Question

C++

The Caesar cipher is one of the earliest known and simplest ciphers. It is a type of substitution cipher in which each letter in the plaintext is 'shifted' a certain number of places down the alphabet.

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H	E	L	L	O		W	O	R	L	D	Normal Text
B	Y	F	F	I		Q	I	R	M	X	Encrypted Text

You are tasked to create an encryption program which takes a key word. The keyword will fill the first part of the shifted array. The remaining letters will fill the end of the array. It should look like this:

Key=”TOY

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You should have an interactive menu with the following options:

1 Enter Key word //takes new key word and re-aligns shifted array

2 Encrypt message//takes a message of 140 characters and encrypts message

3 Decrypt message//takes an encrypted massage of 140 characters and decrypts it

4 Quit//exits program;

A couple tips:

Use the copy function from the <cstring> library not the <algorithm> library. If you use the algorithm library you need to make a change in visual studios precompiler settings or you will get an error. You cannot assign arrays using the ‘=’ operator; only individual indices of the array can be assigned this way.

Take one case or the other for input. All Caps or all lowercase. You can use “toupper(<char>)” to set all letters to upper case: http://cpp.sh/7gwx

/* toupper example */

#include <stdio.h>

#include <iostream>

#include <ctype.h>//for toupper

using namespace std;

int main ()

{

int i=0;

char str[]="Test String.\n";

char c;

while (str[i])

{

    c=str[i];

    str[i]=toupper(c);

    i++;

}

std::cout<<str<<endl;

return EXIT_SUCCESS;

}

When encrypting/decrypting, don’t change characters that are not in your Array set like punctuation and spaces

Answer 1

/* C++ program to encrypt and decrypt the message using a meeu*/
#include <iostream>
#include <stdio.h>
#include <string.h>
#include <ctype.h>

using namespace std;

int checkLetter(char key[],char letter);
void lowercase(char s[]);
int find(char E[],char c);

//main function
int main()
{
int j,avail,keylen,option=1;
char E[27],D[27],letter;
char key[27];
char normal[140],encrypted[140];
//menu
while(option !=4)
{
cout<<endl<<"\n1: Enter the key :";
cout<<endl<<"\n2: Encrypt message :";
cout<<endl<<"\n3: Decrypt message :";
cout<<endl<<"4: Quit Program";
cout<<endl<<"Enter option(1,2,3,4)";
cin>>option;
switch(option)
{
case 1: //enter key and align the decryption string
cout<<"Enter tke key : ";
cin>>key;
lowercase(key);
strcpy(E,key);
letter='a';
for(j=strlen(key);j<26;j++)
{
avail=checkLetter(key,letter);
if(avail==0)
{
E[j]=letter;
E[j+1]='\0';
letter++;
}
else
{
letter++;
j--;
}
}
E[26]='\0';

case 2: // enter the mesggage for encryption
cout<<endl<<"enter the message for encryption: ";
cin.ignore();
fgets(normal, 140, stdin);
//gets(normal);
lowercase(normal); //convert normal text into lower case
int i;
for(i=0;normal[i]!='\0';i++)
{
if(normal[i]>=97 && normal[i]<= 122)
encrypted[i]=E[normal[i]-97];
else
encrypted[i]=normal[i];
}
encrypted[i]='\0';
cout<<endl<<"Encrypted to : "<<encrypted;
break;

case 3: // enter the mesggage for decryption
cout<<endl<<"enter the message for decryption : ";
cin.ignore();
fgets(encrypted, 140, stdin);
//gets(encrypted);
lowercase(encrypted);
for( i=0;encrypted[i]!='\0';i++)
{
if(encrypted[i]>=97 && encrypted[i]<= 122)
{
int loc=find(E,encrypted[i]);// returns the index of a character in encypted message in E
normal[i]=loc+97;;
}
else
normal[i]=encrypted[i];
}
normal[i]='\0';
cout<<endl<<"Decrypted to : "<<normal;
break;
}
}

return 0;

}

//this function returns the letter from alphabet is already
// exist in then key or not
int checkLetter(char key[],char letter)
{
int i=0;
while(key[i]!='\0')
{
if(key[i]==letter)
return 1;
i++;
}
return 0;
}

//This function converts a string s to lower case letter
void lowercase(char s[])
{
for(int i=0;s[i]!='\0';i++)
{
s[i]=tolower(s[i]);
}
}

//This function returns the location of a character in string E
int find(char E[],char c)
{
for(int i=0;E[i]!='\0';i++)
{
if(E[i]==c)
return i;
}
return -1;
}

/*
output:

1: Enter the key :

2: Encrypt message :

3: Decrypt message :
4: Quit Program
Enter option(1,2,3,4)1
Enter tke key : top

enter the message for encryption: HELLO WORLD

Encrypted to : ebiil wlqia

1: Enter the key :

2: Encrypt message :

3: Decrypt message :
4: Quit Program
Enter option(1,2,3,4)2

enter the message for encryption: HELLO WORLD

Encrypted to : ebiil wlqia

1: Enter the key :

2: Encrypt message :

3: Decrypt message :
4: Quit Program
Enter option(1,2,3,4)3

enter the message for decryption : HELLO WORLD

Decrypted to : khoob wbsog

1: Enter the key :

2: Encrypt message :

3: Decrypt message :
4: Quit Program
Enter option(1,2,3,4)3

enter the message for decryption : ebiil wlqia

Decrypted to : hello world

1: Enter the key :

2: Encrypt message :

3: Decrypt message :
4: Quit Program
Enter option(1,2,3,4)4

*/