Add test for hashing of at_hash with all supported algorithms

test/OidcClient.Tests/CryptoHelperTests.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Text;
+using FluentAssertions;
+using IdentityModel;
+using IdentityModel.OidcClient;
+using Xunit;
+
+public class CryptoHelperTests
+{
+    [Theory]
+    [InlineData("asdf", "RS256")]
+    [InlineData("asdf", "RS384")]
+    [InlineData("asdf", "RS512")]
+    public void ComputeHash_should_compute_correct_hashes_for_all_signature_algorithms(string data, string algorithmName)
+    {
+        var sut = new CryptoHelper(new OidcClientOptions());
+        var algorithm = sut.GetMatchingHashAlgorithm(algorithmName);
+
+        var hash = algorithm.ComputeHash(Encoding.ASCII.GetBytes(data));
+
+        var bytesInLeftHalf = algorithm.HashSize / 16; // Divide by 8 for bytes and then 2 to get just half.
+
+        var leftHalf = new byte[bytesInLeftHalf];
+        Array.Copy(hash, leftHalf, bytesInLeftHalf);
+
+        var hashString = Base64Url.Encode(leftHalf);
+
+        sut.ValidateHash(data, hashString, algorithmName).Should().BeTrue();
+    }
+
+}