## 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