Cyber Security Expo
 
Basics of Cryptography by Luser on 04/09/02

I expect you have read one or to papers on cryptography...like everyone else You read, looked confused, felt confused, and a few probably ran away and Cried. Why? Because the encryption used today"s is industrial strength. That"s Because its has to be as our lives rely on information, and so do businesses, So encryption has to stronger than an iron sponge soaked in super glue to keep Our secrets safe.

So any papers on the subject start at the top because that is the standard today. But I will start at the beginning (that"s original) so you can learn the Basics of cryptography and build from there.


The overview

Here is a simple diagram of how the encryption system works...


Plain text - what you wrote
|
Encryption
|
Cipher text - what it looks like after encryption...so no one can read it
|
Decryption
|
Plain text - again

So there are five stages of encryption...some of you will now be shouting at Your screens that I am wrong...as there are keys and stuff...I will talk about This in another paper as this is advance stuff :o,


Monoalphabetic substitution

The very basic of cryptography is that horrible long word written above you. What I mean is that you replace the letters of the alphabet with other Letters.

For example...

a b c d e f g h i j k l m n o p q r s t u v w x y z = plain text
c d e f g h i j k l m n o p q r s t u v w x y z a b = cipher text

So if i wanted to write the message "script kiddies like tuna" i would write...

Script kiddies like tuna
uetkrv mkffkgu nkmg vwpc

Looks like good encryption doesn"t it? Could you crack it? Don"t worry...i will Tell you how later :o) This method of simply shifting the alphabet is called the Caesar Cipher...guess who it was invented by :o? To crack this encryption is easy...because once you have found one letter you Can just unshift the cipher text. For example this cipher is just shifted two Spaces to the right. So this encryption is weak. We can make it slightly Stronger by assigning each letter another random letter...like this.

a b c d e f g h i j k l m n o p q r s t u v w x y z = plain text
w q r e t y u i o p a s d f g h j k l z x c v b n m = cipher text

This is slightly stronger as when you get one letter...it doesn"t unlock the Rest. So how do you crack it?


Cracking a monoalphabetic substitution cipher

That is a very long scary word...even to me :-|
So how do you crack this *cough* super strong cipher? We use a thing called Frequency analysis...because you see in the English language each letter has a personality...and appears a certain number of times. For example the most Common letters in the English language are E, T and A. so if i took a piece have Cipher text...and counted all the letters and took a percentage...and compared To the average percentage...i can decrypt the letter.

Here is the table of frequency analysis for the English language...

a - 8.2%
b - 1.5%
c - 2.8%
d - 4.3%
e - 12.7%
f - 2.2%
g - 2.0%
h - 6.1%
i - 7.0%
j - 0.2%
k - 0.8%
l - 4.0%
m - 2.4%
n - 6.7%
o - 7.5%
p - 1.9%
q - 0.1%
r - 6.0%
s - 6.3%
t - 9.1%
u - 2.8%
v - 1.0%
w - 2.4%
x - 0.2%
y - 2.0%
z - 0.1%

So if i had a piece of encrypted text...i could count all the letters...then i would assume the most common letter in the cipher text...say p...is really e...and the next most common say l... could be t. You do this all the way thought and you should start to see words forming. There is alot of guess work and fill in the blanks. Also trail and error as the most common letter in the text may not be e...but a...so you have to think on you feet.

The only problem with frequency analysis is that the encrypted text has to at least be over a hundred words...because if it is short the averages are all screwed...and using frequency analysis would be hard or impossible as some letters would not match there percentages.

Another way to crack it would be to look for cribs. For example we have the following encrypted word-

lppg


What could it be?? Frequency analysis wont work as it is too short. But look...a double letter. We can use this as a crib as there are only some letters which can be doubled, these are...

ss
ee
tt
ff
ll
mm
oo

So the middle two letters must one of these pairs...we can narrow it down as there are few or no words in the English language which and four letters long and have to constanents in the middle... so it must be either-

oo
ee

So we have narrowed down a letter to one of two possibilities...together with the rest of the cipher text there would be more clues to these letters identity. Another cribe is single letters...take this for example-

plmk r qsrf

This cipher text has a single letter in...and this letter must be either A or I.


Other cribs

The most common three letter words in the English language are-

and
the

the most common two letter words are-

of
to
in
it
is
be
as
at
so
we
he
by
or
on
do
if
me
my
up
an
go
no
us
am


Conclusion

Well there you go...you should hopefully understand the very basic of encyption... I will explain other forms of encryption in other papers...

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