1. Ref: STAL03More Concepts of Cryptography and Cryptanalysis 1 Reference –William Stallings, Cryptography and Network Security, 3rd Edition, Prentice Hall.

2. Ref: STAL03More Concepts of Cryptography and Cryptanalysis 2 Characterization of Cryptographic Systems 1. Type of Operations Used –Subsittution –Transposition –Product Systems--Multiple Stages of Substituion and Transposition

3. Ref: STAL03More Concepts of Cryptography and Cryptanalysis 3 Characterization of Cryptographic Systems 2. Number of Keys –Same single or secret key –Different keys for sender and receiver 3. Processing Technique –Block cipher--one block at a time. –Stream cipher--processes elements continuously, one element at a time.

4. Ref: STAL03More Concepts of Cryptography and Cryptanalysis 4 Code Breaking Techniques Cryptanalysis –Relies on the nature of the algorithm and perhaps some knowledge of the characteristics of the plaintext. –Attempts to discover the ciphertext or the key. Brute-force Attacks –Attacker tries all possible keys. –On average, half of all possible keys must be tried.

5. Ref: STAL03More Concepts of Cryptography and Cryptanalysis 5 Definitions of Security An encryption scheme is unconditionally secure if the ciphertext generated by the scheme does not contain enough information to determine the corresponding plaintext, no matter the amount of ciphertext available. Besides the one-time pad, there is no algorithm that is unconditionally secure.

6. Ref: STAL03More Concepts of Cryptography and Cryptanalysis 6 Definitions of Security (p.2) An encryption algorithm is said to be computationally secure if: –The cost of breaking the cipher exceeds the value of the encrypted information. –The time required to break the cipher exceeds the useful lifetime of the information.