Pascal (Lazarus/Delphi)

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.

Pascal (Lazarus/Delphi)

program ChilkatDemo;

// Demonstrates using the Chilkat Pascal wrapper via the C bridge DLL.
// Builds as a console application under Lazarus (FPC) or Delphi.

{$IFDEF FPC}
  {$MODE DELPHI}
{$ENDIF}
{$APPTYPE CONSOLE}

uses
  {$IFDEF UNIX}
  cthreads,
  {$ENDIF}
  SysUtils,
  CkDllLoader,
  Chilkat.Dh,
  Chilkat.Crypt2;

// ---------------------------------------------------------------------------

procedure RunDemo;
var
  success: Boolean;
  dhBob: TDh;
  dhAlice: TDh;
  p: string;
  g: Integer;
  eBob: string;
  eAlice: string;
  kBob: string;
  kAlice: string;
  crypt: TCrypt2;
  sessionKey: string;
  iv: string;
  cipherText64: string;
  plainText: string;

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 := TDh.Create;
  dhAlice := TDh.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 }
  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.
  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) then
    begin
      WriteLn('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 := dhBob.CreateE(256);

  //  Alice does the same:

  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":
  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:
  WriteLn('Bob''s shared secret:');
  WriteLn(kBob);
  WriteLn('Alice''s shared secret (should be equal to Bob''s)');
  WriteLn(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 := TCrypt2.Create;

  crypt.EncodingMode := 'hex';
  crypt.HashAlgorithm := 'md5';

  sessionKey := crypt.HashStringENC(kBob);

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

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

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

  iv := crypt.HashStringENC(sessionKey);

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

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

  //  Encrypt some text:

  crypt.EncodingMode := 'base64';
  cipherText64 := crypt.EncryptStringENC('The quick brown fox jumps over the lazy dog');
  WriteLn(cipherText64);

  plainText := crypt.DecryptStringENC(cipherText64);

  WriteLn(plainText);


  dhBob.Free;
  dhAlice.Free;
  crypt.Free;

end;

// ---------------------------------------------------------------------------

begin

  try
    RunDemo;
  except
    on E: Exception do
      WriteLn('Unhandled exception: ', E.ClassName, ': ', E.Message);
  end;

  WriteLn;
  {$IFDEF MSWINDOWS}
  WriteLn('Press Enter to exit...');
  ReadLn;
  {$ENDIF}
end.