Update: Added RelayServer Logic

RelayServer/Services/ChannelCryptoService.cs

@@ -0,0 +1,44 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace RelayServer.Services;
+
+public sealed class ChannelCryptoService
+{
+    public string GenerateKey()
+    {
+        return Convert.ToBase64String(RandomNumberGenerator.GetBytes(32));
+    }
+
+    public (string cipherText, string nonce, string tag) Encrypt(string plainText, string keyBase64)
+    {
+        var key = Convert.FromBase64String(keyBase64);
+        var nonce = RandomNumberGenerator.GetBytes(12);
+        var plainBytes = Encoding.UTF8.GetBytes(plainText);
+        var cipherBytes = new byte[plainBytes.Length];
+        var tag = new byte[16];
+
+        using var aes = new AesGcm(key, 16);
+        aes.Encrypt(nonce, plainBytes, cipherBytes, tag);
+
+        return (
+            Convert.ToBase64String(cipherBytes),
+            Convert.ToBase64String(nonce),
+            Convert.ToBase64String(tag)
+        );
+    }
+
+    public string Decrypt(string cipherTextBase64, string nonceBase64, string tagBase64, string keyBase64)
+    {
+        var key = Convert.FromBase64String(keyBase64);
+        var nonce = Convert.FromBase64String(nonceBase64);
+        var tag = Convert.FromBase64String(tagBase64);
+        var cipherBytes = Convert.FromBase64String(cipherTextBase64);
+        var plainBytes = new byte[cipherBytes.Length];
+
+        using var aes = new AesGcm(key, 16);
+        aes.Decrypt(nonce, cipherBytes, tag, plainBytes);
+
+        return Encoding.UTF8.GetString(plainBytes);
+    }
+}