Still working on getting client credential flow complete, most of the request works but still working on generating the JWT.

IdentityShroud.Core/Security/Jwt/JwtSignatureGenerator.cs

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+using System.Buffers.Text;
+using System.Security.Cryptography;
+using System.Text;
+using System.Text.Json;
+using Microsoft.AspNetCore.WebUtilities;
+
+namespace IdentityShroud.Core;
+
+public static class JwtSignatureGenerator
+{
+    /// <summary>
+    /// Generates a JWT signature using RS256 algorithm
+    /// </summary>
+    /// <param name="headerBase64Url">Base64Url encoded header</param>
+    /// <param name="payloadBase64Url">Base64Url encoded payload</param>
+    /// <param name="privateKey">RSA private key (PEM format or RSA parameters)</param>
+    /// <returns>Base64Url encoded signature</returns>
+    public static string GenerateRS256Signature(string headerBase64Url, string payloadBase64Url, RSA privateKey)
+    {
+        // Combine header and payload with a period
+        string dataToSign = $"{headerBase64Url}.{payloadBase64Url}";
+
+        // Convert to bytes
+        byte[] dataBytes = Encoding.UTF8.GetBytes(dataToSign);
+
+        // Sign the data using RSA-SHA256
+        byte[] signatureBytes = privateKey.SignData(dataBytes, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+
+        // Convert signature to Base64Url encoding
+        string signature = WebEncoders.Base64UrlEncode(signatureBytes);
+
+        return signature;
+    }
+
+    public static string GenerateCompleteJwt(string headerBase64Url, string payloadBase64Url, RSA privateKey)
+    {
+        string signature = GenerateRS256Signature(headerBase64Url, payloadBase64Url, privateKey);
+        return $"{headerBase64Url}.{payloadBase64Url}.{signature}";
+    }
+
+}
+
+public static class JwtCreator
+{
+
+    public static byte[] CreateEncodedJwt(ReadOnlySpan<byte> payloadUtf8, IJwtSignatureProvider signatureProvider)
+    {
+        MemoryStream memStream = new();
+        Utf8JsonWriter writer = new(memStream);
+        WriteJwtHeader(writer, signatureProvider);
+        writer.Flush();
+        memStream.Seek(0, SeekOrigin.Begin);
+
+        int headerBase64Length = Base64Url.GetEncodedLength((int)memStream.Length);
+        int payloadBase64Length = Base64Url.GetEncodedLength(payloadUtf8.Length);
+        int signatureBase64Length = Base64Url.GetEncodedLength(signatureProvider.GetSignatureLength());
+        int totalLength = headerBase64Length + 1 + payloadBase64Length + 1 + signatureBase64Length;
+
+        var completeJwt = new byte[totalLength];
+
+        // 
+        var byteArray = new byte[memStream.Length];
+        memStream.ReadExactly(byteArray, 0, (int)memStream.Length);
+        int written = Base64Url.EncodeToUtf8(byteArray, completeJwt);
+
+        if (written != headerBase64Length)
+            throw new Exception("expected header length did not match bytes written");
+        
+        completeJwt[headerBase64Length] = (byte)'.';
+        
+        written = Base64Url.EncodeToUtf8(payloadUtf8, completeJwt.AsSpan().Slice(headerBase64Length + 1, payloadBase64Length));
+        
+        if (written != payloadBase64Length)
+            throw new Exception("expected payload length did not match bytes written");
+        
+        completeJwt[headerBase64Length + 1 + payloadBase64Length] = (byte)'.';
+
+
+        Span<byte> signature = stackalloc byte[signatureProvider.GetSignatureLength()];
+        signatureProvider.CalculateSignature(
+            completeJwt.AsSpan().Slice(0, headerBase64Length + 1 + payloadBase64Length),
+            signature);
+        
+        written = Base64Url.EncodeToUtf8(signature, completeJwt.AsSpan()
+            .Slice(headerBase64Length + 1 + payloadBase64Length + 1));
+        
+        if (written != signatureBase64Length)
+            throw new Exception("expected signature length did not match bytes written");
+
+        return completeJwt;
+    }
+    
+    private static void WriteJwtHeader(Utf8JsonWriter writer, IJwtSignatureProvider signatureProvider)
+    {
+        writer.WriteStartObject();
+        writer.WriteString("typ"u8, "JWT"u8);
+        signatureProvider.WriteJwtHeaderFields(writer);
+        writer.WriteEndObject();
+    }
+}