Java

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.

Chilkat2 Java

import com.chilkatsoft.chilkat2.ChilkatException;
import com.chilkatsoft.chilkat2.Crypt2;
import com.chilkatsoft.chilkat2.Dh;

public class ChilkatExample {

    public static void main(String[] args) {
        // 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.
        Dh dhBob = new Dh();
        Dh dhAlice = new 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 }
        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.
        String p = dhBob.getP();
        int g = dhBob.getG();

        // 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.
        try {
            dhAlice.setPG(p, g);
        } catch (ChilkatException e) {
            System.out.println("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).
        String eBob = dhBob.createE(256);

        // Alice does the same:
        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.

        // Bob computes the shared secret from Alice's "E":
        String kBob = dhBob.findK(eAlice);

        // Alice computes the shared secret from Bob's "E":
        String 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:
        System.out.println("Bob's shared secret:");
        System.out.println(kBob);
        System.out.println("Alice's shared secret (should be equal to Bob's)");
        System.out.println(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:
        Crypt2 crypt = new Crypt2();

        crypt.setEncodingMode("hex");
        crypt.setHashAlgorithm("md5");

        String sessionKey = crypt.hashStringENC(kBob);

        System.out.println("128-bit Session Key:");
        System.out.println(sessionKey);

        // Encrypt something...
        crypt.setCryptAlgorithm("aes");
        crypt.setKeyLength(128);
        crypt.setCipherMode("cbc");

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

        // AES uses a 16-byte IV:
        System.out.println("Initialization Vector:");
        System.out.println(iv);

        crypt.setEncodedKey(sessionKey, "hex");
        crypt.setEncodedIV(iv, "hex");

        // Encrypt some text:

        crypt.setEncodingMode("base64");
        String cipherText64 = crypt.encryptStringENC("The quick brown fox jumps over the lazy dog");
        System.out.println(cipherText64);

        String plainText = crypt.decryptStringENC(cipherText64);

        System.out.println(plainText);
    }
}