NdsDsiKeyScrambler.cs

using System.Text;
 
namespace NdsForge;

public static class NdsDsiKeyScrambler
{
    private static readonly byte[] ScramblerConstant =
        Convert.FromHexString("FFFEFB4E295902582A680F5F1A4F3E79");

    public static byte[] DeriveNormalKey(ReadOnlySpan<byte> keyX, ReadOnlySpan<byte> keyY)
    {
        ValidateKey(keyX, nameof(keyX));
        ValidateKey(keyY, nameof(keyY));
        Span<byte> sum = stackalloc byte[16];
        int carry = 0;
        for (int index = 0; index < sum.Length; index++)
        {
            int value = (keyX[index] ^ keyY[index]) + ScramblerConstant[^(index + 1)] + carry;
            sum[index] = (byte)value;
            carry = value >> 8;
        }
 
        return RotateLeft(sum, 42);
    }

    public static byte[] CreateModcryptKeyX(string gameCode)
    {
        ArgumentNullException.ThrowIfNull(gameCode);
        if (gameCode.Length != 4 || gameCode.Any(static value => value is < ' ' or > '~'))
        {
            throw new ArgumentException("A modcrypt game code must contain exactly four printable ASCII characters.", nameof(gameCode));
        }
 
        var keyX = new byte[16];
        "Nintendo"u8.CopyTo(keyX);
        Encoding.ASCII.GetBytes(gameCode, keyX.AsSpan(8, 4));
        for (int index = 0; index < 4; index++)
        {
            keyX[12 + index] = keyX[11 - index];
        }
 
        return keyX;
    }

    public static byte[] DeriveModcryptNormalKey(NdsHeader header)
    {
        ArgumentNullException.ThrowIfNull(header);
        NdsDsiHeader dsi = header.Dsi ??
            throw new ArgumentException("Secure modcrypt key derivation requires a DSi-family header.", nameof(header));
        return DeriveNormalKey(CreateModcryptKeyX(header.GameCode), dsi.Arm9iHmac.Span[..16]);
    }

    private static byte[] RotateLeft(ReadOnlySpan<byte> value, int bits)
    {
        int byteShift = bits / 8;
        int bitShift = bits % 8;
        var output = new byte[16];
        for (int index = 0; index < output.Length; index++)
        {
            int lowIndex = (index - byteShift) & 15;
            int highIndex = (lowIndex - 1) & 15;
            output[index] = (byte)((value[lowIndex] << bitShift) | (value[highIndex] >> (8 - bitShift)));
        }
 
        return output;
    }

    private static void ValidateKey(ReadOnlySpan<byte> value, string parameterName)
    {
        if (value.Length != 16)
        {
            throw new ArgumentException("A DSi scrambler key component must contain exactly sixteen bytes.", parameterName);
        }
    }
}