Using C++

#include

#include

using namespace std;

// get the ciphered text

string encrypt(string str, int);

// decrypt the cipher text

string decrypt(string str, int);

int main()

{

printf("Enter shift +/- 26: ");

int n;

cin>>n;

printf(" Enter plaintext message (A-Z only, no spaces): ");

string str;

cin>>str;

// get the ciphered text

string cipher = encrypt(str, n);

// decrypt the cipher text

string plaintext = decrypt(cipher, -n);

cout<<" ciphertext: "<<cipher<<" plaintext: "<<plaintext;

return 0;

}

// get the ciphered text

string encrypt(string str, int n)

{

int i;

for( i = 0 ; i < str.length() ; i++ )

{

// get the ascii code

int ascii = (int)str[i];

ascii = ascii + n;

if(ascii < 65)

ascii = 91 - ( 65 - ascii );

else if(ascii > 90)

ascii = 64 + (ascii - 90);

// update the character with the ciphered text

str[i] = (char)ascii;

}

return str;

}

// decrypt the cipher text

string decrypt(string str, int n)

{

int i;

for( i = 0 ; i < str.length() ; i++ )

{

// get the ascii code

int ascii = (int)str[i];

ascii = ascii + n;

if(ascii < 65)

ascii = 91 - ( 65 - ascii );

else if(ascii > 90)

ascii = 64 + (ascii - 90);

// update the character with the ciphered text

str[i] = (char)ascii;

}

return str;

}

You are now to extend the above program to take as inputs files. The program should be able to read a file and encode or decode it as needed.

For the sake of simplicity, we assume that you only need to change letters [A-Z] in the file. You can safely ignore other letters in the file (i.e., keep those as is.)

Encrypting a file to cyphertext

Encrypt a file in.txt containing plaintext to a file out.txt containing ciphertext using shift . Flag -e here refers to encryption.

$ cf -e  in.txt out.txt 

Example

Consider f1.txt

HELLO WORLD THIS IS AMAZING WHY IS THIS SO AMAZING I HAVE NO IDEA 11231 

After running the following command

$ ./cf -e 3 f1.txt f2.txt File f2.txt looks like

KHOOR ZRUOG WKLV LV DPDCLQJ ZKB LV WKLV VR DPDCLQJ L KDYH QR LGHD 11231 

Decrypting a file to plaintext

Decrypting a file in.txt containing ciphertext to a file out.txt containing plaintext using shift . Flag -d here refers to decryption.

$ cf -d  in.txt out.txt 

Example

After running the following command

$ ./cf -d 3 f2.txt f3.txt File f3.txt looks like

HELLO WORLD THIS IS AMAZING WHY IS THIS SO AMAZING I HAVE NO IDEA 11231