Caesar Encryption and decryption C++

Question

I would like to know how to limit the ASCII range of encryption from 32 - 126.

For our task we are supposed to convert string to char and encrypt/decrypt each individual char.

I'm currently using this for encryption

int value = (value-32+shift)%95+32 //value is the current ascii value of a                 
                                   //character
                                   //the first shift is given by the user with regards 
                                   //to how many shifts he wants to do to the right

and this for decryption

int value = (value-32-shift)%95+32

my encryption is working perfectly (when I quote out the decryption function) but my decryption is not working how it is supposed to.

extra note: We only have to do a right shift when encoding, a single string is given for our entire program to encrypt and decrypt ("This is C++")

Give shift: 3
Wklv#lv#F..
DECODE 
Wklv#lv#F..  //must be 'THIS is C++'
ENCODE       //shift=15
This is C++  //must be 'cwx#/x#/R::'  
DECODE 
EYZdpZdp4{{  //must be 'THIS is C++'
ENCODE      //shift=66
cwx#/x#/R::  //must be "8LMWbMWb'mm"
DECODE 
This is C++
ENCODE       //shift=94
cwx#/x#/R::  //must be 'SGHR~hr~B**'
DECODE 
This is C++

Note: in process of adding more code description

We need a [mcve]. Please note the "Minimal" there. We only want to see *one* fixed shift, the output you expect, and the output you get. — Martin Bonner supports Monica, Oct 20 '16 at 09:15

score 2 · Accepted Answer · edited May 23 '17 at 10:34

Your problem is explained in Modulo operator with negative values. I'm not sure this is an exact duplicate. The problem is that decoding a cipher character like '!' with a shift of more than one (say "3")

int value = (value-32-shift)%95+32
          = ('!'-32-3)%95+32
          = (33-32-3)%95+32
          = (-2)%95 + 32
          = -2 + 32
          = 30

Oops. You need to use:

int value = (value-32+(95-shift))%95+32

score 2 · Answer 2 · edited May 23 '17 at 12:34

2

The probelm is that (value-32-shift) can become negative. The modulo operation does not 'wrap around', but actually 'mirrors' around zero (see this question and answer if you want to know why). To ensure that your value stays positive, add 95 before doing the modulo operation:

int value = (value-32-shift+95)%95+32

edited May 23 '17 at 12:34

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answered Oct 20 '16 at 09:21

maddin45

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Caesar Encryption and decryption C++

2 Answers2