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<TITLE>Generating a Secret Key Using the Diffie-Hellman Key Agreement Algorithm (Java Developers Almanac Example)
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<h1>The Java Developers Almanac 1.4</h1>
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<A HREF="../index.html">List of Packages</A>
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<A HREF="../javax.crypto/pkg.html">javax.crypto</A><font color="#666666" class="xsmall-font">
[14 examples]
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<B><A HREF="../javax.crypto/pkg.html#Key Agreement">Key Agreement</A></B><font color="#666666" class="xsmall-font">
[2 examples]
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<h3>e471. Generating a Secret Key Using the Diffie-Hellman Key Agreement Algorithm</h3>
Two parties use a <font color="#0066ff"><i>key agreement protocol</i></font> to generate identical
secret keys for encryption without ever having to transmit the secret
key. The protocol works by both parties agreeing on a set of values
(a prime, a base, and a private value) which are used to generate a
key pair. <a href="../javax.crypto/GenDhParams.html" class="eglink"><b>e470</b> Generating a Parameter Set for the Diffie-Hellman Key Agreement Algorithm</a> demonstrates how to
generate the set of values.
<P> This example uses the set of values and generates a key
pair. The public key is then exchanged with the other party and
the secret key is generated.
<pre> // Retrieve the prime, base, and private value for generating the key pair.
// If the values are encoded as in
// <a href="../javax.crypto/GenDhParams.html" class="eglink"><b>e470</b> Generating a Parameter Set for the Diffie-Hellman Key Agreement Algorithm</a>,
// the following code will extract the values.
String[] values = <font color="#0066ff"><i>valuesInStr</i></font>.split(",");
BigInteger p = new BigInteger(values[0]);
BigInteger g = new BigInteger(values[1]);
int l = Integer.parseInt(values[2]);
try {
// Use the values to generate a key pair
KeyPairGenerator keyGen = KeyPairGenerator.getInstance("DH");
DHParameterSpec dhSpec = new DHParameterSpec(p, g, l);
keyGen.initialize(dhSpec);
KeyPair keypair = keyGen.generateKeyPair();
// Get the generated public and private keys
PrivateKey privateKey = keypair.getPrivate();
PublicKey publicKey = keypair.getPublic();
// Send the public key bytes to the other party...
byte[] publicKeyBytes = publicKey.getEncoded();
// Retrieve the public key bytes of the other party
publicKeyBytes = <font color="#0066ff"><i>...</i></font>;
// Convert the public key bytes into a PublicKey object
X509EncodedKeySpec x509KeySpec = new X509EncodedKeySpec(publicKeyBytes);
KeyFactory keyFact = KeyFactory.getInstance("DH");
publicKey = keyFact.generatePublic(x509KeySpec);
// Prepare to generate the secret key with the private key and public key of the other party
KeyAgreement ka = KeyAgreement.getInstance("DH");
ka.init(privateKey);
ka.doPhase(publicKey, true);
// Specify the type of key to generate;
// see <a href="../javax.crypto/ListKeyGen.html" class="eglink"><b>e458</b> Listing All Available Symmetric Key Generators</a>
String algorithm = <font color="#0066ff"><i>"DES"</i></font>;
// Generate the secret key
SecretKey secretKey = ka.generateSecret(algorithm);
// Use the secret key to encrypt/decrypt data;
// see <a href="../javax.crypto/DesString.html" class="eglink"><b>e462</b> Encrypting a String with DES</a>
} catch (java.security.InvalidKeyException e) {
} catch (java.security.spec.InvalidKeySpecException e) {
} catch (java.security.InvalidAlgorithmParameterException e) {
} catch (java.security.NoSuchAlgorithmException e) {
}
</pre>
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<b>Related Examples</b></font></td>
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e470. <a class="eglink" href="GenDhParams.html?l=rel">
Generating a Parameter Set for the Diffie-Hellman Key Agreement Algorithm
</a>
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See also:
<a class="eglink" href="/egs/javax.crypto/pkg.html?l=rel#Encrypting%20and%20Decrypting">
Encrypting and Decrypting
</a>
<a class="eglink" href="/egs/javax.crypto/pkg.html?l=rel#MAC">
MAC
</a>
<a class="eglink" href="/egs/javax.crypto/pkg.html?l=rel#Symmetric%20Keys">
Symmetric Keys
</a>
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