Question

I am using the AES methods here: http://msdn.microsoft.com/en-us/library/system.security.cryptography.rijndaelmanaged.aspx

I want to have a string value that I will convert to byte array and pass it to the AES encrypt method. How many characters should the string be to produce the correct byte array size that the method expects?

static byte[] encryptStringToBytes_AES(string plainText, byte[] Key, byte[] IV)
    {
        // Check arguments.
        if (plainText == null || plainText.Length <= 0)
            throw new ArgumentNullException("plainText");
        if (Key == null || Key.Length <= 0)
            throw new ArgumentNullException("Key");
        if (IV == null || IV.Length <= 0)
            throw new ArgumentNullException("Key");

        // Declare the stream used to encrypt to an in memory
        // array of bytes.
        MemoryStream msEncrypt = null;

        // Declare the RijndaelManaged object
        // used to encrypt the data.
        RijndaelManaged aesAlg = null;

        try
        {
            // Create a RijndaelManaged object
            // with the specified key and IV.
            aesAlg = new RijndaelManaged();
            aesAlg.Key = Key;
            aesAlg.IV = IV;

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

            // Create the streams used for encryption.
            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);
                }
            }

        }
        finally
        {

            // Clear the RijndaelManaged object.
            if (aesAlg != null)
                aesAlg.Clear();
        }

        // Return the encrypted bytes from the memory stream.
        return msEncrypt.ToArray();

    }

Answer 1

The size of the plain text does not matter. Just make sure you use the exact same IV and Key along with the encrypted bytes in the decryptStringFromBytes_AES(byte[] cipherText, byte[] Key, byte[] IV) method. That will return back to you the entered plain text.

For example:

string plain_text = "Cool this works";
byte[] iv = new byte[] { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
                                           0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F};
byte[] key = new byte[] { 0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77,
                                           0x88, 0x99, 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF };
byte[] encrytped_text = encryptStringToBytes_AES(plain_text, key, iv);
string plain_text_again = decryptStringFromBytes_AES(encrypted_text, key, iv);

Here you should see that plain-text and plain-text-again are the same. Now go ahead and change plain_text to anything you want and see that this works fine.

The default values for RijndaelManaged are:
BlockSize: 128
KeySize: 256
Mode: CipherMode.CBC
Padding: PaddingMode.PKCS7

The valid IV sizes are:
128, 192, 256 bits (This is the BlockSize, make sure to set it to size IV you are using)
The valid Key sizes are:
128, 192, 256 bits (This is the KeySize, make sure to set it to the size key you are using)

This means that the byte[] iv can be 16, 24, or 32 bytes (in my above example its 16 bytes) and the byte[] key can also be 16, 24, or 32 bytes (in my above example its 16 bytes).

Hope that helps.