Transcript
@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
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