NdsDsiRsaPublicKey.cs

using System.Security.Cryptography;
 
namespace NdsForge;

public sealed class NdsDsiRsaPublicKey
{
    private readonly byte[] _modulus;
    private readonly byte[] _exponent;

    public NdsDsiRsaPublicKey(ReadOnlySpan<byte> modulus, ReadOnlySpan<byte> exponent)
    {
        if (modulus.Length != 128 || exponent.IsEmpty)
        {
            throw new ArgumentException("A DSi RSA key requires a 128-byte modulus and a non-empty exponent.");
        }
 
        _modulus = modulus.ToArray();
        _exponent = exponent.ToArray();
    }

    public static NdsDsiRsaPublicKey FromRsa(RSA rsa)
    {
        ArgumentNullException.ThrowIfNull(rsa);
        if (rsa.KeySize != 1024)
        {
            throw new ArgumentException("DSi header signatures require a 1024-bit RSA key.", nameof(rsa));
        }
 
        RSAParameters parameters = rsa.ExportParameters(includePrivateParameters: false);
        return new(parameters.Modulus!, parameters.Exponent!);
    }

    public ReadOnlyMemory<byte> Modulus => _modulus.ToArray();

    public ReadOnlyMemory<byte> Exponent => _exponent.ToArray();

    public bool VerifyHeader(ReadOnlySpan<byte> signedHeader, ReadOnlySpan<byte> signature)
    {
        ValidateBuffers(signedHeader, signature);
        using RSA rsa = RSA.Create();
        rsa.ImportParameters(new() { Modulus = _modulus, Exponent = _exponent });
#pragma warning disable CA5350, CA5387 // DSi headers mandate RSA-SHA1 with PKCS#1 v1.5; this type is format-specific.
        return rsa.VerifyData(signedHeader, signature, HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);
#pragma warning restore CA5350, CA5387
    }

    private static void ValidateBuffers(ReadOnlySpan<byte> signedHeader, ReadOnlySpan<byte> signature)
    {
        if (signedHeader.Length != 0xE00 || signature.Length != 128)
        {
            throw new ArgumentException("DSi RSA verification requires a 0xE00-byte header prefix and 128-byte signature.");
        }
    }
}