What is Fundamental Cryptographic Principles?

Answer 1

1. All encrypted messages must contain some redundancy, i.e. information not needed to understand the message, making a passive intruder attacking less likely. Passive intruder means misuse the information without understanding the information.

   For example, credit card number sent over the internet would be accompanied by other information such as credit card number, or date of birth of the card holder. This reduces the possibility of making up a credit card number.

2. Some measure must be taken to prevent active intruders from playing back old messages.

   The longer an active intruder holds the encrypted message, the more likely he would be able to break it. UNIX password file is a good example. It used to be that the UNIX password is accessible for any one with an account on that host. Intruders can actually get a copy of this file, and try to decrypt the password. Now the file is not accessible from ordinary users.