Caesar's Cipher Code

Question

#include <stdio.h>

void caesar (char cipher[], int shift);

int main () {

char cipher[50];
int shift;

  printf("Enter text to be encrypted IN CAPITAL LETTERS ONLY: ");
  scanf("%s", cipher);

  printf("How many shifts do you prefer? 1-10 only: ");
  scanf("%d", &shift);

  caesar (cipher, shift);

  return 0;
}

void caesar (char cipher[], int shift) {
  int i = 0;

  while (cipher[i] != '\0') {
    if ((cipher[i] += shift) >= 65 && (cipher[i] += shift) <= 90) {
      cipher[i] += (shift);
    } else {
      cipher[i] += (shift - 25); 
    }
    i++;
  }
  printf("%s", cipher);
}

I'm starting to get encrypted outputs but i'm afraid there's something wrong with my statements.

For Example:

• Input: ABCD, 1 shift
• Output: DEFG <= which is actually 3 shifts.

Answer 1

Change

if ((cipher[i] += shift) >= 65 && (cipher[i] += shift) <= 90) ...

to

if ((cipher[i] + shift) >= 65 && (cipher[i] + shift) <= 90) ...

since += modifies cipher[i] .

Answer 2

(I got here via http://codereview.stackexchange.com , so I'm still wearing a code review hat).

With code that manipulates letters, I find it easier to understand if it uses actual letters in the source, rather than numeric codes. So I recommend changing

  cipher[i] += (shift - 25); 

to something like

  cipher[i] += (shift - ('Z' - 'A')); 

Most people doing Caesar ciphers convert only the letters, and pass through punctuation, numbers, spaces, etc. unchanged. You might consider including the standard character library

#include <ctype.h>

and using the functions isalpha(), islower(), isupper() -- in particular, changing

if ((cipher[i]) >= 'A' && (cipher[i]) <= 'Z') {

to something like

if (isupper(cipher[i])) {

.

Answer 3

void caesar (char cipher[], int shift) {
  int i = 0;

  while (cipher[i] != '\0') {
    if (cipher[i] >= 'A' && cipher[i]<='Z') {
        char newletter = cipher[i] - 'A';
        newletter += shift;
        newletter = newletter % 26;
        cipher[i] = newletter + 'A';
    }
    i++;
  }
  printf("%s", cipher);
}

This will ignore anything that is not an uppercase letter.

If the letters had codes 0-25, it would be very easy to do a shift because we could save a condition using just a remainder of 26. Which is what i did, i subtract 'A', so that the A letter will be 0, then i add the shift and calculate the remainder, so that if you add 1 to a 'Z', you will get an 'A' again and so on. And in the end i had to add 'A' again, because in ASCII, the 'A' is not really 0.

Answer 4

try replacing

if ((cipher[i] += shift) >= 65 && (cipher[i] += shift) <= 90) {
  cipher[i] += (shift);
 } else {
  cipher[i] += (shift - 25); 
 }

with

if ((cipher[i] += shift) >= 65 && (cipher[i] = shift) <= 90) {
 // do nothing
 } else {
  cipher[i] = 'A' + ('Z' - cipher[i]) -1; 
 }

using "+=" will modify value whenever its evaluated. this was evaluated 3 times in your code, that's why it gives 3 shifts !

Answer 5

Small working version, no shift range except negative values:

#include <stdio.h>

void caesar (char cipher[], int shift);

int main () {

  char cipher[50];
  int shift;

  printf("Plaintext (Caps only): ");
  scanf("%s", cipher);

  printf("Shift: ");
  scanf("%d", &shift);

  caesar (cipher, shift);

  return 0;
}

void caesar (char cipher[], int shift) {

  for(int i=0; cipher[i] != '\0'; i++)
    cipher[i] = 65 + (cipher[i]-65+shift)%26;
  printf("Cipher text: %s\n", cipher);
}

Answer 6

I modified LtWorf to match my current project.

shift range : -9 ~ +9

void caesar(char cipher[], int shift) {
  int i = 0;

  while (cipher[i] != '\0') {
    if (cipher[i] >= 'A' && cipher[i]<='Z') {
        char newletter = cipher[i] - 'A' + 26;
        newletter += shift;
        newletter = newletter % 26;
        cipher[i] = newletter + 'A';
    } else if (cipher[i] >= '0' && cipher[i]<='9') {
        char newletter = cipher[i] - '0' + 10;
        newletter += shift;
        newletter = newletter % 10;
        cipher[i] = newletter + '0';
    }
    i++;
  }
  printf("%s\n", cipher);
}

Answer 7

Caesar Cipher Code for C++

#include<iostream>
#include<cctype>
using namespace std;

char cipher(char c, int k)
{
    if(isupper(c))
        return char( 'A' + (c - 'A' + k ) % 26);

    else if(islower(c))
        return char( 'a' + (c - 'a' + k ) % 26);

    else
        return c;
}

int main()
{
    int k;
    string str;
    cin>>str>>k;
    int l = str.size();

    for(int i=0; i<l; i++)
        cout<<cipher(str[i], k);
}