View
213
Download
1
Category
Preview:
Citation preview
Ref: STAL03 More Concepts of Cryptography and Cryptanalysis
1
More Concepts of Cryptography and Cryptanalysis
• Reference– William Stallings, Cryptography and Network
Security, 3rd Edition, Prentice Hall.
Ref: STAL03 More 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
Ref: STAL03 More 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.
Ref: STAL03 More 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.
Ref: STAL03 More 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.
Ref: STAL03 More 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.
Recommended