C# RSA module: Get public key as number

Question

I'm trying to use the System.Security.Cryptography.RSA class in C#. I want to my C# script to send the rsa public key to my server after it created one. I wrote the server in python, so I need the public key as a number, but honestly I have no idea how to do that. My code:

RSA rsaObj = RSA.Create();
RSAParameters rsaParas = rsaObj.ExportParameters(false);
byte[] eByte = rsaParas.Exponent;
byte[] nByte = rsaParas.Modulus;

So now I have e (public key exponent) and N (modulus) as bytes. I can convert them to a Base64String with:

String eB64 = Convert.ToBase64String(eByte);
String nB64 = Convert.ToBase64String(nByte);

Now I have strings like that:

wGFd7jRNC09H/TvgcYU5BduqKtAkRQHoHw6lNakfcGAu8Y/qKg+s6jmlq8N9UWqgSKK87Rrfw8pf9ne9wqxVpSO8LHW0WS9/V2IxcX0fzVJPe+L1HhT5cXGrFHMWOyYKkZB+NaIJUIlZUY5JWyX6SdPwt8QBcHpuY/jXSMg8rqU=

I don't know how get the real numbers out if that. It looks like I'm the only one with this problem, because I couldn't find anything about this.

What _"real numbers"_ are? **You had bytes** (real numbers?) and you converted them to base64 encoded string... — Adriano Repetti, Oct 15 '14 at 15:31
@AdrianoRepetti Sorry for not being clear. With "real numbers" I mean a string out of numbers or an int (if the key isn't too big for that). — Jan Gimmler, Oct 15 '14 at 15:44
@AdrianoRepetti Nope, that is not working. Invalid arguments. — Jan Gimmler, Oct 15 '14 at 16:01
Invalid arguments...where? It returns a string where each byte of eByte is converted to a numeric string — Adriano Repetti, Oct 15 '14 at 16:23
@AdrianoRepetti "error CS1502: The best overloaded method match for `string.Join(string, string[])' has some invalid arguments" "error CS1503: Argument `#2' cannot convert `byte[]' expression to type `string[]'" are appearing in the line with this code: String example = string.join(" ", eByte); — Jan Gimmler, Oct 15 '14 at 16:30
Apparently RSA.ToXmlString(false) will return XML representation of public key, which can be sent to the server and then reconstructed with RSA.FromXmlString(string). — divanov, Oct 16 '14 at 08:43

score 0 · Accepted Answer · edited Jun 20 '20 at 09:12

I'm not sure I understand exactly what you want but you may find this useful.

        var eByteString = eByte.Aggregate(string.Empty, (current, n) => current + n.ToString("000"));
        var nByteString = nByte.Aggregate(string.Empty, (current, n) => current + n.ToString("000"));

or you can do it without extensions like:

        //convert to decimal string
        var eByteString = string.Empty;
        foreach (byte b in eByte)
            eByteString = eByteString + b.ToString("000");

        var nByteString = string.Empty;
        foreach (byte b in nByte)
            nByteString = nByteString + b.ToString("000");

the result would be something like:

eByteString : 001000001

nByteString : 211110203128097225245156190057058005219074254110126137008162017185117098093007197199208188134068102171169132057218155254193154082182076214158032164159255142114232250019158058134077184026065093207059162082149098217128015167051030117204095144063074166219072006156228075166154194003029165048078213089138010151145220240249163048191014154160090207050215127170028113070212074123108011170137

These two lines convert both eByte and nByte to decimal representation of the byte array (every 3 character represents a single byte to make converting the string to original byte array possible).

to convert it back to byte array in c# you can use provided extension method:

static class Extensions
{
    public static IEnumerable<String> SplitParts(this string text, int length)
    {
        for (var i = 0; i < text.Length; i += length)
            yield return text.Substring(i, Math.Min(length, text.Length - i));
    }
}

and conversion would be like:

        var arr = eByteString.SplitParts(3).Select(s => Convert.ToByte(s)).ToArray();

here is the complete sample for convering to decimal string and back to byte array:

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

namespace Test
{

    static class Extensions
    {
        public static IEnumerable<String> SplitParts(this string text, int length)
        {
            for (var i = 0; i < text.Length; i += length)
                yield return text.Substring(i, Math.Min(length, text.Length - i));
        }
    }

    static class Program
    {
        static void Main()
        {
            RSA rsaObj = RSA.Create();
            RSAParameters rsaParas = rsaObj.ExportParameters(false);
            byte[] eByte = rsaParas.Exponent;
            byte[] nByte = rsaParas.Modulus;

            //convert to decimal string
            var eByteString = eByte.Aggregate(string.Empty, (current, n) => current + n.ToString("000"));
            var nByteString = nByte.Aggregate(string.Empty, (current, n) => current + n.ToString("000"));

            //convert back to array
            var arrEByteString = eByteString.SplitParts(3).Select(s => Convert.ToByte(s)).ToArray();
            var arrNByteString = nByteString.SplitParts(3).Select(s => Convert.ToByte(s)).ToArray();
        }
    }
}

My compiler couldn't find the method "Aggregate". Do I have to import something? — Jan Gimmler, Oct 15 '14 at 18:47
aggregate is an extension method, add `using System.Linq;` and you need .Net 3.5 or higher — user3473830, Oct 15 '14 at 18:49
Furthermore I don't know whether that's the right answer to my question. I need the variables N (private & public key modulus) and e (public key exponent). The bytes are the only thing I can get from the .NET RSA implementation. Do your this strings represent the values I can calculate with (encrypt my data)? — Jan Gimmler, Oct 15 '14 at 18:56
There we got the problem: I am using Unity Engine which uses NET 2.0... Are there any alternatives? — Jan Gimmler, Oct 15 '14 at 18:58
@JanGimmler, I updated the code for alternative method, and yes you can use it to decrypt/encrypt (one way since you just exported public key), you should call `ImportParameter` and add back the parameters to crypto provider, for more information have a look at http://stackoverflow.com/questions/17128038/c-sharp-rsa-encryption-decryption-with-transmission — user3473830, Oct 15 '14 at 19:09
This is much worse than the base64 encoding he started with. — President James K. Polk, Oct 15 '14 at 23:09
I don't know why the answer get down voted,I never said this is an efficient or correct way to transfer the keys, but the answer is based on collaboration with @JanGimmler who asked the question. — user3473830, Oct 16 '14 at 09:15
The problem is that I don't know a more efficient way to transfer the key when just one side is using the .NET implementation. — Jan Gimmler, Oct 17 '14 at 21:18

C# RSA module: Get public key as number

1 Answers1