Fixed #568 -- Document that users should use urandom for all their random numbers

1 files changed, 20 insertions, 0 deletions

diff --git a/docs/random-numbers.rst b/docs/random-numbers.rst
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+Random number generation
+========================
+
+When generating random data for use in cryptographic operations, such as an
+initialization vector for encryption in
+:class:`~cryptography.hazmat.primitives.ciphers.modes.CBC` mode, you do not
+want to use the standard :mod:`random` module APIs. This is because they do not
+provide a cryptographically secure random number generator, resulting in
+various security issues in different algorithms.
+
+Therefore, it is our recommendation to always use your operating system's
+provided random number generator, which is available as ``os.urandom()``. For
+example, if you need 16 bytes of random data for an initialization vector, you
+can obtain them with:
+
+.. doctest::
+
+    >>> import os
+    >>> os.urandom(16)
+    '...'