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Rationale (Design Decisions)

Reasoning Behind the Order of Preferred Random Data Sources

The order is:

  1. libsodium if available
  2. fread() /dev/urandom if available
  3. mcrypt_create_iv($bytes, MCRYPT_DEV_URANDOM)
  4. COM('CAPICOM.Utilities.1')->GetRandom()

If libsodium is available, we get random data from it. This is the preferred method on all OSes, but libsodium is not very widely installed, so other fallbacks are available.

Next, we read /dev/urandom (if it exists). This is the preferred file to read for random data for cryptographic purposes for BSD and Linux. This step is skipped on Windows, because someone could create a C:\dev\urandom file and PHP would helpfully (but insecurely) return bytes from it.

Despite strongly urging people not to use mcrypt in their projects (because libmcrypt is abandonware and the API puts too much responsibility on the implementor) we prioritize mcrypt_create_iv() with MCRYPT_DEV_URANDOM above the remaining implementations.

The reason is simple: mcrypt_create_iv() is part of PHP's ext/mcrypt code, and is not part libmcrypt. It actually does the right thing:

  • On Unix-based operating systems, it reads from /dev/urandom which (unlike /dev/random) is the sane and correct thing to do.
  • On Windows, it reads from CryptGenRandom, which is an exclusively Windows way to get random bytes.

If we're on Windows and don't have access to mcrypt, we use CAPICOM.Utilities.1.

As of random_compat 2.0, we no longer fall through to OpenSSL.