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(Unicode C) Tips on Matching Encryption with another SystemThis example provides tips on matching encryption results produced by another system.
#include <C_CkCrypt2W.h> void ChilkatSample(void) { HCkCrypt2W crypt; const wchar_t *ivHex1; const wchar_t *ivHex2; const wchar_t *keyHex; // This example assumes the Chilkat API to have been previously unlocked. // See Global Unlock Sample for sample code. crypt = CkCrypt2W_Create(); // Let's examine 256-bit AES encryption in CBC mode. // CBC mode is Cipher Block Chaining, and it uses an IV (initialization vector) CkCrypt2W_putCryptAlgorithm(crypt,L"aes"); CkCrypt2W_putCipherMode(crypt,L"cbc"); CkCrypt2W_putKeyLength(crypt,256); CkCrypt2W_putPaddingScheme(crypt,0); ivHex1 = L"000102030405060708090A0B0C0D0E0F"; ivHex2 = L"FF0102030405060708090A0B0C0D0E0F"; CkCrypt2W_SetEncodedIV(crypt,ivHex1,L"hex"); keyHex = L"000102030405060708090A0B0C0D0E0F101112131415161718191A1B1C1D1E1F"; CkCrypt2W_SetEncodedKey(crypt,keyHex,L"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. CkCrypt2W_putEncodingMode(crypt,L"hex"); // Encrypt something small: wprintf(L"%s\n",CkCrypt2W_encryptStringENC(crypt,L"Hello")); // The result is 5B827AB3B4F9F2292C2B74C8A6C99A3D // This 16 bytes -- exactly one AES encryption block. // Let's change only the padding scheme. CkCrypt2W_putPaddingScheme(crypt,3); // Encrypt again: wprintf(L"%s\n",CkCrypt2W_encryptStringENC(crypt,L"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: wprintf(L"-- Only the padding scheme differs --\n"); CkCrypt2W_putPaddingScheme(crypt,0); wprintf(L"%s\n",CkCrypt2W_encryptStringENC(crypt,L"HelloHelloHelloHelloHelloHelloHello")); CkCrypt2W_putPaddingScheme(crypt,3); wprintf(L"%s\n",CkCrypt2W_encryptStringENC(crypt,L"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: CkCrypt2W_putPaddingScheme(crypt,0); CkCrypt2W_putCipherMode(crypt,L"cbc"); wprintf(L"-- Only the IV differs, CBC mode produces different output. --\n"); CkCrypt2W_SetEncodedIV(crypt,ivHex1,L"hex"); wprintf(L"%s\n",CkCrypt2W_encryptStringENC(crypt,L"HelloHelloHelloHelloHelloHelloHello")); CkCrypt2W_SetEncodedIV(crypt,ivHex2,L"hex"); wprintf(L"%s\n",CkCrypt2W_encryptStringENC(crypt,L"HelloHelloHelloHelloHelloHelloHello")); CkCrypt2W_putCipherMode(crypt,L"ecb"); wprintf(L"-- Only the IV differs, ECB does not use the IV. The outputs are the same. --\n"); CkCrypt2W_SetEncodedIV(crypt,ivHex1,L"hex"); wprintf(L"%s\n",CkCrypt2W_encryptStringENC(crypt,L"HelloHelloHelloHelloHelloHelloHello")); CkCrypt2W_SetEncodedIV(crypt,ivHex2,L"hex"); wprintf(L"%s\n",CkCrypt2W_encryptStringENC(crypt,L"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. CkCrypt2W_Dispose(crypt); } |
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