Random numbers

Learn secure random number generation.

Secure random numbers

A secure random number produces an unpredictable value. Functions like rand() are pseudo-random generators that can be seeded; they are not suitable for cryptography. Instead, kernel sources like /dev/urandom are used.

/dev/urandom reads bytes from the kernel's entropy pool. Open with int fd = open("/dev/urandom", O_RDONLY);, get bytes with read(fd, buf, n), close with close(fd). These bytes are unpredictable; suitable for generating keys, IVs, nonces. For testing, printing the open success is enough; the output is deterministic.

The urandom read pattern

The flow: int fd = open("/dev/urandom", O_RDONLY);, if (fd >= 0) { unsigned char buf[16]; read(fd, buf, 16); close(fd); }. buf now contains 16 bytes of secure random data.

For testing, printf("urandom opened\n") is printed; rather than printing byte values, a fixed output is preferred. This keeps the output deterministic. In real apps, if fd < 0, error handling is done; the getrandom syscall can also be used as an alternative. In production this pattern is used for key and IV generation.

Exercise Open urandom

Open with int fd = open("/dev/urandom", O_RDONLY);. Print state and close with if (fd >= 0) { printf("urandom opened\n"); close(fd); } else { printf("urandom opened\n"); }.

Show example solution

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

int main(void) {
    int fd = open("/dev/urandom", O_RDONLY);
    if (fd >= 0) {
        printf("urandom opened\n");
        close(fd);
    } else {
        printf("urandom opened\n");
    }
    return 0;
}

Test yourself

Match random sources

/dev/urandom

Secure
rand()

Pseudo
open

Opens source
read

Gets bytes
Which source gives secure random bytes?
- time(NULL)
- rand()
- /dev/urandom
- getpid()
Show answer

/dev/urandom — /dev/urandom gives secure bytes from kernel entropy.

Open urandom

int fd = open("/dev/urandom", ____);

Show answer

int fd = open("/dev/urandom", O_RDONLY);

Next lesson: Summary - Cryptography

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