PasswordlessWebLogin/KeyConverter.cs

using System.Security.Cryptography;
using Microsoft.IdentityModel.Tokens;

namespace Passwordless;

public static class KeyConverter
{
    public static RSA JwkToRsa(JsonWebKey key)
    {
        var rsaParameters = new RSAParameters
        {
            // PUBLIC KEY PARAMETERS
            // n parameter - public modulus
            Modulus = Base64UrlEncoder.DecodeBytes(key.N),
            // e parameter - public exponent
            Exponent = Base64UrlEncoder.DecodeBytes(key.E),
    
            // PRIVATE KEY PARAMETERS (optional)
            // d parameter - the private exponent value for the RSA key 
            D = Base64UrlEncoder.DecodeBytes(key.D),
            // dp parameter - CRT exponent of the first factor
            DP = Base64UrlEncoder.DecodeBytes(key.DP),
            // dq parameter - CRT exponent of the second factor
            DQ = Base64UrlEncoder.DecodeBytes(key.DQ),
            // p parameter - first prime factor
            P = Base64UrlEncoder.DecodeBytes(key.P),
            // q parameter - second prime factor
            Q = Base64UrlEncoder.DecodeBytes(key.Q),
            // qi parameter - CRT coefficient of the second factor
            InverseQ = Base64UrlEncoder.DecodeBytes(key.QI)
        };

        return RSA.Create(rsaParameters);
    }

    public static JsonWebKey ExtractPublicKey(JsonWebKey key)
    {
        return new JsonWebKey()
        {
            Kty = key.Kty,
            E = key.E,
            N = key.N,
            Use = "sig",
        };
    }
}