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


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[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:
The beguiling ideas about science quoted here were gleaned from
 
Coded message follows:
D&kI0 .¢1N,FYK.GbgC$w?F-2Rj(]95db).2;6~gOhtM_|63$^qr~@_q@,;JPM5
X3&wL5cT`8-8Tnorg1Q1@1fgfoQ\)WccQlTFgV,d¤.LD:3Y]g2zNwRPM5Lg[T~cV$'
rg1BOpr*1d¤?+|s_,MOJ VDe1zX¤3Tk.qD?_<[8U¢?\?O*<+$<-My&(14vxI:}!
YsHpRGvO$q+,b@bE=5&¤=q+,>V "%I. ]yOm#*9c` \)Wri]I)k5Jr;BT<Ei*]k
seixe?+|'YsHQl{-"J4n.EfZ9P'3\S7GNd70

wi5      Adr2z#um~q9ZEB¢7+VYsHGd¤~K[~*."HKX\29[ucGn7L[q^6;jN=J|/\x
      wI-])^'r{tR;?@R.O5tF_^0Q:0;iF@x`/

wi5      0pF`$]L(2lkUW3¤;jNLQva¤z!Io,`6sVRc'fMy&&¤=4n.EfZkL82z#Po1
      uxC5D_isL8GUv.RgAbl2PqYM&¤=C>*m F,WdE\#Dk$&z/.wO¢u_cGnE\#
      )?¢m+=w:¤|¢pHxXb\^``a_bqx`/

wi5      Iy"vxI*]kuQ!3]h0%yW3¤{:_zlHbO1i(¢*]k;BTs*p ayVDeg2z2bc
      S.5EC 3\Sq /#>R/U6Q7¤3dEKE>"6pCT`W3¤YsH:sUv9xepx&Kg#I'C_G4I¤
      U`¤*]k;BTs*p ay+xhUBkk\jm#&Yp{&¤b$W\gV,8_-

wi5      Iy"aPnBg[{zA"=sI5j~K[*j`2u9Q"ts*pq+,)tt8w)q^6mg8.;c ayVDe
      :d%$^q|ot1n[r=hW3¤l,L2pPkBCFFnGzEz7S/wEYsHMiF$^q}A!Fz)n20

wi5      ^@^{zAYsHW\,hFb-'4sMX]F I0 hc\sQ@`XiQl{or-&¤=`g%)Ar
      b!RlkU$]"NwRPM5 ayz7Sp@0`g%s]wbl}D¢yf3VEC uL8E|w$v]RtZ&¤=x`/

wi5      B3)Rj-qG`oEx/¢$.;c2WcO;Z1)>7+VSj3t1c80HwpmcGnw+y+|'3%d.ke
      wI-/¢$;jNHzF#sa4iEQ)(R@>}WW fb$kfvxITL!v LN~wTf/

wi5      f;"M`#Y¢J'Eo7+Vd.:P^Y'EoDKm?t<vjfm+=l,#TKqh!v)yMphNq&[
      }~L6RC3QF/wEcGnd\?)yM;jN¢0W

wi5      ¢^su&sBL<4n.l(,<~nK'qYsHVmr3Y]HzF#saEfZ¢(A(Evn_`gW4YsHFb=
      @8D@s`?n`#>R;jNJhRyk@x`/

wi5      <'w:%|C$w$^qfe~'_&m?VJ¤;Vr[!d<!YO(14vxILFy]F }%N¤\yLFy
      >gZ-"J|63hc\/¢$YsH>SigAbNxcTf/

wi5      ]<`Rz&Bg[{zAc8aYCL+|'I=$¤\y Ph`g%s]wQ6) ("H$CUh vL.YsH
      hVqJTNkXVH2:zOtK+0m#nEB¢ys2[|O)uy]F 3z*Y>9m&T ayxfim[:,xox`/



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[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.