Implement secure_random() function using OpenSSL

[gavv]

There are a few RTP/RTCP fields that are required to be populated using cryptographically secure PRNG. Currently we fill them using regular PRNG (core::fast_random), which will become a security issue when we add encryption.

We need to implement a new function core::secure_random:

ROC_ATTR_NODISCARD bool secure_random(void* buf, size_t bufsz);

We already have OpenSSL support, so we can implement secure_random() using RAND module from OpenSSL.

Since OpenSSL can be disabled at build time via --disable-openssl scons option, we should actually add two implementations:

• in roc_core/target_openssl - main implementation, using OpenSSL
• in roc_core/target_nocsprng - alternative fallback implementation that uses fast_random()

Scons will automatically use target_openssl when OpenSSL is enabled, and target_nocsprng when it's disabled (this feature is already implemented in SConstruct). See here about target directories.

Then we should switch the following classes/functions from fast_random() to secure_random():

• rtp::Identity
• rtp::Sequencer
• fec::Writer
• core::uuid_generare()

(Each one has a few calls to fast_random()).

We also should add simple tests for secure_random() similar to fast_random().

[lovvik]

Hello, @Yannise-A and I are students at Paris 8 University. We're taking an Open source development course this semester, and would like to work on this, to begin with.

[gavv]

@lovvik @Yannise-A you're both welcome, thank you :)