 Tech Note #35: How Encryption and Digital Signatures Work
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1999 Bionic Buffalo Corporation; All Rights Reserved.
19 May 1999
http://www.tatanka.com
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If the key is 1, then chose the next letter: A becomes B, B becomes C, C becomes D, and so
on.

If the key is 2, then chose the letter two letters later: A becomes C, B becomes D, C
becomes E, and so on.

For other keys, use the key to skip the specified number of letters.
If the original message is
CATS HAVE FUR
then the encrypted version depends on the key as follows:
(key = 1) DBUT IBWF GVS
(key = 2) ECVU JCXG HWT
(key = 3) FDWV KDYH IXU
...and so forth
To decrypt the message, the recipient uses the key to shift the letters of the alphabet backwards
by the correct number.
This example is contrived and (obviously) too simple to use in real life. The key would be easy
to find, just by trying 25 possible numbers. However, it illustrates two points:

the total number of keys possible is 25; this is known as the size of the key space

the same key is used to encrypt the message, as is used to decrypt the message; such a
scheme is called a symmetric cryptosystem
In real life, the scrambling mechanisms are much more complex, and the key spaces are much
larger:

banks communicate with ATMs using a key space of 2
56
(about 72,058,000,000,000,000)
keys

web browsers on the internet secure traffic using a key space of 2
128
keys (a number 39 digits
long)
Key spaces are normally measured in bits. The DES algorithm used by ATMs has a 56-bit key
space, since there are 2
56
possible keys. The scheme used by web browsers is considered to
have a 128-bit key space.