ROT13 encoder/decoder

rot13.c

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv) {
    int ch = '\0';
    while((ch = fgetc(stdin)) != EOF)
    {
      /* for every char in stdin */
      unsigned char c = (unsigned char)ch;
      if(
	(c >= 'a' && c <= 'z') ||
	(c >= 'A' && c <= 'Z')
      )
      {
	/* if letter */
	unsigned char abs;
	if(c >= 'a' && c <= 'z')	/* for every small letter */
	  abs = c - 'a';
	else if(c >= 'A' && c <= 'Z')	/* for every big letter */
	  abs = c - 'A';
	abs = abs + 13;			/* dot rot13 */
	if(abs >= 26)
	  abs -= 26;			/* wrap */
	if(c >= 'a' && c <= 'z')	/* for every small letter */
	  c = abs + 'a';
	else if(c >= 'A' && c <= 'Z')	/* for every big letter */
	  c = abs + 'A';
	
      };
      printf("%c",c);
    }
    return 0;
}