How to get n value from RSA public key XML file .net core c#

Question

What is n value of JWKS end point.

How to get that n value from RSA public key XML file in .net core c#.

Here is my RSA public key.

<RSAKeyValue><Modulus>w2EmXqTfj7LDHtN8/H6I1XAOW5bnH2j0IYDK2VSg0kd+TtCJDh/rogV/ouxGBYoyr6pEtuOQsQyIYcUWektRL+/hVHPZzw9VTNtxGpgYZoVjFH1TB+acJ5wz7eOAybLdEHA4/F7A9VyvlRDir6AuuXcqF4yU0G3Ew21bFzRT7S0hDMMmHCsKwvi33lFkWkMFNnXVwQMdWu59jgksMvTQDWpFveUBr1E4R9FRYmrQPe4hMrjijlmS86xBTn3Qasd6qnT2uW137uwtC2aaqWS/Q8BGBbDpOxiMxffrGLyr3Xos1dOCgqx+oE/ZW6+CGhC+F4My/2EZNciyGhtt4N5abQ==</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>

score 1 · Accepted Answer · answered Dec 13 '21 at 08:39

Modulus is n, Exponent is e, see also here

With a few lines, using LINQ to XML, you can extract the value:

using System;
using System.Xml.Linq;

namespace RSAXML
{
    class Program
    {
        static void Main(string[] args)
        {

            string RSAKey = "<RSAKeyValue><Modulus>w2EmXqTfj7LDHtN8/H6I1XAOW5bnH2j0IYDK2VSg0kd+TtCJDh/rogV/ouxGBYoyr6pEtuOQsQyIYcUWektRL+/hVHPZzw9VTNtxGpgYZoVjFH1TB+acJ5wz7eOAybLdEHA4/F7A9VyvlRDir6AuuXcqF4yU0G3Ew21bFzRT7S0hDMMmHCsKwvi33lFkWkMFNnXVwQMdWu59jgksMvTQDWpFveUBr1E4R9FRYmrQPe4hMrjijlmS86xBTn3Qasd6qnT2uW137uwtC2aaqWS/Q8BGBbDpOxiMxffrGLyr3Xos1dOCgqx+oE/ZW6+CGhC+F4My/2EZNciyGhtt4N5abQ==</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>";

            XElement RSAKeyXML = XElement.Parse(RSAKey);
            
            string Modulus = RSAKeyXML.Element("Modulus").Value.ToString();

            Console.WriteLine(Modulus);
        }
    }
}

Result:

w2EmXqTfj7LDHtN8/H6I1XAOW5bnH2j0IYDK2VSg0kd+TtCJDh/rogV/ouxGBYoyr6pEtuOQsQyIYcUWektRL+/hVHPZzw9VTNtxGpgYZoVjFH1TB+acJ5wz7eOAybLdEHA4/F7A9VyvlRDir6AuuXcqF4yU0G3Ew21bFzRT7S0hDMMmHCsKwvi33lFkWkMFNnXVwQMdWu59jgksMvTQDWpFveUBr1E4R9FRYmrQPe4hMrjijlmS86xBTn3Qasd6qnT2uW137uwtC2aaqWS/Q8BGBbDpOxiMxffrGLyr3Xos1dOCgqx+oE/ZW6+CGhC+F4My/2EZNciyGhtt4N5abQ==

In this Q/A you can see how to import the key based on Modulus and Exponent.

Could you convert to integer and show which kind of endianness is used? Oh, it's unsigned big endian it seems, when I look at the value of n — Maarten Bodewes, Dec 13 '21 at 08:45

How to get n value from RSA public key XML file .net core c#

1 Answers1