Caesar's cipher algorithm

caesar.c

#include <stdio.h>

/**********************************************************************
 *
 * caesar.c
 * Caesar's cipher algorithm
 * Encrypts messages using Caesar's cipher.
 * <@pedrol2b>
 **********************************************************************/

char *encode(char str[], int key)
{
  int i;
  char buff[256];
  for (i = 0; str[i] != '\0'; i++)
  {
    if (str[i] >= 'a' && str[i] <= 'z')
      buff[i] = (str[i] - 'a' + key) % 26 + 'a';
    else if (str[i] >= 'A' && str[i] <= 'Z')
      buff[i] = (str[i] - 'A' + key) % 26 + 'A';
    else
      buff[i] = str[i];

    str[i] = buff[i];
  }

  return str;
}

char *decode(char str[], int key)
{
  int i;
  char buff[256];
  for (i = 0; str[i] != '\0'; i++)
  {
    if (str[i] >= 'a' && str[i] <= 'z')
      buff[i] = (str[i] - 'a' - key) % 26 + 'a';
    else if (str[i] >= 'A' && str[i] <= 'Z')
      buff[i] = (str[i] - 'A' - key) % 26 + 'A';
    else
      buff[i] = str[i];

    str[i] = buff[i];
  }

  return str;
}

int main(void)
{
  int option, key = 0;
  char str[256];
  printf("Caesar Cipher Algorithm\n");
  printf("---------------------\n");
  printf("Options:\n");
  printf("1. Encrypt a text\n");
  printf("2. Decrypt a text\n");
  printf("---------------------\n");
  printf("Usage: First enter the option number, then the key, and finally the text to encrypt/decrypt.\n");
  printf("Example: To encrypt the text 'hello' with a key of 3, you would enter:\n");
  printf("1\n3\nhello\n");
  printf("---------------------\n");
  printf("Please enter your option, key, and text:\n");
  scanf("%d\n%d\n%s", &option, &key, str);

  switch (option)
  {
  case 1:
    printf("%s\n", encode(str, key));
    break;
  case 2:
    printf("%s\n", decode(str, key));
    break;
  default:
    printf("Invalid option.\n");
  }

  return 0;
}