CSAW CTF 2015 Qualification Round - Crypto 500

CTF

#include <stdio.h>
#include <string>
int main()
{
    FILE * fp = fopen( "input", "rb");
    FILE * wp = fopen( "test.pdf", "wb");
    static unsigned char buffer[769180];
    unsigned char iv[4];
    unsigned char tmp[4];
    /*
    pdf = 0x25, 0x50, 0x44, 0x46
    iv = 0x43, 0x41, 0x53, 0x48
    header = 0x24, 0x58, 0x4d, 0x54
    key = pdf ^ iv ^ header
    */
    unsigned char key[4] = { 0x42, 0x49, 0x5a, 0x5a };
    fread( buffer, sizeof(buffer), 1, fp );
    iv[0] = 'C';
    iv[1] = 'A';
    iv[2] = 'S';
    iv[3] = 'H';
    for ( size_t i = 0; i < sizeof(buffer) / 4; ++i )
    {
        for ( size_t j = 0; j < 4; ++j )
        {
            tmp[j] = buffer[i * 4 + j] ^ iv [j] ^ key[j];
            iv[j] = buffer[i * 4 + j];
        }
        fwrite( tmp, 4, 1, wp );
    }
    fclose(fp);
    fclose(wp);
}