The computer has a processor, which contains cores. Each core contains FPU (Floating Point Unit), ALU (Arithmetic and Logic Unit), registers (very quick memory 1-3 cycles to access), caches (L1, L2, L3 for each core or shared in the whole CPU). The computer also has RAM (80 cycles) and mass-storage memory (milliseconds).

There is 4 big challenges in security:

- Authorisation: who has access ?
- Identification: how to find out who they are ?
- Authentication: how to verify their claims ?
- Accountability: who did what ?

Policy definition and enforcement

a process / document to minimize privacy risk

- OCR recognition
- text: segmentation of characters
- sound, image, text, understanding, logic,
unfakable

human ability

Find false randomness (not a proof to true randomness)

- entropy
- compression
- chi-square
- arithmetic mean
- montecarlo estimation for PI
- serial correlation coefficient)

https://www.fourmilab.ch/random/

Example: Entropy = 7.980627 bits per character.

```
Optimum compression would reduce the size
of this 51768 character file by 0 percent.
Chi square distribution for 51768 samples is 1542.26, and randomly
would exceed this value less than 0.01 percent of the times.
Arithmetic mean value of data bytes is 125.93 (127.5 = random).
Monte Carlo value for Pi is 3.169834647 (error 0.90 percent).
Serial correlation coefficient is 0.004249 (totally uncorrelated = 0.0).
```

- requires redundancy in different instruction sets
- equivalent instructions to encode some bits
- used also as watermarking (weak stealth so bad steganography - average robustness)