# CS458 Lecture Notes - Public-Key Cryptography, Checksum, Symmetric-Key Algorithm

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Published on 16 Oct 2011
School
University of Waterloo
Department
Computer Science
Course
CS458
Professor
Module 4 - Summary
Cryptology: science that studies both
- Cryptography: Making Secret Messages
- Cryptanalysis: breaking secret Messages
Cryptography major components
1- Confidentiality: prevent eve from reading Alice’s messages
2- Integrity: prevent Mallory from modifying Alice’s messages
3- Authenticity: prevent Mallory form impersonating Alice
Kerckhoffs’ Principle : the security of a cryptosystem should not rely on a secret
that’s hard ( or expensive ) to change
Note:
- A strong cryptosystem is one where the most eve can do is try all the
keys used to specify which encryption method was used ( “computation”
security )
- Assumptions of what eve might know
o Know the algorithm ( aka the class of the encryption method )
o Know part of the plain text
o Know a number of corresponding plaintext/ciphertext
- Goal of cryptography is to make sure the information transfer is not the
weakest link ( send secure messages over an insecure medium )
Symmetric-key Encryption
- Simplest and used for years , but hard part is exchanging keys
- Unbreakable, simple idea, hard to use correctly , “information-theoretic”
security
- Description:
o The key is random bitstring of the same length as the message
o The encrypt and decrypt functions are just XOR
- Note:
o Key must be truly random not pseudorandom
o Key must never be used more than once
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Types of Symmetric Ciphers
1- Stream Ciphers
- Description:
o Same as One-Time Pad but with pseudorandom keystream ,
operate on the message one bit a time
- Pro
o Very fast, used to send a lot of data securely
- Con
o Tricky to use, using the same key to encrypt two different
messages might be used to exploit the encryption
- Example: RC4 ( bad examples WEP and PPTP )
2- Block Ciphers
- Description:
o Operate on the message one block at a time ( 64/128 bit
long)
- Mode of operation: the choice of what to do with multiple blocks
o Electronic Code Book: encrypt each successive block
separately, but if there were repeated blocks in the plain
text, you’ll see repeating patterns in the cipher text
o Cipher Block Chaining (CBC) Counter (CTR) modes, needs
an initial Value ( IV ) which acts as a salt no patterns will be
exposed
- Example: AES
Public-Key Cryptography
- its asymmetric, one key for locking, another key for unlocking
- examples: RSA , ElGamal , ECC
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