magicless_bug_example.c

#include <stdlib.h>    // malloc, free, exit
#include <stdio.h>     // fprintf, perror, fopen, etc.

#define ZSTD_STATIC_LINKING_ONLY
#include <zstd.h>

#include "common.h"

int main() {
    ZSTD_CCtx* cctx = ZSTD_createCCtx();
    ZSTD_DCtx* dctx = ZSTD_createDCtx();

    const char* src = "arbitrary";
    const size_t srcSize = 9;
    size_t cBuffSize = ZSTD_compressBound(srcSize);
    void* const cBuff = malloc_orDie(cBuffSize);

    // Use dict id 16592821 = 0xfd2fb5 = high 3 bytes of ZSTDv07_MAGICNUMBER
    // zstd --train --dictID=16592821 --maxdict=256 -B100 ~/silesia.tar
    const char* dictFileName = "dictionary";
    void* const dict = malloc_orDie(1000);
    const size_t dictSize = loadFile_orDie(dictFileName, dict, 1000);
    const ZSTD_CDict* const cdict = ZSTD_createCDict(dict, dictSize, 3);

    // Preconditions for corruption using ZSTDv07_MAGICNUMBER.
    // Try commenting out any of these lines, and decompression will succeed!
    // * Magicless format enabled
    // * Checksum enabled
    // * Reference a dictionary with dictID = 0xfd2fb5
    // Note: conditions are different for ZSTDv06_MAGICNUMBER, etc.
    CHECK_ZSTD(ZSTD_CCtx_setParameter(cctx, ZSTD_c_format, ZSTD_f_zstd1_magicless));
    CHECK_ZSTD(ZSTD_CCtx_refCDict(cctx, cdict));
    CHECK_ZSTD(ZSTD_CCtx_setParameter(cctx, ZSTD_c_checksumFlag, 1));
    CHECK_ZSTD(ZSTD_DCtx_setParameter(dctx, ZSTD_d_format, ZSTD_f_zstd1_magicless));

    printf("Trying to compress...\n");
    size_t const cSize = ZSTD_compress2(cctx, cBuff, cBuffSize, src, srcSize);
    CHECK_ZSTD(cSize);
    printf("Compression succeeded!\n");

    // Try hexdump -C compressed.zst, look at the first 4 bytes :)
    saveFile_orDie("compressed.zst", cBuff, cSize);

    printf("Trying to decompress...\n");
    void* const dBuff = malloc_orDie(srcSize);
    CHECK_ZSTD(ZSTD_decompress_usingDict(dctx, dBuff, srcSize, cBuff, cSize, dict, dictSize));
    printf("Decompression succeeded!\n");

    free(cBuff);
    free(dBuff);
    free(dict);
}