A simple replacement code can be created by using a 13-letter phrase with no repeated letters, such as the following:
The letters below the 13-letter phrase are the other 13 letters of the alphabet, in alphabetic order.
Any given message can be encoded or decoded by finding each given letter among the 26 on the chart, and replacing it with the letter above or below it.
A different 13-letter phrase has been used in forming the key to encyphering a sentence, below. Again, of course that key phrase has no repeating letters.
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
is, in this code:
PQS HGCIJ LXMZF NMR KGOTE MDSX PQS BYWA VMU
As was to be expected the replacements are mutual: for example, N codes F and F codes N.
What was the 13-letter phrase used in forming the key to this code?
From the 2013 Mensa 365 Brain Puzzlers for 2013 calendar, puzzle for May 1, by Mark Danna and Fraser Simpson, Workman Publishing, New York, NY.
Well, that was a fun problem.
1) By inspection, the letter pairs are
2) I initially guessed that all the vowels were
in the phrase (which can't be far wrong).
Then, they would be in the following sequence:
3) Note the pair WZ. It must be immediately before
or after the A, and along with A these would be
the last two letters in the phrase. I guess
that the phrase ends in AW, not ZA, making it
3) Note the pair RX. It must be pretty close to the
end, also, either immediately before the A or the E.
Noting the PT pair, I guess before the A, because
a phrase ending in TXEAW or XEPAW seems wrong.
This makes it
4) I note pair DV next. If V is in the bottom, the
phrase ends in DRAW, which is a word. Next guess:
5) Maybe the U is not in the phrase? That would make
the letters before DRAW either TEG or EPG.
I don't like either.
6) How about the TP pair? Is it ..EPDRAW or T..EDRAW?
The latter, because the H fits between T and E.
Still guessing, but feeling more confident that it is
7) The next highest letter we haven't accounted for is
N, and that comes with and F.
If N is on the bottom, then we have
8) And this is enough for me to recognize the phrase.
The I is on the bottom, not the top.
Edited on May 16, 2013, 7:20 pm