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Diffie-Hellman Key Exchange (DH)

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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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Local $bSuccess = 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.
$oDhBob = ObjCreate("Chilkat.Dh")
$oDhAlice = ObjCreate("Chilkat.Dh")

;  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 }
$oDhBob.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).

Local $sP
Local $iG
;  Bob will now send P and G to Alice.
$sP = $oDhBob.P
$iG = $oDhBob.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.
$bSuccess = $oDhAlice.SetPG($sP,$iG)
If ($bSuccess <> True) Then
    ConsoleWrite("P is not a safe prime" & @CRLF)
    Exit
EndIf

;  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).
Local $sEBob
$sEBob = $oDhBob.CreateE(256)

;  Alice does the same:
Local $sEAlice
$sEAlice = $oDhAlice.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.

Local $sKBob
Local $sKAlice

;  Bob computes the shared secret from Alice's "E":
$sKBob = $oDhBob.FindK($sEAlice)

;  Alice computes the shared secret from Bob's "E":
$sKAlice = $oDhAlice.FindK($sEBob)

;  Amazingly, kBob and kAlice are identical and the expected
;  length (260 characters).  The strings contain the hex encoded bytes of
;  our shared secret:
ConsoleWrite("Bob's shared secret:" & @CRLF)
ConsoleWrite($sKBob & @CRLF)
ConsoleWrite("Alice's shared secret (should be equal to Bob's)" & @CRLF)
ConsoleWrite($sKAlice & @CRLF)

;  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:
$oCrypt = ObjCreate("Chilkat.Crypt2")

$oCrypt.EncodingMode = "hex"
$oCrypt.HashAlgorithm = "md5"

Local $sessionKey
$sessionKey = $oCrypt.HashStringENC($sKBob)

ConsoleWrite("128-bit Session Key:" & @CRLF)
ConsoleWrite($sessionKey & @CRLF)

;  Encrypt something...
$oCrypt.CryptAlgorithm = "aes"
$oCrypt.KeyLength = 128
$oCrypt.CipherMode = "cbc"

;  Use an IV that is the MD5 hash of the session key...
Local $sIv
$sIv = $oCrypt.HashStringENC($sessionKey)

;  AES uses a 16-byte IV:
ConsoleWrite("Initialization Vector:" & @CRLF)
ConsoleWrite($sIv & @CRLF)

$oCrypt.SetEncodedKey $sessionKey,"hex"
$oCrypt.SetEncodedIV $sIv,"hex"

;  Encrypt some text:
Local $sCipherText64

$oCrypt.EncodingMode = "base64"
$sCipherText64 = $oCrypt.EncryptStringENC("The quick brown fox jumps over the lazy dog")
ConsoleWrite($sCipherText64 & @CRLF)

Local $sPlainText
$sPlainText = $oCrypt.DecryptStringENC($sCipherText64)

ConsoleWrite($sPlainText & @CRLF)