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package org.spongycastle.crypto.agreement.srp;
import java.math.BigInteger;
import java.security.SecureRandom;
import org.spongycastle.crypto.CryptoException;
import org.spongycastle.crypto.Digest;
/**
* Implements the server side SRP-6a protocol. Note that this class is stateful, and therefore NOT threadsafe.
* This implementation of SRP is based on the optimized message sequence put forth by Thomas Wu in the paper
* "SRP-6: Improvements and Refinements to the Secure Remote Password Protocol, 2002"
*/
public class SRP6Server
{
protected BigInteger N;
protected BigInteger g;
protected BigInteger v;
protected SecureRandom random;
protected Digest digest;
protected BigInteger A;
protected BigInteger b;
protected BigInteger B;
protected BigInteger u;
protected BigInteger S;
public SRP6Server()
{
}
/**
* Initialises the server to accept a new client authentication attempt
* @param N The safe prime associated with the client's verifier
* @param g The group parameter associated with the client's verifier
* @param v The client's verifier
* @param digest The digest algorithm associated with the client's verifier
* @param random For key generation
*/
public void init(BigInteger N, BigInteger g, BigInteger v, Digest digest, SecureRandom random)
{
this.N = N;
this.g = g;
this.v = v;
this.random = random;
this.digest = digest;
}
/**
* Generates the server's credentials that are to be sent to the client.
* @return The server's public value to the client
*/
public BigInteger generateServerCredentials()
{
BigInteger k = SRP6Util.calculateK(digest, N, g);
this.b = selectPrivateValue();
this.B = k.multiply(v).mod(N).add(g.modPow(b, N)).mod(N);
return B;
}
/**
* Processes the client's credentials. If valid the shared secret is generated and returned.
* @param clientA The client's credentials
* @return A shared secret BigInteger
* @throws CryptoException If client's credentials are invalid
*/
public BigInteger calculateSecret(BigInteger clientA) throws CryptoException
{
this.A = SRP6Util.validatePublicValue(N, clientA);
this.u = SRP6Util.calculateU(digest, N, A, B);
this.S = calculateS();
return S;
}
protected BigInteger selectPrivateValue()
{
return SRP6Util.generatePrivateValue(digest, N, g, random);
}
private BigInteger calculateS()
{
return v.modPow(u, N).multiply(A).mod(N).modPow(b, N);
}
}
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