Here is exercise #3.

[1.] Decode the following phrase, which was encoded using a affine
encoding cipher on a 27-letter alphabet (the letters a-z and a blank).
Note that a=0.

 xgb dkjvmbkffcmtkrv m lkaerask mdbmwkfmezremzkmxrcm kkmfvkrx mxgvkmaskrvsc


-----------------------------------------------------------------------------
[2.]
The message below has been encoded using a linear cipher, A*v, where A is a
3x3 matrix.  The mesage was coded in a 97-character alphabet (given below),
where the first character (a blank) is given the numeric code 0.  You are
given the decoding of the first sentence.  Line breaks have been preserved.
Decrypt the message.

Alphabet:=convert([seq(i,i=32..126),162,164],bytes);
 
First line of message:
How many surrealists does it take to screw in a lightbulb?
 
Coded message follows:
4)lb|ypZ:2o1,10W=Kpy¤l!V0?]q-\d4x78E87\/¤/5{d%Abg>f5P&¤,0m,A
         *bC^sN=[4

@5;1W>yqjXYZQ>WmT~JiT3jF¤#q¢fMrb/%!Nj$+XYZMolB<SstZr7G3o=+a!$?&Rc_
!^]K}vf0G?"B0dzjzO\vJ2N7SrpA_`P%F2,AB9@SNn$F;(1gX0G=lv,JLRl;
@$O¤5[zdI Y_

hO7};pt{@kf>5{d4qp.H-4of5dzZ 2C\dB9@K~0nhNy8+!vg.&|Nku_cq(zKaZ.
'~B 'En.b

xqj};p`;vXI;NL8!`{qBq0dz!¢u0dz


 -------------------------------------------------------------------------
[3.] Recall that a Vign\`ere cipher can be interpreted as a Caesar-like
cipher on n-vectors, where n is the length of the key phrase.  Can
any affine encipherment on digraphs (two-character codes) be interpreted as
a an affine matrix encipherment on 2-vectors?  
If your answer is yes, prove it.  If no, give a counter-example which cannot
be so interpreted.