先日日本を訪問中のマレーシアの高校生向けに
模擬授業を行いました．
On Mathematics in Cryptography
Ciphers are techniques for hiding the meaning of a message that only
the your co-worker can read it.
For example, you want to send a message to your friend with
writing it on a card. Imagine the situation that you have send
with helps of some other students. They can see it, when they take
it from the previous student.
This is something that you would like to avoid.
So what can you do ?
Well, one thing you can do it to change message according to a rule that
you and your friend agreed with.
This kind of technique is generally called cryptography, and each rule is
called a cipher.
Let me introduce some more technical terms.
The sentence that you would like to hide is called a plain text.
By applying an encryption algorithm on the plain text, you can obtain a
cipher text.
We note that this process requires not only an encryption algorithm but
also an extra information called key
The text will be sent to your friend, that is an appropriate receiver.
The cipher text may be intercepted by an enemy.
But the enemy will not be able to decipher it, because they do not know
how to do it.
Your friend will decipher it to obtain the original plain text with using the
algorithm and a that you and he have already agreed with.
Here is an example of a cipher.
Julius Caeser, a general of the Roman Empire used a cipher using the
algorithm that shifts the alphabet three times.
For example this plain text “this is a pen” is ciphered to “wklv lv d shq”
Well some of you may see how can get variety of Caesar ciphers by
changing the numbers of shifts among 1 to 25.
In this case the algorithm is the rule to change each alphabet by shifts,
and the key is the number of shifts that are applied to each alphabet.
(As you can guess, this is a weak algorithm.)
