Cryptography > Introduction (15 min.)      

Learn how to use and navigate through the tutorial such as

- using the interactive web pages 

- challenging yourself by breaking ciphers yourself   

- using CTRL-A to check hidden answers

Welcome to the world of disguising information. Historically, cryptography - the science of encoding and decoding information - was mainly used for military purposes. You may recall the ENIGMA machine used by the Germans in WW2 to communicate secretly. Today, encryption has become crucial for secure electronic communication such as credit card transactions over the Internet, email privacy, etc. Can we trust these secure channels? Do they provide sufficient security or do we jeopardize ruining our checking account? You will learn which of the many encryption methods are secure and which ones you better don't rely on.   

This tutorial offers an interactive introduction to cryptography. It allows you to encode, to decode and to break secret messages using interactive web pages. Additionally, you will have the chance to "explore" newly introduced ciphers for yourself. By encoding, decoding and breaking a variety of words on your own you will gain a good command of a wide variety of encryption methods. Starting with simple classical ciphers and ending with today's most prevalent "RSA" Cipher, your ultimate exercise will be to perform the encoding and decoding of each cipher using paper and pencil. In that way, you will  

Let's consider the classical "Caesar Cipher" as a concrete example: Simply encode "safe" by hitting the "ENCODE ==>" button. Then, the encoded text appears on the right. To vary the messages to encode, replace "safe" by another word or sentence. Hint: words such as "aaaaa" , "aabbccdd" or "aabbxxyyzz" often help to reveal the underlying encoding method. Go ahead and try it now. 

To decode, insert the encoded text at "Recipient reads" and hit the "DECODE==>" button. Once you have grasped the encryption method, use paper and pencil to encode words and sentences yourself. You will find exercises on each page that you can use to test if you really understand the encryption method.   

Sender writes:  Cipher text

Recipient reads: Plain text

Naturally, I will explain how each cipher works and how it can - if at all - be broken. Most ciphers that we will study can be broken. Surely, nobody uses breakable ciphers to encrypt sensitive information. However, in order to understand today's unbreakable ciphers (i.e. the famous "RSA-Cipher") we have to study the classical ciphers first. Each of the classical ciphers adds a little piece to our cryptographic insights that will eventually enable us to understand the RSA Cipher - the ultimate cipher in this tutorial. Therefore, follow the order of the ciphers in this tutorial: go from left to right, from top to bottom in the head menu. 

Every exercise has an answer written next to it. However, it is hidden so that you don't see it right away. I am using Steganography - The Science of Concealing Information - by placing white letters on a white background. To make the answers visible simply press Ctrl-A simultaneously. This highlights the entire text. Test it here --> Do you know any other Steganography tricks?   

Each cipher starts off with a "cipher challenge" that can be solved - guaranteed - if only you encode a sufficient number of messages and observe closely how they are encoded.  By doing so you also time-travel through the history of cryptography. You will start off with classical encryption methods used by the Greek as well as today's encryption methods that assure that your credit card number remains secret when purchasing Christmas gifts over the internet, and that your email can only be read by the intended recipient.  

Going through this tutorial and learning the history of cryptography should be enjoyable. No prior knowledge is required. You just have to know how to add and multiply numbers. I will explain everything beyond this within the tutorial. 

My suggestion to profit most from this tutorial: Challenge yourself, try to figure out as many ciphers as possible by yourself. If only you try hard enough - I promise you - you can. The benefit: A "Cipher Challenge" awaits you at the end of the tutorial. You will find 8 cipher texts that were encoded using the learned encryption methods. Each can be broken with the acquired knowledge in this tutorial. If you are able to crack all of them email me your answers at . Prizes await you. 


Start the cryptography tutorial now.    

Related web sources:

Read Encarta's Introduction to Cryptography

Enigma and the Codebreakers


PBS Online

Introduction to Cryptography

Yahoo's Encryption & Security


RSA Securities - Introduction to cryptography 

Free Newsletter

FAQ - Cryptography




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