vectorized ROT-47 cipher

Author: Peter-Juhasz

docs/performance-improvements.md

@@ -51,6 +51,18 @@ Payload length: 44 characters
 
 Measured speed up: **14x**
 
+## ROT-47 cipher
+In version 2, a fast path was added for shifting ASCII characters:
+
+|      | Method                    | Mean      | Error    | StdDev   | Allocated |
+|------|-------------------------- |----------:|---------:|---------:|----------:|
+|v1    | Rot47Cipher_General       | 23.393 ns | 0.0773 ns | 0.0723 ns |         - |
+|**v2**| Rot47Cipher_Ascii_Avx2128 |  7.873 ns | 0.0778 ns | 0.0728 ns |         - |
+
+Payload length: 44 characters
+
+Measured speed up: **3x**
+
 ## Frequency analysis
 The old v1 implementation was based on a very simple, but expensive functional LINQ implementation. In the new version, memory allocation was greatly reduced:
 

perf/Science.Cryptography.Ciphers.Benchmarks/Program.cs

@@ -1,4 +1,4 @@
 using BenchmarkDotNet.Running;
 
-BenchmarkRunner.Run<AtbashBenchmarks>();
+BenchmarkRunner.Run<Rot47Benchmarks>();
 //BenchmarkRunner.Run(typeof(Program).Assembly);

perf/Science.Cryptography.Ciphers.Benchmarks/Rot47Benchmarks.cs

@@ -0,0 +1,29 @@
+using BenchmarkDotNet.Attributes;
+
+using Science.Cryptography.Ciphers.Specialized;
+using System.Collections.Generic;
+using System.Linq;
+using System;
+using Science.Cryptography.Ciphers;
+
+[MemoryDiagnoser]
+public class Rot47Benchmarks
+{
+	private static readonly Rot47Cipher General = new();
+	private static readonly AsciiRot47Cipher Optimized = new();
+
+	private const string Plaintext = "The quick brown fox jumps over the lazy dog.";
+	private static readonly char[] Output = new char[64];
+
+	[Benchmark]
+	public void Atbash()
+	{
+		General.Encrypt(Plaintext, Output, out _);
+	}
+
+	[Benchmark]
+	public void SlowXor_I64_K32()
+	{
+		Optimized.Encrypt(Plaintext, Output, out _);
+	}
+}

