SPMS_API/SPMS.Infrastructure/Security/E2EEService.cs

using Microsoft.Extensions.Configuration;
using SPMS.Application.Interfaces;
using SPMS.Domain.Common;
using SPMS.Domain.Exceptions;

namespace SPMS.Infrastructure.Security;

public class E2EEService : IE2EEService
{
    private const int RsaEncryptedKeySize = 256;
    private const int AesIvSize = 16;
    private const int MinPayloadSize = RsaEncryptedKeySize + AesIvSize;

    private readonly string _privateKeyPem;

    public E2EEService(IConfiguration configuration)
    {
        var keyPath = configuration["E2EE:PrivateKeyPath"];
        if (!string.IsNullOrEmpty(keyPath) && File.Exists(keyPath))
        {
            _privateKeyPem = File.ReadAllText(keyPath);
        }
        else
        {
            _privateKeyPem = configuration["E2EE:PrivateKeyPem"] ?? string.Empty;
        }
    }

    public E2EEDecryptResult DecryptRequest(byte[] encryptedPayload)
    {
        if (string.IsNullOrEmpty(_privateKeyPem))
            throw SpmsException.BadRequest("E2EE 키가 설정되지 않았습니다.");

        if (encryptedPayload.Length < MinPayloadSize)
            throw SpmsException.BadRequest("잘못된 암호화 페이로드입니다.");

        var encryptedKey = encryptedPayload[..RsaEncryptedKeySize];
        var iv = encryptedPayload[RsaEncryptedKeySize..(RsaEncryptedKeySize + AesIvSize)];
        var encryptedBody = encryptedPayload[(RsaEncryptedKeySize + AesIvSize)..];

        var aesKey = RsaEncryption.Decrypt(encryptedKey, _privateKeyPem);

        var decryptedBody = encryptedBody.Length > 0
            ? AesEncryption.Decrypt(encryptedBody, aesKey, iv)
            : [];

        return new E2EEDecryptResult
        {
            DecryptedBody = decryptedBody,
            AesKey = aesKey
        };
    }

    public byte[] EncryptResponse(byte[] plainResponse, byte[] aesKey)
    {
        var iv = AesEncryption.GenerateIv();
        var encrypted = AesEncryption.Encrypt(plainResponse, aesKey, iv);

        var result = new byte[iv.Length + encrypted.Length];
        Buffer.BlockCopy(iv, 0, result, 0, iv.Length);
        Buffer.BlockCopy(encrypted, 0, result, iv.Length, encrypted.Length);
        return result;
    }
}