DES Vector Initialization in C #

Question

I am trying to decrypt (using the DES algorithm) data coming from a third party in C # code. Here are many examples in other places that have helped me. The problem is that I do not know what to use for the “initialization vector”.

A third party provided a tool for use on the command line (DES.EXE, which I believe is an assembly of the libdes v4.01 library), which only requires that you provide an encryption key. So, I can decrypt using the tool. However, I would prefer not to create a process to run the tool from my code.

My question is how can I generate / find this initialization vector. I am 99.9% sure that this can be done if you look at other posts, but I can not understand this. I talked with a third party and they said that they do not support this approach. Any help would be greatly appreciated.

Answer 1

I spent some time (~ 2 days) to deal with the same problem. "Rewriting" des.exe in C #. In the end, I got the libdes sources and reconstructed the logic.

The initialization vector is all (8) zeros. That is, a new byte [8].

However, as a rule, you can convert a string password into an 8-byte long key. If you use simple DES (not Triple DES), this code may do the trick for you:

public class LibDesPasswordConvertor { public byte[] PasswordToKey(string password) { if (password == null) throw new ArgumentNullException("password"); if (password == "") throw new ArgumentException("password"); var key = new byte[8]; for (int i = 0; i < password.Length; i++) { var c = (int)password[i]; if ((i % 16) < 8) { key[i % 8] ^= (byte)(c << 1); } else { // reverse bits eg 11010010 -> 01001011 c = (((c << 4) & 0xf0) | ((c >> 4) & 0x0f)); c = (((c << 2) & 0xcc) | ((c >> 2) & 0x33)); c = (((c << 1) & 0xaa) | ((c >> 1) & 0x55)); key[7 - (i % 8)] ^= (byte)c; } } AddOddParity(key); var target = new byte[8]; var passwordBuffer = Encoding.ASCII.GetBytes(password).Concat(new byte[8]).Take(password.Length + (8 - (password.Length % 8)) % 8).ToArray(); var des = DES.Create(); var encryptor = des.CreateEncryptor(key, key); for (int x = 0; x < passwordBuffer.Length / 8; ++x) { encryptor.TransformBlock(passwordBuffer, 8 * x, 8, target, 0); } AddOddParity(target); return target; } private void AddOddParity(byte[] buffer) { for (int i = 0; i < buffer.Length; ++i) { buffer[i] = _oddParityTable[buffer[i]]; } } private static byte[] _oddParityTable = { 1, 1, 2, 2, 4, 4, 7, 7, 8, 8, 11, 11, 13, 13, 14, 14, 16, 16, 19, 19, 21, 21, 22, 22, 25, 25, 26, 26, 28, 28, 31, 31, 32, 32, 35, 35, 37, 37, 38, 38, 41, 41, 42, 42, 44, 44, 47, 47, 49, 49, 50, 50, 52, 52, 55, 55, 56, 56, 59, 59, 61, 61, 62, 62, 64, 64, 67, 67, 69, 69, 70, 70, 73, 73, 74, 74, 76, 76, 79, 79, 81, 81, 82, 82, 84, 84, 87, 87, 88, 88, 91, 91, 93, 93, 94, 94, 97, 97, 98, 98,100,100,103,103,104,104,107,107,109,109,110,110, 112,112,115,115,117,117,118,118,121,121,122,122,124,124,127,127, 128,128,131,131,133,133,134,134,137,137,138,138,140,140,143,143, 145,145,146,146,148,148,151,151,152,152,155,155,157,157,158,158, 161,161,162,162,164,164,167,167,168,168,171,171,173,173,174,174, 176,176,179,179,181,181,182,182,185,185,186,186,188,188,191,191, 193,193,194,194,196,196,199,199,200,200,203,203,205,205,206,206, 208,208,211,211,213,213,214,214,217,217,218,218,220,220,223,223, 224,224,227,227,229,229,230,230,233,233,234,234,236,236,239,239, 241,241,242,242,244,244,247,247,248,248,251,251,253,253,254,254}; } 

(Some pieces of libdes code were reused, although I had to determine part of the DES checksum myself.

The final error is that libds uses a non-standard padding mechanism. It's almost like an ISO, but the last byte is not the number of bytes added, but 8 is the number. I set the Padding property to None and handled the pad myself.

Answer 2

See http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation for a description of the various modes of operation used in block ciphers. You do not need an initialization vector (IV) if you use ECB mode, which simply encrypts a block of messages with a block. You may need to check out the DES library to configure ECB mode / disable others.

If they do not use ECB mode, you need IV; you cannot “guess” it. IV in most cases should not be secret, so do not interfere with its movement.