First commit... it kinda works, but it's missing most things

.gitignore

@@ -0,0 +1,8 @@
+.vscode/
+bin/
+obj/
+.vs/
+*.csproj.user
+launchSettings.json
+.idea/
+.DS_Store

Helpers/CryptoHelper.cs

@@ -0,0 +1,31 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace ImportDataAsRelay.Helpers;
+
+public static class CryptoHelper
+{
+    public static string GetSHA256Digest(string content)
+    {
+        using var sha256 = SHA256.Create();
+        var hashValue = sha256.ComputeHash(Encoding.UTF8.GetBytes(content));
+        return Convert.ToBase64String(hashValue);
+    }
+
+    public static string ConvertPemToBase64(string filename)
+    {
+        var rsa = RSA.Create();
+        rsa.ImportFromPem(File.ReadAllText(filename).ToCharArray());
+        return Convert.ToBase64String(rsa.ExportRSAPrivateKey());
+    }
+
+    public static string Sign(string stringToSign)
+    {
+        using var rsaProvider = new RSACryptoServiceProvider();
+        using var sha256 = SHA256.Create();
+        rsaProvider.ImportRSAPrivateKey(Convert.FromBase64String(Environment.GetEnvironmentVariable("PRIVATE_KEY")), out _);
+        
+        var signature = rsaProvider.SignData(Encoding.UTF8.GetBytes(stringToSign), sha256);
+        return Convert.ToBase64String(signature);
+    }
+}

Helpers/MastodonHelper.cs

@@ -0,0 +1,54 @@
+using System.Net.Http.Headers;
+
+namespace ImportDataAsRelay.Helpers;
+
+public static class MastodonHelper
+{
+    private static string TargetHost = Environment.GetEnvironmentVariable("TARGET_HOST");
+    private static string RelayHost = Environment.GetEnvironmentVariable("RELAY_HOST");
+    
+    public static async Task EnqueueStatusToFetch(string statusUrl)
+    {
+        var client = new HttpClient();
+
+        var date = DateTime.UtcNow;
+        
+        var content = $@"{{
+    ""@context"": ""https://www.w3.org/ns/activitystreams"",
+    ""actor"": ""https://{RelayHost}/actor"",
+    ""id"": ""https://{RelayHost}/activities/23af173e-e1fd-4283-93eb-514f1e5e5408"",
+    ""object"": ""{statusUrl}"",
+    ""to"": [
+        ""https://{RelayHost}/followers""
+    ],
+    ""type"": ""Announce""
+}}";
+        var digest = CryptoHelper.GetSHA256Digest(content);
+        var requestContent = new StringContent(content);
+
+        requestContent.Headers.Add("Digest", "SHA-256=" + digest);
+
+        var stringToSign = $"(request-target): post /inbox\ndate: {date.ToString("R")}\nhost: {TargetHost}\ndigest: SHA-256={digest}\ncontent-length: {content.Length}";
+        var signature = CryptoHelper.Sign(stringToSign);
+        requestContent.Headers.Add("Signature", $@"keyId=""https://{RelayHost}/actor#main-key"",algorithm=""rsa-sha256"",headers=""(request-target) date host digest content-length"",signature=""{signature}""");
+
+        requestContent.Headers.ContentType = new MediaTypeHeaderValue("application/activity+json");
+        client.DefaultRequestHeaders.Date = date;
+
+        var response = await client.PostAsync($"https://{TargetHost}/inbox", requestContent);
+        try
+        {
+            response.EnsureSuccessStatusCode();
+        }
+        catch (Exception e)
+        {
+            Console.WriteLine("Error: " + e.Message);
+            Console.WriteLine("Status code: " + response.StatusCode);
+
+            var body = await response.Content.ReadAsStringAsync();
+            Console.WriteLine("Response content: " + body);
+
+            throw;
+        }
+    }
+}

ImportDataAsRelay.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Jil" Version="2.17.0" />
+  </ItemGroup>
+
+</Project>

Program.cs

@@ -0,0 +1,53 @@
+using ImportDataAsRelay.Helpers;
+using Jil;
+
+var interestingTagsEverywhere = new[] { "dotnet", "csharp" };
+var sources = new Dictionary<string, string[]>
+{
+    ["hachyderm.io"] = new [] { "hachyderm" },
+    ["mastodon.social"] = Array.Empty<string>(),
+    ["dotnet.social"] = Array.Empty<string>(),
+};
+
+var client = new HttpClient();
+
+foreach (var (site, specificTags) in sources)
+{
+    var tags = specificTags.Concat(interestingTagsEverywhere).ToList();
+    foreach (var tag in tags)
+    {
+        Console.WriteLine($"Fetching tag #{tag} from {site}");
+        var response = await client.GetAsync($"https://{site}/tags/{tag}.json");
+        try
+        {
+            response.EnsureSuccessStatusCode();
+        }
+        catch (Exception e)
+        {
+            Console.WriteLine($"Error fetching tag, status code: {response.StatusCode}. Error: {e.Message}");
+            continue;
+        }
+
+        var json = await response.Content.ReadAsStringAsync();
+        var data = JSON.Deserialize<TagResponse>(json, Options.CamelCase);
+
+        foreach (var statusLink in data.OrderedItems)
+        {
+            Console.WriteLine($"Bringing in {statusLink}");
+            try
+            {
+                await MastodonHelper.EnqueueStatusToFetch(statusLink);
+                await Task.Delay(500);
+            }
+            catch (Exception e)
+            {
+                Console.WriteLine($"{e.Message}");
+            }
+        }
+    }
+}
+
+public class TagResponse
+{
+    public string[] OrderedItems { get; private set; }
+}