Click here to download caesar.c.

/***************************************************
Ceasar-Shift Encryption

Write a program that reads a key value from the
user (i.e. a number between 0 and 26) and a message
consisting solely of lowercase letters terminated
by a '.', and prints the Ceasar Shift encryption
of the message using the key.

Ceasar Shift Encryption is decribed in a problem
from Unit 2, so check out the lecture notes there.
***************************************************/
#include "si204.h"

int main() {
  // Read in key value
  fputs("Enter key value: ", stdout);
  int key = readnum(stdin);

  // Read in message
  fputs("Enter message (terminated by a .): ", stdout);
  char nextlet;

  // read the first letter
  nextlet = readchar(stdin);

  // Keep encrypting until you see a .
  while (nextlet != '.') {
    // turn letter to upper-case if it's lower-case
    if (nextlet >= 'a') {
      nextlet -= 'a' - 'A';
    }

    // Compute distance for original letter
    int dist = nextlet - 'A';

    // Compute distance for encrypted letter
    int ed = (dist + key) % 26;

    // Compute encrypted letter
    char e = 'A' + ed;

    // Write out encrypted letter
    fputc(e, stdout);

    // read next letter
    nextlet = readchar(stdin);
  }

  fputs(".\n", stdout);

  return 0;
}