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

namespace WxAgent.Core;

public static class SqlCipherPageVerifier
{
    public const int PageSize = 4096;
    public const int SaltSize = 16;
    public const int KeySize = 32;
    public const int HmacSize = 64;

    public static bool Verify(ReadOnlySpan<byte> page, ReadOnlySpan<byte> key, uint pageNumber = 1)
    {
        if (page.Length != PageSize || key.Length != KeySize || pageNumber == 0)
        {
            return false;
        }

        Span<byte> hmacSalt = stackalloc byte[SaltSize];
        for (var index = 0; index < SaltSize; index++)
        {
            hmacSalt[index] = (byte)(page[index] ^ 0x3a);
        }

        Span<byte> hmacKey = stackalloc byte[KeySize];
        Rfc2898DeriveBytes.Pbkdf2(key, hmacSalt, hmacKey, 2, HashAlgorithmName.SHA512);

        var authenticatedLength = PageSize - SaltSize - HmacSize;
        var authenticated = new byte[authenticatedLength + sizeof(uint)];
        page.Slice(SaltSize, authenticatedLength).CopyTo(authenticated);
        BinaryPrimitives.WriteUInt32LittleEndian(authenticated.AsSpan(authenticatedLength), pageNumber);

        var computed = HMACSHA512.HashData(hmacKey, authenticated);
        var valid = CryptographicOperations.FixedTimeEquals(computed, page[^HmacSize..]);
        CryptographicOperations.ZeroMemory(hmacKey);
        CryptographicOperations.ZeroMemory(authenticated);
        CryptographicOperations.ZeroMemory(computed);
        return valid;
    }

    public static bool VerifyHexKey(ReadOnlySpan<byte> page, string hexKey, uint pageNumber = 1)
    {
        if (hexKey.Length != KeySize * 2)
        {
            return false;
        }

        byte[] key;
        try
        {
            key = Convert.FromHexString(hexKey);
        }
        catch (FormatException)
        {
            return false;
        }

        try
        {
            return Verify(page, key, pageNumber);
        }
        finally
        {
            CryptographicOperations.ZeroMemory(key);
        }
    }
}