An Intro To encryption with polyalphabetical cyphering

The basis of encryption can be broken down into two main actions: replacing plaintext with coded text and rearranging text so it is not readable. Polyalphabetical cyphers are a basic form of encryption which replaces plaintext with coded text in an attempt to hide the message being transmitted. To explain polyalphabetical cyphers we should first examine monoalphabetical cyphers.

this table provides a monoalphabetical cypher to code a message by replacing the messages A with the coded g and so forth

A g
B a
C r
D v
E h
F s
G u
H e
I t
J q
K p
L w
M b
N c
O z
P f
Q y
R n
S x
T o
U d
V j
W m
X k
Y i
Z l

taking a message such as THEMESSAGETOSEND and coding it into oehbhxxguhozxhcv

this method of coding is not very secure, the problem is that the message shows too much of itself

in the sample above we have a message with 16 chars
h appears 4 times or 25% of the message, since the letter E appears often in spoken English one could assume that E may be replaced with h in the message

- - E - E - - - - E - - - E - -
also x appears beside another x, so language is again consulted for letters that match together perhaps S would be attempted

- - E - ESS - - - E - - SE - -

already the message is almost half showing, so a need for a more secure method brought polyalphabetic cypers

consider the same message: THEMESSAGETOSEND
we will use three tables this time to code the message, the tables will be used in a order based upon a KEY

KEYKEYKEYKEYKEYK
THEMESSAGETOSEND

the letters with a K above them will be replaced with the letters in the K table, ect..

this method allows our message to hide repeated letters and to lower the frequency of some letters over others. Each table complements the others inorder to minimize the frequency of one letter over another in the coded message

this method is more secure than using one table but still is not uncrackable, the flaw is in the message coded. English has many three character chucks such as "ing" these patterns can be seen in the coded message if enough data is gathered. Once a pattern of three or more repeating characters is found the process of decoding the message moves easily. The factors of the distance between repeating patterns reflect the number of tables or letters in the key, if the coded method is divided into groups based upon the length of the key those groups would be the same as monoalphabetic coded messages.

Although these methods are far from secure, they help with other methods to make messages difficult to uncover.

Nice tut. .. esp. 4 noobz like me..

Quote:

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KEYKEYKEYKEYKEYK
THEMESSAGETOSEND

the letters with a K above them will be replaced with the letters in the K table, ect..

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I was wondering how the decoding worked.. how the original message could be retrieved...
I persume that the k-table is a table like.
A g
B a
C r
D v
E h
F s
G u
H e
I t
J q
K p
L w
M b
N c
O z
P f
Q y
R n
S x
T o
U d
V j
W m
X k
Y i
Z l
..Like aforesaid, e and y should also have a table.

Hope I get u rite.

Yes. Lets say a user inputs this message: I have mail
What would then happen is that the entered text would be altered by the polyalphabetic cypher, which is GOOGLE. G O L E each have its own table for conversions. Therefore it would be encrypted as this:

I g
H o
A o
V g
E l
M e
A g
I o
L o

As you can see this is a very weak encryption because it repeats words for conversion. However a more effective approach to encryption would be like the following:
Each character that is possible for user entry has its own table. First a user enters the text intended to be encrypted, and then a key. The ability to create ones own key can create a much more difficult encryption to crack. For instance: I have mail is encrypted by my own key which is #%Fs94!_SC~.

The key could be a pointer to a bidimensional array. On one side [0][x] there could be the characters to be replaced, on the other side ([y][0]) the characters of the user-defined key. The intersection of the two would yield a replacement character [not exclusively alphanumeric].

The generation of the table can be either through permutations or through a reversible algorithm [since decryption is important too :)]

http://www.amazon.com/exec/obidos/tg...89519?v=glance

For a fairly comprehensive overview of encryption from Ancient Egypt to the Quantum age, read the book from the above link.

http://www.amazon.co.uk/exec/obidos/...&mode=books-uk

For a more varied pallette there are a further 1252 books here.................

http://www.amazon.com/exec/obidos/tg...89519?v=glance

And for those that like a good old fashioned read with their crypto, may I recommend The Cryptonomicon, Fantastic book, lots of ground covered, kept me reading till V late each night till finished...........

