python project

In  this  project,  you  will  be  modifying  the  Caesar  Cipher  program  you  completed  earlier  in  the   semester.  We  will  use  multiple  shift  amounts  in  an  attempt  to  make  our  “encryption”  harder  to   crack  and  also  make  our  program  more  general  by  reading  the  message  from  a  file.     Caesar  Cipher  Encryption  Review   The  Caesar  Cipher  is  simple  way  to  “encrypt”  alphabetic  letters  (aside:  don’t  try  encrypt   anything  important  with  the  Caesar  Cipher).  All  remaining  punctuation  symbols,  numeric  digits,   or  other  characters  (spaceing)  remain  unchanged.       Encrypting  a  message  using  the  Caesar  Cipher  involves  replacing  each  letter  in  a  message  with   the  letter  k  places  further  down  the  alphabet,  wrapping  around  at  the  end  of  the  alphabet  when   necessary.  With  k  =  0,  each  letter  is  replaced  by  itself.  With  k  =  20,  each  letter  is  shifted  20  places   down  the  alphabet.    To  decode  or  decrypt  the  text  you  simply  shift  the  encrypted  letter  in  the   opposite  direction  by  k  places  and  wrap  around  as  necessary.     The  letters  in  the  Caesar  Cipher  alphabet  start  at  0  and  continue  through  25,  with  the  letter  “A”   being  0  and  “Z”  being  25.    If  the  user  were  to  choose  k  =  3,  the  letter  “A”  (0)  would  be  replaced  by   the  letter  “D”  (3),  while  the  letter  “B”  (1)  would  be  replaced  by  the  letter  “E”  (4).    If  a  letter   appears  towards  the  end  of  the  alphabet,  the  alphabet  simply  wraps  around  and  starts  again.  So   the  letter  “Z”  (25)  would  be  replaced  by  the  letter  “C”  (25  +  3  =  28),  which  is  2  after  wrapping   around.  The  “wrap  around”  math  is  accomplished  simply  using  the  “modulo”  operator  in  Python   (%)  which  returns  the  remainder  after  integer  division.       If  x  is  the  position  of  some  letter  we  are  trying  to  encrypt,  it  should  be  replaced  by  the  letter  at   the  position  denoted  by  (x  +  k)  %  26,  which  will  be  a  number  between  0  and  25.  For  the  above   example,  25  +  3  =  28,  and  28  %  26  =  2,  or  “C”.  To  decrypt  you  simply  subtract  k  instead  of  adding:   (x  -­‐  k)  %  26.    To  reverse  the  previous  example  we  take  the  result  “C”  =  2,  with  a  shift  of  3,  so  (2-­‐3)   %  26  =  25  giving  us  back  the  letter  “Z”.       Shifting  the  Letters  in  Python     Recall  (from  Chapter  4)  that  our  alphabet  in  Python  is  encoded  using  UTF-­‐8  (see  the  table  in  the   slides  or  book),  from  Python’s  perspective  “A”  is  not  0.  Instead,  an  upper  case  “A”  is  represented   by  the  number  65,  “B”  by  66,  and  so  on,  with  capital  “Z”  represented  by  90.    While  a  lower  case   “a”  is  represented  by  the  number  97,  “b”  by  98,  and  so  on,  with  “z”  represented  by  122.  Your   program  will  have  to  account  for  this  when  shifting  the  letters  using  the  formula  above.  Note:   your  program  should  not  change  the  case  of  the  original  text  or  message.     To  obtain  the  UTF-­‐8  values,  you  will  need  to  use  the  built-­‐in  ord(char)  function,  which  accepts   a  string  containing  a  single  letter  and  returns  the  integer  value  of  the  letter.  You  will  also  need  to   use  the  built-­‐in  chr(int)  function  which  accepts  an  integer  and  returns  the  corresponding   Unicode/ASCII  letter.    The  methods  isupper()  and  islower()  may  also  be  useful.   Further,  any  remaining  punctuation  symbols,  numeric  digits,  or  other  characters  (spaces)  should   be  unmodified  and  remain  unchanged  in  the  message.  Chapter  4  contains  examples  of  how  to   identify  and  skip  these  characters  while  encoding,  as  a  hint  you  will  need  to  import string.

