Knots/Knots/Services/EncryptionService.cs

using System.Security.Cryptography;
using System.Text;

namespace Knots.Services;

public class EncryptionService
{
    private const int NonceSize = 12; // AesGcm.NonceByteSizes.MaxSize
    private const int TagSize = 16;   // AesGcm.TagByteSizes.MaxSize

    // Génère une clé AES-256 (32 octets) encodée en Base64
    public string GenerateKey()
        => Convert.ToBase64String(RandomNumberGenerator.GetBytes(32));

    // Chiffre → renvoie Base64(nonce + tag + ciphertext)
    public string Encrypt(string plainText, string base64Key)
    {
        byte[] key = Convert.FromBase64String(base64Key);
        byte[] plain = Encoding.UTF8.GetBytes(plainText);

        byte[] nonce = RandomNumberGenerator.GetBytes(NonceSize);
        byte[] cipher = new byte[plain.Length];
        byte[] tag = new byte[TagSize];

        using AesGcm aes = new(key, TagSize);
        aes.Encrypt(nonce, plain, cipher, tag);

        byte[] result = new byte[NonceSize + TagSize + cipher.Length];
        Buffer.BlockCopy(nonce, 0, result, 0, NonceSize);
        Buffer.BlockCopy(tag, 0, result, NonceSize, TagSize);
        Buffer.BlockCopy(cipher, 0, result, NonceSize + TagSize, cipher.Length);

        return Convert.ToBase64String(result);
    }

    // Déchiffre Base64(nonce + tag + ciphertext)
    public string Decrypt(string base64Cipher, string base64Key)
    {
        byte[] key = Convert.FromBase64String(base64Key);
        byte[] data = Convert.FromBase64String(base64Cipher);

        byte[] nonce = new byte[NonceSize];
        byte[] tag = new byte[TagSize];
        byte[] cipher = new byte[data.Length - NonceSize - TagSize];

        Buffer.BlockCopy(data, 0, nonce, 0, NonceSize);
        Buffer.BlockCopy(data, NonceSize, tag, 0, TagSize);
        Buffer.BlockCopy(data, NonceSize + TagSize, cipher, 0, cipher.Length);

        byte[] plain = new byte[cipher.Length];
        using AesGcm aes = new(key, TagSize);
        aes.Decrypt(nonce, cipher, tag, plain);

        return Encoding.UTF8.GetString(plain);
    }
}