sifstring.c

/*
 * some methods in love live school idol festival have obfuscated strings. not
 * sure if this is a thing built into java or if it's some known obfuscator,
 * but either way this is a small tool to decipher them
 *
 * the pseudocode will usually look something like this
 *
 * ```c
 * v6 = "ke|RarsmsnRapeawi";
 * v7 = 2;
 * do
 * {
 *     v761[0] = ((*v6 - 1) ^ 1) - 1;
 *     v761[1] = ((v6[1] - 1) ^ 2) - 1;
 *     v761[2] = ((v6[2] - 1) ^ 3) - 1;
 *     v761[3] = ((v6[3] - 1) ^ 4) - 1;
 *     v761[4] = ((v6[4] - 1) ^ 5) - 1;
 *     v761[5] = ((v6[5] - 1) ^ 6) - 1;
 *     v761[6] = ((v6[6] - 1) ^ 7) - 1;
 *     v761[7] = ((v6[7] - 1) ^ 8) - 1;
 *     v761[8] = ((v6[8] - 1) ^ 9) - 1;
 *     v761[9] = ((v6[9] - 1) ^ 0xA) - 1;
 *     v761[10] = ((v6[10] - 1) ^ 0xB) - 1;
 *     v761[11] = ((v6[11] - 1) ^ 0xC) - 1;
 *     v761[12] = ((v6[12] - 1) ^ 0xD) - 1;
 *     v761[13] = ((v6[13] - 1) ^ 0xE) - 1;
 *     v761[14] = ((v6[14] - 1) ^ 0xF) - 1;
 *     v761[15] = ((v6[15] - 1) ^ 0x10) - 1;
 *     LOBYTE(v762) = ((v6[16] - 1) ^ 0x11) - 1;
 *     BYTE1(v762) = ((v6[17] - 1) ^ 0x12) - 1;
 *     --v7;
 *     v6 = v761;
 * }
 * while ( v7 );
 * ```
 *
 * 2 would be the number of rounds here
 *
 * # license
 * this is free and unencumbered software released into the public domain.
 * refer to the attached UNLICENSE or http://unlicense.org/
 */

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

#define info(fmt, ...) fprintf(stderr, fmt "\n", __VA_ARGS__);
#define inf1(msg) info("%s", msg)

int main(int argc, char* argv[])
{
    size_t n;
    char* str;
    size_t rounds = 2;

    if (argc < 2)
    {
        info("usage: %s encrypted_string nrounds", argv[0]);
        inf1("encrypted_string: the encrypted string, or - to read from stdin");
        inf1("nrounds: number of rounds, defaults to 2. you get this by "
            "reading the disassembly or by trial and error");
        return 1;
    }

    if (!strcmp(argv[1], "-"))
    {
        str = (char*)malloc(0x10000);
        if (!str) {
            perror("malloc");
            return 1;
        }

        if (!(n = fread(str, 1, 0xFFFF, stdin)) && !feof(stdin))
        {
            perror("fread");
            return 1;
        }

        str[n] = 0;
    }

    else {
        str = argv[1];
    }

    if (argc >= 3) rounds = strtoul(argv[2], 0, 10);

    for (n = 0; n < rounds; ++n)
    {
        char* p;

        for (p = str; *p; ++p) {
            *p = ((*p - 1) ^ (p - str + 1)) - 1;
        }
    }

    puts(str);

    return 0;
}