Simple Ciphers - Modern Cryptanalysis: Techniques for Advanced Code Breaking - page 3

WKHTX LFNEU RZQUR PDQIR AMXPS HGRYH UWKHO DCBRV WURJR WKGRJ
When encrypted, the lack of the correct spaces in the ciphertext means nothing to either party. After decryp-
tion, though, when the party has plaintext with few spaces in the correct place, the inconvenience is usually
minor, as most people can read the message anyway. The added security of removing all spaces from the plain-
text before encryption is worth the small added difficulty in reading the message. The spaces added at regular
intervals add no new information to the data stream and are therefore safe to keep.
With these examples, it is easier to see exactly what is meant by the term monoalphabetic. Essentially, to
use a monoalphabetic cipher, we only need to consult a single lookup table. This will contrast shortly with other
techniques, which consult multiple tables.
1.2 Keying
The Caesar cipher has a prominent flaw: Anyone who knows the cipher can immediately decrypt the message.
This was not a concern to Caesar 2,000 years ago, as having the message in writing often provided sufficient
subterfuge, considering the high illiteracy of the general population. However, the simplicity of the cipher al-
lowed field commanders to be able to send and receive encrypted messages with relative ease, knowing that
even if a message was intercepted and the enemy was literate, the opposition would have little hope of discov-
ering the content.
As time progressed, more people became aware of the algorithm, and its security was therefore lessened.
However, a natural evolution of the Caesar cipher is to change the way the letters are transformed into other
letters, by using a different ordering of the alphabet.
But easily communicating an alphabet between two parties is not necessarily so easy. There are 26! =
403,291,461,126,605,635,584,000,000 different possible arrangements of a standard 26-letter alphabet, mean-
ing that both sides would need to know the encrypting alphabet that the other was using in order to decrypt the
message. If the two parties first agree on an alphabet as a key, then, since they both know the algorithm, either
can send messages that the other can receive. However, if they number the alphabets individually, they would
have an 89-bit key (since 26! ≈ 2 89 ), which is difficult to work with. Instead, most cryptographers would typic-
ally use a few simple transformations to the alphabet, and have a much smaller key.
For example, the most common method is simply to shift the letters of the output alphabet to the right or left
by a certain number of positions. In this way, the Caesar cipher can be viewed as having a shift of +3. There are
then 26 different keys possible, and it should be fairly easy for two parties to exchange such keys. Moreover,
such a short key would also be easy to remember.
Other common transformations are typically a combination of the shifting operation above and another
simple operation, such as reversing the order of the output alphabet [4].
1.2.1 Keyed Alphabets
To increase the number of alphabets available for easy use, a popular keying method is to use a keyword, such
as swordfish , to generate an alphabet. An alphabet can be derived, for example, by removing the letters in
the keyword from the alphabet, and appending this modified alphabet to the end of the keyword. Thus, the al-
phabet generated by swordfish would be