CS1064                                                                                                                                                                                                                                                                                                          Spring  2017

Implementation  and  Differences  from  Homework  5   Your  program  should  begin  by  prompting  the  user  for  an  input  file  (which  contains  the  message),   5  shift  amounts  (all  integers),  and  whether  they  would  like  to  encrypt  or  decrypt,  then  our   implementation  will  add  some  variation  to  the  basic  algorithm  to  make  it  a  little  harder  to   decode.  When  encrypting:     (1)  We  will  read  the  text  data  from  the  file  line-­‐by-­‐line,  and  all  of  the  transformations  described   below  be  should  applied  line-­‐by-­‐line.    Before  doing  the  transformations  below  you  should  use   strip()  to  remove  the  whitespace  at  the  ends  of  a  line.   (2)  Like  before,  you’ll  replace  each  “e”  character  in  the  original  message  with  the  letters  “zw”.     If  the  file  contains  “Hello World!”,  the  resulting  message  will  be  “Hzwllo World!”.   (3)  Also  like  before,  after  replacing  the  “e”  characters,  you’ll  add  the  word  “hokie”  to  the   beginning,  middle  (the  middle  is  length//2),  and  the  end  of  the  message.  Continuing  with   example  above  the  message  that’s  actually  encrypted  using  the  Caesar  Cipher  is   “hokieHzwllohokie World!hokie”.  This  should  be  done  for  each  line  in  the  file.     Once  you  have  the  altered  message  then  we  will  perform  a  slightly  modified  Caesar  Cipher   encoding  as  described  below  and  print  the  result.  Like  above  this  transformation  is  applied  line-­‐ by-­‐line  and  shouldn’t  “cross”  lines.       Rather  than  have  a  single  shift,  instead  we  will  have  5  shift  amounts  entered  by  the  user.  The   shifts  are  applied  character  by  character.  So  if  the  user  enters  shift  amounts:  3 4 5 6 7  and   the  first  line  of  the  file  contains  “Hello  World!”,  your  program  will  first  transform  it  into   “hokieHzwllohokie World!hokie”  then:     “h”  would  be  shifted  by  3  characters.   “o”  would  be  shifted  by  4  characters.   “k”  would  be  shifted  by  5  characters.   “i”  would  be  shifted  by  6  characters.   “e”  would  be  shift  by  7  characters.     We  then  start  back  at  the  first  shift  amount:     “H”  would  be  shifted  by  3  characters.   “z”  would  be  shifted  by  4  characters.   “w”  would  be  shifted  by  5  characters.   “l”  would  be  shifted  by  6  characters.   “l”  would  be  shift  by  7  characters.     The  space,  exclamation  point,  and  anything  else  that’s  not  a  letter  isn’t  shifted,  but  it  does  “take   up”  a  shift  amount.  So  if  the  space  character  would  have  been  shifted  by  3  characters,  the  next   character  would  be  shifted  by  4.  This  continues  until  the  end  of  the  line  is  reached.  The  whole   process  starts  over  with  shift  amount  3  on  the  next  line  in  the  file.

CS1064                                                                                                                                                                                                                                                                                                          Spring  2017

When  decrypting,  perform  each  step  in  reverse  order.  Decrypt  the  message  using  the  Caesar   Cipher  above,  then  remove  the  “hokie”  strings,  then  change  any  occurrence  of  “zw”  to  “e”.     You  may  assume  the  all  occurrences  of  “zw”  in  the  result  are  really  “e”  characters,  and  further   you  may  assume  the  “hokie”  strings  are  in  the  encrypted  message  at  the  appropriate  place,   however  there  may  be  other  occurrences  of  “hokie”  in  the  original  message.     Finally,  your  program  must  be  able  to  encrypt/decrypt  an  arbitrary  number  of  messages.  When   you’ve  completed  the  encrypting/decrypting  a  message  the  user  should  be  asked  if  they  want  to   enter  another  message.    Your  program  should  continue  until  the  user  types  “N”.     While  it’s  not  a  requirement  for  this  project,  I’d  strongly  recommend  writing  function(s)   to  help  break  down  this  project.  For  example,  you  might  write  a  function  that  shifts  a  single   letter  and  use  in  your  main  program.     Input  Files  and  Sample  Execution   Here’s  a  sample  input  file,  quote.txt:     Though Birnam Wood be come to Dunsinane, And thou opposed, being of no woman born, Yet I will try the last. Before my body I throw my warlike shield. Lay on, Macduff; And damned be him that first cries “Hold, enough!” — MacBeth

