The History of Cryptography (3) — Caesar Cipher
by FOX on 02:59 PM, under History
The alternative of transposition is substitution. One of the earliest description of encryption by substitution can be dated back to first century B.C. During Gallic Wars, the great emperor Julius Caesar, invented the famous Caesar’s Cypher.
Caesar used secret writing so frequently that Valerius Probus wrote an entire treatise on his ciphers, unfortunately it’s not survived any more. However, thanks to Suetonius’ Lives of the Caesar LVI, written in the second century A.D. We do have a detailed description of one of the types of substitution cipher used by Caesar. The emperor simply switch each letter with another one which is three place down from the original one in alphabet. This is called Caesar shift Cipher, or simply Caesar Cipher. A Cipher is the name given to any form of cryptographic substitution in which each letter is replaced by another letter or symbol. The following code illustrated how Caesar Cipher works
| Plan alphabet | 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 |
| Cipher alphabet | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | A | B | C |
Plain-text: how are you
cipher-text: KRZ DUH BRX
Each distinct cipher can be considered as a general encryption method with two 2 elements, a key and a protocol (algorithm). In Caesar Cipher, key is 3, protocol is substituting each letter in the plain alphabet with a corresponding letter in cipher alphabet.
The beauty of this kind of cipher is that it is easy to implement but difficult to break. It is very easy for sender and intent receipt to negotiate a key but infeasible for enemies to check every possibilities, known as brute force attack.
In fact an even simpler key is possible comparing randomly rearranging the plain alphabet to cipher alphabet. The sender chooses a keyword or keyphrase. For example, to use FOXDIE as a keyword. We just simply pick up the six letters, put them at beginning of alphabet, then we get our new cipher alphabet.
| Plan alphabet | 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 |
| Cipher alphabet | F | O | X | D | I | E | A | B | C | G | H | J | K | L | M | N | P | Q | R | S | T | U | V | W | Y | Z |
The advantage of building cipher alphabet like this is that it is very easy to remember the keyword or keyphrase, then hence the cipher alphabet. This is important because if the sender has to keep the cipher alphabet on a piece of paper(at that time, normal people like a messenger would not know how to read or write), the enemy can capture the paper and discover the key.
