Junk bytes before payload in C# UTF8 to AES to Base64 conversion

Question

I have been trying to implement proper IV practice in methods to encrypt and decrypt a UTF-8 string with AES which is then returned as a Base64 string. Using this question as a reference, I have prepended the generated IV to the byte array before the Base64 conversion. I'm having an issue where the decrypt method returns the UTF-8 string with exactly fifty characters of random junk (encryption artifacts?). I don't believe the issue is with the encryption because the decrypt method does consistently return the encrypted string. I think the problem is with one of the other conversion steps but I'm having trouble seeing where this might be coming from. Any help would be wildly appreciated.

Encrypt method

    public static string EncryptString(string input, string key)
    {
        using (var aes = new AesCryptoServiceProvider())
        {

            aes.Key = System.Convert.FromBase64String(key);
            aes.Mode = CipherMode.CBC;
            aes.Padding = PaddingMode.PKCS7;

            byte[] rawData = Encoding.UTF8.GetBytes(input); 
            // IV is the 16 byte AES Initialization Vector 
            aes.GenerateIV();

            using (var encryptor = aes.CreateEncryptor(aes.Key, aes.IV)) 
            {
                using (var ms = new MemoryStream())
                {
                    ms.Write(aes.IV, 0, aes.IV.Length); // aes.IV.Length should be 16
                    using (var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
                    {
                        cs.Write(rawData, 0, rawData.Length);
                        cs.FlushFinalBlock();
                    }
                    byte[] encryptedData = ms.ToArray();

                    // this will hold the IV prepended to the encrypted data
                    byte[] output = new byte[aes.IV.Length + encryptedData.Length];

                    Array.Copy(aes.IV, output, aes.IV.Length); // save the iv
                    Array.Copy(encryptedData, 0, output, aes.IV.Length, encryptedData.Length);    // save the data

                    // now encode the whole thing as base 64
                    return System.Convert.ToBase64String(output);

                }
            }

        }

    }

Decrypt method

    public static string DecryptString(string input, string key)
    {

        using (var aes = new AesCryptoServiceProvider())
        {
            aes.Key = Convert.FromBase64String(key);
            aes.Mode = CipherMode.CBC;
            aes.Padding = PaddingMode.PKCS7;

            byte[] rawData = Convert.FromBase64String(input);
            byte[] IV = new byte[16];  // aes.IV.Length should be 16

            Array.Copy(rawData, IV, IV.Length);
            using (var ms = new MemoryStream())
            {
                using (var cs = new CryptoStream(ms, aes.CreateDecryptor(aes.Key, IV), CryptoStreamMode.Write))
                {
                    using (var binaryWriter = new BinaryWriter(cs))
                    {
                        binaryWriter.Write(rawData,IV.Length ,rawData.Length - IV.Length);
                    }
                }
                return Encoding.UTF8.GetString(ms.ToArray());
            }
        }
    }

My test

    static void Main(string[] args)
    {
        string payload = "My super secret string";
        string key = "tR4mPn7mBQ8G6HWusyFnGk/gqdd/enWiUTr7YbhNrJg=";

        Console.WriteLine(payload);
        Console.WriteLine(key);

        Console.WriteLine("");
        string encrypted = EncryptString(payload, key);
        Console.WriteLine(encrypted);

        Console.WriteLine("");
        string decrypted = DecryptString(encrypted, key);
        Console.WriteLine(decrypted);

        Console.WriteLine(decrypted.Length.ToString() + " " + encrypted.Length.ToString());

        Console.ReadKey();

    }

Edit to add - this is an example of the output:

�XQ��=F�]�D�?�My super secret string

Examine the output as hex and you will see the issue, Base64 is for computers, not humans. — zaph, Sep 01 '17 at 21:00

score 3 · Accepted Answer · answered Sep 01 '17 at 20:49

You are writing the IV to the output twice in EncryptString. First you have:

ms.Write(aes.IV, 0, aes.IV.Length); // aes.IV.Length should be 16

which is the start of encryptedData. You then copy the IV and encryptedData (which already includes the IV) into a new byte array:

// this will hold the IV prepended to the encrypted data
byte[] output = new byte[aes.IV.Length + encryptedData.Length];

Array.Copy(aes.IV, output, aes.IV.Length); // save the iv
Array.Copy(encryptedData, 0, output, aes.IV.Length, encryptedData.Length);    // save the data

This doubling of the IV is what is causing the extra bytes.

You don’t need to do the second copying. Just convert encryptedData to base 64 directly and return that:

return System.Convert.ToBase64String(encryptedData);

This is correct. I figured it was a simple mistake. My tired Friday eyes just couldn't catch it. — Nick Van Brunt, Sep 05 '17 at 13:11

Junk bytes before payload in C# UTF8 to AES to Base64 conversion

1 Answers1