Network security
Network security

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Network security

7.4 Nonces

This third method of freshness indication uses an unpredictable value in a challenge–response sequence. The sequence of events is illustrated in Figure 11. Bob wants to communicate with Alice but she needs reassurance that his message is not an old one that is simply being replayed. She generates some random number, which she encrypts and sends to Bob. He then binds the decrypted version of the random number to his message to Alice. On receipt she checks that the returned number is indeed the one she recently issued and sent to Bob. This number, which is used only once by Alice, is called a nonce (derived from ‘number used once’). The term ‘nonce’ is also often used in a wider sense to indicate any freshness indicator.

Figure 11 Using a nonce as a freshness indicator
T823_1

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371