(Android™) 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 Android™ Downloads Android™ Java Libraries Android C/C++ Libraries

// Important: Don't forget to include the call to System.loadLibrary
// as shown at the bottom of this code sample.
package com.test;

import android.app.Activity;
import com.chilkatsoft.*;

import android.widget.TextView;
import android.os.Bundle;

public class SimpleActivity extends Activity {

  private static final String TAG = "Chilkat";

  // Called when the activity is first created.
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // 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.
    CkDh dhBob = new CkDh();
    CkDh dhAlice = new CkDh();

    // 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).

    String p;
    int g;
    // Bob will now send P and G to Alice.
    p = dhBob.p();
    g = dhBob.get_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.
    boolean success = dhAlice.SetPG(p,g);
    if (success != true) {
        Log.i(TAG, "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;
    eBob = dhBob.createE(256);

    // Alice does the same:
    String eAlice;
    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.

    String kBob;
    String kAlice;

    // 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:
    Log.i(TAG, "Bob's shared secret:");
    Log.i(TAG, kBob);
    Log.i(TAG, "Alice's shared secret (should be equal to Bob's)");
    Log.i(TAG, 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:
    CkCrypt2 crypt = new CkCrypt2();

    crypt.put_EncodingMode("hex");
    crypt.put_HashAlgorithm("md5");

    String sessionKey;
    sessionKey = crypt.hashStringENC(kBob);

    Log.i(TAG, "128-bit Session Key:");
    Log.i(TAG, sessionKey);

    // Encrypt something...
    crypt.put_CryptAlgorithm("aes");
    crypt.put_KeyLength(128);
    crypt.put_CipherMode("cbc");

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

    // AES uses a 16-byte IV:
    Log.i(TAG, "Initialization Vector:");
    Log.i(TAG, iv);

    crypt.SetEncodedKey(sessionKey,"hex");
    crypt.SetEncodedIV(iv,"hex");

    // Encrypt some text:
    String cipherText64;

    crypt.put_EncodingMode("base64");
    cipherText64 = crypt.encryptStringENC("The quick brown fox jumps over the lazy dog");
    Log.i(TAG, cipherText64);

    String plainText;
    plainText = crypt.decryptStringENC(cipherText64);

    Log.i(TAG, plainText);

  }

  static {
      System.loadLibrary("chilkat");

      // Note: If the incorrect library name is passed to System.loadLibrary,
      // then you will see the following error message at application startup:
      //"The application <your-application-name> has stopped unexpectedly. Please try again."
  }
}