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When
you read about cryptography (the science of secrecy), you will encounter lots
of jargon, as well as different words that all seem to mean roughly the same thing.
This short, rather boring section is supposed to clarify some of the jargon. | |
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Definitions
Code:
A system for hiding the meaning of a message by replacing each word or phrase
in the original message with another character or set of characters. The list
of replacements is contained within a so-called codebook. A code has no built
in flexibility, other than re-writing the codebook. (An alternative definition
of a code is any form of encryption which has no built in flexibility.) To protect
a message in this way is called encoding. Cipher:
Any general system for hiding the meaning of a message by replacing each letter
in the original message with another letter. To protect a message in this way
is called enciphering. Each cipher can be split into two halves – the algorithm
and the key. The key gives a cipher some built in flexibility. Encrypt:
A term that covers encoding and enciphering. Encicode:
The process of encoding a message followed by enciphering. Multiple layers of
encryption is also known as superencipherment. Key:
The flexible component of a cipher. The cipher is a general algorithm that is
specified by the key. For example, substitution is a general algorithm that is
specified by a key, which is the substitution for each letter. Rival groups can
use the same substitution cipher, but they will choose different keys so that
they can not read each other’s messages.
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| The
Enigma Cipher To
put some of the definitions into context, let’s use the Enigma cipher machine
as an example. It is definitely a cipher, because it encrypts at the level of
letters and the algorithm depends on a flexible key that is chosen by the sender. The
key is the set-up of the machine – rotor orientations, plug selections, etc. all
determine the encipherment. The receiver must have an Enigma machine, but must
also know the key in order to decipher the message. |
Simon
Singh and his Second World War
German Enigma cipher machine.
Encryption
depends on picking a key | |
The More Keys The Better A
secure cipher system must have a wide range of potential keys. For example, if
the sender uses the Caesar shift cipher to encrypt a message, then encryption
is relatively weak because there are only 25 potential keys, i.e., the 25 possible
shifts of the alphabet. From the enemy's point of view, if they intercept the
message and suspect that the algorithm being used is the Caesar shift, then they
merely have to check the 25 possibilities. The
following table outlines four ciphers and the number of possible keys for each
one.
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Cipher
| Number
of Keys and How To Calculate the Number of Keys |
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Caesar
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25 Keys
The
alphabet can be shifted 1, 2, 3 ... or 25 places, but shifting a letter 26 places
takes it back to its original position, leaving the alphabet unchanged. Also,
shifting a letter 27 places is the same as shifting it 1 place. |
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Kama-sutra
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7,905,853,580,625 Keys
To
construct the cipher alphabet, the letter A could be paired with any of the remaining
25 letters. Another letter could be paired with any of the remaining 23 letters.
Another letter could be paired with any of the remaining 21 letters, and so on.
The total number of permutations is 25 x 23 x 21 x ... ...x 1
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Pigpen
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1
Key
There is only one
pig pen grid. |
General Monoalphabetic |
403,291,461,126,605,635,584,000,000
Keys
To
construct the cipher alphabet, the first letter could be any of the 26 letters.
The second letter could be any of the remaining 25 letters. The third letter could
be any of the remaining 24 letters, and so on. The total number of permutations
is 26 x 25 x 24 x ... ...x 1 (otherwise written as 26!) |
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