(Swift 3,4,5...) 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.

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func chilkatTest() {
    // 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.
    let dhBob = CkoDh()!
    let dhAlice = CkoDh()!

    // 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).

    var p: String?
    var g: Int
    // Bob will now send P and G to Alice.
    p = dhBob.p
    g = dhBob.g.intValue

    // 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.
    var success: Bool = dhAlice.setPG(p, g: g)
    if success != true {
        print("P is not a safe prime")
        return
    }

    // 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).
    var eBob: String?
    eBob = dhBob.createE(256)

    // Alice does the same:
    var eAlice: String?
    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.

    var kBob: String?
    var kAlice: String?

    // 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:
    print("Bob's shared secret:")
    print("\(kBob!)")
    print("Alice's shared secret (should be equal to Bob's)")
    print("\(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:
    let crypt = CkoCrypt2()!

    crypt.encodingMode = "hex"
    crypt.hashAlgorithm = "md5"

    var sessionKey: String?
    sessionKey = crypt.hashStringENC(kBob)

    print("128-bit Session Key:")
    print("\(sessionKey!)")

    // Encrypt something...
    crypt.cryptAlgorithm = "aes"
    crypt.keyLength = 128
    crypt.cipherMode = "cbc"

    // Use an IV that is the MD5 hash of the session key...
    var iv: String?
    iv = crypt.hashStringENC(sessionKey)

    // AES uses a 16-byte IV:
    print("Initialization Vector:")
    print("\(iv!)")

    crypt.setEncodedKey(sessionKey, encoding: "hex")
    crypt.setEncodedIV(iv, encoding: "hex")

    // Encrypt some text:
    var cipherText64: String?

    crypt.encodingMode = "base64"
    cipherText64 = crypt.encryptStringENC("The quick brown fox jumps over the lazy dog")
    print("\(cipherText64!)")

    var plainText: String?
    plainText = crypt.decryptStringENC(cipherText64)

    print("\(plainText!)")

}