
    {
        var encryptor = aes.CreateEncryptor(aes.Key, aes.IV);
        return encryptor.TransformFinalBlock(plainText, 0, plainText.Length);
    }
}
private static byte[] AES_decrypt_block(byte[] plainText, byte[] key)
{
    using (AesManaged aesAlg = Create(key))
    {
        var decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);
        return decryptor.TransformFinalBlock(plainText, 0, plainText.Length);
    }
}

string-to-byte:

private static byte[] StringToBytes(string s)
{
    var length = BitConverter.GetBytes(s.Length);
    var raw = Encoding.UTF8.GetBytes(s);
    var full = new byte[sizeof(int) + raw.Length];

    Array.Copy(length, full, sizeof(int));
    Array.Copy(raw, 0, full, sizeof(int), raw.Length);

    return full;
}
private static string BytesToString(byte[] input)
{
    var length = BitConverter.ToInt32(input, 0);

    return Encoding.UTF8.GetString(input, sizeof(int), length);
}

verification:

var key = UTF8Encoding.UTF8.GetBytes("1234567890123456");
var plainBytes = StringToBytes("textt");
var encryptedBytes = AES_encrypt_block(plainBytes, key);
var decryptedBytes = AES_decrypt_block(encryptedBytes, key);
var decryptedString = BytesToString(decryptedBytes);

Answer 2

You should change it;

encryptor.TransformBlock(plainText, 0, plainText.Length, output_buffer, 0);

to

var output = encryptor.TransformFinalBlock(plainText, 0, plainText.Length);

and you should specify the PaddingMode ;

aesAlg.Padding = PaddingMode.PKCS7;

Entire method looks like;

private static byte[] AES_encrypt_block(byte[] plainText, byte[] Key)
{
    using (AesManaged aesAlg = new AesManaged())
    {
        aesAlg.Mode = CipherMode.ECB;
        aesAlg.BlockSize = 128;
        aesAlg.KeySize = 128;
        aesAlg.Padding = PaddingMode.PKCS7;
        aesAlg.Key = Key;

        // Create a decrytor to perform the stream transform.
        ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);
        return encryptor.TransformFinalBlock(plainText, 0, plainText.Length);
    }
}

If you use the TransformBlock , the input text length should be 16 . Because it transform the encyrption per block. So, you should use TransformFinalBlock instead of it. Also, PaddingMode should be set for shorter message data block.

EDIT

To perform decryption ;

private static byte[] AES_Decrypt_block(byte[] cipherText, byte[] Key)
{
    using (AesManaged aesAlg = new AesManaged())
    {
        aesAlg.Mode = CipherMode.ECB;
        aesAlg.BlockSize = 128;
        aesAlg.KeySize = 128;
        aesAlg.Padding = PaddingMode.PKCS7;
        aesAlg.Key = Key;
        ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);
        return decryptor.TransformFinalBlock(cipherText, 0, cipherText.Length);
    }
}

Usage

byte[] plainText = UTF8Encoding.UTF8.GetBytes("textt");
byte[] key = UTF8Encoding.UTF8.GetBytes("1234567890123456");
var encrypted = AES_encrypt_block(plainText, key);
var decrypted = AES_Decrypt_block(encrypted, key);
Console.WriteLine(UTF8Encoding.UTF8.GetString(decrypted));