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Diffstat (limited to 'docs/hazmat/primitives/symmetric-encryption.rst')
-rw-r--r-- | docs/hazmat/primitives/symmetric-encryption.rst | 29 |
1 files changed, 26 insertions, 3 deletions
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst index 4d0703bb..9d18ce50 100644 --- a/docs/hazmat/primitives/symmetric-encryption.rst +++ b/docs/hazmat/primitives/symmetric-encryption.rst @@ -159,6 +159,9 @@ Weak Ciphers ``192``, or ``256`` bits in length. This must be kept secret. + +.. _symmetric-encryption-modes: + Modes ~~~~~ @@ -173,9 +176,29 @@ 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 + something 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) |