(PureBasic) 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.

Chilkat PureBasic Module Download Chilkat PureBasic Module

IncludeFile "CkCrypt2.pb"
IncludeFile "CkDh.pb"

Procedure ChilkatExample()

    ; 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.i = CkDh::ckCreate()
    If dhBob.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    dhAlice.i = CkDh::ckCreate()
    If dhAlice.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    ; 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::ckUseKnownPrime(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).

    p.s
    g.i
    ; Bob will now send P and G to Alice.
    p = CkDh::ckP(dhBob)
    g = CkDh::ckG(dhBob)

    ; 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.
    success.i = CkDh::ckSetPG(dhAlice,p,g)
    If success <> 1
        Debug "P is not a safe prime"
        CkDh::ckDispose(dhBob)
        CkDh::ckDispose(dhAlice)
        ProcedureReturn
    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).
    eBob.s
    eBob = CkDh::ckCreateE(dhBob,256)

    ; Alice does the same:
    eAlice.s
    eAlice = CkDh::ckCreateE(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.

    kBob.s
    kAlice.s

    ; Bob computes the shared secret from Alice's "E":
    kBob = CkDh::ckFindK(dhBob,eAlice)

    ; Alice computes the shared secret from Bob's "E":
    kAlice = CkDh::ckFindK(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:
    Debug "Bob's shared secret:"
    Debug kBob
    Debug "Alice's shared secret (should be equal to Bob's)"
    Debug 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.i = CkCrypt2::ckCreate()
    If crypt.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    CkCrypt2::setCkEncodingMode(crypt, "hex")
    CkCrypt2::setCkHashAlgorithm(crypt, "md5")

    sessionKey.s
    sessionKey = CkCrypt2::ckHashStringENC(crypt,kBob)

    Debug "128-bit Session Key:"
    Debug sessionKey

    ; Encrypt something...
    CkCrypt2::setCkCryptAlgorithm(crypt, "aes")
    CkCrypt2::setCkKeyLength(crypt, 128)
    CkCrypt2::setCkCipherMode(crypt, "cbc")

    ; Use an IV that is the MD5 hash of the session key...
    iv.s
    iv = CkCrypt2::ckHashStringENC(crypt,sessionKey)

    ; AES uses a 16-byte IV:
    Debug "Initialization Vector:"
    Debug iv

    CkCrypt2::ckSetEncodedKey(crypt,sessionKey,"hex")
    CkCrypt2::ckSetEncodedIV(crypt,iv,"hex")

    ; Encrypt some text:
    cipherText64.s

    CkCrypt2::setCkEncodingMode(crypt, "base64")
    cipherText64 = CkCrypt2::ckEncryptStringENC(crypt,"The quick brown fox jumps over the lazy dog")
    Debug cipherText64

    plainText.s
    plainText = CkCrypt2::ckDecryptStringENC(crypt,cipherText64)

    Debug plainText


    CkDh::ckDispose(dhBob)
    CkDh::ckDispose(dhAlice)
    CkCrypt2::ckDispose(crypt)


    ProcedureReturn
EndProcedure