Freebase oAuth2 Bad Request(400) .Net

Question

I read the tutorial in https://developers.google.com/accounts/docs/OAuth2ServiceAccount

and tried using their example but keep getting 400 bad request. this is my code:

 ClaimSet cs = new ClaimSet()
        {
            aud = "https://www.googleapis.com/oauth2/v3/token",
            iss = "1070248278615-hoq0meaunarl9hj8t9klg4gqkohlme9u@developer.gserviceaccount.com",
            exp = GetTime(DateTime.UtcNow.AddHours(1)).ToString(),
            iat = GetTime(DateTime.UtcNow).ToString(),
            scope = "https://www.googleapis.com/auth/freebase"
        };

        //get the signed JWT
        var signedJwt = JsonWebToken.Encode(cs);           


public static string Encode(object payload, JwtHashAlgorithm algorithm = JwtHashAlgorithm.RS256)
        {
            return Encode(payload, Encoding.UTF8.GetBytes(PrivateKey), algorithm);
        }

    public static string Encode(object payload, byte[] keyBytes, JwtHashAlgorithm algorithm)
    {
        var segments = new List<string>();
        var header = new { alg = algorithm.ToString(), typ = "JWT" };

        byte[] headerBytes = Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(header, Formatting.None));
        byte[] payloadBytes = Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(payload, Formatting.None));

        segments.Add(Base64UrlEncode(headerBytes));
        segments.Add(Base64UrlEncode(payloadBytes));

        var stringToSign = string.Join(".", segments);

        var bytesToSign = Encoding.UTF8.GetBytes(stringToSign);

        byte[] signature = HashAlgorithms[algorithm](keyBytes, bytesToSign);
        segments.Add(Base64UrlEncode(signature));

        return string.Join(".", segments.ToArray());
    }
        using (var wb = new WebClient())
        {
            var url = "https://www.googleapis.com/oauth2/v3/token/";
            wb.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
            var data2 = new NameValueCollection();
            data2["grant_type"] = "urn:ietf:params:oauth:grant-type:jwt-bearer";
            data2["assertion"] = signedJwt;
            var response2 = wb.UploadValues(url, "POST", data2);
        }

Now after getting the access token I try to write to freebase: using the following tutorial I saw that I should Get verb: https://developers.google.com/accounts/docs/OAuth2ServiceAccount#creatinganaccount

    var url = "https://www.googleapis.com/freebase/v1/mqlwrite";
                wb.QueryString.Add("lang", "/lang/en");
                wb.QueryString.Add("query", "%5B%7B%0A%20%20%22mid%22%3A%20%22%2Fm%2F011840dm%22%2C%0A%20%20%22%2Fcommon%2Ftopic%2Ftopic_equivalent_webpage%22%3A%20%7B%0A%20%20%20%20%22connect%22%3A%20%22insert%22%2C%0A%20%20%20%20%22value%22%3A%20%22http%3A%2F%2Fwww.imdb.com%2Fname%2Fnm4963898%2F%22%0A%20%20%7D%0A%7D%5D");
                wb.Headers.Add("Authorization", "Bearer " + accesstoken);
                var ResponseBytes = wb.DownloadString(url);

Appreciate the help :)

Answer 1

You're including the Content-Type in the POST data but it should be presented as part of the HTTP headers as in:

wb.Headers.Add("Content-Type","application/x-www-form-urlencoded");

However, using UploadValues this will be the default setting anyway unless overridden.

Other than that, UploadValues will automatically URL-encode your values, so you should present them in their raw form; so for the grant type that would mean:

data2["grant_type"] = "urn:ietf:params:oauth:grant-type:jwt-bearer";

Edit1:
Also, your JWT uses the wrong aud claim since it is set to https://accounts.google.com/o/oauth2/token instead of https://www.googleapis.com/oauth2/v3/token and is expired since July 2nd 2012.

Edit2: You must also post to the URL without the trailing slash and get the iat and exp timestamps correct. Successfully tested code using Newtonsoft.Json:

public class GoogleServiceAccountBearerJWTSample
{
    private static string Base64UrlEncode(byte[] input)
    {
        var output = Convert.ToBase64String(input);
        output = output.Split('=')[0]; // Remove any trailing '='s
        output = output.Replace('+', '-'); // 62nd char of encoding
        output = output.Replace('/', '_'); // 63rd char of encoding
        return output;
    }

    public static string Encode(object payload, AsymmetricAlgorithm rsa) {
        var segments = new List<string>();
        var header = new { alg = "RS256", typ = "JWT" };
        byte[] headerBytes = Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(header, Formatting.None));
        byte[] payloadBytes = Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(payload, Formatting.None));
        segments.Add(Base64UrlEncode(headerBytes));
        segments.Add(Base64UrlEncode(payloadBytes));
        var stringToSign = string.Join(".", segments.ToArray());
        var bytesToSign = Encoding.UTF8.GetBytes(stringToSign);

        // VARIANT A - should work on non-SHA256 enabled systems
        var rs = rsa as RSACryptoServiceProvider;
        var cspParam = new CspParameters
        {
            KeyContainerName = rs.CspKeyContainerInfo.KeyContainerName,
            KeyNumber = rs.CspKeyContainerInfo.KeyNumber == KeyNumber.Exchange ? 1 : 2
        };
        var aescsp = new RSACryptoServiceProvider(cspParam) { PersistKeyInCsp = false };
        var signature = aescsp.SignData(bytesToSign, "SHA256");
        // END OF VARIANT A

        // VARIANT B - works on FIPS SHA256 enabled systems
        // var pkcs1 = new RSAPKCS1SignatureFormatter(rsa);
        // pkcs1.SetHashAlgorithm("SHA256");
        // var signature = pkcs1.CreateSignature(new SHA256Managed().ComputeHash(bytesToSign));
        // END OF VARIANT B

        segments.Add(Base64UrlEncode(signature));
        return string.Join(".", segments.ToArray());
   }

   public static void Main()
   {    
        var utc0 = new DateTime(1970,1,1,0,0,0,0, DateTimeKind.Utc);
        var issueTime = DateTime.UtcNow;

        var iat = (int)issueTime.Subtract(utc0).TotalSeconds;
        var exp = (int)issueTime.AddMinutes(55).Subtract(utc0).TotalSeconds; // Expiration time is up to 1 hour, but lets play on safe side

        var payload = new {
            iss = "xxxxxxxxxxxxxxxxxx@developer.gserviceaccount.com",
            aud = "https://www.googleapis.com/oauth2/v3/token",
            scope = "https://www.googleapis.com/auth/freebase",
            exp = exp,
            iat = iat
        };

        var certificate = new X509Certificate2("google-client.p12", "notasecret");
        var signedJwt = Encode(payload, certificate.PrivateKey);

        //System.Console.WriteLine(signedJwt);

        using (var wb = new WebClient())
        {
            var url = "https://www.googleapis.com/oauth2/v3/token";
            var data2 = new NameValueCollection();
            data2["grant_type"] = "urn:ietf:params:oauth:grant-type:jwt-bearer";
            data2["assertion"] = signedJwt;
            var response2 = wb.UploadValues(url, "POST", data2);
            System.Console.WriteLine(Encoding.UTF8.GetString(response2));
        }
    }
}