programming-for-dummies

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Book IV: Algorithms
each letter with the third letter from the current letter. So the letter A would
be replaced by the letter D, the letter B by the letter E, and so on. In this
case, the method is the same, but the implementation is slightly different
while being only marginally harder to decipher.
Although substitution ciphers are easy to implement, they’re also easy to
break. After you know to replace a letter in the ciphertext by another letter
that’s shifted by a specific distance in the alphabet (such as the third letter),
you can easily break the code. One way to avoid this problem is to use a onetime
pad, which consists of a series of random numbers that tell how far to
shift the next letter in a message. So a one-time pad might contain three
random numbers, like this:
2 7 –3
The first number (2) tells the algorithm to shift the first letter of the text by
two letters. So if the first three letters of the message are SAM, the first
letter, S, would get replaced by the second letter from S in the alphabet,
which is U.
The second number (7) tells the algorithm to shift the second letter by
seven letters. So the letter A gets replaced by the seventh letter down, which
is H. Finally, the third number (3) tells the algorithm to shift the third letter
by the third letter down, so the letter M gets replaced by the letter P. Now
the entire message SAM gets encrypted as the ciphertext UHP.
The one-time pad gets its name because the random series of numbers are
used only once. Now it’s virtually impossible for anyone to discover how the
letters are substituted because the replacement letters are picked at random.
The only way to decipher this ciphertext is to get a copy of the one-time pad.
Of course, the one-time pad has its drawbacks. To work, both parties need a
copy of the same one-time pad. If you could transfer a copy of the one-time
pad securely, you might as well transfer the message you’re delivering
instead. Also, one-time pads can be used only once. If they’re used more
than once, someone can eventually guess the random pattern of letters.
Even worse is that a one-time pad must specify how far to shift each letter in
a message. If you’re encrypting a message consisting of 1,000 letters, you
need a one-time pad to specify how to shift all 1,000 letters. If you’re encrypting
a message consisting of 10,000 letters, you need a one-time pad that
specifies how to shift all 10,000 letters.
Given these problems, one-time pads are generally impractical for normal use.
A slight variation of the one-time pad is the use of a password. A password acts
like a one-time pad; instead of defining how to alter each individual character