[edit]The UK's own Diana Dors (50's 60's starlet / sex goddess) encrypted the location of her fortune, with a hand generated code, that lasted until only recently.

http://www.channel4.com/culture/micr...code/code.html

Also the American starlet / sex goddess Hedy Lamarr got inspiration from a piano keyboard (black and white keys ?) to come up with AND patent 'frequncy hopping'

Quote:

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Although Wi-Fi is the latest New New Thing, in another sense it's something rather old. Classic-movie buffs, take note: Wi-Fi technology was patented during World War II by Hollywood starlet Hedy Lamarr and composer George Antheil, so that radio-guided torpedoes couldn't be jammed by the enemy. As an alternative to using one radio signal to direct the torpedo, their idea (U.S. patent 2,292,387) involved what they called "frequency hopping," constantly changing the radio signal between a large number of bandwidths, which is a basis for today's Wi-Fi communications

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http://www.turner-invest.com/index.c...detail/ID/1153

At times it would appear that EVERYONE is at IT (it) {No: Oh please yourself}[/edit]

ps: nice little tut, maybe a bit TOO little.

Nice intro, a couple of add ons to anyone interested. hidden is right on track with the letter frequency gig, first step in breaking a simple code like the example. Frequency of letters in english from highest are as follows:

E T N O R I A S (remembere the word SENORITA)

Easier to spot than the tri graph info like ING are repeated letters, or doubles like TT in letter, or LL in yellow. Very easy to pick out if a simple cipher like the example is being used.

Hope this helps to demonstrate how telling and important a letter frequency count can be, not only to breaking a code but also in determining it's complexity as well.

There is another way of doing polyalpebetic ciphers. It may be the same idea though.

You have a key word KEY and the message Break this cipher

you arrange the tables according to the keyword and a regular alphabet

KLMNOPQRSTUVWXYZABCDEFGHIJ
EFGHIJKLMNOPQRSTUVWXYZABCD
YZABCDEFGHIJKLMNOPQRSTUVWX
ABCDEFGHIJKLMNOPQRSTUVWXYZ and then take your message

BREAK THIS CIPHER

You encrypt by taking the first line with the first letter (B becomes L) the second letter with the second line (R becomes V), third with the third line, fourth with the first line and so on. Decryption is done in the opposite direction.

The weakness with the fequency count. You know how often E is used usually in the english language T is used some perecent of the time. You take every other letter in the count and keep trying different key lengths until you get "E" to come out the most, "T" the second most and so on until the fequency count looks like a monoalphebic cipher. This type of nondigital encryption is effective with short messages. The longer the message the better the possibility of breaking it.

There are other techniques that you can use to obscure it, like reversing alphabets, adding null characters, and using longer words or changing keywords several times through the message depending whether you need a speedy decryption or strong security.

Breaking it is tedious but possible. Remember that the cipher should be at least strong enough to keep the information secret for as long as the info is valuable.

This method is vulnerable to a tool like crank.
http://crank.sourceforge.net

A perfect tutorial to show how weak polyalphabetical ciphering is, and how it should never be used to "encrypt" anything.

Check out DES, AES, RSA, PGP for better encryption.


This rules:
http://axcrypt.sourceforge.net/

Quote:

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Originally posted here by MK19
Breaking it is tedious but possible. Remember that the cipher should be at least strong enough to keep the information secret for as long as the info is valuable.

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That's the basic rule for any type of encryption. For a code to be effective [appropriate] in a given situation the theoretical time required to break the encryption [brute-forcing or otherwise] has to be greater than the time the information is required as ciphertext. Of course the longer the better, especially if the algorithm is susceptible to being reversed. At that point the time required to break the cipher is exponentially smaller than brute forcing because there is a mathematical modelling for decryption [without a key].

In some cases [like personal fortunes or the pyramids] the algorithm is used/created to have a 'resistance' period that is as long as possible. Generally speaking, most digital encryption algorithms are written with the purpose of being unbreakable for as long as possible. However with increasing computing power there is an exponential drop in the period required for brute forcing even some hard algorithms.

I would have to say that this is the magick decoder ring of ciphers, but on the other hand the fact that it is so easy to understand how it works that it is a good starting point.