## Saturday 7 February 2015

### CoderBobo

Referring back to one of Ireland's smartest young mathematicians, coder Sarah Flannery, I thought I'd lash out a puzzle for the weekend. Secret codes include Caesar’s Code which shifts each letter along the alphabet by a fixed number.  Here is an encoding key using a shift of three letters:
Clear: A B C D E F G H I J K L M
Coded: D E F G H I J K L M N O P
Clear: N O P Q R S T U V W X Y Z
Coded: Q R S T U V W X Y Z A B C

And here is an encoded message:
WKH  PRQHB  LV  LQ  WKH  WRRO  VKHG
Or you can use a numerical code:
15’130108151425  0919  01  191625
Or you could combine the two:
1312161602  110422  04  051210  17182208
For something more complicated, but more secure, try the Playfair Code, which depends on both parties knowing a keyword (in this case “house”).  The keyword is written first into a 5x5 grid, the rest of the letters of the alphabet are written in order afterwards, omitting the letters from the keyword, and letting I and J share a square.  If the same letter appears more than once in your keyword, leave out the second occurrence.
H O U S E
A B C D F
G I K L M
N P Q R T
V W X Y Z

To encode, the letters of the message are paired off (with double letters separated by X).
Then follow three simple rules.  1) If the pair is on the same row take the letter to the right. 2) If the pair is in the same column take the letter below. With both those rules you wrap the row or column in a circle so QX becomes XU and CF becomes DA. 3) If the letters are in neither the same row nor column, they are at the corners of an imaginary rectangle so take the letters at the other corners of that rectangle. TH becomes NE Thus:
YO UR ME SX SA GE RE CE IV ED
becomes:
WS SQ TF UY HD MH TS FU GW SF
Here is a political message based on the first word:
MP CF FO TB TR CT DG AQ
We really should communicate more.