1 files changed, 67 insertions, 0 deletions
diff --git a/core/src/main/java/org/spongycastle/crypto/generators/DHKeyGeneratorHelper.java b/core/src/main/java/org/spongycastle/crypto/generators/DHKeyGeneratorHelper.java
new file mode 100644
index 00000000..6982d222
--- /dev/null
+++ b/core/src/main/java/org/spongycastle/crypto/generators/DHKeyGeneratorHelper.java
@@ -0,0 +1,67 @@
+package org.spongycastle.crypto.generators;
+
+import java.math.BigInteger;
+import java.security.SecureRandom;
+
+import org.spongycastle.crypto.params.DHParameters;
+import org.spongycastle.math.ec.WNafUtil;
+import org.spongycastle.util.BigIntegers;
+
+class DHKeyGeneratorHelper
+{
+    static final DHKeyGeneratorHelper INSTANCE = new DHKeyGeneratorHelper();
+
+    private static final BigInteger ONE = BigInteger.valueOf(1);
+    private static final BigInteger TWO = BigInteger.valueOf(2);
+
+    private DHKeyGeneratorHelper()
+    {
+    }
+
+    BigInteger calculatePrivate(DHParameters dhParams, SecureRandom random)
+    {
+        int limit = dhParams.getL();
+
+        if (limit != 0)
+        {
+            int minWeight = limit >>> 2;
+            for (;;)
+            {
+                BigInteger x = new BigInteger(limit, random).setBit(limit - 1);
+                if (WNafUtil.getNafWeight(x) >= minWeight)
+                {
+                    return x;
+                }
+            }
+        }
+
+        BigInteger min = TWO;
+        int m = dhParams.getM();
+        if (m != 0)
+        {
+            min = ONE.shiftLeft(m - 1);
+        }
+
+        BigInteger q = dhParams.getQ();
+        if (q == null)
+        {
+            q = dhParams.getP();
+        }
+        BigInteger max = q.subtract(TWO);
+
+        int minWeight = max.bitLength() >>> 2;
+        for (;;)
+        {
+            BigInteger x = BigIntegers.createRandomInRange(min, max, random);
+            if (WNafUtil.getNafWeight(x) >= minWeight)
+            {
+                return x;
+            }
+        }
+    }
+
+    BigInteger calculatePublic(DHParameters dhParams, BigInteger x)
+    {
+        return dhParams.getG().modPow(x, dhParams.getP());
+    }
+}