using System.Buffers.Binary;
using System.Security.Cryptography;

namespace SwBLL.OfflineRollout;

// ============================================================
// RolloutPackageCrypto — see IRolloutPackageCrypto.
//
// Container format, byte-for-byte identical to
// FrameworkBLL.Promotion.PromotionPackageCrypto (deliberate — one format across both directions
// this platform moves encrypted content, not two independently-maintained schemes):
//   [1 byte]   FormatVersion (=1)
//   [4 bytes]  BE length of the RSA-OAEP-SHA256-encrypted AES key (N)
//   [N bytes]  RSA-encrypted AES-256 key
//   [12 bytes] AES-GCM nonce
//   [16 bytes] AES-GCM tag
//   [remaining bytes] AES-GCM ciphertext
//
// The recipient's private key never touches this deployment — only the public half (registered
// via SW.MSWCLIENTDATABASEKEY) is ever seen here. Decrypt happens exclusively at the on-prem/BYOC
// install, in the standalone apply tool (Phase 4), which has its own independent implementation
// of the read/decrypt half — this class is deliberately encrypt-only.
// ============================================================
public class RolloutPackageCrypto : IRolloutPackageCrypto
{
    private const byte FormatVersion   = 1;
    private const int  AesKeySizeBytes = 32; // AES-256
    private const int  NonceSizeBytes  = 12; // AES-GCM standard nonce size
    private const int  TagSizeBytes    = 16; // AES-GCM standard tag size

    public Task<byte[]> EncryptPackageAsync(byte[] plaintext, string recipientPublicKeyPem, CancellationToken ct)
    {
        if (plaintext is null || plaintext.Length == 0)
            throw new ArgumentException("Plaintext package content is required.", nameof(plaintext));
        if (string.IsNullOrWhiteSpace(recipientPublicKeyPem))
            throw new ArgumentException("RecipientPublicKeyPem is required.", nameof(recipientPublicKeyPem));

        using var rsa = RSA.Create();
        rsa.ImportFromPem(recipientPublicKeyPem);

        var aesKey = RandomNumberGenerator.GetBytes(AesKeySizeBytes);
        var nonce  = RandomNumberGenerator.GetBytes(NonceSizeBytes);
        var ciphertext = new byte[plaintext.Length];
        var tag = new byte[TagSizeBytes];

        using (var aes = new AesGcm(aesKey, TagSizeBytes))
            aes.Encrypt(nonce, plaintext, ciphertext, tag);

        var encryptedAesKey = rsa.Encrypt(aesKey, RSAEncryptionPadding.OaepSHA256);

        using var output = new MemoryStream();
        using (var writer = new BinaryWriter(output))
        {
            writer.Write(FormatVersion);
            writer.Write(BinaryPrimitives.ReverseEndianness(encryptedAesKey.Length));
            writer.Write(encryptedAesKey);
            writer.Write(nonce);
            writer.Write(tag);
            writer.Write(ciphertext);
        }
        return Task.FromResult(output.ToArray());
    }
}