How to decrypt this file:

https://drive.google.com/open?id=1gZCujDYmEL-FZ5o_xBUS-Ng5979HSsZ7

Given that the only information available about it are:

1- it has been created on 21/3/2018, 8:31:47 PM

2- the plaintext of the file is propably starting with "\documentclass[12pt]{article}"

3- the code used for encryption is in C, and used a pseudo-random number generator (The Linear Congruence rand():

/* The ISO/IEC 9899:1990 edition of the C standard */

#define RAND_MAX 32767

static unsigned long int next = 1;

int rand(void) // RAND_MAX assumed to be 32767

{

next = next * 1103515245 + 12345;

return (unsigned int)(next/65536) % 32768;

}

void srand(unsigned int seed)

{

next = seed;

}

#include

#include

//Return a byte at a time of the rand() keystream

char randchar() {

static int key;

static int i = 0;

i = i % 4;

if (i == 0) key = rand();

return ((char *)(&key))[i++];

}

int main(int argc, const char* argv[]) {

static char randstate[64];

srand(time(NULL));

FILE *input, *output;

input = fopen("Homework1b.tex", "r");

output = fopen("Homework1b.tex.enc", "w");

int c,rc;

while ((c = fgetc(input)) != EOF) {

rc=randchar();

printf("c=%d (%c) and rc=%d ",c,c,rc);

fputc(c^rc,output);

}

fclose(input);

fclose(output);

}

I need the code as well as a description of the solution.