Example to validate firebase token in C#

TokenFirebaseVerify.cs

using System;
using System;
using System.Collections.Generic;
using System.IdentityModel.Tokens.Jwt;
using System.Linq;
using System.Net.Http;
using System.Security.Claims;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Threading.Tasks;
using Microsoft.IdentityModel.Tokens;

namespace ConsoleApp2
{
    class Program
    {
        static HttpClient client = new HttpClient();

        static void Main()
        {
            string encodedJwt = "TOKEN";
            string firebaseProjectId = "FIREBASEID";
            RunAsync(encodedJwt, firebaseProjectId).Wait();
            Console.ReadKey();
        }

        static async Task RunAsync(string encodedJwt, string firebaseProjectId)
        {
            
            // 1. Get Google signing keys
            client.BaseAddress = new Uri("https://www.googleapis.com/robot/v1/metadata/");
            var response = await client.GetAsync("x509/securetoken@system.gserviceaccount.com");
            if (!response.IsSuccessStatusCode) { return; }
            var x509Data = await response.Content.ReadAsAsync<Dictionary<string, string>>();
            var keys = x509Data.Values.Select(CreateSecurityKeyFromPublicKey).ToArray();
            // 2. Configure validation parameters

            var parameters = new TokenValidationParameters
            {
                ValidIssuer = "https://securetoken.google.com/" + firebaseProjectId,
                ValidAudience = firebaseProjectId,
                IssuerSigningKeys = keys,
            };
            // 3. Use JwtSecurityTokenHandler to validate signature, issuer, audience and lifetime
            var handler = new JwtSecurityTokenHandler();
            var principal = handler.ValidateToken(encodedJwt, parameters, out var token);
            var jwt = (JwtSecurityToken)token;

            // 4.Validate signature algorithm and other applicable valdiations
            if (jwt.Header.Alg != SecurityAlgorithms.RsaSha256)
            {
                throw new SecurityTokenInvalidSignatureException(
                    "The token is not signed with the expected algorithm.");
            }

            foreach (var claim in principal.Claims)
            {
                Console.WriteLine($"{claim.Type}::{claim.Value}");
            }

            var teste = principal.Claims.Where(x => x.Type == "user_id").Select(x => x.Value).FirstOrDefault();
            Console.WriteLine(teste);
        }
        static SecurityKey CreateSecurityKeyFromPublicKey(string data)
        {
            return new X509SecurityKey(new X509Certificate2(Encoding.UTF8.GetBytes(data)));
        }
    }
}