Update: Full E2EE + Scripts

RelayServer/Services/E2EeHelper.cs

@@ -0,0 +1,78 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace RelayServer.Services;
+
+public static class E2EeHelper
+{
+    public static (string publicKey, string privateKey) GenerateRsaKeyPair()
+    {
+        using var rsa = RSA.Create(2048);
+
+        return (
+            Convert.ToBase64String(rsa.ExportSubjectPublicKeyInfo()),
+            Convert.ToBase64String(rsa.ExportPkcs8PrivateKey())
+        );
+    }
+
+    public static EncryptedPayload EncryptForRecipient(string plainText, string recipientPublicKeyBase64)
+    {
+        byte[] aesKey = RandomNumberGenerator.GetBytes(32);
+        byte[] nonce = RandomNumberGenerator.GetBytes(12);
+        byte[] plainBytes = Encoding.UTF8.GetBytes(plainText);
+        byte[] cipherBytes = new byte[plainBytes.Length];
+        byte[] tag = new byte[16];
+
+        using (var aes = new AesGcm(aesKey, 16))
+        {
+            aes.Encrypt(nonce, plainBytes, cipherBytes, tag);
+        }
+
+        byte[] encryptedKey;
+        using (var rsa = RSA.Create())
+        {
+            rsa.ImportSubjectPublicKeyInfo(Convert.FromBase64String(recipientPublicKeyBase64), out _);
+            encryptedKey = rsa.Encrypt(aesKey, RSAEncryptionPadding.OaepSHA256);
+        }
+
+        return new EncryptedPayload
+        {
+            CipherText = Convert.ToBase64String(cipherBytes),
+            Nonce = Convert.ToBase64String(nonce),
+            Tag = Convert.ToBase64String(tag),
+            EncryptedKey = Convert.ToBase64String(encryptedKey)
+        };
+    }
+
+    public static string DecryptForRecipient(EncryptedPayload payload, string recipientPrivateKeyBase64)
+    {
+        byte[] aesKey;
+        using (var rsa = RSA.Create())
+        {
+            rsa.ImportPkcs8PrivateKey(Convert.FromBase64String(recipientPrivateKeyBase64), out _);
+            aesKey = rsa.Decrypt(Convert.FromBase64String(payload.EncryptedKey), RSAEncryptionPadding.OaepSHA256);
+        }
+
+        byte[] plainBytes = new byte[Convert.FromBase64String(payload.CipherText).Length];
+
+        using (var aes = new AesGcm(aesKey, 16))
+        {
+            aes.Decrypt(
+                Convert.FromBase64String(payload.Nonce),
+                Convert.FromBase64String(payload.CipherText),
+                Convert.FromBase64String(payload.Tag),
+                plainBytes
+            );
+        }
+
+        return Encoding.UTF8.GetString(plainBytes);
+    }
+}
+
+public class EncryptedPayload
+{
+    public required string CipherText { get; set; }
+    public required string Nonce { get; set; }
+    public required string Tag { get; set; }
+    public required string EncryptedKey { get; set; }
+}