G4G0-1/Semester 1/Computer Systems Internals & Linux/Week 6/Week 6 Computer Systems.md

Lecture 1 (10:00)

Caesar Cipher

Dictionary

• Precalculate encrypted versions of likely words, and when one appears, test the key on the whole message.

Plaintext

• If you can see certain English commonalities in the cipher, you can make an educated guess. ex. If one letter appears alone, it is most likely an "A" or an "I".

DES - Data Encryption Standard

• Cascaded Transposition.
• Substitution Ciphers.
• Initially proposed as 128-bit, US Gov enforced 56-bit. Wonder why????
  • 2^128 - 3.4x10^38 combinations. 5x10^19 years to do half at 100 billion keys per second.
  • 1^56 - 7.2x10^16 combinations. 4 days to do half at 100 billion keys per second.

Current State

• (1999) Triple DES - 112bit -> 128bit standard
• (2002) AES - 128bit -> 256bit

Assessment

4pm 27th Nov Deadline

• 50% of mark, questions on blackboard