ANTOOZA-BACKEND/Infrastructure/Kyc/Nice/NiceCryptoInvariants.cs

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

namespace Infrastructure.Kyc.Nice;

/// <summary>
/// <see cref="NiceCrypto"/> 의 invariants self-check.
///
/// dev 환경 부팅 시 1회 실행해서 PBKDF2 출력 길이, AES-GCM 라운드트립, HMAC 라운드트립이
/// 깨지지 않았음을 확인. 라이브러리/런타임 업데이트로 silently 동작이 바뀌는 회귀를 방지.
///
/// 진짜 NICE 호환성 검증은 별도 phase (xUnit + NICE 공식 PHP/Node.js sample 의 골든벡터) 에서.
/// </summary>
public static class NiceCryptoInvariants
{
    /// <summary>
    /// 실패 시 <see cref="CryptographicException"/> throw.
    /// </summary>
    public static void EnsureOrThrow()
    {
        EnsureBase64UrlRoundTrip();
        EnsureDeriveKeysShape();
        EnsureHmacRoundTrip();
        EnsureAesGcmRoundTrip();
    }

    private static void EnsureBase64UrlRoundTrip()
    {
        // 패딩 길이 0/1/2 byte 모두 라운드트립
        var samples = new[]
        {
            new byte[] { 0x00 },
            new byte[] { 0x01, 0x02 },
            new byte[] { 0xFF, 0xFE, 0xFD },
            Encoding.UTF8.GetBytes("hello world")
        };
        foreach (var s in samples)
        {
            var encoded = NiceCrypto.Base64UrlEncode(s);
            if (encoded.Contains('+') || encoded.Contains('/') || encoded.EndsWith('='))
            {
                throw new CryptographicException($"Base64UrlEncode 가 URL-safe + no-padding 규칙을 위반: '{encoded}'");
            }
            var decoded = NiceCrypto.Base64UrlDecode(encoded);
            if (!decoded.AsSpan().SequenceEqual(s))
            {
                throw new CryptographicException("Base64Url 라운드트립 실패.");
            }
        }
    }

    private static void EnsureDeriveKeysShape()
    {
        // 임의 vector — NICE 호환 vector 아님. 길이/결정성만 검증.
        var (aesKey, hmacKey) = NiceCrypto.DeriveKeys("ticket-self-check", "transaction-id-self-check", 1000);
        if (aesKey.Length != 32)
        {
            throw new CryptographicException($"DeriveKeys: AES key length = {aesKey.Length} (expected 32).");
        }
        if (hmacKey.Length != 32)
        {
            throw new CryptographicException($"DeriveKeys: HMAC key length = {hmacKey.Length} (expected 32).");
        }
        // 같은 입력 → 같은 출력 (deterministic)
        var (aesKey2, hmacKey2) = NiceCrypto.DeriveKeys("ticket-self-check", "transaction-id-self-check", 1000);
        if (!aesKey.AsSpan().SequenceEqual(aesKey2) || !hmacKey.AsSpan().SequenceEqual(hmacKey2))
        {
            throw new CryptographicException("DeriveKeys 가 결정론적이지 않습니다.");
        }
    }

    private static void EnsureHmacRoundTrip()
    {
        var hmacKey = new byte[32];
        RandomNumberGenerator.Fill(hmacKey);
        const string encData = "AbCdEfGhIjKlMnOpQrStUvWxYz0123456789";
        var integrity = NiceCrypto.ComputeIntegrity(hmacKey, encData);
        if (!NiceCrypto.VerifyIntegrity(hmacKey, encData, integrity))
        {
            throw new CryptographicException("HMAC ComputeIntegrity ↔ VerifyIntegrity 라운드트립 실패.");
        }
        // 다른 키로는 fail
        var otherKey = new byte[32];
        RandomNumberGenerator.Fill(otherKey);
        if (NiceCrypto.VerifyIntegrity(otherKey, encData, integrity))
        {
            throw new CryptographicException("HMAC VerifyIntegrity 가 잘못된 키를 통과시켰습니다.");
        }
    }

    private static void EnsureAesGcmRoundTrip()
    {
        var aesKey = new byte[32];
        RandomNumberGenerator.Fill(aesKey);

        const string plaintext = "{\"name\":\"홍길동\",\"birthdate\":\"19800101\",\"gender\":\"1\"}";

        // NICE 와 동일한 16 byte IV + ciphertext+tag 레이아웃 (BouncyCastle 기반).
        var encData = NiceCrypto.EncryptAesGcm(aesKey, plaintext);
        var decrypted = NiceCrypto.DecryptAesGcm(aesKey, encData);
        if (decrypted != plaintext)
        {
            throw new CryptographicException($"AES-GCM 라운드트립 실패. expected='{plaintext}', got='{decrypted}'");
        }

        // 위변조 검출: 임의 1 byte 변경 시 throw
        var tamperedBytes = NiceCrypto.Base64UrlDecode(encData);
        tamperedBytes[tamperedBytes.Length - 1] ^= 0x01;
        var tampered = NiceCrypto.Base64UrlEncode(tamperedBytes);
        try
        {
            NiceCrypto.DecryptAesGcm(aesKey, tampered);
            throw new CryptographicException("AES-GCM 위변조된 데이터를 무사 통과시켰습니다.");
        }
        catch (Org.BouncyCastle.Crypto.InvalidCipherTextException)
        {
            // 정상 — GCM tag 가 위변조 감지
        }
    }
}