\font\largerbold = cmbx10 scaled \magstep1
\font\largerrm =cmr10 scaled \magstep1
\nopagenumbers
\font\sc=cmcsc10.tfm
\magnification=\magstep1
%{\parindent 0pt
\rightline{\sevenrm January 26, 2000}
\centerline{\largerbold Boxes ...}
\bigskip\medskip
\noindent{\bf Background}
\medskip

I try to write a course diary recording what happened during each
class meeting. Part of the diary entry for our second class meeting
follows:

\medskip

{\narrower

I then displayed a different kind of encryption scheme, by writing a
rectangular 3 by 5 array on the board, and writing horizontally the
message {\tt the giraffe hops}, padding it appropriately on the end
with an {\tt x} and reading off vertically the message {\tt
trhhaoefpgfsiex} (see the illustration below).

{\tt
\centerline{thegi}
\centerline{raffe}
\centerline{hopsx}

}
I described how Bob would decipher, insisting that his instructions be
as unambiguous as possible and as similar to Alice's as possible. So
the key in this case was the pair of numbers (3 \& 5) which Alice used
and which Bob reversed.

This is one example of a transposition method, where letters were
permuted or interchanged.

}

\bigskip

\noindent{\bf Reality (?)}

\medskip

Alice might send the messages below to Bob using this system. What are
the plaintexts corresponding to each of the ciphertexts? I have tried
to make this exercise a bit realistic by allowing Alice to commit some
simple mistakes. In each case the message has been split into groups
of 5 letters, which is conventional in this field. Briefly explain any
problems you find and describe how you solved them.  Please hand in
your work Monday. People may work together and hand in joint
solutions. Of course you should indicate the members of your group. 

\bigskip
%\noindent 
{\bf  Message A} The key is $4 \; \&\;  6$. The message is 
\smallskip
\centerline{\tt twash fncea dosli inltd osix}
\medskip

%\noindent 
{\bf  Message B} The key is $3 \; \&\;  9$. The message is 
\smallskip
\centerline{\tt tsnhf iergr ohizt veenr tio}
\medskip

%\noindent 
{\bf  Message C} The key is $4 \; \&\;  10$. The message is 
\smallskip
\centerline{\tt wtump wdlar hhnet owbrx eecsh riemx nsoue llwex}
\medskip

%\noindent 
{\bf  Message D} The key is 
$5\hskip -.07in 6 \hskip -.07in 7\hskip -.07in 2 \hskip -.07in h
\; \&\;  
11\hskip -.07in 3\hskip -.07in 4\hskip -.07in x$. The message is 
\smallskip
\centerline{\tt tehhy hheef eowwi wwoie olonr liddc vnsie egasx sinvx
andex rttrx} 
\medskip

%\noindent 
{\bf  Message E} The key is $6\; \&\; 4$. The message is 
\smallskip
\centerline{\tt segwu uhlsi cnoin cnoin shxvn omfx}
\bigskip

\centerline{\fiverm Hints are available electronically. Hints are
available electronically. Hints are available electronically. Hints
are available electronically. Hints are available electronically. Hints are available electronically.}


\vfil\eject\end


the snow falls and it is cold

thesnowfallsanditiscold

4 6

thesno
wfalls
anditi
scoldx

twashfnceadosliinltdosix

twash fncea dosli inltd osix


twas
hfnc
eado
slii
nltd
osix

3 9

the river will be frozen tonight where will the fish go

theriverisfrozentonight

theriveri
sfrozento
night

tsnhfiergrohiztveenrtio

tsnhf iergr ohizt veenr tio

tsn
hfi
erg
roh
izt
vee
nrt
io

NO PADDING!

4 10

when the suncomes up the world will be warmer

whenthesuncomesuptheworldwillbewarmer

when
thes
unco
mesu
pthe
worl
dwil
lbew
arme
rxxx

wtumpwdlarhhnetowbrxeecshriemxnsouellwex

wtump wdlar hhnet owbrx eecsh riemx nsoue llwex

wtumpwdlar
hhnetowbrx
eecshriemx
nsouellwex

TRANSPOSE USED!

5 11

the wolves are howling in the woods and the wind is very fierce

thewolvesarehowlinginthewoodsandthewindisveryfierce

thewolvesar
ehowlingint
hewoodsandt
hewindisver
yfiercexxxx

tehhy hheef eowwi wwoie olonr liddc vnsie egasx sinvx andex rttrx

tehhy 
hheef 
eowwi 
wwoie 
olonr 
liddc 
vnsie 
egasx 
sinvx 
andex 
rttrx

NO KEY GIVEN!

6 4

shoveling snow is great fun

shovelingsnowismuchfun

shov
elin
gsno
wism
uchf
unxx

segwu uhlsi cnoin shxvn omfx

segwuu
hlsicn
oinshx
vnomfx

segwu uhlsi cnoin cnoin shxvn omfx

DUPLICATE GROUP!
\noindent Hand in \underbar{either} assignment \#1 or assignment \#2
on Monday; people may work together.
\medskip
\noindent {\bf \#1 Technical writing}
\medskip

Write instructions for encrypting and decrypting with this system
using complete English sentences. Your instructions should be valid
for {\it any}\/ key. Keys to this system are pairs of integers. Be
clear and unequivocal. Shorter is better, but precision and
correctness are most important. (I think this is {\it very}\/
difficult!)

\medskip \noindent{\bf \#2 Decoding some messages} \medskip 



