c# - Decryption Exception - length of the data to decrypt is invalid

Question

I am working in a C# application. We have common methods to store data on a file. These methods encrypt the data and store them on the file system. when we need the data, ReadData method decrypts the data and returns me plain text.

This code works fine in normal cases if size of the text in small. but for a example text given below, the decryption code is throwing exception - length of the data to decrypt is invalid.

The exception occurs at line

        // close the CryptoStream
        x_cryptostream.Close();

I tried different ways but no luck. Can some pls help.

Why am I encrypting already encrypted data - I am just trying to store in a file using common method of the huge application. The common methods storedata(key,data) nad readdata(key) do the encryption/decryption I can't avoid.

   public static byte[] Decrypt(byte[] ciphertext, string Key, string IV)
    {
        byte[] k = Encoding.Default.GetBytes(Key);
        byte[] iv = Encoding.Default.GetBytes(IV);

        // create the encryption algorithm
        SymmetricAlgorithm x_alg = SymmetricAlgorithm.Create("Rijndael");
        x_alg.Padding = PaddingMode.PKCS7;

        // create an ICryptoTransform that can be used to decrypt data
        ICryptoTransform x_decryptor = x_alg.CreateDecryptor(k, iv);

        // create the memory stream
        MemoryStream x_memory_stream = new MemoryStream();

        // create the CryptoStream that ties together the MemoryStream and the 
        // ICryptostream
        CryptoStream x_cryptostream = new CryptoStream(x_memory_stream,
        x_decryptor, CryptoStreamMode.Write);

        // write the ciphertext out to the cryptostream
        x_cryptostream.Write(ciphertext, 0, ciphertext.Length);

        // close the CryptoStream
        x_cryptostream.Close();

        // get the plaintext from the MemoryStream
        byte[] x_plaintext = x_memory_stream.ToArray();

Below is the code of encrypt method.

        public static byte[] Encrypt(string strplain, string Key, string IV)
        {
        byte[] k = Encoding.Default.GetBytes(Key);
        byte[] iv = Encoding.Default.GetBytes(IV);

        byte[] plaintext = Encoding.Default.GetBytes(strplain);

        // create the encryption algorithm
        SymmetricAlgorithm x_alg = SymmetricAlgorithm.Create("Rijndael");
        x_alg.Padding = PaddingMode.PKCS7;
        // create an ICryptoTransform that can be used to encrypt data
        ICryptoTransform x_encryptor = x_alg.CreateEncryptor(k, iv);

        // create the memory stream
        MemoryStream x_memory_stream = new MemoryStream();

        // create the CryptoStream that ties together the MemoryStream and
        // the ICryptostream
        CryptoStream x_cryptostream = new CryptoStream(x_memory_stream,
        x_encryptor, CryptoStreamMode.Write);

        // write the plaintext out to the cryptostream
        x_cryptostream.Write(plaintext, 0, plaintext.Length);

        // close the CryptoStream
        x_cryptostream.Close();

        // get the ciphertext from the MemoryStream
        byte[] x_ciphertext = x_memory_stream.ToArray();

        // close memory stream
        x_memory_stream.Close();

        // convert from array to string
        string cipher_Tx = Encoding.Default.GetString(x_ciphertext, 
            0, x_ciphertext.Length);

        x_encryptor.Dispose();

        x_alg.Clear();
        byte[] cipher = Encoding.Default.GetBytes(cipher_Tx);

        return cipher;
    }  

Answer 1

Your problem is string cipher_Tx = Encoding.Default.GetString(x_ciphertext, 0, x_ciphertext.Length);.

x_ciphertext is not a valid byte representation of text, it has many unpresentable characters and when you do your byte[] to string conversion you are loosing information. The correct way to do it is use a string format that is designed to represent binary data using something like Convert.ToBase64String(byte[]) and Convert.FromBase64String(string).

string cipher_Tx = Convert.ToBase64String(x_ciphertext)

x_encryptor.Dispose();

x_alg.Clear();
byte[] cipher = Convert.FromBase64String(cipher_Tx)

That being said, there is a lot of other "odd" things about your code, for example you don't use using statements and you really should. Also that whole conversion to string and back is totally unnecessary, just return x_ciphertext. There may be other problems with the code too (like where did the strings for Key and IV come from) and many other best practices (like you should be generating a random IV and writing it out in to the output and the key should be generated using a key derivation function not straight from user text), but I stopped checking after I found the string conversion issue.