1 files changed, 46 insertions, 0 deletions
diff --git a/core/src/main/java/org/spongycastle/pqc/math/ntru/polynomial/ModularResultant.java b/core/src/main/java/org/spongycastle/pqc/math/ntru/polynomial/ModularResultant.java
new file mode 100644
index 00000000..7e8031fe
--- /dev/null
+++ b/core/src/main/java/org/spongycastle/pqc/math/ntru/polynomial/ModularResultant.java
@@ -0,0 +1,46 @@
+package org.spongycastle.pqc.math.ntru.polynomial;
+
+import java.math.BigInteger;
+
+import org.spongycastle.pqc.math.ntru.euclid.BigIntEuclidean;
+
+/**
+ * A resultant modulo a <code>BigInteger</code>
+ */
+public class ModularResultant
+    extends Resultant
+{
+    BigInteger modulus;
+
+    ModularResultant(BigIntPolynomial rho, BigInteger res, BigInteger modulus)
+    {
+        super(rho, res);
+        this.modulus = modulus;
+    }
+
+    /**
+     * Calculates a <code>rho</code> modulo <code>m1*m2</code> from
+     * two resultants whose <code>rho</code>s are modulo <code>m1</code> and <code>m2</code>.<br/>
+     * </code>res</code> is set to <code>null</code>.
+     *
+     * @param modRes1
+     * @param modRes2
+     * @return <code>rho</code> modulo <code>modRes1.modulus * modRes2.modulus</code>, and <code>null</code> for </code>res</code>.
+     */
+    static ModularResultant combineRho(ModularResultant modRes1, ModularResultant modRes2)
+    {
+        BigInteger mod1 = modRes1.modulus;
+        BigInteger mod2 = modRes2.modulus;
+        BigInteger prod = mod1.multiply(mod2);
+        BigIntEuclidean er = BigIntEuclidean.calculate(mod2, mod1);
+
+        BigIntPolynomial rho1 = (BigIntPolynomial)modRes1.rho.clone();
+        rho1.mult(er.x.multiply(mod2));
+        BigIntPolynomial rho2 = (BigIntPolynomial)modRes2.rho.clone();
+        rho2.mult(er.y.multiply(mod1));
+        rho1.add(rho2);
+        rho1.mod(prod);
+
+        return new ModularResultant(rho1, null, prod);
+    }
+}