An Alphabet is a set of symbols used to write down thoughts, speech, actions, news, declarations, messages etc. Different spoken languages have different alphabets to write them down. We are generally used to the "Roman" Alphabet ABCDEFGHIJKLMNOPQRSTUVWXYZ and its lower case equivalents, in which these forum messages are written and displayed.

A Code is, usually, a means of converting the symbols of an alphabet into a different format, often for transmission purposes. Thus MORSE CODE is a means of presenting the alphabet as sequences of dots and dashes for transmission by the simplest of radio or wire signals OFF/ON. The Telegraph Code was much the same thing but designed for use by machines. it is a sequence of ONs and OFFs each of fixed time length and the presence or absence of the signal in those fixed intervals constitutes the code. The original had 5 intervals with variable On/Offs (we call these BITs) and so could have 32 different codes, 26 used for the letters of our Roman alphabet and the others used for punctuation symbols.

Later telegraph codes have 7 or 8 bits, so can code 128 or 256 symbols. The relationship between codes and symbols were standardised long ago first as ASCII (American Signal Code for Information Interchange) and with more recent additions to the specifications is the 8-bit code that is used in our computers to represent alphabetic symbols and punctuation.

Codes are usually open knowledge, thus anyone has access to the code definition so anyone can translate a coded message into the alphabet to read it.

However a second use of our word CODE is to create a 'secret' dictionary whereby a single word or number is used to stand for an unrelated concept. This enables secret communication between individuals who have copies of the Code List (usually called a code-book), often used for wartime command signals.
So if I synthesise an example, we might have a code-book which contains among many others:

grave = 0
gross = 1
eyelid = 3
lout = 7
glider = cargo ship
ruffian = destroyer
shoe = battleship
label = attack
oblong = dawn

The code-book system is clumsy and rendered useless if the 'enemy' captures a copy of the code-book as it is not easy to re-issue a new one to say a submarine at sea.

A CYPHER is a means of deliberately confusing the usual symbols (or codes) of an alphabet to render the message unreadable to anyone who does not know the KEY, which is the information about what confusion method was used. The simplest cypher dates back to Roman times and was used by Julius Caeser so it is known by that name. It consists of shifting the symbols of the alphabet along by a fixed number.

For example if the shift is 2 we can write down an encoding decoding sheet as

ABCDEFGHIJKLMNOPQRSTUVWXYZ
CDEFGHIJKLMNOPQRSTUVWXYZAB

To encypher (or encrypt) a message we write it down (plaintext). Then underneath each letter we write the letter shown below in the table above. thus:
VENI VIDI VICI becomes ciphertext: XGPK XKFK XKEK
to decrypt you perform the opposite action. Write down the ciphertext then find each letter on the bottom row of the table and write down the letter above it.

The Caeser cipher is easily broken either by knowing the average proportions of the number of times letters appear in the language of the plaintext or by brute force, there are only 26 possible coding sheets.

Bill.

Revised on 26 Nov 2012