using System;
using System.IO;
using System.Security.Cryptography;

namespace BirdsiteLive.Cryptography
{
    //https://gist.github.com/ststeiger/f4b29a140b1e3fd618679f89b7f3ff4a
    //https://gist.github.com/valep27/4a720c25b35fff83fbf872516f847863
    //https://gist.github.com/therightstuff/aa65356e95f8d0aae888e9f61aa29414
    //https://stackoverflow.com/questions/52468125/export-rsa-public-key-in-der-format-and-decrypt-data
    public class RsaGenerator
    {
        public string GetRsa()
        {
            var rsa = RSA.Create();

            var outputStream = new StringWriter();
            var parameters = rsa.ExportParameters(true);
            using (var stream = new MemoryStream())
            {
                var writer = new BinaryWriter(stream);
                writer.Write((byte)0x30); // SEQUENCE
                using (var innerStream = new MemoryStream())
                {
                    var innerWriter = new BinaryWriter(innerStream);
                    innerWriter.Write((byte)0x30); // SEQUENCE
                    EncodeLength(innerWriter, 13);
                    innerWriter.Write((byte)0x06); // OBJECT IDENTIFIER
                    var rsaEncryptionOid = new byte[]
                    {
                        0x2a,
                        0x86,
                        0x48,
                        0x86,
                        0xf7,
                        0x0d,
                        0x01,
                        0x01,
                        0x01
                    };
                    EncodeLength(innerWriter, rsaEncryptionOid.Length);
                    innerWriter.Write(rsaEncryptionOid);
                    innerWriter.Write((byte)0x05); // NULL
                    EncodeLength(innerWriter, 0);
                    innerWriter.Write((byte)0x03); // BIT STRING
                    using (var bitStringStream = new MemoryStream())
                    {
                        var bitStringWriter = new BinaryWriter(bitStringStream);
                        bitStringWriter.Write((byte)0x00); // # of unused bits
                        bitStringWriter.Write((byte)0x30); // SEQUENCE
                        using (var paramsStream = new MemoryStream())
                        {
                            var paramsWriter = new BinaryWriter(paramsStream);
                            //EncodeIntegerBigEndian(paramsWriter, parameters.Modulus); // Modulus
                            //EncodeIntegerBigEndian(paramsWriter, parameters.Exponent); // Exponent
                            var paramsLength = (int)paramsStream.Length;
                            EncodeLength(bitStringWriter, paramsLength);
                            bitStringWriter.Write(paramsStream.GetBuffer(), 0, paramsLength);
                        }
                        var bitStringLength = (int)bitStringStream.Length;
                        EncodeLength(innerWriter, bitStringLength);
                        innerWriter.Write(bitStringStream.GetBuffer(), 0, bitStringLength);
                    }
                    var length = (int)innerStream.Length;
                    EncodeLength(writer, length);
                    writer.Write(innerStream.GetBuffer(), 0, length);
                }

                var base64 = Convert
                    .ToBase64String(stream.GetBuffer(), 0, (int)stream.Length)
                    .ToCharArray();
                // WriteLine terminates with \r\n, we want only \n
                outputStream.Write("-----BEGIN PUBLIC KEY-----\n");
                for (var i = 0; i < base64.Length; i += 64)
                {
                    outputStream.Write(base64, i, Math.Min(64, base64.Length - i));
                    outputStream.Write("\n");
                }
                outputStream.Write("-----END PUBLIC KEY-----");
            }

            return outputStream.ToString();
        }

        private static void EncodeLength(BinaryWriter stream, int length)
        {
            if (length < 0)
                throw new ArgumentOutOfRangeException("length", "Length must be non-negative");
            if (length < 0x80)
            {
                // Short form
                stream.Write((byte)length);
            }
            else
            {
                // Long form
                var temp = length;
                var bytesRequired = 0;
                while (temp > 0)
                {
                    temp >>= 8;
                    bytesRequired++;
                }
                stream.Write((byte)(bytesRequired | 0x80));
                for (var i = bytesRequired - 1; i >= 0; i--)
                {
                    stream.Write((byte)(length >> (8 * i) & 0xff));
                }
            }
        }
    }
}