TLSharp/TLSharp.Core/MTProto/Crypto/RSA.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Security.Cryptography;

namespace TLSharp.Core.MTProto.Crypto
{
    internal class RSAServerKey
    {
        private readonly BigInteger e;

        private string fingerprint;
        private readonly BigInteger m;

        public RSAServerKey(string fingerprint, BigInteger m, BigInteger e)
        {
            this.fingerprint = fingerprint;
            this.m = m;
            this.e = e;
        }

        public byte[] Encrypt(byte[] data, int offset, int length)
        {
            using (var buffer = new MemoryStream(255))
            using (var writer = new BinaryWriter(buffer))
            {
                using (SHA1 sha1 = new SHA1Managed())
                {
                    var hashsum = sha1.ComputeHash(data, offset, length);
                    writer.Write(hashsum);
                }

                buffer.Write(data, offset, length);
                if (length < 235)
                {
                    var padding = new byte[235 - length];
                    new Random().NextBytes(padding);
                    buffer.Write(padding, 0, padding.Length);
                }

                var ciphertext = new BigInteger(1, buffer.ToArray()).ModPow(e, m).ToByteArrayUnsigned();

                if (ciphertext.Length == 256)
                {
                    return ciphertext;
                }
                {
                    var paddedCiphertext = new byte[256];
                    var padding = 256 - ciphertext.Length;
                    for (var i = 0; i < padding; i++)
                        paddedCiphertext[i] = 0;
                    ciphertext.CopyTo(paddedCiphertext, padding);
                    return paddedCiphertext;
                }
            }
        }
    }

    public class RSA
    {
        private static readonly Dictionary<string, RSAServerKey> serverKeys = new Dictionary<string, RSAServerKey>
        {
            {
                "216be86c022bb4c3",
                new RSAServerKey("216be86c022bb4c3",
                    new BigInteger(
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
                        16), new BigInteger("010001", 16))
            }
        };

        public static byte[] Encrypt(string fingerprint, byte[] data, int offset, int length)
        {
            var fingerprintLower = fingerprint.ToLower();
            if (!serverKeys.ContainsKey(fingerprintLower))
                return null;

            var key = serverKeys[fingerprintLower];

            return key.Encrypt(data, offset, length);
        }
    }
}