Visual FoxPro

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.

Visual FoxPro

LOCAL lnSuccess
LOCAL loDhBob
LOCAL loDhAlice
LOCAL p
LOCAL g
LOCAL lcEBob
LOCAL lcEAlice
LOCAL lcKBob
LOCAL lcKAlice
LOCAL loCrypt
LOCAL lcSessionKey
LOCAL lcIv
LOCAL lcCipherText64
LOCAL lcPlainText

lnSuccess = 0

* 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.
loDhBob = CreateObject('Chilkat.Dh')
loDhAlice = CreateObject('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 }
loDhBob.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 = loDhBob.P
g = loDhBob.G

* Alice calls SetPG to set P and G.  SetPG checks
* the values to make sure it's a safe prime and will
* return 0 if not.
lnSuccess = loDhAlice.SetPG(p,g)
IF (lnSuccess <> 1) THEN
    ? "P is not a safe prime"
    RELEASE loDhBob
    RELEASE loDhAlice
    CANCEL
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).

lcEBob = loDhBob.CreateE(256)

* Alice does the same:

lcEAlice = loDhAlice.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":
lcKBob = loDhBob.FindK(lcEAlice)

* Alice computes the shared secret from Bob's "E":
lcKAlice = loDhAlice.FindK(lcEBob)

* Amazingly, kBob and kAlice are identical and the expected
* length (260 characters).  The strings contain the hex encoded bytes of
* our shared secret:
? "Bob's shared secret:"
? lcKBob
? "Alice's shared secret (should be equal to Bob's)"
? lcKAlice

* 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:
loCrypt = CreateObject('Chilkat.Crypt2')

loCrypt.EncodingMode = "hex"
loCrypt.HashAlgorithm = "md5"

lcSessionKey = loCrypt.HashStringENC(lcKBob)

? "128-bit Session Key:"
? lcSessionKey

* Encrypt something...
loCrypt.CryptAlgorithm = "aes"
loCrypt.KeyLength = 128
loCrypt.CipherMode = "cbc"

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

lcIv = loCrypt.HashStringENC(lcSessionKey)

* AES uses a 16-byte IV:
? "Initialization Vector:"
? lcIv

loCrypt.SetEncodedKey(lcSessionKey,"hex")
loCrypt.SetEncodedIV(lcIv,"hex")

* Encrypt some text:

loCrypt.EncodingMode = "base64"
lcCipherText64 = loCrypt.EncryptStringENC("The quick brown fox jumps over the lazy dog")
? lcCipherText64

lcPlainText = loCrypt.DecryptStringENC(lcCipherText64)

? lcPlainText

RELEASE loDhBob
RELEASE loDhAlice
RELEASE loCrypt