Unicode C
Unicode C
Diffie-Hellman Key Exchange (DH)
See more Diffie-Hellman Examples
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.
Chilkat Unicode C Downloads
#include <C_CkDhW.h>
#include <C_CkCrypt2W.h>
void ChilkatSample(void)
{
BOOL success;
HCkDhW dhBob;
HCkDhW dhAlice;
const wchar_t *p;
int g;
const wchar_t *eBob;
const wchar_t *eAlice;
const wchar_t *kBob;
const wchar_t *kAlice;
HCkCrypt2W crypt;
const wchar_t *sessionKey;
const wchar_t *iv;
const wchar_t *cipherText64;
const wchar_t *plainText;
success = FALSE;
// 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.
dhBob = CkDhW_Create();
dhAlice = CkDhW_Create();
// 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 }
CkDhW_UseKnownPrime(dhBob,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).
// Bob will now send P and G to Alice.
p = CkDhW_p(dhBob);
g = CkDhW_getG(dhBob);
// Alice calls SetPG to set P and G. SetPG checks
// the values to make sure it's a safe prime and will
// return FALSE if not.
success = CkDhW_SetPG(dhAlice,p,g);
if (success != TRUE) {
wprintf(L"P is not a safe prime\n");
CkDhW_Dispose(dhBob);
CkDhW_Dispose(dhAlice);
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).
eBob = CkDhW_createE(dhBob,256);
// Alice does the same:
eAlice = CkDhW_createE(dhAlice,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.
// Bob computes the shared secret from Alice's "E":
kBob = CkDhW_findK(dhBob,eAlice);
// Alice computes the shared secret from Bob's "E":
kAlice = CkDhW_findK(dhAlice,eBob);
// Amazingly, kBob and kAlice are identical and the expected
// length (260 characters). The strings contain the hex encoded bytes of
// our shared secret:
wprintf(L"Bob's shared secret:\n");
wprintf(L"%s\n",kBob);
wprintf(L"Alice's shared secret (should be equal to Bob's)\n");
wprintf(L"%s\n",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:
crypt = CkCrypt2W_Create();
CkCrypt2W_putEncodingMode(crypt,L"hex");
CkCrypt2W_putHashAlgorithm(crypt,L"md5");
sessionKey = CkCrypt2W_hashStringENC(crypt,kBob);
wprintf(L"128-bit Session Key:\n");
wprintf(L"%s\n",sessionKey);
// Encrypt something...
CkCrypt2W_putCryptAlgorithm(crypt,L"aes");
CkCrypt2W_putKeyLength(crypt,128);
CkCrypt2W_putCipherMode(crypt,L"cbc");
// Use an IV that is the MD5 hash of the session key...
iv = CkCrypt2W_hashStringENC(crypt,sessionKey);
// AES uses a 16-byte IV:
wprintf(L"Initialization Vector:\n");
wprintf(L"%s\n",iv);
CkCrypt2W_SetEncodedKey(crypt,sessionKey,L"hex");
CkCrypt2W_SetEncodedIV(crypt,iv,L"hex");
// Encrypt some text:
CkCrypt2W_putEncodingMode(crypt,L"base64");
cipherText64 = CkCrypt2W_encryptStringENC(crypt,L"The quick brown fox jumps over the lazy dog");
wprintf(L"%s\n",cipherText64);
plainText = CkCrypt2W_decryptStringENC(crypt,cipherText64);
wprintf(L"%s\n",plainText);
CkDhW_Dispose(dhBob);
CkDhW_Dispose(dhAlice);
CkCrypt2W_Dispose(crypt);
}