Encryption AES 256 Input Plaintext Length Issue

Question

From my security team, they want me to use AES256 key strength and CBC mode. My code only works when I enter a input plaintext of 32 letters in length now after changing to 256 CBC and block size to 128.

If I enter "This is a test" (not 32 characters long), I receive:

System.Security.Cryptography.CryptographicException: The input data is not a complete block.

If I enter: " ABCDEFGHIJKLMNOPQRSTUVWXYZ000000", works!

What code do I need to make this work with "This is a test" as input.

Code Below:

public byte[] EncryptStringToByte(string plainText, byte[] key, byte[]  vector)
{               
byte[] encrypted;                
using (AesCryptoServiceProvider aes = new AesCryptoServiceProvider())
{
    aes.BlockSize = 128;
    aes.KeySize = 256;
    aes.Mode = CipherMode.CBC;
    aes.Padding = PaddingMode.None;
    aes.Key = key;
    aes.IV = vector;

    // Create a decrytor to perform the stream transform.
    ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);

    // Create the streams used for encryption. 
    using (MemoryStream msEncrypt = new MemoryStream())
    {
        using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
        {
            using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
            {

                //Write all data to the stream.
                swEncrypt.Write(plainText);
            }
            encrypted = msEncrypt.ToArray();
        }
    }
    }
// Return the encrypted bytes from the memory stream. 
return encrypted;
}

score 3 · Accepted Answer · answered Mar 20 '15 at 18:25

AES is a block cipher, so it only works on plaintexts that have exactly the size of one block. A mode of operation like CBC enables you to encrypt plaintexts that are a multiple of the block size. To encrypt plaintexts of arbitrary length a padding mode must be used.

A common mode used for block ciphers is PKCS#5/PKCS#7:

aes.Padding = PaddingMode.PKCS7;

Encryption AES 256 Input Plaintext Length Issue

1 Answers1