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(PHP Extension) Tips on Matching Encryption with another SystemThis example provides tips on matching encryption results produced by another system.
<?php // The version number (9_5_0) should match version of the Chilkat extension used, omitting the micro-version number. // For example, if using Chilkat v9.5.0.48, then include as shown here: include("chilkat_9_5_0.php"); // This example assumes the Chilkat API to have been previously unlocked. // See Global Unlock Sample for sample code. $crypt = new 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->put_CryptAlgorithm('aes'); $crypt->put_CipherMode('cbc'); $crypt->put_KeyLength(256); $crypt->put_PaddingScheme(0); $ivHex1 = '000102030405060708090A0B0C0D0E0F'; $ivHex2 = 'FF0102030405060708090A0B0C0D0E0F'; $crypt->SetEncodedIV($ivHex1,'hex'); $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->put_EncodingMode('hex'); // Encrypt something small: print $crypt->encryptStringENC('Hello') . "\n"; // The result is 5B827AB3B4F9F2292C2B74C8A6C99A3D // This 16 bytes -- exactly one AES encryption block. // Let's change only the padding scheme. $crypt->put_PaddingScheme(3); // Encrypt again: print $crypt->encryptStringENC('Hello') . "\n"; // 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 --' . "\n"; $crypt->put_PaddingScheme(0); print $crypt->encryptStringENC('HelloHelloHelloHelloHelloHelloHello') . "\n"; $crypt->put_PaddingScheme(3); print $crypt->encryptStringENC('HelloHelloHelloHelloHelloHelloHello') . "\n"; // 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->put_PaddingScheme(0); $crypt->put_CipherMode('cbc'); print '-- Only the IV differs, CBC mode produces different output. --' . "\n"; $crypt->SetEncodedIV($ivHex1,'hex'); print $crypt->encryptStringENC('HelloHelloHelloHelloHelloHelloHello') . "\n"; $crypt->SetEncodedIV($ivHex2,'hex'); print $crypt->encryptStringENC('HelloHelloHelloHelloHelloHelloHello') . "\n"; $crypt->put_CipherMode('ecb'); print '-- Only the IV differs, ECB does not use the IV. The outputs are the same. --' . "\n"; $crypt->SetEncodedIV($ivHex1,'hex'); print $crypt->encryptStringENC('HelloHelloHelloHelloHelloHelloHello') . "\n"; $crypt->SetEncodedIV($ivHex2,'hex'); print $crypt->encryptStringENC('HelloHelloHelloHelloHelloHelloHello') . "\n"; // 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. ?> |
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