Cryptography: Classical Encryption Techniques

1. List and briefly define types of cryptanalytic attacks based on what is known to the 
attacker. 


• Ciphertext only. One possible attack under these circumstances is the brute-force approach of trying all possible keys. If the key space is very large, this becomes impractical. Thus, the opponent must rely on an analysis of the ciphertext itself, generally applying various statistical tests to it.  
• Known plaintext. The analyst may be able to capture one or more plaintext messages as well as their encryptions. With this knowledge, the analyst may be able to deduce the key on the basis of the way in which the known plaintext is transformed.  
• Chosen plaintext. If the analyst is able to choose the messages to encrypt, the analyst may deliberately pick patterns that can be expected to reveal the structure of the key.

2. What is the difference between an unconditionally secure cipher and a computationally secure cipher? 
An encryption scheme is unconditionally secure if the ciphertext generated by the scheme does not contain enough information to determine uniquely the corresponding plaintext, no matter how much ciphertext is available. 
An encryption scheme is said to be computationally secure if:

1. the cost of breaking the cipher exceeds the value of the encrypted information, and
2. the time required to break the cipher exceeds the useful lifetime of the information.

3. What are two problems with the one-time pad?

1. There is the practical problem of making large quantities of random keys. Any heavily used system might require millions of random characters on a regular basis. Supplying truly random characters in this volume is a significant task. 
2. Even more daunting is the problem of key distribution and protection. For every message to be sent, a key of equal length is needed by both sender and receiver. Thus, a mammoth key distribution problem exists.