Lecture5 of Computing SecurityI 2018-2019

Lecture 5 Outlines:
• Kerckhoff’s Principles.
• Cryptanalysis.
• Cryptographic Attacks.
• Security of Algorithms.

1. Kerckhoff’s Principles
In the 19th century, a Dutch cryptographer A. Kerckhoff furnished the requirements of a good cryptosystem. Kerckhoff stated that a cryptographic system should be secure even if everything about the system, except the key, is public knowledge. The design principles defined by Kerckhoff for cryptosystem are:
1. Encryption and decryption transformation must be efficient for all keys.
2. The system must be easy to use.
3. The key should be easily communicable, memorable, and changeable.
4. The security of the system must depend only on the secrecy of the key and not on the secrecy of the algorithm E or D.
5. It should be computationally infeasible for a cryptanalyst to determine the deciphering transformation Dk from intercepted ciphertext C ,even if the corresponding plaintext M is known.
In modern era, Kerckhoff principles became essential guidelines for designing algorithms in modern cryptography.

2. Cryptanalysis.
The whole point of cryptography is to keep the plaintext (or the key, or both) secret from eavesdroppers (also called adversaries, attackers, interceptors, interlopers, intruders, opponents, or simply the enemy). Eavesdroppers are assumed to have complete access to the communications between the sender and receiver.
Cryptanalysis (also called Code breaking) is the science of recovering the plaintext of a message without access to the key. Successful cryptanalysis may recover the plaintext or the key. It also may find weaknesses in a cryptosystem that eventually lead to the previous results. (The loss of a key through no cryptanalytic means is called a compromise).
An attempted cryptanalysis is called an Attack. A fundamental assumption in cryptanalysis according to Kerckhoffs is that the secrecy must reside entirely in the key, and the cryptanalyst has complete details of the cryptographic algorithm and implementation.