How to decrypt a file in C# that was encrypted on Unix/Solaris using AES-128

I have been supplied with a file that was encrypted on a Unix Solaris 10/SunOS 5.10 machine using the $encrypt method. I believe this to have been done using an aes algorithm.

I need to decrypt this file using C# on a windows machine. I have been supplied with a 16 byte symmetric key but I am unsure how to proceed.

Several code examples I've found mention an IV, block size, padding, cipher mode etc. Unfortunately, I do not know what these are and my research on the standard Unix encrypt method hasn't returned anything useful. I believe I have been supplied with everything I would need to decrypt this on Unix (although I do not have the means to test) - but need this to work on Windows with C#.

Files will be supplied periodically (with the same key), so I need a solution that will work on an on-going basis.

Any helps would be greatly appreciated.

Thanks.

UPDATE:

Thanks to @Maarten (see comments) I now have a working solution:

RijndaelManaged objAlgorithm = new RijndaelManaged();
//set the mode, padding and block size
objAlgorithm.Padding = PaddingMode.PKCS7;
objAlgorithm.Mode = CipherMode.CBC;
objAlgorithm.KeySize = 128;
objAlgorithm.BlockSize = 128;

byte[] key = File.ReadAllBytes(@"PATH_TO_KEY_FILE");
byte[] inputBytes = File.ReadAllBytes(@"PATH_TO_INPUT");

byte[] format = new byte[4];
byte[] iterations = new byte[4];
byte[] IV = new byte[objAlgorithm.BlockSize / 8];
byte[] salt = new byte[16];
byte[] cipherText = new byte[inputBytes.Length - format.Length - iterations.Length - IV.Length - salt.Length];

// Split the input array
Array.Copy(inputBytes, 0, format, 0, format.Length);
Array.Copy(inputBytes, format.Length, iterations, 0, iterations.Length);
Array.Copy(inputBytes, (format.Length + iterations.Length), IV, 0, IV.Length);
Array.Copy(inputBytes, (format.Length + iterations.Length + IV.Length), salt, 0, salt.Length);
Array.Copy(inputBytes, (format.Length + iterations.Length + IV.Length + salt.Length), cipherText, 0, cipherText.Length);

Byte[] outputBytes = cipherText;
string plaintext = string.Empty;

using (MemoryStream memoryStream = new MemoryStream(outputBytes))
{
    using (CryptoStream cryptoStream = new CryptoStream(memoryStream, objAlgorithm.CreateDecryptor(key, IV), CryptoStreamMode.Read))
    {
        using (StreamReader srDecrypt = new StreamReader(cryptoStream))
        {
            try
            {
                int iReadBytes = cryptoStream.Read(outputBytes, 0, outputBytes.Length);
                //plaintext = Encoding.UTF8.GetString(outputBytes,0,outputBytes.Length);

                byte[] finalBytes = new byte[iReadBytes];
                Array.Copy(outputBytes, 0, finalBytes, 0, iReadBytes);

                File.WriteAllBytes(DirectoryPath + strOriginalFileName.Replace(".out", ".xml"), finalBytes);
            }
            catch (Exception ex)
            {
                //Handle Error
            }
        }
    }
}

Solution

You'll have to use AES with CBC padding and PKCS#7 padding, source on docs.oracle.com/..../encrypt-1.html:

Algorithms

The supported algorithms are displayed with their minimum and maximum key sizes in the -l option. These algorithms are provided by the cryptographic framework. Each supported algorithm is an alias of the PKCS #11 mechanism that is the most commonly used and least restricted version of a particular algorithm type. For example, des is an alias to CKM_DES_CBC_PAD and arcfour is an alias to CKM_RC4. Algorithm variants with no padding or ECB are not supported.

This tells you to expect CBC and PKCS#7 padding for AES as well.

Furthermore, you'd have to parse the ciphertext structure that is output by encrypt (it has been defined as a stable interface which means it should not change between versions of encrypt:

The output file of encrypt and the input file for decrypt contains the following information:

Output format version number, 4 bytes in network byte order. The current version is 1.

Iterations used in key generation function, 4 bytes in network byte order.

IV (ivlen bytes)[1]. iv data is generated by random bytes equal to one block size.

Salt data used in key generation (16 bytes).

Cipher text data.

As the iterations and salt are not required for decryption using a key file I expect them to be either absent or zeroized.

This should be enough information to decrypt in any environment that you control.