1 files changed, 93 insertions, 0 deletions
diff --git a/core/src/main/java/org/spongycastle/crypto/generators/DHParametersHelper.java b/core/src/main/java/org/spongycastle/crypto/generators/DHParametersHelper.java
new file mode 100644
index 00000000..24813191
--- /dev/null
+++ b/core/src/main/java/org/spongycastle/crypto/generators/DHParametersHelper.java
@@ -0,0 +1,93 @@
+package org.spongycastle.crypto.generators;
+
+import java.math.BigInteger;
+import java.security.SecureRandom;
+
+import org.spongycastle.math.ec.WNafUtil;
+import org.spongycastle.util.BigIntegers;
+
+class DHParametersHelper
+{
+    private static final BigInteger ONE = BigInteger.valueOf(1);
+    private static final BigInteger TWO = BigInteger.valueOf(2);
+
+    /*
+     * Finds a pair of prime BigInteger's {p, q: p = 2q + 1}
+     * 
+     * (see: Handbook of Applied Cryptography 4.86)
+     */
+    static BigInteger[] generateSafePrimes(int size, int certainty, SecureRandom random)
+    {
+        BigInteger p, q;
+        int qLength = size - 1;
+        int minWeight = size >>> 2;
+
+        for (;;)
+        {
+            q = new BigInteger(qLength, 2, random);
+
+            // p <- 2q + 1
+            p = q.shiftLeft(1).add(ONE);
+
+            if (!p.isProbablePrime(certainty))
+            {
+                continue;
+            }
+
+            if (certainty > 2 && !q.isProbablePrime(certainty - 2))
+            {
+                continue;
+            }
+
+            /*
+             * Require a minimum weight of the NAF representation, since low-weight primes may be
+             * weak against a version of the number-field-sieve for the discrete-logarithm-problem.
+             * 
+             * See "The number field sieve for integers of low weight", Oliver Schirokauer.
+             */
+            if (WNafUtil.getNafWeight(p) < minWeight)
+            {
+                continue;
+            }
+
+            break;
+        }
+
+        return new BigInteger[] { p, q };
+    }
+
+    /*
+     * Select a high order element of the multiplicative group Zp*
+     * 
+     * p and q must be s.t. p = 2*q + 1, where p and q are prime (see generateSafePrimes)
+     */
+    static BigInteger selectGenerator(BigInteger p, BigInteger q, SecureRandom random)
+    {
+        BigInteger pMinusTwo = p.subtract(TWO);
+        BigInteger g;
+
+        /*
+         * (see: Handbook of Applied Cryptography 4.80)
+         */
+//        do
+//        {
+//            g = BigIntegers.createRandomInRange(TWO, pMinusTwo, random);
+//        }
+//        while (g.modPow(TWO, p).equals(ONE) || g.modPow(q, p).equals(ONE));
+
+
+        /*
+         * RFC 2631 2.2.1.2 (and see: Handbook of Applied Cryptography 4.81)
+         */
+        do
+        {
+            BigInteger h = BigIntegers.createRandomInRange(TWO, pMinusTwo, random);
+
+            g = h.modPow(TWO, p);
+        }
+        while (g.equals(ONE));
+
+
+        return g;
+    }
+}