Add basic HTTP signature validation (implementation works but needs to be integrated into the rest of the thing)

Iceshrimp.Backend/Controllers/SignatureTestController.cs

@@ -0,0 +1,43 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Net;
+using System.Net.Mime;
+using Iceshrimp.Backend.Controllers.Schemas;
+using Iceshrimp.Backend.Core.Database;
+using Iceshrimp.Backend.Core.Database.Tables;
+using Iceshrimp.Backend.Core.Federation.Cryptography;
+using Iceshrimp.Backend.Core.Helpers;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+
+namespace Iceshrimp.Backend.Controllers;
+
+[ApiController]
+[Produces("application/json")]
+[Route("/inbox")]
+public class SignatureTestController : Controller {
+	[HttpPost]
+	[Consumes(MediaTypeNames.Application.Json)]
+	public async Task<IActionResult> Inbox() {
+		var sig = new HttpSignature(Request, ["(request-target)", "digest", "host", "date"]);
+	
+		//TODO: fetch key from db (duh)
+		
+		const string key = """
+		                   -----BEGIN PUBLIC KEY-----
+		                   MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAtCFuaufkSCpsDZ2twSrH
+		                   GAFcJTGQ7ZspaFekVM7gBP1GQ/jfjwO3qT9fMgbsCuQXNTIw0U9zlsTIPB91yNPw
+		                   w5UpbqQ3dnpnYnwXg1BsqfX7EOLR1Dlnw6dk+5yeginJsNno15SRQ7CDqbEXj7Nc
+		                   lhNOGgU+LaXHhN59Paye3sfsvUHu4fmTp/rALWGPl/Rvx7RVRcR76CcTfTaHPYdb
+		                   OQAtqPJBfWgHpPLAUjRypzZoN/ExMgiCbFuxI7UFNNXxU3te8GNZaaob8bSwyUB6
+		                   Xuq7Rw+Me3eYiDxrYHQ99ZytsgoHBNVrVh/X7wIl0AlpjyWeGug3uIUjXR0twuGj
+		                   wwIDAQAB
+		                   -----END PUBLIC KEY-----
+		                   """;
+		
+		return Ok(new ErrorResponse {
+			StatusCode = 200,
+			Error = "null",
+			Message = sig.Verify(key).ToString()
+		});
+	}
+}

Iceshrimp.Backend/Core/Federation/Cryptography/HttpSignature.cs

@@ -0,0 +1,59 @@
+using System.Data;
+using System.Security.Cryptography;
+using System.Text;
+
+namespace Iceshrimp.Backend.Core.Federation.Cryptography;
+
+public class HttpSignature {
+	private readonly string _keyId;
+	private readonly string _algo;
+	private readonly byte[] _signature;
+	private readonly byte[] _signatureData;
+
+	public HttpSignature(HttpRequest request, IEnumerable<string> requiredHeaders) {
+		if (!request.Headers.TryGetValue("signature", out var sigHeader))
+			throw new ConstraintException("Signature string is missing the signature header");
+
+		//TODO: does this break for requests without a body?
+		var digest = SHA256.HashDataAsync(request.BodyReader.AsStream()).AsTask();
+
+		var sig = sigHeader.ToString().Split(",")
+		                 .Select(s => s.Split('='))
+		                 .ToDictionary(p => p[0], p => (p[1] + new string('=', p.Length - 2)).Trim('"'));
+		
+		var signatureBase64 = sig["signature"] ?? throw new ConstraintException("Signature string is missing the signature field");
+
+		_keyId     = sig["keyId"] ?? throw new ConstraintException("Signature string is missing the keyId field");
+		_algo      = sig["algorithm"] ?? throw new ConstraintException("Signature string is missing the algorithm field");
+		_signature = Convert.FromBase64String(signatureBase64);
+
+		var headers = sig["headers"].Split(" ") ??
+		              throw new ConstraintException("Signature data is missing the headers field");
+
+		var sb = new StringBuilder();
+		if (!requiredHeaders.All(headers.Contains))
+			throw new ConstraintException("Request is missing required headers");
+
+		//TODO: handle additional params, see https://github.com/Chocobozzz/node-http-signature/blob/master/lib/parser.js#L294-L310
+		foreach (var header in headers) {
+			sb.Append($"{header}: ");
+			sb.AppendLine(header switch {
+				"(request-target)" => $"{request.Method.ToLower()} {request.Path}",
+				_                  => request.Headers[header]
+			});
+		}
+
+		_signatureData = Encoding.UTF8.GetBytes(sb.ToString(0, sb.Length - 1)); // remove trailing newline
+
+		//TODO: this definitely breaks if there's no body
+		//TODO: check for the SHA256= prefix instead of blindly removing the first 8 chars
+		if (Convert.ToBase64String(digest.Result) != request.Headers["digest"].ToString().Remove(0, 8)) 
+			throw new ConstraintException("Request digest mismatch");
+	}
+
+	public bool Verify(string key) {
+		var rsa = RSA.Create();
+		rsa.ImportFromPem(key);
+		return rsa.VerifyData(_signatureData, _signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+	}
+}