From 8ed651e717537ea69c987b78f6ef9f8d336bb734 Mon Sep 17 00:00:00 2001 From: Alex Gaynor Date: Thu, 7 Nov 2013 13:24:31 -0800 Subject: Be really explicit about what's good and bad --- docs/hazmat/primitives/symmetric-encryption.rst | 26 ++++++++++++++++++++++--- 1 file changed, 23 insertions(+), 3 deletions(-) (limited to 'docs/hazmat') diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst index 7d3b072d..1aeb2a56 100644 --- a/docs/hazmat/primitives/symmetric-encryption.rst +++ b/docs/hazmat/primitives/symmetric-encryption.rst @@ -163,9 +163,29 @@ Modes to be kept secret (they can be included in a transmitted message). Must be the same number of bytes as the - ``block_size`` of the cipher. Do not - reuse an ``initialization_vector`` with - a given ``key``. + ``block_size`` of the cipher. Each time + someting is encrypted a new + ``initialization_vector`` should be + generated. Do not reuse an + ``initialization_vector`` with + a given ``key``, and particularly do + not use a constant + ``initialization_vector``. + + A good construction looks like: + + .. code-block:: pycon + + >>> import os + >>> iv = os.urandom(16) + >>> mode = CBC(iv) + + While the following is bad and will leak information: + + .. code-block:: pycon + + >>> iv = "a" * 16 + >>> mode = CBC(iv) .. class:: CTR(nonce) -- cgit v1.2.3