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.

7.2 Time stamps

A digital time stamp is analogous to a conventional postmark on an envelope: it provides some check of when a message was sent. Returning to the example of Alice and Bob, Alice could add the time and date to her communication to Bob. If she encrypts this with her own private key, or with a key that is known only to Alice and Bob, then Bob may feel reassured that Alice's message is not an old one that has been recorded and replayed.

Activity 11

Look back to Section 3.4, which introduced some types of active attack. If the encrypted message and the encrypted time stamp were sent together, could Bob be truly sure of the freshness of the message?

Answer

No. The exchange could be subject to a message replay attack. An eavesdropper could separate the encrypted message from the encrypted time stamp, and substitute a different message in place of the original one. (This could be a previously recorded encrypted message sent from Alice to Bob.)

To prevent this kind of message replay attack, the message and the time stamp need to be bound together in some way. One method of doing this is to encrypt them together. Only those in possession of the decryption key can then separate the two elements.

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