Login.m

#import "NSString+SHA1HMAC.h"

NSString *client_time = [NSString stringWithFormat:@"%f", NSDate.date.timeIntervalSince1970];
NSString *client_digest = [client_time SHA1HMACWithKey:kCliendSecret];
NSDictionary *parameters = @{@"grant_type":@"password",
                             @"client_id":kCliendId,
                             @"client_time":client_time,
                             @"client_digest":client_digest,
                             @"username":self.emailField.text,
                             @"password":self.passwordField.text};

patches.rb

module Doorkeeper
  module OAuth
    class Client
      module Methods
        def from_obfuscated_params(request)
          client_id, client_time, client_digest = request.parameters.values_at(:client_id, :client_time, :client_digest)
          if Time.at(client_time.to_f).between?(15.minutes.ago, 15.minutes.from_now) and app = Doorkeeper::Application.by_uid(client_id)
            calculated_digest = OpenSSL::HMAC::hexdigest(OpenSSL::Digest::SHA1.new, app.secret, client_time)
            if Rack::Utils.secure_compare(client_digest, calculated_digest)
              return [client_id, app.secret]
            end
          end
          [client_id, nil]
        end
      end
    end
  end
end