Tenho este codigo e me da a excepção specified initialization vector (iv) does not match the block size for this algorithm, eu precciso encriptar e desencriptar com AES de 256 bites e usando o vector de inicialização, como resolver. Ai vai o meu codigo.

private void EncryptFile( string sourceFile, string targetFile, string key)
{
    AesManage aes = new AesManage();

    using ( MD5CryptoServiProvider md5 = new MD5CryptoServiProvider()){
        AES.KeySize = md5.HashSize;
        AES.BlockSize = md5.HashSize();
        AES.IV = md5.ComputeHash(ASCIIEncoding.ASCII.GetBytes("stick some string in here"));
        AES.Key = md5.ComputeHash(ASCIIEncoding.ASCII.GetBytes(key));
    }    

    using(FileStream reader = new FileStream(sourceFile, FileMode.Open,                         FileAcess.Read) )
{

    using(FileStream writer = new FileStream(sourceFile, FileMode.OpenOrCreate, FileAcess.Write){

            using(CryptoStream cs = new CryptoStream(writer, AES.CreateEncryptor(), CryptoStreamMode.Write){

            int bufferSize = 4096;
            byte[] buffer = new byte[bufferSize];
            int bytesRead;

            do
            {
                bytesRead = reader.Read(buffer, 0, bufferSize);
                if(bytesRead != 0)
                {
                    cs.Write(buffer, 0, bytesRead);
                }
            }
            while(bytesRead != 0);
            cs.FlushFinalBlock();
        }
        }
    }

}