Verify Firebase JWT in c# .net

Question

I am trying to verify a json web token obtained by a firebase android client and passed to a server running .net

Following the answer here I created these methods to validate the token and extract the uid:

public static async Task<string> GetUserNameFromTokenIfValid(string jsonWebToken)
    {
        const string FirebaseProjectId = "testapp-16ecd";
        try
        {
            // 1. Get Google signing keys
            HttpClient client = new HttpClient();
            client.BaseAddress = new Uri("https://www.googleapis.com/robot/v1/metadata/");
            HttpResponseMessage response = await client.GetAsync("x509/[email protected]");
            if (!response.IsSuccessStatusCode) { return null; }
            var x509Data = await response.Content.ReadAsAsync<Dictionary<string, string>>();
            SecurityKey[] keys = x509Data.Values.Select(CreateSecurityKeyFromPublicKey).ToArray();

            // Use JwtSecurityTokenHandler to validate the JWT token
            JwtSecurityTokenHandler tokenHandler = new JwtSecurityTokenHandler();

            // Set the expected properties of the JWT token in the TokenValidationParameters
            TokenValidationParameters validationParameters = new TokenValidationParameters()
            {
                ValidAudience = FirebaseProjectId,
                ValidIssuer = "https://securetoken.google.com/" + FirebaseProjectId,
                ValidateIssuerSigningKey = true,
                IssuerSigningKeys = keys
            };
            SecurityToken validatedToken;
            ClaimsPrincipal principal = tokenHandler.ValidateToken(jsonWebToken, validationParameters, out validatedToken);
            var jwt = (JwtSecurityToken)validatedToken;
            return jwt.Subject;

        }
        catch (Exception e)
        {
            return null;
        }
    }

    static SecurityKey CreateSecurityKeyFromPublicKey(string data)
    {
        return new X509SecurityKey(new X509Certificate2(Encoding.UTF8.GetBytes(data)));
    }

When I run the code I get the response:

{"IDX10501: Signature validation failed. Unable to match 'kid': 'c2154b0435d58fc96a4480bd7655188fd4370b07', \ntoken: '{"alg":"RS256","typ":"JWT","kid":"c2154b0435d58fc96a4480bd7655188fd4370b07"}......

Calling https://www.googleapis.com/robot/v1/metadata/x509/[email protected] does return a certificate with a matching id:

{
 "c2154b0435d58fc96a4480bd7655188fd4370b07": "-----BEGIN CERTIFICATE-----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\nfg/tm7J+MGH5NH5MwjO+CI4fA3NoGOuEzF1vb7/hNdU=\n-----END CERTIFICATE-----\n"

I have successfully validated this token using the Java call (made in kotlin)

FirebaseAuth.getInstance().verifyIdToken(idToken).addOnSuccessListener { decodedToken ->
    val uid = decodedToken.uid
}

See here. Which may help you. Otherwise, as a workaround, you can specify your own key resolver, like this — Rob
– Rob ♦, Commented Nov 22, 2016 at 3:19

Miguel Laranjeira · Accepted Answer · 2018-03-13 06:17:36Z

1

I'm sure by now you have figured out the solution for this, but for future people who come across this question.

Set the KeyId for the X509SecurityKey

x509Data.Select(cert => new X509SecurityKey(new X509Certificate2(Encoding.UTF8.GetBytes(cert.Value)))
            {
                KeyId = cert.Key
            })
            .ToArray()

This will allow the TokenValidationParameters to look up which issuerKey to validate against.

answered Mar 13, 2018 at 6:17

Miguel Laranjeira

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