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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//  Create two separate instances of the DH object.
Chilkat.Dh dhBob = new Chilkat.Dh();
Chilkat.Dh dhAlice = new Chilkat.Dh();

bool success;

//  Unlock the component once at program startup...
success = dhBob.UnlockComponent("Anything for 30-day trial");
if (success != true) {
    MessageBox.Show(dhBob.LastErrorText);
    return;
}

//  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).

string p;
int g;
//  Bob will now send P and G to Alice.
p = dhBob.P;
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.
success = dhAlice.SetPG(p,g);
if (success != true) {
    MessageBox.Show("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).
string eBob;
eBob = dhBob.CreateE(256);

//  Alice does the same:
string eAlice;
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.

string kBob;
string kAlice;

//  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:
textBox1.Text += "Bob's shared secret:" + "\r\n";
textBox1.Text += kBob + "\r\n";
textBox1.Text += "Alice's shared secret (should be equal to Bob's)" + "\r\n";
textBox1.Text += kAlice + "\r\n";

//  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.

//  Here's how you would use Chilkat Crypt (a separate Chilkat component) to
//  produce the session key:
Chilkat.Crypt2 crypt = new Chilkat.Crypt2();
success = crypt.UnlockComponent("Anything for 30-day trial.");
if (success != true) {
    MessageBox.Show(crypt.LastErrorText);
    return;
}

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

string sessionKey;
sessionKey = crypt.HashStringENC(kBob);

textBox1.Text += "128-bit Session Key:" + "\r\n";
textBox1.Text += sessionKey + "\r\n";

//  Encrypt something...
crypt.CryptAlgorithm = "aes";
crypt.KeyLength = 128;
crypt.CipherMode = "cbc";

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

//  AES uses a 16-byte IV:
textBox1.Text += "Initialization Vector:" + "\r\n";
textBox1.Text += iv + "\r\n";

crypt.SetEncodedKey(sessionKey,"hex");
crypt.SetEncodedIV(iv,"hex");

//  Encrypt some text:
string cipherText64;

crypt.EncodingMode = "base64";
cipherText64 = crypt.EncryptStringENC("The quick brown fox jumps over the lazy dog");
textBox1.Text += cipherText64 + "\r\n";

string plainText;
plainText = crypt.DecryptStringENC(cipherText64);

textBox1.Text += plainText + "\r\n";