sophos - level1

// by petter wahlman, http://www.twitter.com/badeip

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <ctype.h>
#include <string.h>

#define ROTATION 13
static const char b64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

void decodeblock(unsigned char *input, char *output)
{
    unsigned char out[4];
    out[0] = input[0] << 2 | input[1] >> 4;
    out[1] = input[1] << 4 | input[2] >> 2;
    out[2] = input[2] << 6 | input[3] >> 0;
    out[3] = '\0';
    strncat(output, (const char *)out, sizeof(out));
}

char *unbase(char *input)
{
    int c, phase, i;
    unsigned char in[4];
    char *output;
    char *p;

    output = calloc(1, strlen(input) << 1);
    phase = 0;
    i = 0;
    while(input[i]) {
        c = input[i];
        if('=' == c) {
            decodeblock(in, output);
            break;
        }
        p = strchr(b64, c);
        if (p) {
            in[phase] = p - b64;
            phase = (phase + 1) % 4;
            if (phase == 0) {
                decodeblock(in, output);
                in[0]=in[1]=in[2]=in[3]=0;
            }
        }
        i++;
    }
    return output;
}

int rotate(char c, int pos)
{
    char alpha[pos << 1];
    int i;

    for (i = 0; i < sizeof(alpha); i++)
        alpha[i] = 'a' + i;

    return (c < alpha[0] || c > alpha[sizeof(alpha)-1]) ? c : alpha[(c + (sizeof(alpha) >> 1) - alpha[0]) % sizeof(alpha)];
}

void revert(char *str)
{
    char c;
    int i;

    for (i = 0; i < (strlen(str) >> 1); i++) {
        c = str[i];
        str[i] = str[strlen(str)-i - 1];
        str[strlen(str) -i - 1] = c;
    }
}

int main(int argc, char **argv)
{
    char challenge[] = "=ImYndmbn1ieiBnLmJWdjJmZ";
    char *unbased;
    int i, r;

    revert(challenge);
    unbased = unbase(challenge);

    for (i = 0; i < strlen(unbased); i++) {
        r = rotate(unbased[i], 13);
        if (r < 1)
            break;
        printf("%c", r);
    }
    putchar('\n');

    free(unbased);

    return 0;
}