C#

Tips on Matching Encryption with another System

See more Encryption Examples

This example provides tips on matching encryption results produced by another system. C#

//  This example assumes the Chilkat API to have been previously unlocked.
//  See Global Unlock Sample for sample code.

Chilkat.Crypt2 crypt = new Chilkat.Crypt2();

//  Let's examine 256-bit AES encryption in CBC mode.
//  CBC mode is Cipher Block Chaining, and it uses an IV (initialization vector)
crypt.CryptAlgorithm = "aes";
crypt.CipherMode = "cbc";
crypt.KeyLength = 256;
crypt.PaddingScheme = 0;
string ivHex1 = "000102030405060708090A0B0C0D0E0F";
string ivHex2 = "FF0102030405060708090A0B0C0D0E0F";
crypt.SetEncodedIV(ivHex1,"hex");
string keyHex = "000102030405060708090A0B0C0D0E0F101112131415161718191A1B1C1D1E1F";
crypt.SetEncodedKey(keyHex,"hex");

//  Matching encryption requires all of the above settings to be matched exactly.
//  Let's get our output in hex format so we can easily see the values of the encrypted bytes.
crypt.EncodingMode = "hex";

//  Encrypt something small:
Debug.WriteLine(crypt.EncryptStringENC("Hello"));
//  The result is 5B827AB3B4F9F2292C2B74C8A6C99A3D
//  This 16 bytes -- exactly one AES encryption block.

//  Let's change only the padding scheme.
crypt.PaddingScheme = 3;

//  Encrypt again:
Debug.WriteLine(crypt.EncryptStringENC("Hello"));
//  The result is entirely different: 469C28CC576069F807891FEE2DE76D68

//  The padding scheme only affects the very last block of output.  Therefore,
//  if all settings match except for the padding scheme, we're unable to
//  know if we encrypt a very small amount of data. However, if we encrypt
//  a larger amount of data, the single difference becomes apparent:
Debug.WriteLine("-- Only the padding scheme differs --");
crypt.PaddingScheme = 0;
Debug.WriteLine(crypt.EncryptStringENC("HelloHelloHelloHelloHelloHelloHello"));
crypt.PaddingScheme = 3;
Debug.WriteLine(crypt.EncryptStringENC("HelloHelloHelloHelloHelloHelloHello"));

//  Now examine the outputs:
//  F6A201F8E0B6595FA20E4A212A2AD9A5046DAF29E8B35AD15CEE56A1A69F2A3A7B347A7C15E26E7A6760533C7A8E0D44
//  F6A201F8E0B6595FA20E4A212A2AD9A5046DAF29E8B35AD15CEE56A1A69F2A3A292CA61D03A85E1AC39B50D4DA71691E
//  We can see the output matches except for the last block, which is affected by the padding scheme.

//  If we are able to easily use ECB mode w/ the other system
//  we are trying to match, then eliminate the IV from the picture.
//  If the encryption matches in ECB mode, but not in CBC mode,
//  then we know all correct except for the IV.
//  For example, you can see how the IV changes everything with CBC mode,
//  but it's not used in ECB mode:
crypt.PaddingScheme = 0;
crypt.CipherMode = "cbc";
Debug.WriteLine("-- Only the IV differs, CBC mode produces different output. --");
crypt.SetEncodedIV(ivHex1,"hex");
Debug.WriteLine(crypt.EncryptStringENC("HelloHelloHelloHelloHelloHelloHello"));
crypt.SetEncodedIV(ivHex2,"hex");
Debug.WriteLine(crypt.EncryptStringENC("HelloHelloHelloHelloHelloHelloHello"));

crypt.CipherMode = "ecb";
Debug.WriteLine("-- Only the IV differs, ECB does not use the IV.  The outputs are the same. --");
crypt.SetEncodedIV(ivHex1,"hex");
Debug.WriteLine(crypt.EncryptStringENC("HelloHelloHelloHelloHelloHelloHello"));
crypt.SetEncodedIV(ivHex2,"hex");
Debug.WriteLine(crypt.EncryptStringENC("HelloHelloHelloHelloHelloHelloHello"));

//  If we can eliminate the padding scheme and IV from the degrees of freedom,
//  then the only remaining likely differences are (1) the secret key,
//  and (2) the input data itself.

//  The secret key is composed of binary bytes of exactly KeyLength bits.
//  For 256-bit AES encrytion, the key length is 256, and therefore the 
//  secret key is exactly 32 bytes.  (32 * 8 bits/byte = 256 bits)
//  If the secret key is derived from an arbitrary password string, then one must
//  exactly duplicate the derivation scheme (such as PBKDF2, for example)
//  The input bytes to the derivation scheme must also match.  For example,
//  is it the utf-8 byte representation of the password string that is used
//  as the starting point for the derivation, or perhaps utf-16, or ANSI (1 byte per char)?

//  Likewise, if the data being encrypted is a string, what byte representation of
//  the string is being encrypted?  If the bytes presented to the encryptor are different,
//  then the output is different.