Basic (working!) roundtrip LD signature implementation

Iceshrimp.Backend/Controllers/AuthController.cs

@@ -40,7 +40,7 @@ public class AuthController(DatabaseContext db) : Controller {
 			UserId    = user.Id,
 			Active    = !profile.TwoFactorEnabled,
 			CreatedAt = new DateTime(),
-			Token     = IdHelpers.GenerateRandomString(32)
+			Token     = CryptographyHelpers.GenerateRandomString(32)
 		});
 
 		var session = res.Entity;

Iceshrimp.Backend/Core/Federation/ActivityStreams/LDHelpers.cs

@@ -56,4 +56,5 @@ public static class LDHelpers {
 	public static JObject? Compact(JToken?      json) => JsonLdProcessor.Compact(json, DefaultContext, Options);
 	public static JArray?  Expand(JToken?       json) => JsonLdProcessor.Expand(json, Options);
 	public static string   Canonicalize(JArray  json) => JsonLdProcessor.Canonicalize(json);
+	public static string   Canonicalize(JObject json) => JsonLdProcessor.Canonicalize([json]);
 }

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

@@ -0,0 +1,12 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace Iceshrimp.Backend.Core.Federation.Cryptography;
+
+public static class DigestHelpers {
+	public static async Task<string> Sha256Digest(string input) {
+		var bytes = Encoding.UTF8.GetBytes(input);
+		var data  = await SHA256.HashDataAsync(new MemoryStream(bytes));
+		return Convert.ToHexString(data).ToLowerInvariant();
+	}
+}

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

@@ -0,0 +1,107 @@
+using System.Security.Cryptography;
+using System.Text;
+using Iceshrimp.Backend.Core.Federation.ActivityStreams;
+using Iceshrimp.Backend.Core.Helpers;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+using J = Newtonsoft.Json.JsonPropertyAttribute;
+using JC = Newtonsoft.Json.JsonConverterAttribute;
+using VC = Iceshrimp.Backend.Core.Federation.ActivityStreams.Types.ValueObjectConverter;
+
+namespace Iceshrimp.Backend.Core.Federation.Cryptography;
+
+public static class LdSignature {
+	public static async Task<bool> Verify(JArray activity, string key) {
+		foreach (var act in activity) {
+			var options = act["https://w3id.org/security#signature"];
+			if (options?.ToObject<SignatureOptions[]>() is not { Length: 1 } signatures) return false;
+			var signature = signatures[0];
+			if (signature.Type is not ["_:RsaSignature2017"]) return false;
+			if (signature.Signature is null) return false;
+
+			var signatureData = await GetSignatureData(act, options);
+			if (signatureData is null) return false;
+
+			var rsa = RSA.Create();
+			rsa.ImportFromPem(key);
+			var verify = rsa.VerifyData(signatureData, Convert.FromBase64String(signature.Signature),
+			                            HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+			if (!verify) return false;
+		}
+
+		return true;
+	}
+
+	public static async Task<JToken> Sign(JToken data, string key, string? creator) {
+		var options = new SignatureOptions {
+			Created = DateTime.Now,
+			Creator = creator,
+			Nonce   = CryptographyHelpers.GenerateRandomHexString(16),
+			Type    = ["_:RsaSignature2017"],
+			Domain  = null,
+		};
+
+		var signatureData = await GetSignatureData(data, options);
+		if (signatureData == null) throw new NullReferenceException("Signature data must not be null");
+
+		var rsa = RSA.Create();
+		rsa.ImportFromPem(key);
+		var signatureBytes = rsa.SignData(signatureData, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+		var signature = Convert.ToBase64String(signatureBytes);
+
+		options.Signature = signature;
+		
+		if (data is not JObject obj) throw new Exception();
+		obj.Add("https://w3id.org/security#signature", JToken.FromObject(options));
+		
+		return obj;
+	}
+
+	private static Task<byte[]?> GetSignatureData(JToken data, SignatureOptions options) =>
+		GetSignatureData(data, LDHelpers.Expand(JObject.FromObject(options))!);
+
+	private static async Task<byte[]?> GetSignatureData(JToken data, JToken options) {
+		if (data is not JObject inputData) return null;
+		if (options is not JArray { Count: 1 } inputOptionsArray) return null;
+		if (inputOptionsArray[0] is not JObject inputOptions) return null;
+
+		inputOptions.Remove("@id");
+		inputOptions.Remove("@type");
+		inputOptions.Remove("https://w3id.org/security#signatureValue");
+
+		inputData.Remove("https://w3id.org/security#signature");
+
+		var canonicalData    = LDHelpers.Canonicalize(inputData);
+		var canonicalOptions = LDHelpers.Canonicalize(inputOptions);
+
+		var dataHash    = await DigestHelpers.Sha256Digest(canonicalData);
+		var optionsHash = await DigestHelpers.Sha256Digest(canonicalOptions);
+
+		return Encoding.UTF8.GetBytes(optionsHash + dataHash);
+	}
+
+	private class SignatureOptions {
+		[J("@type")]    public required List<string> Type    { get; set; }
+
+		[J("https://w3id.org/security#signatureValue")]
+		[JC(typeof(VC))]
+		public string? Signature { get; set; }
+
+		[J("http://purl.org/dc/terms/creator", NullValueHandling = NullValueHandling.Ignore)]
+		[JC(typeof(VC))]
+		public string? Creator { get; set; }
+
+		[J("https://w3id.org/security#nonce", NullValueHandling = NullValueHandling.Ignore)]
+		[JC(typeof(VC))]
+		public string? Nonce { get; set; }
+
+		[J("https://w3id.org/security#domain", NullValueHandling = NullValueHandling.Ignore)]
+		[JC(typeof(VC))]
+		public string? Domain { get; set; }
+
+		[J("https://w3id.org/security#created", NullValueHandling = NullValueHandling.Ignore)]
+		[JC(typeof(VC))]
+		//FIXME: is this valid? it should output datetime in ISO format
+		public DateTime? Created { get; set; }
+	}
+}

Iceshrimp.Backend/Core/Helpers/CryptographyHelpers.cs

@@ -0,0 +1,11 @@
+using System.Security.Cryptography;
+
+namespace Iceshrimp.Backend.Core.Helpers;
+
+public static class CryptographyHelpers {
+	public static string GenerateRandomString(int length) =>
+		Convert.ToBase64String(RandomNumberGenerator.GetBytes(length));
+
+	public static string GenerateRandomHexString(int length) =>
+		Convert.ToHexString(RandomNumberGenerator.GetBytes(length)).ToLowerInvariant();
+}

Iceshrimp.Backend/Core/Helpers/IdHelpers.cs

@@ -13,8 +13,4 @@ public static class IdHelpers {
 		var timestamp = time.ToBase36().PadLeft(8, '0');
 		return timestamp + cuid;
 	}
-
-	public static string GenerateRandomString(int length) {
-		return Convert.ToBase64String(RandomNumberGenerator.GetBytes(length));
-	}
 }