Added RandomNumberGeneratorExtensions.GetBigPrime

CompactCryptoGroupAlgebra.Tests/RandomNumberGeneratorExtensionsTests.cs

@@ -1,6 +1,6 @@
 // CompactCryptoGroupAlgebra - C# implementation of abelian group algebra for experimental cryptography
 
-// SPDX-FileCopyrightText: 2022 Lukas Prediger <lumip@lumip.de>
+// SPDX-FileCopyrightText: 2022-2024 Lukas Prediger <lumip@lumip.de>
 // SPDX-License-Identifier: GPL-3.0-or-later
 // SPDX-FileType: SOURCE
 
@@ -85,9 +85,9 @@ public void TestRandomWithLengthNonByteBoundary()
             var length = NumberLength.FromBitLength(17);
 
             var leadingOneRngBuffer = ((BigInteger.One << (3 * 8)) - 1).ToByteArray(); // 3 bytes of ones
-            var leadingZeroRngBuffer = ((BigInteger.One << (length.InBits - 2))).ToByteArray(); // only bit 16 set
+            var leadingZeroRngBuffer = (BigInteger.One << (length.InBits - 2)).ToByteArray(); // only bit 16 set
             var expectedFirst = (BigInteger.One << length.InBits) - 1;
-            var expectedSecond = ((BigInteger.One << (length.InBits - 2)) ^ (BigInteger.One << (length.InBits - 1)));
+            var expectedSecond = (BigInteger.One << (length.InBits - 2)) ^ (BigInteger.One << (length.InBits - 1));
 
             bool firstCall = true;
             var rngMock = new Mock<RandomNumberGenerator>(MockBehavior.Strict);
@@ -151,5 +151,36 @@ public void TestRandomWithLengthByteBoundary()
 
             rngMock.Verify(rng => rng.GetBytes(It.IsAny<byte[]>()), Times.Exactly(2));
         }
+
+        [Test]
+        [TestCase(3)]  // ((prime - 3) | 1) mod 6 == ((prime - 2) | 1) == 5
+        [TestCase(4)]  // ((prime - 4) | 1) mod 6 == 3
+        [TestCase(6)]  // ((prime - 6) | 1) mod 6 == 1
+        public void TestRandomBigPrime(int subtrahend)
+        {
+            var length = NumberLength.FromBitLength(128);
+            var prime = BigPrime.CreateWithoutChecks(BigInteger.Parse("340282366920938463463374607431768211297"));
+
+            var rngResponse = (prime - subtrahend).ToByteArray();
+
+            Random random = new Random(0);
+            bool firstCall = true;
+
+            var rngMock = new Mock<RandomNumberGenerator>(MockBehavior.Strict);
+            rngMock.Setup(rng => rng.GetBytes(It.IsAny<byte[]>()))
+                   .Callback<byte[]>(buffer =>
+                   {
+                       if (firstCall)
+                           Buffer.BlockCopy(rngResponse, 0, buffer, 0, Math.Min(rngResponse.Length, buffer.Length));
+                       else
+                           random.NextBytes(buffer);
+                       firstCall = false;
+                   });
+
+            var result = rngMock.Object.GetBigPrime(length);
+
+            Assert.AreEqual(prime, result);
+        }
+
     }
 }

CompactCryptoGroupAlgebra/RandomNumberGeneratorExtensions.cs

@@ -1,6 +1,6 @@
 // CompactCryptoGroupAlgebra - C# implementation of abelian group algebra for experimental cryptography
 
-// SPDX-FileCopyrightText: 2022 Lukas Prediger <lumip@lumip.de>
+// SPDX-FileCopyrightText: 2022-2024 Lukas Prediger <lumip@lumip.de>
 // SPDX-License-Identifier: GPL-3.0-or-later
 // SPDX-FileType: SOURCE
 
@@ -59,7 +59,7 @@ public static BigInteger GetBigIntegerBetween(
         /// Returns a random positive <see cref="BigInteger"/> with the given bit length.
         /// </summary>
         /// <param name="randomNumberGenerator">Random number generator.</param>
-        /// <param name="length">The bit length of the generator number.</param>
+        /// <param name="length">The bit length of the generated number.</param>
         /// <returns>The random <see cref="BigInteger"/>.</returns>
         public static BigInteger GetBigIntegerWithLength(
             this RandomNumberGenerator randomNumberGenerator, NumberLength length
@@ -74,5 +74,40 @@ public static BigInteger GetBigIntegerWithLength(
             candidate |= BigInteger.One << (length.InBits - 1); // ensure msb is set to achieve desired length
             return candidate;
         }
+
+        /// <summary>
+        /// Returns a random <see cref="BigPrime"/> with the given bit length.
+        /// </summary>
+        /// <param name="randomNumberGenerator">Random number generator.</param>
+        /// <param name="length">The bit length of the generated prime number.</param>
+        /// <returns>The random <see cref="BigPrime"/>.</returns>
+        public static BigPrime GetBigPrime(
+            this RandomNumberGenerator randomNumberGenerator, NumberLength length
+        )
+        {
+            BigInteger primeCandidate = randomNumberGenerator.GetBigIntegerWithLength(length);
+
+            // Ensure primeCandidate is odd.
+            primeCandidate |= 1;
+
+            // Ensure primeCandidate mod 6 == 1 or 5. Any prime p can only have p mod 6 == 1 or p mod 6 == 5.
+            // Relying on bit operations to avoid any branching statements.
+            var residue = primeCandidate % 6;
+            primeCandidate += residue & 2;  // <-> primeCandidate += (residue == 3) ? 2 : 0;
+            residue += residue & 2; // <-> residue += (residue == 3) ? 2 : 0;
+            int step = 2 << (int)((residue ^ (residue >> 2)) & 1); // <-> step = (residue == 1) ? 4 : 2;
+            int shiftDir = -1 + (step & 2); // <-> shiftDir = (step == 2) ? 1 : -1;
+
+            while (!PrimalityTest.IsProbablyPrime(primeCandidate, randomNumberGenerator))
+            {
+                primeCandidate += step;
+
+                // Below is equivalent to step = (step == 2) ? 4 : 2;
+                step = (step << shiftDir) | (step >> -shiftDir);
+                shiftDir = -shiftDir;
+            }
+
+            return BigPrime.CreateWithoutChecks(primeCandidate);
+        }
     }
 }