Dart

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.

Dart

import 'package:chilkat/chilkat.dart';

void main() {
  // 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.
  final dhBob = CkDh();
  final dhAlice = CkDh();

  // 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(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.
  final p = dhBob.p;
  final g = dhBob.g;

  // 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.
  try {
    dhAlice.setPG(p, g);
  } on ChilkatException {
    print('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).
  final eBob = dhBob.createE(256);

  // Alice does the same:
  final eAlice = dhAlice.createE(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":
  final kBob = dhBob.findK(eAlice);

  // Alice computes the shared secret from Bob's "E":
  final 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:
  print('Bob\'s shared secret:');
  print(kBob);
  print('Alice\'s shared secret (should be equal to Bob\'s)');
  print(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:
  final crypt = CkCrypt2();

  crypt.encodingMode = 'hex';
  crypt.hashAlgorithm = 'md5';

  final sessionKey = crypt.hashStringENC(kBob);

  print('128-bit Session Key:');
  print(sessionKey);

  // Encrypt something...
  crypt.cryptAlgorithm = 'aes';
  crypt.keyLength = 128;
  crypt.cipherMode = 'cbc';

  // Use an IV that is the MD5 hash of the session key...
  final iv = crypt.hashStringENC(sessionKey);

  // AES uses a 16-byte IV:
  print('Initialization Vector:');
  print(iv);

  crypt.setEncodedKey(sessionKey, 'hex');
  crypt.setEncodedIV(iv, 'hex');

  // Encrypt some text:

  crypt.encodingMode = 'base64';
  final cipherText64 = crypt.encryptStringENC('The quick brown fox jumps over the lazy dog');
  print(cipherText64);

  final plainText = crypt.decryptStringENC(cipherText64);

  print(plainText);
}