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

namespace CareFix.Api.Security;

/// <summary>AES-256-GCM encryption for hospital SQL passwords. Layout: nonce(12) | tag(16) | ciphertext.</summary>
public sealed class CredentialVault
{
    private readonly byte[] _key;

    public CredentialVault(IOptions<CareFixOptions> options)
    {
        try { _key = Convert.FromBase64String(options.Value.MasterKey); }
        catch (FormatException) { throw new InvalidOperationException("CareFix:MasterKey must be base64 (32 random bytes)."); }
        if (_key.Length != 32) throw new InvalidOperationException("CareFix:MasterKey must decode to exactly 32 bytes.");
    }

    public byte[] Encrypt(string plain)
    {
        var nonce = RandomNumberGenerator.GetBytes(12);
        var pt = Encoding.UTF8.GetBytes(plain);
        var ct = new byte[pt.Length];
        var tag = new byte[16];
        using var aes = new AesGcm(_key, 16);
        aes.Encrypt(nonce, pt, ct, tag);
        return [.. nonce, .. tag, .. ct];
    }

    public string Decrypt(byte[] blob)
    {
        if (blob.Length < 28) throw new CryptographicException("Stored credential is corrupt.");
        var pt = new byte[blob.Length - 28];
        using var aes = new AesGcm(_key, 16);
        aes.Decrypt(blob.AsSpan(0, 12), blob.AsSpan(28), blob.AsSpan(12, 16), pt);
        return Encoding.UTF8.GetString(pt);
    }
}