1 files changed, 138 insertions, 0 deletions
diff --git a/core/src/main/java/org/bouncycastle/crypto/signers/DSASigner.java b/core/src/main/java/org/bouncycastle/crypto/signers/DSASigner.java
new file mode 100644
index 00000000..a96cef07
--- /dev/null
+++ b/core/src/main/java/org/bouncycastle/crypto/signers/DSASigner.java
@@ -0,0 +1,138 @@
+package org.bouncycastle.crypto.signers;
+
+import org.bouncycastle.crypto.CipherParameters;
+import org.bouncycastle.crypto.DSA;
+import org.bouncycastle.crypto.params.DSAKeyParameters;
+import org.bouncycastle.crypto.params.DSAParameters;
+import org.bouncycastle.crypto.params.DSAPrivateKeyParameters;
+import org.bouncycastle.crypto.params.DSAPublicKeyParameters;
+import org.bouncycastle.crypto.params.ParametersWithRandom;
+
+import java.math.BigInteger;
+import java.security.SecureRandom;
+
+/**
+ * The Digital Signature Algorithm - as described in "Handbook of Applied
+ * Cryptography", pages 452 - 453.
+ */
+public class DSASigner
+    implements DSA
+{
+    DSAKeyParameters key;
+
+    SecureRandom    random;
+
+    public void init(
+        boolean                 forSigning,
+        CipherParameters        param)
+    {
+        if (forSigning)
+        {
+            if (param instanceof ParametersWithRandom)
+            {
+                ParametersWithRandom    rParam = (ParametersWithRandom)param;
+
+                this.random = rParam.getRandom();
+                this.key = (DSAPrivateKeyParameters)rParam.getParameters();
+            }
+            else
+            {
+                this.random = new SecureRandom();
+                this.key = (DSAPrivateKeyParameters)param;
+            }
+        }
+        else
+        {
+            this.key = (DSAPublicKeyParameters)param;
+        }
+    }
+
+    /**
+     * generate a signature for the given message using the key we were
+     * initialised with. For conventional DSA the message should be a SHA-1
+     * hash of the message of interest.
+     *
+     * @param message the message that will be verified later.
+     */
+    public BigInteger[] generateSignature(
+        byte[] message)
+    {
+        DSAParameters   params = key.getParameters();
+        BigInteger      m = calculateE(params.getQ(), message);
+        BigInteger      k;
+        int                  qBitLength = params.getQ().bitLength();
+
+        do 
+        {
+            k = new BigInteger(qBitLength, random);
+        }
+        while (k.compareTo(params.getQ()) >= 0);
+
+        BigInteger  r = params.getG().modPow(k, params.getP()).mod(params.getQ());
+
+        k = k.modInverse(params.getQ()).multiply(
+                    m.add(((DSAPrivateKeyParameters)key).getX().multiply(r)));
+
+        BigInteger  s = k.mod(params.getQ());
+
+        BigInteger[]  res = new BigInteger[2];
+
+        res[0] = r;
+        res[1] = s;
+
+        return res;
+    }
+
+    /**
+     * return true if the value r and s represent a DSA signature for
+     * the passed in message for standard DSA the message should be a
+     * SHA-1 hash of the real message to be verified.
+     */
+    public boolean verifySignature(
+        byte[]      message,
+        BigInteger  r,
+        BigInteger  s)
+    {
+        DSAParameters   params = key.getParameters();
+        BigInteger      m = calculateE(params.getQ(), message);
+        BigInteger      zero = BigInteger.valueOf(0);
+
+        if (zero.compareTo(r) >= 0 || params.getQ().compareTo(r) <= 0)
+        {
+            return false;
+        }
+
+        if (zero.compareTo(s) >= 0 || params.getQ().compareTo(s) <= 0)
+        {
+            return false;
+        }
+
+        BigInteger  w = s.modInverse(params.getQ());
+
+        BigInteger  u1 = m.multiply(w).mod(params.getQ());
+        BigInteger  u2 = r.multiply(w).mod(params.getQ());
+
+        u1 = params.getG().modPow(u1, params.getP());
+        u2 = ((DSAPublicKeyParameters)key).getY().modPow(u2, params.getP());
+
+        BigInteger  v = u1.multiply(u2).mod(params.getP()).mod(params.getQ());
+
+        return v.equals(r);
+    }
+
+    private BigInteger calculateE(BigInteger n, byte[] message)
+    {
+        if (n.bitLength() >= message.length * 8)
+        {
+            return new BigInteger(1, message);
+        }
+        else
+        {
+            byte[] trunc = new byte[n.bitLength() / 8];
+
+            System.arraycopy(message, 0, trunc, 0, trunc.length);
+
+            return new BigInteger(1, trunc);
+        }
+    }
+}