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Forum Home --- Satellite Communication Tutorials --- Tutorials - Course 5 - Digital Measurements & Tests
Encryption and Bletchley Park
Location: United Kingdom
Topic: Encryption and Bletchley Park
Posted: 14/August/2008 at 16:37
Encryption and Bletchley Park - Codes & Code Breakers
Data Communications usually require some form of encryption in order to keep the data safe from interception.
Encryption is the term used to encode some data (or in simpler terms some text) so that it is no longer readable. In order to read the data it must first be decoded. The process involves a key which is only known by the sender of the data and the receiver of the data.
This key is (pardon the pun) the key to encryption. The term crypto is short for cryptography which is the process of encryption and decryption.
There follows a simple demonstration and tutorial in encryption.
The Caesar cipher, named after Julius Caesar, is one of the best examples of early cryptography. Caeser is thought to have used it even if he didn't actually invent it.
Here's how it works:
Take two pieces of paper and write out the alphabet at the top of each piece of paper:
Next write your message:
MEET ME AT TEN TONIGHT
Line up the two alphabets so that they are one or more letters out:
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
For every letter in your message appearing on the top line, write down the corresponding letter on the bottom line.
MEET ME AT TEN TONIGHT becomes KCCR KC YR RCL RMLGEFR
Your message is now Encrypted. Encryption uses a Key which in our case is two, move two letters along in the alphabet, and using the key to encrypt the data creates an Algorithm. In this case substitute a letter with another letter which is two along the alphabet.
If the person recieving our message has our key and uses the algorithm they can decrypt our message.
The Algorithm becomes move two letters back along the alphabet, the key being two. Also reversing the rows of the algorithm makes life a little easier.
Y Z A B C D E F G H I J K L M N O P Q R S T U V W X
KCCR KC YR RCL RMLGEFR becomes MEET ME AT TEN TONIGHT
This is fairly easy to crack if you guess the simple algorithm. There are only 25 possible keys as a key value of 0 or 26 has the effect of no encryption at all.
This is a very basic form of encryption but using much more complex algorithms and keys this method is essentially unchanged and is still in use today. The algorithms are extremely complicated and the keys very long so thats why the codes are virtually impossible to crack.
So what have we done to encrypt some text?
The original message is called Plaintext and the encrypted message Ciphertext.
So to decrypt this message we need:
Using the same algorithm and key to encrypt and decrypt is called Symetric Encryption.
There are many algorithms available which are far harder to break than the Caesar cipher such as Rijndahl, Blowfish, RC2, RC4, Triple DES, CAST. These algorithms use key sizes that are enormous by comparison to our Caesar cipher. For more information on encryption and the code breakers who helped win world war 2 - visit the Bletchley Park - Station X website. Bletchley Park in England is was the home home of the first ever computer. Now known as GCHQ, Bletchley Park was instrumental in cracking the Enigma Machine.
In order to improve on the Caeser cipher we just need a new key and algorithm. To get this we must think about how we can make the code more complex. If the first letter is substituted with the 5th letter of the alphabet away from the original letter and the 2nd letter substituted with the 18th and so on we can build up a long key and complex algorithm.
The key may be 5, 18, 14, 17, 18, 8, 4, 6, 2 repeated.
The algorithm would be substitute each letter with the corresponding key letter of the alphabet.
HELLO GEOFF becomes MWZCG OIUHK
If we included the space in our encryption, life would only get harder for the code breakers.
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 SPACE
HELLO GEOFF becomes MWZBGHKKQKX which is much harder to decifer.
Using binary data and simple XOR logic the following is the basic principle for most modern encryption methods.
In this example 10101010 is they key and 10111001 is the plaintext data.
See also : http://en.wikibooks.org/wiki/Cryptography for an in depth explanation.
You will find that encryption and decryption algorithms involve a lot of maths which can be quite daunting. But you are unlikely to ever have to build your own algorithm as there are already commercial companies that provide devices that encrypt data and decrypt data. All you are likely to have to do is change the key occassionally, using a swipe card or other method of key loading, and also understand how the devices work. Hopefully now you have some idea of the latter.
RC4* Secure Talk Software
Now you can try this out for yourself because we have written a simple chat program for you.
Using our software on two computers connected via a network or the internet the text you send is Encrypted using an RC4 type of algorithm and is decrypted at the other end. To connect you need the IP address of the other computer which will auto answer. The Tx & Rx lights flash as the data is sent and received and you can talk to two computers at once with the Dual View mode.
You can see the encrypted text in small data windows and can even double encrypt the data with your own keys. You must be able to communicate through at least one port on your PC. The default port is 1700 but this can be changed.
This software is available for download in the Satcom Tools section.
* RC4 is a trade secret of RSA Data Security and as such the algorithm used in Secure Talk is merely based upon the RC4 algoritm and can not be said to be an actual RC4 algorithm since no one outside of RSA knows about RC4 in sufficient detail. The source code for RC4 was allegedly posted on the internet anonymously but since the source has never been authenticated algorthims based on it can not be directly related to RC4. The Algorithm used by Secure Talk is nonetheless a very strong encryption algorithm.
Edited by Satcoms UK - 16/April/2015 at 22:40
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