src/Science.Cryptography.Ciphers.Specialized/Optimized/AsciiRot47Cipher.cs

@@ -0,0 +1,110 @@
+using System;
+using System.Composition;
+using System.Numerics;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using System.Runtime.Intrinsics;
+using System.Runtime.Intrinsics.X86;
+
+using TVector = System.Runtime.Intrinsics.Vector256<short>;
+
+namespace Science.Cryptography.Ciphers.Specialized;
+
+/// <summary>
+/// Represents the Atbash cipher.
+/// </summary>
+[Export("ASCII-ROT-47", typeof(ICipher))]
+public class AsciiRot47Cipher : ReciprocalCipher
+{
+	private static readonly TVector VectorOfSpace = Vector256.Create((short)' ');
+	private static readonly TVector VectorOf47 = Vector256.Create((short)47);
+	private static readonly TVector VectorOf126 = Vector256.Create((short)126);
+	private static readonly TVector VectorOf33 = Vector256.Create((short)33);
+	private static readonly TVector VectorOf94 = Vector256.Create((short)94);
+
+	protected override void Crypt(ReadOnlySpan<char> text, Span<char> result, out int written)
+	{
+		if (result.Length < text.Length)
+		{
+			throw new ArgumentException("Size of output buffer is insufficient.", nameof(result));
+		}
+
+		var totalVectorizedLength = 0;
+
+		// process vectorized
+		if (Avx2.IsSupported && Vector256.IsHardwareAccelerated)
+        {
+            var vectorCount = text.Length / TVector.Count;
+            totalVectorizedLength = vectorCount * TVector.Count;
+            var inputAsShort = MemoryMarshal.Cast<char, short>(text);
+            var outputAsShort = MemoryMarshal.Cast<char, short>(result);
+            for (int offset = 0; offset < totalVectorizedLength; offset += TVector.Count)
+            {
+                var inputBlock = Vector256.LoadUnsafe(ref MemoryMarshal.GetReference(inputAsShort[offset..]));
+                var outputBlock = CryptBlockAvx2(inputBlock);
+                outputBlock.StoreUnsafe(ref MemoryMarshal.GetReference(outputAsShort[offset..]));
+            }
+        }
+
+		// process the remaining input
+		if (totalVectorizedLength < text.Length)
+		{
+			var remainingInput = text[totalVectorizedLength..];
+			var remainingOutput = result[totalVectorizedLength..];
+			CryptSlow(remainingInput, remainingOutput);
+		}
+
+		written = text.Length;
+	}
+
+	internal static void CryptSlow(ReadOnlySpan<char> text, Span<char> result)
+	{
+		for (int i = 0; i < text.Length; i++)
+		{
+			var ch = text[i];
+			if (ch == ' ')
+			{
+				result[i] = ' ';
+				continue;
+			}
+
+			int value = ch + 47;
+
+			if (value > 126)
+			{
+				value -= 94;
+			}
+			else if (value < 33)
+			{
+				value += 94;
+			}
+
+			result[i] = (char)value;
+		}
+	}
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	private static TVector CryptBlockAvx2(TVector input)
+	{
+		// whitespace mask
+		var spaceMask = Avx2.CompareEqual(VectorOfSpace, input);
+
+		// add 47
+		var transformed = Avx2.Add(input, VectorOf47);
+
+		// subtract 94 if greater than 126
+		var greaterThan126Mask = Avx2.CompareGreaterThan(transformed, VectorOf126);
+		var subtracted = Avx2.Subtract(transformed, VectorOf94);
+		transformed = Avx2.BlendVariable(transformed, subtracted, greaterThan126Mask);
+
+		// add 94 if less than 33
+		var lessThan33Mask = Avx2.CompareGreaterThan(VectorOf33, transformed);
+		var added = Avx2.Add(transformed, VectorOf94);
+		transformed = Avx2.BlendVariable(transformed, added, lessThan33Mask);
+
+		// restore whitespace
+		transformed = Avx2.BlendVariable(transformed, VectorOfSpace, spaceMask);
+
+		return transformed;
+	}
+}

tests/Science.Cryptography.Ciphers.Tests/Ciphers/AsciiRot47CipherTests.cs

@@ -0,0 +1,34 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+using Science.Cryptography.Ciphers.Specialized;
+using System;
+
+namespace Science.Cryptography.Ciphers.Tests;
+
+[TestClass]
+public class AsciiRot47CipherTests
+{
+	[TestMethod]
+	public void Rot47()
+	{
+		var cipher = new Rot47Cipher();
+
+		const string plaintext = "The quick brown fox jumps over the lazy dog";
+		const string ciphertext = "%96 BF:4< 3C@H? 7@I ;F>AD @G6C E96 =2KJ 5@8";
+
+		Assert.AreEqual(ciphertext, cipher.Encrypt(plaintext));
+		Assert.AreEqual(plaintext, cipher.Decrypt(ciphertext));
+	}
+
+	[TestMethod]
+	public void AsciiRot47()
+	{
+		var cipher = new AsciiRot47Cipher();
+
+		const string plaintext = "The quick brown fox jumps over the lazy dog";
+		const string ciphertext = "%96 BF:4< 3C@H? 7@I ;F>AD @G6C E96 =2KJ 5@8";
+
+		Assert.AreEqual(ciphertext, cipher.Encrypt(plaintext));
+		Assert.AreEqual(plaintext, cipher.Decrypt(ciphertext));
+	}
+}

tests/Science.Cryptography.Ciphers.Tests/Ciphers/CipherTests.cs

@@ -103,18 +103,6 @@ public void Bacon()
 		Assert.AreEqual(plaintext, cipher.Decrypt(ciphertext), true);
 	}
 
-	[TestMethod]
-	public void Rot47()
-	{
-		var cipher = new Rot47Cipher();
-
-		const string plaintext = "My string!";
-		const string ciphertext = "|J DEC:?8P";
-
-		Assert.AreEqual(ciphertext, cipher.Encrypt(plaintext));
-		Assert.AreEqual(plaintext, cipher.Decrypt(ciphertext));
-	}
-
     [TestMethod]
     public void Autokey()
     {