In contrast to modern cryptographic algorithms, most classical ciphers can be practically computed and solved by hand.
The term includes the simple systems used since Greek and Roman times, the elaborate Renaissance ciphers, World War II cryptography such as the Enigma machine and beyond.
In contrast, modern strong cryptography relies on new algorithms and computers developed since the 1970s.
All spaces and repeated letters are removed from a word or phrase, which the encoder then uses as the start of the cipher alphabet.
This makes the message much harder to decode because the codebreaker would have to figure out both cipher alphabets.
Another example of a polyalphabetic substitution cipher that is much more difficult to decode is the Vigenère square, an innovative encoding method.
The pigpen cipher uses a grid system or lines and dots to establish symbols for letters.
In a transposition cipher, the letters themselves are kept unchanged, but their order within the message is scrambled according to some well-defined scheme.
In a columnar cipher, the original message is arranged in a rectangle, from left to right and top to bottom.
Next, a key is chosen and used to assign a number to each column in the rectangle to determine the order of rearrangement.
For example, during England's Civil War Royalist Sir John Trevanian was aided in his escape from a Puritan castle in Colchester by this message:[3]WORTHIE SIR JOHN, HOPE, THAT IS YE BESTE COMFORT OF YE AFFLICTED, CANNOT MUCH, I FEAR ME, HELP YOU NOW.
I KNOW THAT, IF DETHE COMES, IF ORDINARY MEN FEAR IT, IT FRIGHTS NOT YOU, ACCOUNTING IT FOR A HIGH HONOUR, TO HAVE SUCH A REWARDE OF YOUR LOYALTY.
I FEAR NOT THAT YOU WILL GRUDGE ANY SUFFERINGS; ONLY IF BIE SUBMISSIONS YOU CAN TURN THEM AWAY, TIS THE PART OF A WISE MAN.
RESTINGE YOUR SERVANT TO COMMAND.The third letter after each punctuation reveals "Panel at East end of Chapel slides".
Classical ciphers do not satisfy these much stronger criteria and hence are no longer of interest for serious applications.