ROT13

tr '[a-m][n-z][A-M][N-Z]' '[n-z][a-m][N-Z][A-M]'

tr A-Za-z N-ZA-Mn-za-m

perl -pe'y/a-zA-Z/n-za-mN-ZA-M/'

import sys
print"".join([chr(x/32*32+1+(x%32+12)%26if 64<x<91or 96<x<123 else x)for x in map(ord,sys.stdin.read())])

main(c,b){while((c=getchar())>=0)b=c&96,putchar(isalpha(c)?b+1+(c-b+12)%26:c);}

#include <ctype.h>
#include <stdio.h>

int main(void)
{
    int c, base;

    while ((c = getchar()) >= 0) {
        if (isalpha(c)) {
            base = (c & 96) + 1; /* First letter of the upper or lower case. */
            c = base + (c - base + 13) % 26;
        }
        putchar(c);
    }

    return 0;
}