using System.Security.Cryptography;
using System.Text;
using Microsoft.Extensions.Options;

namespace TeamUp.Modules.Integrations.Security;

internal sealed class EncryptionOptions
{
    public const string SectionName = "Encryption";

    /// <summary>Deployment master secret. A 32-byte AES key is derived from it (SHA-256).</summary>
    public string MasterKey { get; set; } = string.Empty;
}

internal interface ISecretProtector
{
    string Protect(string plaintext);

    string Unprotect(string protectedValue);
}

/// <summary>
/// AES-256-GCM authenticated encryption with a key derived from the deployment master secret.
/// Output blob = nonce(12) ‖ tag(16) ‖ ciphertext, base64-encoded.
/// </summary>
internal sealed class AesGcmSecretProtector : ISecretProtector
{
    private const int NonceSize = 12;
    private const int TagSize = 16;
    private readonly byte[] _key;

    public AesGcmSecretProtector(IOptions<EncryptionOptions> options)
    {
        var masterKey = options.Value.MasterKey;
        if (string.IsNullOrWhiteSpace(masterKey))
        {
            throw new InvalidOperationException("Missing 'Encryption:MasterKey'.");
        }

        _key = SHA256.HashData(Encoding.UTF8.GetBytes(masterKey));
    }

    public string Protect(string plaintext)
    {
        var plain = Encoding.UTF8.GetBytes(plaintext);
        var nonce = RandomNumberGenerator.GetBytes(NonceSize);
        var cipher = new byte[plain.Length];
        var tag = new byte[TagSize];

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

        var blob = new byte[NonceSize + TagSize + cipher.Length];
        Buffer.BlockCopy(nonce, 0, blob, 0, NonceSize);
        Buffer.BlockCopy(tag, 0, blob, NonceSize, TagSize);
        Buffer.BlockCopy(cipher, 0, blob, NonceSize + TagSize, cipher.Length);
        return Convert.ToBase64String(blob);
    }

    public string Unprotect(string protectedValue)
    {
        var blob = Convert.FromBase64String(protectedValue);
        var nonce = blob.AsSpan(0, NonceSize);
        var tag = blob.AsSpan(NonceSize, TagSize);
        var cipher = blob.AsSpan(NonceSize + TagSize);
        var plain = new byte[cipher.Length];

        using var aes = new AesGcm(_key, TagSize);
        aes.Decrypt(nonce, cipher, tag, plain);
        return Encoding.UTF8.GetString(plain);
    }
}