MR01001101 :: cryptography and steganography resources

A brief note on early cryptography and the Caesar Shift for beginners

What is cryptography? Cryptography is the process or skill of communicating in or deciphering secret writings. Its name stems from the Greek kryptós, meaning "hidden" and gráphein meaning "to write".

Why use cryptography? There are many reasons such as confidentiality and security. Cryptography has always gone hand in hand with war and war has probably, like so many things, defined its evolution. As will be discussed later it offers privacy to all of us, at least in principal.

Here are some common terms that are used throughout:

• Cipher - Secret method of writing
• Ciphertext - The encoded text (often shown in CAPITALS)
• Plaintext - The unencoded or original text (often shown in lowercase)
• Encryption - The process of converting plaintext into ciphertext
• Decryption - The process of converting ciphertext into the original plaintext

Now armed with some terms, its time to look at some history.

Around 3000 BC in Egypt the Hieroglphs appeared (oddly, as a fully formed system), but significantly around 1900 BC the most senior scribes began to introduce non-standard heiroglyphs. At the time, these enrypted messages were only understood by the elite priesthood and senior officials. This is claimed to be the earliest form of cryptography.

In some degree, it reappeared loosely as cryptography in 1799, with the discovery of the Rosetta stone (shown left) by French soldiers. Although originally written in plaintext, time had obscurred any knowledge of reading the hieroglphs, therefore encrypting its secrets until 1822, when Jean-François Champollion deciphered the hieroglyphs.

The Caesar Substitution (or shift) Cipher. Julius Caesar, born 100 BC, is said to have used a shift cipher to send military orders and is the origin of the cipher name. At the time _most_ of his enemies were illiterate and would not been able to even understand the plaintext, let alone the cipher text. But all good military leaders respected the power of information and this is why spies and espionage have existed as long as the history of war. Caesar was no exception and this may have been what promted his use of ciphers.

The very early ciphers often used the method of substitution in order to scramble the plaintext into ciphertext. Substitution is the process of exchanging each letter for another in the alphabet. The simplist method is a shift-cipher, which substitutes each letter with another x places along in the alphabet.

For example:

The plaintext thecaesarsubstituioncipherwasshiftedtothreeplaces, when shifted to three places would give the cipher text WKHFDHVDUVXEVWLWXLRQFLSKHUZDVVKLIWHGWRWKUHHSODFHV

Using a plaintext alphabet of abcdefghijklmnopqrstuvwyz and a cipher alphabet (shifted +3) of: DEFGHIJKLMNOPQRSTUVWXYZABC

Therefore, with a shift of 3, a = D and f = I and so on. A shift of the entire alphabet gives 25 unique cipher possibilities, with a shift of 26 bringing you back to your original. Unfortuantely though, before you go and encrypt your most sacred secrets - a maximum of 25 attempts to crack it!!!

The next note will be on more interesting substitution ciphers, plus look out for the notes on deciphering the Caesar cipher...

Monty 19/05/2004 :: Back Home

Did you know...