Firebase 3: creating a custom authentication token using .net and c#

Question

I'm trying to implement Firebase 3 Authentication mechanism using Custom Tokens (as described at https:// firebase.google.com/docs/auth/server/create-custom-tokens).

My server is ASP.NET MVC Application.

So according to the instructions (https://firebase.google.com/docs/server/setup) I've created a service account for my Firebase application and generated a key in '.p12' format.

After that according to instructions here (https://firebase.google.com/docs/auth/server/create-custom-tokens#create_custom_tokens_using_a_third-party_jwt_library) I tried to generate a custom token and sign it using the key received on the previous step. For token generation I used SystemIdentityModel.Tokens.Jwt library from Microsoft, so the code looks like the following:

var now = DateTime.UtcNow;
var tokenHandler = new JwtSecurityTokenHandler();
var key = new X509AsymmetricSecurityKey(new X509Certificate2(p12path, p12pwd));
var signinCredentials = new SigningCredentials(key, "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256", "http://www.w3.org/2001/04/xmlenc#rsa-sha256");
Int32 nowInUnixTimestamp = (Int32)(now.Subtract(new DateTime(1970, 1, 1))).TotalSeconds;

var token = tokenHandler.CreateToken(
            issuer: serviceAccountEmail,
            audience: "https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit",                
            signingCredentials: signinCredentials,
            subject: new ClaimsIdentity(new Claim[]
                    {
                    new Claim("sub", serviceAccountEmail),
                    new Claim("iat", nowInUnixTimestamp.ToString()),
                    new Claim("exp", (nowInUnixTimestamp + (60*60)).ToString()),
                    new Claim("uid", uid)
                    })
            );

var tokenString = tokenHandler.WriteToken(token);

Then tried to sign in user in React Native application using Firebase Javascript SDK, with the following code:

//omitting initialization code
firebase.auth().signInWithCustomToken(firebaseJWT).catch(function(error) {
            console.log('Error authenticating Firebase user. Code: ' + error.code + ' Message: ' + error.message);            
        });

But got an error from Firebase saying:

Error authenticating Firebase user. Code: auth/invalid-custom-token Message: The custom token format is incorrect. Please check the documentation.

Experimenting with adding different claims for token expiration control didn't help either.

Also I tried to generate tokens with "dvsekhvalnov/jose-jwt" library but can't get it working with "RS256" algorithm.

So the question:

Any suggestion on what am I doing wrong?

Answer 1

This pure .NET solution works for me, using the Org.BouncyCastle (https://www.nuget.org/packages/BouncyCastle/) and Jose.JWT (https://www.nuget.org/packages/jose-jwt/) libraries.

I followed these steps:

• In the Firebase console click the 'cog' icon which is top left, next to the project name, and click 'Permissions'.
• At the IAM and Admin page, click 'Service Accounts' on the left
• Click 'Create Service Account' at the top, enter a 'Service Account Name', select 'Project->Editor' in the Role selection, tick the 'Furnish a new private key' checkbox and select JSON
• Click 'Create' and download the Service Account JSON file and keep it safe.

• Open the Service Account JSON file in a suitable text editor and put the values into the following code:

  // private_key from the Service Account JSON file
  public static string firebasePrivateKey=@"-----BEGIN PRIVATE KEY-----\nMIIE...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n...\n-----END PRIVATE KEY-----\n";
  
  // Same for everyone
  public static string firebasePayloadAUD="https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit";
  
  // client_email from the Service Account JSON file
  public static string firebasePayloadISS="[email protected]";
  public static string firebasePayloadSUB="[email protected]";
  
  // the token 'exp' - max 3600 seconds - see https://firebase.google.com/docs/auth/server/create-custom-tokens
  public static int firebaseTokenExpirySecs=3600;
  
  private static RsaPrivateCrtKeyParameters _rsaParams;
  private static object _rsaParamsLocker=new object();
  
  void Main() {
      // Example with custom claims
      var uid="myuserid";
      var claims=new Dictionary<string, object> {
          {"premium_account", true}
      };
      Console.WriteLine(EncodeToken(uid, claims));
  }
  
  public static string EncodeToken(string uid, Dictionary<string, object> claims) {
      // Get the RsaPrivateCrtKeyParameters if we haven't already determined them
      if (_rsaParams == null) {
          lock (_rsaParamsLocker) {
              if (_rsaParams == null) {
                  StreamReader sr = new StreamReader(GenerateStreamFromString(firebasePrivateKey.Replace(@"\n","\n")));
                  var pr = new Org.BouncyCastle.OpenSsl.PemReader(sr);
                  _rsaParams = (RsaPrivateCrtKeyParameters)pr.ReadObject();
              }
          }
      }
  
      var payload = new Dictionary<string, object> {
          {"claims", claims}
          ,{"uid", uid}
          ,{"iat", secondsSinceEpoch(DateTime.UtcNow)}
          ,{"exp", secondsSinceEpoch(DateTime.UtcNow.AddSeconds(firebaseTokenExpirySecs))}
          ,{"aud", firebasePayloadAUD}
          ,{"iss", firebasePayloadISS}
          ,{"sub", firebasePayloadSUB}
      };
  
      return Jose.JWT.Encode(payload, Org.BouncyCastle.Security.DotNetUtilities.ToRSA(_rsaParams), JwsAlgorithm.RS256);
  }
  
  private static long secondsSinceEpoch(DateTime dt) {
      TimeSpan t = dt - new DateTime(1970, 1, 1);
      return (long)t.TotalSeconds;
  }
  
  private static Stream GenerateStreamFromString(string s) {
      MemoryStream stream = new MemoryStream();
      StreamWriter writer = new StreamWriter(stream);
      writer.Write(s);
      writer.Flush();
      stream.Position = 0;
      return stream;
  }
  

To get this working in IIS I needed to change the application's pool identity and set the "load user profile" setting to true.