Original subtitles

1

- [Instructor] As we progress through the course,

2

we'll use our newly gained knowledge

3

to develop a file encryptor.

4

We'll base this on the World War II

5

three rotor Enigma machine.

6

As we can see, the Enigma has a plug board at the front

7

and a set of rotors located in slots.

8

There could be up to eight rotors and for any session,

9

three would be selected and placed in the machine.

10

The rotor consists of wires,

11

which connect two of the 26 characters of the alphabet,

12

therefore providing a full alphabet substitution.

13

For each rotor,

14

the alphabet wheel can be set to start character.

15

So a single rotor could be in one of 26 possible states.

16

The way the Enigma works

17

is that a character is entered on the keyboard,

18

it passes through the plug board,

19

and may be exchanged with another character.

20

Up to 10 characters of the alphabet could be exchanged.

21

The character that comes out of the plug board

22

is then passed to the first rotor.

23

When a character arrives at the first rotor,

24

it substitutes it, and then turns the rotor one position.

25

The substituted character is then passed

26

to the second rotor,

27

which does the same substitution using its coding.

28

It would only step after a complete rotation

29

of the first rotor.

30

After passing through the third rotor,

31

it'S reflected back through the rotors

32

and reverse substituted.

33

The final character is then passed

34

through the plug board,

35

where it may or may not be substituted

36

and becomes the next character in the cipher message.

37

The reflector was the cause of some weakness in Enigma,

38

but it did mean that decryption was very easy.

39

Just run the cipher through the machine

40

and plain text appears.

41

We can see then that our program will need to provide

42

for a set of rotors, which have coded settings,

43

a plug board, and a fixed reflector.

44

We'll allow the user to select three rotors,

45

set the alphabet start position for each rotor,

46

and enter two plug board connections.

47

We'll use an abbreviated alphabet of zero to F

48

to cover each of the hexadecimal characters,

49

and this will allow us to process readable messages

50

as four bit nibbles.

51

The program when run will read a file

52

and process it nibble by nibble,

53

passing each nibble through the plug board and three rotors,

54

and then returning it via the plug board to be written.

55

The first rotor will turn after each nibble,

56

the second one, every 16 nibbles,

57

and the third, once every 256 nibbles.

58

The point at which the second and third rotors turn

59

is determined by what is known

60

as a notch in the previous wheel,

61

which we'll simulate by configuring a notch position.

62

One of the advantages of this design

63

is that if you feed in the cipher using the same settings,

64

then it reverses the ciphering

65

and you get plain text out.