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package sun.security.ssl;
import java.math.BigInteger;
import java.security.*;
import javax.crypto.SecretKey;
import javax.crypto.KeyAgreement;
import javax.crypto.interfaces.DHPublicKey;
import javax.crypto.spec.*;
/**
* This class implements the Diffie-Hellman key exchange algorithm.
* D-H means combining your private key with your partners public key to
* generate a number. The peer does the same with its private key and our
* public key. Through the magic of Diffie-Hellman we both come up with the
* same number. This number is secret (discounting MITM attacks) and hence
* called the shared secret. It has the same length as the modulus, e.g. 512
* or 1024 bit. Man-in-the-middle attacks are typically countered by an
* independent authentication step using certificates (RSA, DSA, etc.).
*
* The thing to note is that the shared secret is constant for two partners
* with constant private keys. This is often not what we want, which is why
* it is generally a good idea to create a new private key for each session.
* Generating a private key involves one modular exponentiation assuming
* suitable D-H parameters are available.
*
* General usage of this class (TLS DHE case):
* . if we are server, call DHCrypt(keyLength,random). This generates
* an ephemeral keypair of the request length.
* . if we are client, call DHCrypt(modulus, base, random). This
* generates an ephemeral keypair using the parameters specified by the server.
* . send parameters and public value to remote peer
* . receive peers ephemeral public key
* . call getAgreedSecret() to calculate the shared secret
*
* In TLS the server chooses the parameter values itself, the client must use
* those sent to it by the server.
*
* The use of ephemeral keys as described above also achieves what is called
* "forward secrecy". This means that even if the authentication keys are
* broken at a later date, the shared secret remains secure. The session is
* compromised only if the authentication keys are already broken at the
* time the key exchange takes place and an active MITM attack is used.
* This is in contrast to straightforward encrypting RSA key exchanges.
*
* @author David Brownell
*/
final class DHCrypt {
// group parameters (prime modulus and generator)
private BigInteger modulus; // P (aka N)
private BigInteger base; // G (aka alpha)
// our private key (including private component x)
private PrivateKey privateKey;
// public component of our key, X = (g ^ x) mod p
private BigInteger publicValue; // X (aka y)
/**
* Generate a Diffie-Hellman keypair of the specified size.
*/
DHCrypt(int keyLength, SecureRandom random) {
try {
KeyPairGenerator kpg = JsseJce.getKeyPairGenerator("DiffieHellman");
kpg.initialize(keyLength, random);
KeyPair kp = kpg.generateKeyPair();
privateKey = kp.getPrivate();
DHPublicKeySpec spec = getDHPublicKeySpec(kp.getPublic());
publicValue = spec.getY();
modulus = spec.getP();
base = spec.getG();
} catch (GeneralSecurityException e) {
throw new RuntimeException("Could not generate DH keypair", e);
}
}
/**
* Generate a Diffie-Hellman keypair using the specified parameters.
*
* @param modulus the Diffie-Hellman modulus P
* @param base the Diffie-Hellman base G
*/
DHCrypt(BigInteger modulus, BigInteger base, SecureRandom random) {
this.modulus = modulus;
this.base = base;
try {
KeyPairGenerator kpg = JsseJce.getKeyPairGenerator("DiffieHellman");
DHParameterSpec params = new DHParameterSpec(modulus, base);
kpg.initialize(params, random);
KeyPair kp = kpg.generateKeyPair();
privateKey = kp.getPrivate();
DHPublicKeySpec spec = getDHPublicKeySpec(kp.getPublic());
publicValue = spec.getY();
} catch (GeneralSecurityException e) {
throw new RuntimeException("Could not generate DH keypair", e);
}
}
static DHPublicKeySpec getDHPublicKeySpec(PublicKey key) {
if (key instanceof DHPublicKey) {
DHPublicKey dhKey = (DHPublicKey)key;
DHParameterSpec params = dhKey.getParams();
return new DHPublicKeySpec(dhKey.getY(), params.getP(), params.getG());
}
try {
KeyFactory factory = JsseJce.getKeyFactory("DH");
return (DHPublicKeySpec)factory.getKeySpec
(key, DHPublicKeySpec.class);
} catch (Exception e) {
throw new RuntimeException(e);
}
}
/** Returns the Diffie-Hellman modulus. */
BigInteger getModulus() {
return modulus;
}
/** Returns the Diffie-Hellman base (generator). */
BigInteger getBase() {
return base;
}
/**
* Gets the public key of this end of the key exchange.
*/
BigInteger getPublicKey() {
return publicValue;
}
/**
* Get the secret data that has been agreed on through Diffie-Hellman
* key agreement protocol. Note that in the two party protocol, if
* the peer keys are already known, no other data needs to be sent in
* order to agree on a secret. That is, a secured message may be
* sent without any mandatory round-trip overheads.
*
* <P>It is illegal to call this member function if the private key
* has not been set (or generated).
*
* @param peerPublicKey the peer's public key.
* @returns the secret, which is an unsigned big-endian integer
* the same size as the Diffie-Hellman modulus.
*/
SecretKey getAgreedSecret(BigInteger peerPublicValue) {
try {
KeyFactory kf = JsseJce.getKeyFactory("DiffieHellman");
DHPublicKeySpec spec =
new DHPublicKeySpec(peerPublicValue, modulus, base);
PublicKey publicKey = kf.generatePublic(spec);
KeyAgreement ka = JsseJce.getKeyAgreement("DiffieHellman");
ka.init(privateKey);
ka.doPhase(publicKey, true);
return ka.generateSecret("TlsPremasterSecret");
} catch (GeneralSecurityException e) {
throw new RuntimeException("Could not generate secret", e);
}
}
}