The  user  input  is  green:     Enter input file: quote.txt Enter shift amounts: 3 4 5 6 7 Encode (E) or Decode (D)? E Result: kspolWltank Goyqer Drsi icltqpha ivpdb ar Iauvmsguca,nvnmj kspolDri aksz vsttygzh, icltqphantn sk ur butdr hvur,nvnmj kspolBdbz L boso yxf xmfd pmurlifya. Hgzjtxgz re esieoronk kspolL ynyra sf afxsloec vlnnvnmjfdoh. Oed vq, Shfhzlm;ltqph kspolDri kdqsfdg gfd lns wlfz immurliwya gwogzw “Orpi, casubjl!”oronk kspol– ShkspolfFecakltqph Go again? (Y/N): Y Enter input file: encoded-quote.txt Enter shift amounts: 3 4 5 6 7 Encode (E) or Decode (D)? D Result: Though Birnam Wood be come to Dunsinane, And thou opposed, being of no woman born, Yet I will try the last. Before my body I throw my warlike shield. Lay on, Macduff; And damned be him that first cries “Hold, enough!” — MacBeth Go again? (Y/N): N

CS1064                                                                                                                                                                                                                                                                                                          Spring  2017

What  to  Submit   For  this  assignment  you  should  submit  your  p2.py  file.  Your  file  must  be  named  p2.py.     This  assignment  will  be  graded  automatically.  Test  your  programs  thoroughly  before  submitting   them.    Make  sure  that  your  programs  produce  correct  results  for  every  logically  valid  test  case   you  can  think  of.    Do  not  waste  submissions  on  untested  code,  or  on  code  that  does  not  compile   with  the  supplied  code  from  the  course  website.     Web-­‐CAT  will  assign  a  score  based  on  runtime  testing  of  your  submission;  your  best  score  will  be   counted;  the  TAs  will  later  verify  that  your  best  submission  meets  the  stated  restrictions,  and   assess  penalties  if  not.     To  submit  this  assignment:

1. Visit  http://web-­‐cat.cs.vt.edu  in  your  web  browser.   2. Enter  your  Virginia  Tech  PID  and  password  in  the  appropriate  fields  on  the  log-­‐in  screen,

and  make  sure  that  Virginia  Tech  is  selected  as  the  institution.  Click  Login.   3. The  Web-­‐CAT  home  screen  will  display  useful  announcements  and  assignments  that  are

currently  accepting  submissions.  Find  the  assignment  that  you  want  to  submit  in  the   table,  and  click  the  “Submit”  button  next  to  it.

4. Click  the  Browse…  button  and  select  the  file  you  want  to  upload.  The  homework   assignments  and  programming  projects  for  this  course  should  be  self-­‐contained  in  a   single  .py  file,  so  you  can  simply  select  that  one  file.

5. Click  the  Upload  Submission  button.  The  next  page  will  ask  you  to  review  your  selection   to  ensure  that  you  have  chosen  the  right  file.  If  everything  looks  correct,  click  Confirm.

The  next  page  will  show  that  your  assignment  is  currently  queued  for  grading,  with  an  estimated   wait  time.  This  page  will  refresh  itself  automatically,  and  when  grading  is  complete  you  will  be   taken  to  a  page  with  your  results.   When  your  results  are  ready,  make  sure  that  you  have  80%  on  the  assignment.  If  you  have   anything  less,  read  the  hints  that  Web-­‐CAT  gave  you  and  make  any  corrections  to  your  code  that   you  need  to  make,  then  submit  again.  Remember  that  for  the  programming  projects  in  this  class   (as  opposed  to  the  homework  assignments),  you  can  submit  up  to  5  days  after  the  due  date,  with   a  10%  penalty  per  day  late.

CS1064                                                                                                                                                                                                                                                                                                          Spring  2017

Pledge   Each  of  your  program  submissions  must  be  pledged  to  conform  to  the  Honor  Code  requirements   for  this  course.    Specifically,  you  must  include  the  following  pledge  statement  in  the  submitted   file:     # <include a description of the purpose of this file/project/package> # # @author <name and surname> (your VT PID) # @date <the date> # # Virginia Tech Honor Code Pledge # On my honor: # # – I have not discussed the Python language code in my program with # anyone other than my instructor or the teaching assistants # assigned to this course. # – I have not used Python language code obtained from another student, # or any other unauthorized source, either modified or unmodified. # – If any Python language code or documentation used in my program # was obtained from another source, such as a text book of course # notes, that has been clearly noted with a proper citation in # the comments of my program. # – I have not designed this program in such a way as to defeat or # interfere with the normal operation of the Web-Cat Server. # # <your name>

Failure  to  include  this  pledge  in  a  submission  will  result  in  the  submission  being

disallowed  during  code  review.

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