In Dorothy Sayers' novel "Have His Carcass", Lord Peter Wimsey describes a
cryptography technique that is simple for encoding and decoding, yet relatively hard to
crack. Your job is to implement this technique.
Here's how it works, in Sayers' (edited) words:
You choose a key-word of six letters or more, none of which recurs. Such as, for
example, SQUANDER. Then you make a diagram of five squares each way and write
the key-word in the squares like this:
Then you fill up the remaining spaces with the rest of the alphabet in order, leaving out
the ones you've already got.
You can't put twenty-six letters in twenty-five spaces, so you pretend you're an ancient
Roman or a medieval monk and treat I and J as one letter. So you get this:
Now let's take a message -- What shall we say? 'All is known, fly at once' -- that classic
hardy perennial. We write it down all of a piece and break it into groups of two letters,
reading from left to right. It won't do to have two of the same letters coming together in a
pair, so where that happens, we shove in a Q, which won't confuse the reader. Now our
AL QL IS KN OW NF LY AT ON CE
If there is an odd letter at the end, we'd add on another Q to square it up. Now we take
our first group, AL. We see that they come at the corners of a rectangle in which the
other corners are SP. So we put down SP for the first two letters of the coded message.
In the same way, QL becomes SM and IS becomes FA.
Ah, but here's KN. They both come on the same vertical line. In this case you take the
letter next below each -- TC. Next comes OW, which translates to MX. Going on, SK,
PV, NP, TU... If your first diagonal went from bottom to top, you must take it the same
way again. ON = TU, NO would be UT.
CE comes on the same horizontal line. In that case, you take the letter to the right of
each. Since there isn't a letter to the right of C, you start again at the beginning of the
line, producing DR. Your coded message stands now:
SP SM FA TC MX SK PV NP TU DR
The input for your program will be a series of keywords and messages to encode,
alternating line-by-line, until the end-of-file flag of 999. Using the above technique, you
are to encode the message, using the word.
ALL IS KNOWN FLY AT ONCE
THE ROOSTER CROWED AT MIDNIGHT
Input will be in upper case and will contain no punctuation. Letters may occur more than
once in the keywords, in which case you must ignore all but the first occurrence of the
Print each message, encrypted, using two letter groups and no punctuation, separated
by a single white space.
SP SM FA TC MX SK PV NP TU DR
Print an I for IJ.
If the last letter is both odd and repeated, treat it as repeated and put the 'Q' before it,
not afterward. That is, ALL becomes ALQL for encoding purposes.
In the unlikely case of two Qs in a row, insert a Z between them. Also, augment oddlength
messages ending with a Q by using a Z. Thus, FAQQAD becomes FAQZQADQ
and HUQ becomes HUQZ for encoding.