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Send your own secret code messages!

This webpage can be used to create secret spy messages that you can send to your friends (and also allow you to convert them back into a readable message!)

To send a coded message to your friend, you must both agree on the two "Code" numbers. These are the numbers that allow you both to make coded messages and be able to convert them back to something that is readable. You must keep your code numbers secret. Never send them along with a text message or somebody else will be able to read your messages ("break" your code).

Type your message into the top text box (you will need to click inside the box first), then enter your two secret numbers into the "Code" boxes (use the TAB key on your keyboard to easily move between the boxes). If your message is in normal writing then click the encipher button and if it is a coded message (you can't read it) then press the decipher button and the message will be displayed in the text box under the buttons with a red background.

Converting a normal message into code is called "enciphering" or "encrypting" the message. Converting a message in code back into something that you can read is called "deciphering" or "decrypting" a message. This system of converting meaningful messages into something that can't be easily read by other people has been used for a long time to pass secrets. The technical term for this process is called "cryptography". Cryptography is very important in the modern world - it protects all our secrets from messages sent during war down to the details of our online shopping transactions. It is the main reason why electronic computers were invented in the first place! They were needed during the second World War to break very difficult secret codes, the most famous of which is called the Enigma Cipher.

How does it work?

The method used to convert from a plain message (called "plaintext") into a coded message (called a "ciphertext") is called a cipher. There are lots of different ciphers, some of which are easier to break that others. Modern ciphers are very complicated and even the fastest computers in the world cannot break them quickly.

The cipher we are using is called a "substitution cipher". In the form that we are using, it is quite easy for a modern computer to break (if you know how!) but before computers it was very commonly used.

A subsitution cipher works by replacing (substituting) each letter of the alphabet by a different one. For example, all letter "a"s are changed to "x" and the letter "x" is not used again. As long as you know which letter was used to convert the message, you can convert it back again.

Our code maker program generates a list of which letters are converted to other letters by using some simple maths. This means that instead of having to remember what each letter is subsituted by, we can just enter a few simple numbers and let the computer work it out.

As mentioned above, our secret code is two numbers. We do this to make the message harder to break because each of the numbers is used to change two different methods that we use to create our subsitution list. Lets look at each part of the secret code and what it does by doing some experiments using the code maker at the top of this page...

First of all, type in all the letters of the alphabet "abcdefghijklmnopqrstuvwxyz". Then enter the code numbers 0 and 0 into the two code boxes. Press the "encipher" button and you should see "abcdefghijklmnopqrstuvwxyz" in the code box. This means the message has not been enciphered (so don't use the codes 0,0 for sending a message!!). Now, enter the number 1 into the first code box and press "encipher" again. This time you should see that the code has worked. The letter "a" is now "b", "b" is now "c", "c" is now "d", and so on. We have replaced each letter by the letter which is 1 place higher in the alphabet. If there is no letter one place higher (in this case there is nothing after "z") then we go back to the start of the alphabet. Try entering a 2 in the first code box - the letters will all be replaced by the letter two places higher in the alphabet. This method is called "shifting".

Interesting fact: the Roman Emperor Julias Ceaser used this method to hide his secret messages!

The second number makes the code much harder to break. It changes the number of letters above the letter to be replaced by a different amount each time. For example "a" will be subsituted by "b" (one place higher), "b" by "d" (two places higher), "c" by "f" (three places higher) and so on. Try changing the second code number in the code maker and see how much more difficult trying to break the code will be.

As I said, a computer can break this code by working out what would happen for each of the code numbers and checking the resulting message against a dictionary to see if the words it has created are real words. Because it can work this out very quickly it can break the code quickly.

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