Secure web browsing

Team-Fly

Secure web browsing

Many people now shop on the web. To do so they almost certainly use a credit card, which means that their credit card details are transmitted across the Internet. Concerns about the security of these details is often listed as one of the main reasons why this form of shopping is not more widely used. In this short section we discuss how credit card details are protected on the web and extend the discussion to other security issues.

Secure web browsing is an essential feature of e-commerce. The Secure Sockets Layer (SSL) and the Transport Layer Security (TLS) are two important protocols that are used to verify the authenticity of websites. They facilitate the use of encryption for sensitive data and help to ensure the integrity of information exchanged between web browsers and websites. We concentrate on SSL.

SSL is an example of a client-server protocol where the web browser is the client and the website is the server. It is the client that initiates a secure communication, while the server responds to the client's request. The most basic function that SSL is used for is the establishment of a channel for sending encrypted data, such as credit card details, from the browser to a chosen site.

Before discussing the protocols we note that web browsers typically hold some cryptographic algorithms together with the public key values of a number of recognized Certification Authorities.

In the initial message from browser to site, often referred to as the 'client hello', the browser has to send the server a list of the cryptographic parameters that it can support. However, although it initializes an exchange of information that enables the use of encryption, that message does not identify the browser to the site. In fact, for many applications, the site is not able to authenticate the browser and the authentication protocol merely authenticates the site to the browser. This often makes sense. If, for instance, an individual wishes to make a purchase using the web browser then it is very important for them to establish that the website they are browsing is authentic. The merchant, on the other hand, may have other means of authenticating the user's identity or may not even care about it. Once the merchant has received a credit card number, for example, it can validate that number directly with the card issues.

The website authenticates itself to the browser by sending its public key certificate which, provided the browser has the appropriate public key, provides the browser with an authentic copy of the site's public key. As part of the establishment of the secure channel, the browser then sends the site a session key for an agreed symmetric algorithm. The session key is encrypted using the site's public key, thereby giving the browser confidence that only the nominated site can use it. Thus SSL provides another everyday example of the type of hybrid key management system discussed in Chapter 8. It also provides an example of the use of a PKI for entity authentication.

Team-Fly

file:///D|/1/4303/Fly0049.html [08.10.2007 12:51:25]

Using a GSM mobile phone

Team-Fly

Using a GSM mobile phone

One of the main attractions for users to have mobile phones is that they offer the ability to roam and to make telephone calls from almost anywhere. However, since the mobile phones are wireless, the phone message is transmitted across the airwaves until it reaches the nearest base station, where it is transferred to the fixed landline. Since intercepting radio signals is likely to be easier than intercepting landline calls, one of the initial security requirements for GSM was that their mobile phones should be no less secure than the conventional fixed telephones. This requirement was satisfied by providing encryption for transmissions from the handset to the nearest base station. Another serious security issue was the problem of the operator being able to identify the phone so that they knew whom to charge. Thus, for GSM, there were the following two major security requirements: confidentiality, which was a customer requirement and user authentication, which was a system requirement.

Each user is issued with a personalized smart card, called a SIM, which contains a 128-bit secret authentication value known only to the operator. This value is then used as the key to a challenge-response authentication protocol using an algorithm, which can be selected by the operator. When the user attempts to make a call, their identity is transmitted to the system operator via a base station. Since the base station does not know the SIM's secret key, and may not even know the authentication algorithm, the central system generates a challenge and sends it, with the response appropriate for the card, to the base station. This enables the base station to check the validity of the response.

In addition to the authentication algorithm the SIM also contains a stream cipher encryption algorithm, which is common throughout the network. This algorithm is used to encrypt the messages from the mobile phone to the base station. The key management for the encryption keys is ingenious and makes use of the authentication protocol. The authentication algorithm accepts a 128-bit challenge and computes a 128-bit response, which depends on the card's authentication key. However only 32 bits are transmitted from the SIM to the base station as the response.

This means that when the authentication process has been completed there are 96 bits of secret information known only to the SIM, the base station, and the host computer. Of these bits, 64 are then allocated for the determination of the encryption key. Note that the encryption key changes every time that authentication takes place.

Team-Fly

file:///D|/1/4303/Fly0050.html [08.10.2007 12:51:26]