AES Encryption -Key Generation with OpenSSL

Question

As a reference and as continuation to the post: how to use OpenSSL to decrypt Java AES-encrypted data?

I have the following questions.

I am using OpenSSL lib

Answer 1

An AES key, and an IV for symmetric encryption, are just bunchs of random bytes. So any cryptographically strong random number generator will do the trick. OpenSSL provides such a random number generator (which itself feeds on whatever the operating system provides, e.g. CryptGenRandom() on Windows or /dev/random and /dev/urandom on Linux). The function is RAND_bytes(). So the code would look like this:

#include <openssl/rand.h>

/* ... */
unsigned char key[16], iv[16];

if (!RAND_bytes(key, sizeof key)) {
    /* OpenSSL reports a failure, act accordingly */
}
if (!RAND_bytes(iv, sizeof iv)) {
    /* OpenSSL reports a failure, act accordingly */
}

Answer 2

Assuming AES-128:

unsigned char key[16];
RAND_bytes(key, sizeof(key));

unsigned char iv[16];
RAND_bytes(iv, sizeof(iv));

The random generator needs to be seeded before using one of those.