Information Security Notes 1 - Classical Crypto System
Information Security Notes 1 - Classical Crypto System
What is Encryption
Encryption is composed of a key and an Encryption algorithm.
- Type of operations used for transforming plaintext to ciphertext
- The number of keys used
- The way in which the plaintext is processed
Encryption and Decryption
- Unencrypted message = plaintext/message
- Encrypted message = cipher/ciphertext
Cryptanalysis
Means attack
- Brute-force
- Tries every possible
- Breaking the algorithm
- Tries to exploit the weakness of the encryption algorithm
How to measured by the following dimensions
- Attacker models
- How strong is the attacker
- Ciphertext only attacks
- Known plaintext attacks
- Chosen plaintext attacks(Attacker can choose plaintext on his own)
- Chosen ciphertext attacks(Attacker can choose the cipher and obtain the plaintext)
- Security Goal
- What Goals does your attacker wants to achieve
- Computationally secure: The cost of breaking the cipher exceeds the value of the encryptited information
- Unconditionally secure: No matter how much time as opponent has, it is impossible for people decrypt. (secure against brute-force)
- Assumptions:
- What is the computational limitation
- Always better to over-estimate the ability of your attackers
- Computation: attacker might have many computing resource(super-computers)
- Network: attacker might have control over the network/communication channel, they can send/drop/inject/view your packet
- Some problems are hard(NP=/=P), no polynomial time solutions
- We generally assume computation requiring 2^80 is unsolvable
Brute-force attack
- Attackers try all possible sets of keys
- By probability, it has to try at least half of them
- We generally assume computation requiring 2^80 is unsolvable
