Posts Tagged ‘ciphers’

A Basic Introduction to Cryptanalysis

When you look at a cryptogram for the first time, they nearly always look rather daunting after all how are you expected to decipher all that code!  However there are certain techniques that can make the task much less daunting and in some senses it is actually quite fun to work them out.

One of the most basic procedures is that of frequency analysis.  In fact without this technique you won’t get very far in understanding any of the procedures behind substitution cryptanalysis. So let’s try and briefly explain what’s behind this technique in an effort to expand our mathematical education.

Cryptanalysis relies on the fact that all letters of a specific language have specific characteristics or personalities.  To the ordinary observer all the letters might look fairly similar, but to the analyst they will know the specific traits and characters of each individual letter.

 

GJXV EHRT NUCOT WMOHY UWGK HWIE

GJXV EHRT NUCOT WMOHY UWGK HWIE

GJXV EHRT NUCOT WMOHY UWGK HWIE

GJXV EHRT NUCOT WMOHY UWGK HWIE

GJXV EHRT NUCOT WMOHY UWGK HWIE

GJXV EHRT NUCOT WMOHY UWGK HWIE

GJXV EHRT NUCOT WMOHY UWGK HWIE

GJXV EHRT NUCOT WMOHY UWGK HWIE

Don’t try and decipher this example though – there’s nothing there.  The cryptanalyst would initially begin by counting each letters frequency and it’s contacts (the contacts are the letters which are adjacent. They would then construct a frequency table based on the text.  This can then be compared with a standard frequency table based on a similar number of words.  This table will list the likely frequency of each individual letter – i.e how often it would be expected to appear.
Unfortunately it is rarely as simple to line up your standard frequency table and the one you created to solve the cryptogram.  They are very unlikely to be identical even for the reason that they will be based on a different number of characters.  However it is surprising how little relative frequencies shift from one piece of text to another.  You will almost always find e,t,a,o,n,r, i, s and h in the high frequency areas whereas d,l,u, c and m will normally be found in the medium frequency group.
It’s useful to see how the basics work in encryption, especially if you use such technology to protect yourself online.  For example I can use a technology to spoof my IP called a Virtual private network which creates an encrypted tunnel between the client, effectively hiding the IP address of my client.
Using frequency you can narrow down to distinct groups each individual letter, often with very accurate results.  However you need more than this to focus more specifically on a possible solution and this is where ’contacts’ are important.  Every letter has a cluster of associations that are likely to occur.  In fact an experienced cryptanalyst can spot these associations almost without thinking when presented with a frequency distribution and a tally chart.
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Ciphers, Encryption and the Internet

The term cipher implies a technique of keeping information secure and unreadable accept to the intended recipient.  It’s often described as ‘secret writing’ – and involved transforming normal text into a cryptogram that is an encrypted message.  This process is described as encipherment and bringing back  the message from it’s encrypted state is known as decipherment.

There are many types of cipher but in general they fall into three major classifications.

Concealment

The concealment cipher involves hiding the true letters of the message by whatever method is available.  Perhaps the most striking part of this cipher as that it is intended to obscure that the message is encrypted or protected in any way.  For instance it could be hidden as a phrase in a normal message or perhaps concealed in an image or graphic.  If the concealment cipher is successful then nobody other than the recipient should be aware of it’s existence.

Substitution 

The  Substitution cipher,  involves replacing the original characters of the message with substitutes and these are arranged in the same order as the originals.  There can of course be many combinations of these substitutions in a single cipher or message.

Transposition 

In this method the true letters of the message are  taken out of their original order and then rearranged.  This arrangement is done by using an agreed pattern or key agreed upon by the correspondents.

Every encryption method will normally fit into one these categories.  This includes all the advanced modern encryption methods like DES or AES.   For example the well used and highly secure Advanced Encryption system used by the US military and the most secure anonymous surfing methods here http://www.anonymous-proxies.org/ – it is actually a form of substitution cipher.  AES was originally known as Rijndael and  the substitution method is quite complicated and uses a series of permutations in order to further secure the plain text.

Over human history encryption has been used to protect communication methods,  however the most important application and developments are now to be seen in the areas of electronic communication.   The pressing urge for effective security online is partly because of the huge amount of data being transmitted electronically but also because of a fundamental weakness of a major infrastructure component.

We are talking of course about HTTP – HyperText Transfer Protocol, which is the primary means of access the billions of pages of content on the internet.  The issue is that this protocol operates completely in clear text which means that the information is not protected in any way.  As with traditional messages, clear text means that anyone can read the content if they have access to it – much like a postcard in a post office sorting office.  It is essential that communication is protected by some form of encryption method which is able to deal with the intense demands of the internet.

 

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