(Objective-C) Tips on Matching Encryption with another System

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

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#import <CkoCrypt2.h>
#import <NSString.h>

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

CkoCrypt2 *crypt = [[CkoCrypt2 alloc] init];

// 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 = [NSNumber numberWithInt:256];
crypt.PaddingScheme = [NSNumber numberWithInt:0];
NSString *ivHex1 = @"000102030405060708090A0B0C0D0E0F";
NSString *ivHex2 = @"FF0102030405060708090A0B0C0D0E0F";
[crypt SetEncodedIV: ivHex1 encoding: @"hex"];
NSString *keyHex = @"000102030405060708090A0B0C0D0E0F101112131415161718191A1B1C1D1E1F";
[crypt SetEncodedKey: keyHex encoding: @"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:
NSLog(@"%@",[crypt EncryptStringENC: @"Hello"]);
// The result is 5B827AB3B4F9F2292C2B74C8A6C99A3D
// This 16 bytes -- exactly one AES encryption block.

// Let's change only the padding scheme.
crypt.PaddingScheme = [NSNumber numberWithInt:3];

// Encrypt again:
NSLog(@"%@",[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:
NSLog(@"%@",@"-- Only the padding scheme differs --");
crypt.PaddingScheme = [NSNumber numberWithInt:0];
NSLog(@"%@",[crypt EncryptStringENC: @"HelloHelloHelloHelloHelloHelloHello"]);
crypt.PaddingScheme = [NSNumber numberWithInt:3];
NSLog(@"%@",[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 = [NSNumber numberWithInt:0];
crypt.CipherMode = @"cbc";
NSLog(@"%@",@"-- Only the IV differs, CBC mode produces different output. --");
[crypt SetEncodedIV: ivHex1 encoding: @"hex"];
NSLog(@"%@",[crypt EncryptStringENC: @"HelloHelloHelloHelloHelloHelloHello"]);
[crypt SetEncodedIV: ivHex2 encoding: @"hex"];
NSLog(@"%@",[crypt EncryptStringENC: @"HelloHelloHelloHelloHelloHelloHello"]);

crypt.CipherMode = @"ecb";
NSLog(@"%@",@"-- Only the IV differs, ECB does not use the IV.  The outputs are the same. --");
[crypt SetEncodedIV: ivHex1 encoding: @"hex"];
NSLog(@"%@",[crypt EncryptStringENC: @"HelloHelloHelloHelloHelloHelloHello"]);
[crypt SetEncodedIV: ivHex2 encoding: @"hex"];
NSLog(@"%@",[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.