Dart

Tips on Matching Encryption with another System

See more Encryption Examples

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

import 'package:chilkat/chilkat.dart';

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

  final crypt = CkCrypt2();

  // 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;
  final ivHex1 = '000102030405060708090A0B0C0D0E0F';
  final ivHex2 = 'FF0102030405060708090A0B0C0D0E0F';
  crypt.setEncodedIV(ivHex1, 'hex');
  final 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:
  print(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:
  print(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:
  print('-- Only the padding scheme differs --');
  crypt.paddingScheme = 0;
  print(crypt.encryptStringENC('HelloHelloHelloHelloHelloHelloHello'));
  crypt.paddingScheme = 3;
  print(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';
  print('-- Only the IV differs, CBC mode produces different output. --');
  crypt.setEncodedIV(ivHex1, 'hex');
  print(crypt.encryptStringENC('HelloHelloHelloHelloHelloHelloHello'));
  crypt.setEncodedIV(ivHex2, 'hex');
  print(crypt.encryptStringENC('HelloHelloHelloHelloHelloHelloHello'));

  crypt.cipherMode = 'ecb';
  print('-- Only the IV differs, ECB does not use the IV.  The outputs are the same. --');
  crypt.setEncodedIV(ivHex1, 'hex');
  print(crypt.encryptStringENC('HelloHelloHelloHelloHelloHelloHello'));
  crypt.setEncodedIV(ivHex2, 'hex');
  print(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.
}