using System.Numerics;
using System.Security.Cryptography;

namespace Encryption;

public static class RSA
{
    // Hash funciton to use. Only cryptographic hash functions can be used.
    private static Func<byte[], byte[]> _hash = (data) => SHA256.HashData(data);
    
    /// <summary>
    /// Encrypt message with (message^e) mod n
    /// </summary>
    /// <param name="publicKey"></param>
    /// <param name="data"></param>
    /// <returns></returns>
    public static byte[] Encrypt(RsaPublicKey publicKey, byte[] data)
    {
        var dataAsBigint = new BigInteger(data);
        return BigInteger.ModPow(dataAsBigint, publicKey.E, publicKey.N).ToByteArray();
    }

    /// <summary>
    /// Decrypt cipher with (cipher^d) mod n
    /// </summary>
    /// <param name="publicKey"></param>
    /// <param name="data"></param>
    /// <returns></returns>
    public static byte[] Decrypt(RsaPrivateKey privateKey, byte[] data)
    {
        var dataAsBigint = new BigInteger(data);
        return BigInteger.ModPow(dataAsBigint, privateKey.D, privateKey.N).ToByteArray();
    }

    public static byte[] Sign(RsaPrivateKey privateKey, byte[] data)
    {
        var dataHash = _hash(data);
        return Decrypt(privateKey, dataHash);
    }

    public static bool Verify(RsaPublicKey publicKey, byte[] data, byte[] signature)
    {
        var dataHash = _hash(data);
        var encryptedSignature = Encrypt(publicKey, signature);

        return dataHash == encryptedSignature;
    }
    
}