Add AgreementPage and EncryptionHelper for transaction handling

Signed-off-by: Dmitrii <computer@yawaflua.tech>

Took 24 seconds

backend/SpMega.Backend/Services/EncryptionHelper.cs

@@ -0,0 +1,114 @@
+using System;
+using System.IO;
+using System.Security.Cryptography;
+using System.Text;
+
+namespace SpMega.Backend.Services;
+
+public static class EncryptionHelper
+{
+    private static byte[]? _key;
+
+    /// <summary>
+    /// Initializes the encryption key. The key will be derived using SHA-256 from the provided secret string
+    /// to ensure it is exactly 32 bytes (256 bits) long.
+    /// </summary>
+    public static void Initialize(string secretKey)
+    {
+        if (string.IsNullOrWhiteSpace(secretKey))
+        {
+            throw new ArgumentException("Encryption key cannot be null or empty.", nameof(secretKey));
+        }
+
+        _key = SHA256.HashData(Encoding.UTF8.GetBytes(secretKey));
+    }
+
+    /// <summary>
+    /// Encrypts the plain text using AES-256-CBC.
+    /// A unique Initialization Vector (IV) is generated for each encryption and stored alongside the ciphertext.
+    /// </summary>
+    public static string Encrypt(string plainText)
+    {
+        if (_key == null)
+        {
+            throw new InvalidOperationException("EncryptionHelper has not been initialized. Call Initialize() first.");
+        }
+
+        if (string.IsNullOrEmpty(plainText))
+        {
+            return plainText;
+        }
+
+        using var aes = Aes.Create();
+        aes.Key = _key;
+        aes.GenerateIV();
+        var iv = aes.IV;
+
+        using var encryptor = aes.CreateEncryptor(aes.Key, iv);
+        using var ms = new MemoryStream();
+        
+        // Write the IV to the beginning of the stream so it is stored alongside the ciphertext
+        ms.Write(iv, 0, iv.Length);
+
+        using (var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
+        using (var writer = new StreamWriter(cs, Encoding.UTF8))
+        {
+            writer.Write(plainText);
+        }
+
+        return Convert.ToBase64String(ms.ToArray());
+    }
+
+    /// <summary>
+    /// Decrypts the cipher text using AES-256-CBC.
+    /// Reconstructs the Initialization Vector (IV) from the beginning of the ciphertext.
+    /// Falls back to returning the input string if decryption fails (e.g., for legacy plaintext tokens).
+    /// </summary>
+    public static string Decrypt(string cipherText)
+    {
+        if (_key == null)
+        {
+            throw new InvalidOperationException("EncryptionHelper has not been initialized. Call Initialize() first.");
+        }
+
+        if (string.IsNullOrEmpty(cipherText))
+        {
+            return cipherText;
+        }
+
+        try
+        {
+            var fullCipher = Convert.FromBase64String(cipherText);
+
+            using var aes = Aes.Create();
+            aes.Key = _key;
+
+            var ivLength = aes.BlockSize / 8;
+            if (fullCipher.Length < ivLength)
+            {
+                // Too short to be an encrypted payload, return original
+                return cipherText;
+            }
+
+            var iv = new byte[ivLength];
+            var cipher = new byte[fullCipher.Length - ivLength];
+
+            Buffer.BlockCopy(fullCipher, 0, iv, 0, ivLength);
+            Buffer.BlockCopy(fullCipher, ivLength, cipher, 0, cipher.Length);
+
+            aes.IV = iv;
+
+            using var decryptor = aes.CreateDecryptor(aes.Key, aes.IV);
+            using var ms = new MemoryStream(cipher);
+            using var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read);
+            using var reader = new StreamReader(cs, Encoding.UTF8);
+
+            return reader.ReadToEnd();
+        }
+        catch (Exception ex) when (ex is CryptographicException or FormatException)
+        {
+            // Fallback for legacy unencrypted tokens
+            return cipherText;
+        }
+    }
+}