1 files changed, 90 insertions, 0 deletions
diff --git a/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/agreement/srp/SRP6Server.java b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/agreement/srp/SRP6Server.java
new file mode 100644
index 000000000..50f6f7cea
--- /dev/null
+++ b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/agreement/srp/SRP6Server.java
@@ -0,0 +1,90 @@
+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);
+    }
+}