(Objective-C) Diffie-Hellman Key Exchange (DH)

Diffie-Hellman key exchange (DH) is a cryptographic protocol that allows two parties that have no prior knowledge of each other to jointly establish a shared secret key.

This example demonstrates how two parties (Alice and Bob) can compute an N-bit shared secret key without the key ever being transmitted.

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

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

// Create two separate instances of the DH object.
CkoDh *dhBob = [[CkoDh alloc] init];
CkoDh *dhAlice = [[CkoDh alloc] init];

// The DH algorithm begins with a large prime, P, and a generator, G.  
// These don't have to be secret, and they may be transmitted over an insecure channel.  
// The generator is a small integer and typically has the value 2 or 5.

// The Chilkat DH component provides the ability to use known
// "safe" primes, as well as a method to generate new safe primes.

// This example will use a known safe prime.  Generating
// new safe primes is a time-consuming CPU intensive task
// and is normally done offline.

// Bob will choose to use the 2nd of our 8 pre-chosen safe primes.  
// It is the Prime for the 2nd Oakley Group (RFC 2409) -- 
// 1024-bit MODP Group.  Generator is 2. 
// The prime is: 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }
[dhBob UseKnownPrime: [NSNumber numberWithInt: 2]];

// The computed shared secret will be equal to the size of the prime (in bits).
// In this case the prime is 1024 bits, so the shared secret will be 128 bytes (128 * 8 = 1024).
// However, the result is returned as an SSH1-encoded bignum in hex string format.
// The SSH1-encoding prepends a 2-byte count, so the result is going  to be 2 bytes
// longer: 130 bytes.  This results in a hex string that is 260 characters long (two chars
// per byte for the hex encoding).

NSString *p = 0;
int g;
// Bob will now send P and G to Alice.
p = dhBob.P;
g = [dhBob.G intValue];

// Alice calls SetPG to set P and G.  SetPG checks
// the values to make sure it's a safe prime and will
// return NO if not.
BOOL success = [dhAlice SetPG: p g: [NSNumber numberWithInt: g]];
if (success != YES) {
    NSLog(@"%@",@"P is not a safe prime");
    return;
}

// Each side begins by generating an "E"
// value.  The CreateE method has one argument: numBits.
// It should be set to twice the size of the number of bits
// in the session key.

// Let's say we want to generate a 128-bit session key
// for AES encryption.  The shared secret generated by the Diffie-Hellman
// algorithm will be longer, so we'll hash the result to arrive at the
// desired session key length.  However, the length of the session
// key we'll utlimately produce determines the value that should be
// passed to the CreateE method.

// In this case, we'll be creating a 128-bit session key, so pass 256 to CreateE.
// This setting is for security purposes only -- the value
// passed to CreateE does not change the length of the shared secret
// that is produced by Diffie-Hellman.  
// Also, there is no need to pass in a value larger
// than 2 times the expected session key length.  It suffices to
// pass exactly 2 times the session key length.

// Bob generates a random E (which has the mathematical
// properties required for DH).
NSString *eBob = 0;
eBob = [dhBob CreateE: [NSNumber numberWithInt: 256]];

// Alice does the same:
NSString *eAlice = 0;
eAlice = [dhAlice CreateE: [NSNumber numberWithInt: 256]];

// The "E" values are sent over the insecure channel.
// Bob sends his "E" to Alice, and Alice sends her "E" to Bob.

// Each side computes the shared secret by calling FindK.
// "K" is the shared-secret.

NSString *kBob = 0;
NSString *kAlice = 0;

// Bob computes the shared secret from Alice's "E":
kBob = [dhBob FindK: eAlice];

// Alice computes the shared secret from Bob's "E":
kAlice = [dhAlice FindK: eBob];

// Amazingly, kBob and kAlice are identical and the expected
// length (260 characters).  The strings contain the hex encoded bytes of
// our shared secret:
NSLog(@"%@",@"Bob's shared secret:");
NSLog(@"%@",kBob);
NSLog(@"%@",@"Alice's shared secret (should be equal to Bob's)");
NSLog(@"%@",kAlice);

// To arrive at a 128-bit session key for AES encryption, Bob and Alice should
// both transform the raw shared secret using a hash algorithm that produces
// the size of session key desired.   MD5 produces a 16-byte (128-bit) result, so
// this is a good choice for 128-bit AES.

// To produce the session key:
CkoCrypt2 *crypt = [[CkoCrypt2 alloc] init];

crypt.EncodingMode = @"hex";
crypt.HashAlgorithm = @"md5";

NSString *sessionKey = 0;
sessionKey = [crypt HashStringENC: kBob];

NSLog(@"%@",@"128-bit Session Key:");
NSLog(@"%@",sessionKey);

// Encrypt something...
crypt.CryptAlgorithm = @"aes";
crypt.KeyLength = [NSNumber numberWithInt:128];
crypt.CipherMode = @"cbc";

// Use an IV that is the MD5 hash of the session key...
NSString *iv = 0;
iv = [crypt HashStringENC: sessionKey];

// AES uses a 16-byte IV:
NSLog(@"%@",@"Initialization Vector:");
NSLog(@"%@",iv);

[crypt SetEncodedKey: sessionKey encoding: @"hex"];
[crypt SetEncodedIV: iv encoding: @"hex"];

// Encrypt some text:
NSString *cipherText64 = 0;

crypt.EncodingMode = @"base64";
cipherText64 = [crypt EncryptStringENC: @"The quick brown fox jumps over the lazy dog"];
NSLog(@"%@",cipherText64);

NSString *plainText = 0;
plainText = [crypt DecryptStringENC: cipherText64];

NSLog(@"%@",plainText);