Homework 4, Coded Messages.

Short Description:

This assignment is worth 40 points.

Goals

• Have written code that employs string manipulation functions.
• Continued to refine your skills working with functions.
• Have employed an enumeration type.

Formal Description

Your program should open the files codes.dat which will contain a series of messages, along with instructions on how to transform these message.

The format of the file is as follows

• The first line for any message will contain the following space separated items
  • The action: either encode or decode.
  • The code name, see the table below
  • Any arguments needed by the code algorithm.
• The next line will contain an integer n, 1 ≤ n ≤ 100, the number of lines of text in the message.
• The next n lines will contain the message.

The possible codes are

• shift: a modified version of the Caesar cipher
• combine: a modified version of the Vigenère cipher
• altCombine: an alternative version of Vigenère

encode shift 23
1
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
decode shift 23
1
QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD
encode combine LEMON
1
ATTACK AT DAWN
decode combine LEMON
1
LXFOPV EF RNHR
encode altCombine ALBERTA
2
CALGARY
Attack At Dawn
decode altCombine ALBERTA
2
CPMCRYY
Aiuwtr At Ozaw

• A message that has not been encoded is called a plain text message or plaintext.
• An encoded message is called the ciphertext.
• An auxiliary phrase used to encode a message is called a key.

  The shift Cipher

  The shift cipher takes a plaintext message and an offset and adds the offset to each letter in the message to produce a ciphertext. Non-alphabetic characters are unchanged.

  To decode, the offset is subtracted from the ciphertext to produce a plaintext. Again non-alphabetic characters are removed.

  When adding or subtracting an offset from a letter, the result should be returned to the alphabetic range. For example 'z'+ 1 become 'a', and 'A'-2 becomes 'Y'.

  Shifting "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG" by 23 will result in the ciphertext of "QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD"

  Decoding, or shifting "QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD" by an offset of -23 will return the original plaintext of "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG".

  Plaintext Letter	T	H	E		Q	U
  Shift Amount	23	23	23	none, whitespace	23	23
  Shift Calculation	"T" + 23	"H" + 23	"E" + 23	none, whitespace	"Q" + 23	"U" + 23
  Ciphertext Letter	Q	E	B		N	R

  The combine Cipher

  The combine cipher takes a plaintext message and a key. The plain text message is shifted by the "offset" of each letter in the key. When the plaintext is longer than the key, the letters of the key are repeated. Non-alphabetic characters are unchanged.

  This offset is the distance from 'a' or 'A'. So the letter 'a' represents an offset of 0, while the letter 'C' is two characters from 'A', so it represents an offset of 2.

  Please note that the key may contain non alphabetic characters including whitespace. Only alphabetic characters should be used when encoding and decoding. The key is guaranteed to have at least on alphabetic character.

  Encoding "ATTACK AT DAWN" with the keyword "LEMON" produces the ciphertext "LXFOPV EF RNHR"

  Decoding "LXFOPV EF RNHR" with the keyword "LEMON" produces the ciphertext "ATACK AT DAWN"

  Plaintext Letter	A	T	T	A	C	K
  Corresponding Key	L	E	M	O	N	L
  Shift Amount	11	4	12	14	13	11
  Shift Calculation	A+11	T+4	T+12	A+14	C+13	K+11
  Ciphertext Letter	L	X	F	O	P	V

  Note, when encoding ATTACK AT DAWN with LEMON, the key will need to used over.

  ATTACK AT DAWN
  LEMONL EM ONLE
  

  The altCombine Cipher

  The altCombine cipher is the same as the combine cipher with the exception that every time an letter position in the keyword is odd, the shift is in a negative direction. Letters encrypted even position letters of the keyword are still shifted in the negative positive direction. Keys start at position 0.

  Encoding "CALGARY" with "ALBERTA" produces "CPMCRYY"

  Decoding "CPMCRYY" with "ALBERTA" produces "CALGARY"

  Plaintext Letter	C	A	L	G	A	R
  Corresponding Key	A	L	B	E	R	T
  Shift Amount	0	11	1	4	17	19
  Key position	0 (+)	1 (-)	2 (+)	3	4	5
  Shift Calculation	C+0	A-11	L+1	G-4	A+17	R-19
  Ciphertext Letter	C	P	M	C	R	Y

  Output

  WHen executing each command, produce a line as follows:

  codetype n lines whith ciphername.

  Where:

  • codetype is either Encoding or Decoding
  • n is the number of lines to encode or decode.
  • ciphername is the name of the cipher (shift, combine, altCombine).
  The print the encoded or decoded lines, one per lline starting with a tab.

  The following is an example of the output when the program is run

  $ hw4
  Encoding 1 lines with shift.
          QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD
  Decoding 1 lines with shift.
          THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
  Encoding 1 lines with combine.
          LXFOPV EF RNHR
  Decoding 1 lines with combine.
          ATTACK AT DAWN
  Encoding 2 lines with altCombine.
          CPMCRYY
          Aiuwtr At Ozaw
          Aiuwtr At Sbse
  Decoding 2 lines with altCombine.
          CALGARY
          Attack At Dawn
  

  Notes

  • Your program should be written so that additional ciphers can be added easily.
  • Your code should be modular in design, with small functions.
  • While the mod command (%) works well for positive integers, it has somewhat strange behavior for negative integers.
    • The following code might be more appropriate to compute value % 15 when value is negative.

    • while (value < 0) {
         value += 15;
      } 

  Other Requirements

  • The code must employ at least one enumeration type.
  • This code must compile without errors or warnings on cslab103.
  • The code must be compiled with the following compiler flags

     -g -O3 -Wpedantic -Wall -Wextra -Wmisleading-indentation -Wunused -Wuninitialized -Wshadow -Wconversion -std=c++17
    

  • The project must include a Makefile,
    • The makefile must
      • Build the entire project
      • Contain a clean target which removes object files and executables.
    • You may use this makefile
      • Save it in your project directory.
      • Your source code should be called hw4.C or hw4.cpp
    • This will produce an executable called hw4
  • Your code should follow the Local Programming Style Guild for C++

  Required Files

  A single tar file containing the source code and makefile for this program. Your tar file should not contain executable code (hw4) or any object files (*.o).

  To create tar file, follow the Working with the Tape Archive Utility (tar)

  Submission

  Email your tar file as an attachment to dbennett@edinboro.edu. Make sure that the message title indicates that this is homework 4 and that your name and section are included in the body of the message.