Cryptology - Unofficial St. Mary's College of California Web Site

Chapter 1
Caesar Ciphers
There are also letters of his to Cicero, as well as to
his intimates on private affairs, and in the latter,
if he had anything confidential to say, he wrote
it in cipher, that is, by so changing the order of
the letters of the alphabet, that not a word could
be made out. If anyone wishes to decipher these,
and get at their meaning, he must substitute the
fourth letter of the alphabet, namely D, for A,
and so with the others.
Suetonius
De Vita Caesarum
(The Lives of the Caesars)
The first true use of secret writing in recorded history is due to Julius Caesar,
at least as explained by the Roman historian Suetonius. There had been
earlier uses of what David Kahn, in his masterwork The Codebreakers: The History
of Secret Writing, calls “proto-cryptography,” such as complex Egyptian
hieroglyphics and certain stories in the biblical book of Jeremiah (see verses
25:26 and 51:41). 1 But it is the Roman general Julius Caesar who apparently
actually invented cryptography, the art and science of designing methods to
send secret messages.
Caesar’s method for making his messages secret is straightforward: replace
every letter in a message with the one three letters down the alphabet. So, as
Suetonius explains, a is replaced by D, b is replaced by E, etc. Of course, Caesar
would have probably sent his message in Latin, but the point is clear.
This rather simple idea, replace the letters in your message by other letters
according to some rule, constitutes a cipher. One enciphers a message to make
it (hopefully!) secret, and deciphers a secret message to make it readable.
1 It is all but impossible to write a book involving the history of cryptography without
making extensive use of The Codebreakers [Kahn]. In fact, most uncredited references in this
book come from [Kahn].
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