Make 'random' seeds cryptographically secure if possible

* configure.ac: Check for "/dev/urandom".

* src/sysdep.c (init_random) [HAVE_DEV_URANDOM]: Read the stream
for the seed from "/dev/urandom".
[WINDOWSNT]: Obtain the stream for the seed from w32 APIs.
* src/fns.c (Frandom): Update the doc string to indicate that
system entropy is used when available.
* src/w32.c: Include wincrypt.h.
(w32_init_crypt_random, w32_init_random): New functions, use the
CryptGenRandom API.
(globals_of_w32): Initialize w32_crypto_hprov handle to zero.
* src/w32.h (w32_init_random): Add prototype.

* doc/lispref/numbers.texi (Random Numbers): Document more details
about 't' as the argument to 'random'.

* etc/NEWS: Mention that '(random t)' now uses a cryptographically
strong seed if possible.

(Bug#22202)

1 files changed, 3 insertions, 1 deletions

diff --git a/doc/lispref/numbers.texi b/doc/lispref/numbers.texi
index 20d3c4290f3..3a9483af967 100644
--- a/doc/lispref/numbers.texi
+++ b/doc/lispref/numbers.texi
@@ -1252,7 +1252,9 @@ any integer representable in Lisp, i.e., an integer between
 (@pxref{Integer Basics}).
 
 If @var{limit} is @code{t}, it means to choose a new seed as if Emacs
-were restarting.
+were restarting.  The new seed will be set from the system entropy, if
+that is available, or from the current time and Emacs process's PID
+(@pxref{System Environment, emacs-pid}) if not.
 
 If @var{limit} is a string, it means to choose a new seed based on the
 string's contents.