1 files changed, 93 insertions, 0 deletions
diff --git a/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/agreement/srp/SRP6Client.java b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/agreement/srp/SRP6Client.java
new file mode 100644
index 000000000..d47ad39ec
--- /dev/null
+++ b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/agreement/srp/SRP6Client.java
@@ -0,0 +1,93 @@
+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 client 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 SRP6Client
+{
+    protected BigInteger N;
+    protected BigInteger g;
+
+    protected BigInteger a;
+    protected BigInteger A;
+
+    protected BigInteger B;
+
+    protected BigInteger x;
+    protected BigInteger u;
+    protected BigInteger S;
+
+    protected Digest digest;
+    protected SecureRandom random;
+
+    public SRP6Client()
+    {
+    }
+
+    /**
+     * Initialises the client to begin new 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 digest The digest algorithm associated with the client's verifier
+     * @param random For key generation
+     */
+    public void init(BigInteger N, BigInteger g, Digest digest, SecureRandom random)
+    {
+        this.N = N;
+        this.g = g;
+        this.digest = digest;
+        this.random = random;
+    }
+
+    /**
+     * Generates client's credentials given the client's salt, identity and password
+     * @param salt The salt used in the client's verifier.
+     * @param identity The user's identity (eg. username)
+     * @param password The user's password
+     * @return Client's public value to send to server
+     */
+    public BigInteger generateClientCredentials(byte[] salt, byte[] identity, byte[] password)
+    {
+        this.x = SRP6Util.calculateX(digest, N, salt, identity, password);
+        this.a = selectPrivateValue();
+        this.A = g.modPow(a, N);
+
+        return A;
+    }
+
+    /**
+     * Generates client's verification message given the server's credentials
+     * @param serverB The server's credentials
+     * @return Client's verification message for the server
+     * @throws CryptoException If server's credentials are invalid
+     */
+    public BigInteger calculateSecret(BigInteger serverB) throws CryptoException
+    {
+        this.B = SRP6Util.validatePublicValue(N, serverB);
+        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()
+    {
+        BigInteger k = SRP6Util.calculateK(digest, N, g);
+        BigInteger exp = u.multiply(x).add(a);
+        BigInteger tmp = g.modPow(x, N).multiply(k).mod(N);
+        return B.subtract(tmp).mod(N).modPow(exp, N);
+    }
+}