1. @agatestudio
Cryptography
M Saepul
Knight
Agate Studio

2. M. Saepul Ramdani
Cryptography

3. What is …
 Crypto = secret
 Graph = writing
Or
Cryptology = science of secret

4. Alice
Bob
M
Chester
Alice and Bob are communicating via insecure channel
(nguping)

5. How
 We need “machine” or “function”
 To modify original message to cipher text.
 And then…
 We need a “secret”, we need key

6. More details
 Encryption  E (m, k) = c;
 Decryption  D (c, k) = m;
D ( E ( m, k ), k ) = m

7. Alice
Bob
c
Chester
Alice and Bob are communicating via insecure channel
(nguping)
E (m, k1) = c
D (c, k2) = m

8. 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.

9. Vigenère cipher
Example:
m = Z
k = S
c = R
m :AGATE
k : MLAML
c : MRAFP

10. Burial at the sea

11. Periodic table

12. 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 theVigenère cipher!
 And also, we have SUCHONG as key
 Why SUCHONG ? Because you should never underestimate
the fallibility of an egomaniac

13. 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
CathodeTube
NB:Actually, that wasn’t good implementation

14. 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

15. 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)

16. 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

17. … back to impractical
 One time pad is a perfect cipher
 As long:|k| >= |M|

18. Closure
 Next time, we’ll discuss more about keys, modern cipher,
how it works, how we use it etc.
 Uh… Question, anyone ?
 Thank you 