BFcipher

BF uses a pure Beaufort algorithm to encrypt text in the visible range of ASCII
characters.  High-bit characters and control characters below <SPACE> are left as-is.
The output is exactly the same size as the input, and all are 7-bit characters. 

A classic Beaufort crypt is presented as a 'tabula recta' of 26 alphabets,
rotating one character per line.  The encryption process requires a key-text and
message-text of the same size; take each character in order, find its place in the
top row of the table, trace the column down to the corresponding key character,
and output the 'encrypted' letter in the first column.  Simple enough for an agent
in hostile territory to do in a hurry.  If the key is well-protected, it's very
secure against a manual attack, although cryptologists say it's easily broken by
sufficient compute power.

If you don't happen to have an alphabet-grid handy, there is an easy mathematical
algorithm for doing encryption/decryption:  subtract the text-letters from the key-letters
to get the encrypted letters.  Decrypt in exactly the same way:  subtract your message
from the key.

So, what does that mean, 'subtract the message from the key'?

Print your key, triple-spaced if it's multiple lines.  Under each letter of the key,
write each letter of the message text.  If you run out of key letters, start repeating
the key from the beginning until you reach the length of the message, but note:
if the key is short by very much, the repetition will cause patterns to appear,
weakening the value of the cipher.

Using the alphabetic ordinal values of the letters, subtract each text value from
its corresponding key value.  If you're doing this manually, you can skip all the
spacing and punctuation, and reduce the character set to uppercase letters to keep
the arithmetic in the 1-26 range.  Subtracting Q (17) from H (8), e.g., will go
negative; add 26 to bring it back in range.  Convert this ordinal value back to its
letter value, and that is the encrypted letter.

The range of visible ASCII text is 32-126, i.e., 95 characters.  If a computer is doing
the work (or even a calculator), it's easy to do the arithmetic over the whole range
of uppercase, lowercase, numbers and punctuation.  After subtracting the message
character from its key character (using ASCII values, not ordinals), add 95 to bring it
into the 32-126 range.

Beaufort is a well-known algorithm, so serious cryptanalysis will easily decode whatever
you hide with this routine.  It's a learning exercise.  Feel free to fiddle with it or
salt the key to make it stronger.
Key:
Text:

Message:
BF: a pure Beaufort cipher.
