Delphi DLL

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.

Delphi DLL

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Dh, Crypt2;

...

procedure TForm1.Button1Click(Sender: TObject);
var
success: Boolean;
dhBob: HCkDh;
dhAlice: HCkDh;
p: PWideChar;
g: Integer;
eBob: PWideChar;
eAlice: PWideChar;
kBob: PWideChar;
kAlice: PWideChar;
crypt: HCkCrypt2;
sessionKey: PWideChar;
iv: PWideChar;
cipherText64: PWideChar;
plainText: PWideChar;

begin
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 := CkDh_Create();
dhAlice := CkDh_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 }
CkDh_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 := CkDh__p(dhBob);
g := CkDh_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 := CkDh_SetPG(dhAlice,p,g);
if (success <> True) then
  begin
    Memo1.Lines.Add('P is not a safe prime');
    Exit;
  end;

// 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 := CkDh__createE(dhBob,256);

// Alice does the same:

eAlice := CkDh__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 := CkDh__findK(dhBob,eAlice);

// Alice computes the shared secret from Bob's "E":
kAlice := CkDh__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:
Memo1.Lines.Add('Bob''s shared secret:');
Memo1.Lines.Add(kBob);
Memo1.Lines.Add('Alice''s shared secret (should be equal to Bob''s)');
Memo1.Lines.Add(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 := CkCrypt2_Create();

CkCrypt2_putEncodingMode(crypt,'hex');
CkCrypt2_putHashAlgorithm(crypt,'md5');

sessionKey := CkCrypt2__hashStringENC(crypt,kBob);

Memo1.Lines.Add('128-bit Session Key:');
Memo1.Lines.Add(sessionKey);

// Encrypt something...
CkCrypt2_putCryptAlgorithm(crypt,'aes');
CkCrypt2_putKeyLength(crypt,128);
CkCrypt2_putCipherMode(crypt,'cbc');

// Use an IV that is the MD5 hash of the session key...

iv := CkCrypt2__hashStringENC(crypt,sessionKey);

// AES uses a 16-byte IV:
Memo1.Lines.Add('Initialization Vector:');
Memo1.Lines.Add(iv);

CkCrypt2_SetEncodedKey(crypt,sessionKey,'hex');
CkCrypt2_SetEncodedIV(crypt,iv,'hex');

// Encrypt some text:

CkCrypt2_putEncodingMode(crypt,'base64');
cipherText64 := CkCrypt2__encryptStringENC(crypt,'The quick brown fox jumps over the lazy dog');
Memo1.Lines.Add(cipherText64);

plainText := CkCrypt2__decryptStringENC(crypt,cipherText64);

Memo1.Lines.Add(plainText);

CkDh_Dispose(dhBob);
CkDh_Dispose(dhAlice);
CkCrypt2_Dispose(crypt);

end;