title	author	date
Pseudo Random Number Generation Lab	Xinyi Li	\today{}

Instruction: https://seedsecuritylabs.org/Labs_16.04/PDF/Crypto_Random_Number.pdf

Task 1

compile time_random.c and run:

gcc time_random.c -o time_random
./time_random

Executing ./time_random several times, it always gives different results:

Because it uses the current time as a random seed to generate random numbers, which guarantee the seed is always different in each run.

Comment out the line srand(time(NULL));, recompile it and run, the numbers generated are the same now:

Task 2

Get the epoch of 2018-04-17 23:08:49 by:

date -d "2018-04-17 23:08:49" +%s

It returns 1524020929. Then we list all possible random numbers generated by time_random.c within the two hours by adding a loop before line 12 in it as time_guess.c:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define KEYSIZE 16

void main()
{
    int i;
    char key[KEYSIZE];
    for (time_t t = 1524020929 - 60 * 60 * 2; t < 1524020929; t++) // within 2h window
    {
        srand(t);
        for (i = 0; i < KEYSIZE; i++)
        {
            key[i] = rand() % 256;
            printf("%.2x", (unsigned char)key[i]);
        }
        printf("\n");
    }
}

get the list:

gcc time_guess.c -o time_guess
time_guess > key_dict.txt

Use a brute-force method to crack the key from key_dict.txt as guess_key.py:

#!/usr/bin/python3
from Crypto.Cipher import AES

data = bytearray.fromhex('255044462d312e350a25d0d4c5d80a34')
ciphertext = bytearray.fromhex('d06bf9d0dab8e8ef880660d2af65aa82')
iv = bytearray.fromhex('09080706050403020100A2B2C2D2E2F2')

with open('key_dict.txt') as f:
    keys = f.readlines()

for k in keys:
    k = k.rstrip('\n')
    key = bytearray.fromhex(k)
    cipher = AES.new(key=key, mode=AES.MODE_CBC, iv=iv)
    guess = cipher.encrypt(data)
    if guess == ciphertext:
        print("find the key:", k)
        exit(0)

print("cannot find the key!")

It finds out the key:

$ chmod u+X guess_key.py
$ guess_key.py
find the key: 95fa2030e73ed3f8da761b4eb805dfd7

Task 3 - 4

cat /dev/random | hexdump

It generates output slowly and almost gets stuck when I keep unmoved. Monitor the entropy in another terminal：

watch -n .1 cat /proc/sys/kernel/random/entropy_avail

When I move the mouse or type something, the value increases fast. Every time it decreases, one line of new random numbers appear. So we can say that /dev/random consumes the available entropy produced by user's behaviors to generate new random numbers. (more details can be found in Wikipedia and this question)

If a server uses /dev/random to generate the random session key with a client. Please describe how you can launch a Denial-Of-Service (DOS) attack on such a server.

Attackers keep asking for establishing connections, which makes the server run out of the available entropy for /dev/random. Then the random number generator is blocked.

Task 5

cat /dev/urandom | hexdump

It keeps printing out random numbers. We truncate the first 1 MB outputs into a file named output.bin:

head -c 1M /dev/urandom > output.bin

Then use ent to evaluate its information density:

ent output.bin

see the documentation. It looks random in most measures.

Use /dev/urandom to generate a 256-bit random number as a session key by read_random_key.c:

#include <stdio.h>
#include <stdlib.h>
#define LEN 32 // 256 bits

int main()
{
    unsigned char *key = (unsigned char *)malloc(sizeof(unsigned char) * LEN);
    FILE *random = fopen("/dev/urandom", "r");
    fread(key, sizeof(unsigned char) * LEN, 1, random);
    fclose(random);
    printf("k = ");
    for (int i = 0; i < LEN; i++)
        printf("%.2x", key[i]);
    printf("\n");
    return 0;
}

Compile:

gcc read_random_key.c -o read_random_key

Run: