Azure Function as Token service

generateSASfunction

    using System.Net;
    using Microsoft.Azure.Devices;
    using Microsoft.Azure.Devices.Common.Security;
    using System.Globalization;
    using System.Security.Cryptography;
    using System.Text;

    public static async Task<HttpResponseMessage> Run(HttpRequestMessage req, TraceWriter log)
    {
        string deviceid = req.GetQueryNameValuePairs()
                    .FirstOrDefault(q => string.Compare(q.Key, "deviceid", true, CultureInfo.InvariantCulture) == 0).Value;
        string hash = req.GetQueryNameValuePairs()
                    .FirstOrDefault(q => string.Compare(q.Key, "hash", true, CultureInfo.InvariantCulture) == 0).Value;
        if (String.IsNullOrEmpty(deviceid)) return req.CreateResponse(HttpStatusCode.BadRequest, "device id missing"); 
        if (String.IsNullOrEmpty(hash)) return req.CreateResponse(HttpStatusCode.BadRequest, "hash missing");

        var resourceUri ="ArduinoDemoHub.azure-devices.net/devices/"+deviceid;

        // taken from IoT Hub user with Connect devices rights (not from Device Explorer)   
        var connectionString = "HostName=ArduinoDemoHub.azure-devices.net;SharedAccessKeyName=iothubowner;SharedAccessKey=cuYBKc42lfJr4oSRGQGQ8IiKWxGQkLre7rprZDZ/ths=";
    
        var registryManager = RegistryManager.CreateFromConnectionString(connectionString);
        var device = await registryManager.GetDeviceAsync(deviceid);
        var key = device.Authentication.SymmetricKey.PrimaryKey;

        HMACSHA256 hmac = new HMACSHA256(Encoding.UTF8.GetBytes("somerandomkeyKJBWyfy4gski"));
        var hashedkey = Convert.ToBase64String(hmac.ComputeHash(Encoding.UTF8.GetBytes(key)));

        if (hashedkey!=hash) return req.CreateResponse(HttpStatusCode.BadRequest, "wrong hash");
           SharedAccessSignatureBuilder sasBuilder = new SharedAccessSignatureBuilder()
            {
                Key = key,
                Target = resourceUri,
                TimeToLive = TimeSpan.FromDays(Convert.ToDouble(7))
            };

            var SAS = sasBuilder.ToSignature();
            return req.CreateResponse(HttpStatusCode.OK, SAS);
    }
    
 //  add dependency in project.json file   
 //  {
 //     "frameworks": {
 //     "net46":{
 //       "dependencies": {
 //         "Microsoft.Azure.Devices": "1.4.1"
 //         }
 //       }
 //     }
 //   }