Cryptography by Epul

@agatestudio

Cryptography

M Saepul

Knight

Agate Studio

M. Saepul Ramdani

Cryptography

What is …

Crypto = secret

Graph = writing

Or

Cryptology = science of secret

Alice

Bob

M

Chester

Alice and Bob are communicating via insecure channel

(nguping)

How

We need “machine” or “function”

To modify original message to cipher text.

And then…

We need a “secret”, we need key

More details

Encryption E (m, k) = c;

Decryption D (c, k) = m;

D ( E ( m, k ), k ) = m

Alice

Bob

c

Chester

Alice and Bob are communicating via insecure channel

(nguping)

E (m, k1) = c

D (c, k2) = m

Example

Vigenère cipher

TheVigenère cipher is a method of encrypting alphabetic text by

using a series of different Caesar ciphers based on the letters of a

keyword. It is a simple form of polyalphabetic substitution.

Vigenère cipher

Example:

m = Z

k = S

c = R

m : AGATE

k : MLAML

c : MRAFP

Burial at the sea

Periodic table

There’s nothing to do with that

chemical structure

(But we need look at the atomic numbers)

Sc-Sc-Ti-P-Li-Cl-Na-Mg-P-Be-Mg

21-21-22-15-3-17-11-12-15-4-12

U-U-V-O-C-Q-K-L-O-D-L

Remember the Vigenère cipher!

And also, we have SUCHONG as key

Why SUCHONG ? Because you should never underestimate

the fallibility of an egomaniac

Next?

U-U-V-O-C-Q-K-L-O-D-L

S-U-C-H-O-N-G-S-U-C-H

C-A-T-H-O-D-E-T-U-B-E

Cathode Tube

NB: Actually, that wasn’t good implementation

When we start use our

implementation?

We won’t and we mustn’t

Because, we need a secure cipher which proven by the expert

mathematically and statistically

Unless, we do it for fun or learning

Perfect but impractical

The cipher called perfect when there’s no clue/information

about key or message on cipher text

Why impractical ?

(We will discuss it after next slide)

One Time Pad

Use XOR operation

1 ^ 1 = 0

1 ^ 0 = 1

0 ^ 1 = 1

0 ^ 0 = 0

Quiz: z = x ^ y ^ x; z = ?

z = y (whether value x and y)

M ^ K = C

… back to impractical

One time pad is a perfect cipher

As long:|k| >= |M|

Closure

Next time, we’ll discuss more about keys, modern cipher,

how it works, how we use it etc.

Uh… Question, anyone ?

Thank you