As an increasing number of organizations deploy services using the Wireless Application Protocol (WAP), they must seriously consider the security issues present in such an environment. This is especially true in systems that involve financial information or sensitive personal data. The WAP specification, created by the WAP Forum, partly addresses such concerns with the introduction of the Wireless Transport Layer Security (WTLS) protocol.

WTLS is based on the Transport Layer Security (TLS) protocol, a derivative of the Secure Sockets Layer (SSL) protocol. The goal of WTLS is very much like that of SSL: to provide privacy and reliability for client-server communications over a network. While SSL primarily provides security over the Internet, WTLS is specific to wireless applications using WAP. The need for WTLS over TLS and SSL is due to the restrictions present in the wireless application environment. Specifically, TLS and SSL don't offer the necessary support for the limited memory and processing capabilities of WAP-enabled phones, support for multiple transport protocol layers, or capabilities to address low-bandwidth environments.

Start with WAP

To gain a better understanding of WTLS, you have to understand the basics of the WAP network architecture. The WAP client, typically a cellular phone, communicates directly with a gateway. A gateway is a proxy server that provides protocol translations, compression of WML/WML Script, and additional services. The gateway receives the request from a WAP client, and translates it into HTTP to communicate with the appropriate content server. In the implementation of a secure WAP system, WTLS encrypts the communication between the WAP client and gateway. The gateway decrypts WTLS and re-encrypts using SSL to connect to the content server. The SSL connection is similar to what you find in a traditional secure Internet application.

The dangers

This process has its weaknesses. The first is that WTLS allows for weak encryption algorithms -- with some WAP clients, users can even disable WTLS encryption entirely. The availability of such options severely limits the security of a WAP application.

Other WTLS security problems were brought to light by Markku-Juhani Saarinen in his paper "Attacks Against the WAP WTLS Protocol," including the chosen plain-text data recovery attack, the datagram truncation attack, the message forgery attack, and the exportable key-search shortcut. (The paper is available at http://www.jyu.fi/~mjos/wtls.pdf.)

Finally, there could be a compromise of the WAP gateway. As the gateway decrypts the data between a WAP client and content server, the data becomes vulnerable if the gateway is compromised. In cases where you're providing your own gateway, you should take steps to prevent this from occurring, via firewalls, operating system hardening, or even physical location security features. However, many companies have yet to realize security concerns exist in WTLS.

Future improvements

Future versions of the WAP specification may address shortcomings in WTLS. A WAP Forum draft called "The WAP Transport Layer E2E Security Specification" presents a scheme to eliminate the need for decryption/re-encryption at the WAP gateway. Here, a WAP client's request is redirected using an XML document. This document supplies the WAP client with instructions on how to make a direct, secure connection with a secondary proxy, which provides direct access to the content server. This method inherently bypasses the WAP client's default gateway.

Widespread implementation and deployment of WAP client security certificates will bring additional improvements. Security certificates digitally identify a WAP client and let it send and receive encrypted information. Currently, the content provider's application must handle digital signing of WAP client requests, or the WML Script Crypto library can create the digital signature.

WTLS is just one piece of a secure WAP application architecture. But you have to understand it fully, along with the full WAP specification, to recognize the pertinent threats, issues, and potential weaknesses.