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

namespace Aegis.Repository.Crypto;

public class ServerSecretLabelKeyProvider : ILabelKeyProvider
{
    private readonly byte[] _serverSecret;

    public ServerSecretLabelKeyProvider(byte[] serverSecret)
    {
        if (serverSecret.Length < 32)
            throw new ArgumentException("Server secret deve ter pelo menos 32 bytes.");

        _serverSecret = serverSecret;
    }

    public byte[] GetLabelKey(string lkKid, int lkVersion)
    {
        // HKDF simples via HMACSHA256 (ok para derivação)
        // info amarra ao kid+version
        var info = Encoding.UTF8.GetBytes($"aegis:lk:{lkKid}:v{lkVersion}");
        return HkdfSha256(_serverSecret, salt: null, info, 32);
    }

    private static byte[] HkdfSha256(byte[] ikm, byte[]? salt, byte[] info, int len)
    {
        salt ??= new byte[32]; // salt zero (ok se ikm for segredo forte)
        using var hmac = new HMACSHA256(salt);
        var prk = hmac.ComputeHash(ikm);

        var okm = new byte[len];
        byte[] t = Array.Empty<byte>();
        var pos = 0;
        byte counter = 1;

        using var hmac2 = new HMACSHA256(prk);
        while (pos < len)
        {
            var input = new byte[t.Length + info.Length + 1];
            Buffer.BlockCopy(t, 0, input, 0, t.Length);
            Buffer.BlockCopy(info, 0, input, t.Length, info.Length);
            input[^1] = counter;

            t = hmac2.ComputeHash(input);

            var take = Math.Min(t.Length, len - pos);
            Buffer.BlockCopy(t, 0, okm, pos, take);

            pos += take;
            counter++;
        }

        CryptographicOperations.ZeroMemory(prk);
        CryptographicOperations.ZeroMemory(t);
        return okm;
    }
}