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[Openstack-security] [Bug 1757300] Fix proposed to heat (stable/queens)


Fix proposed to branch: stable/queens
Review: https://review.openstack.org/555859

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https://bugs.launchpad.net/bugs/1757300

Title:
  RandomString may have less entropy than expected

Status in OpenStack Heat:
  Fix Released
Status in OpenStack Security Advisory:
  Won't Fix

Bug description:
  When generating a random string, once we have selected from the
  various required pools, we continue by selecting a pool at random and
  then selecting a character from that pool at random. This does not
  take into account the differing sizes of the available pools, nor the
  fact that the same character could appear in multiple pools. This
  results in a non-uniform probability distribution of characters.

  For example, in the following resource:

      type: OS::Heat::RandomString
      properties:
        length: 66
        character_classes:
          - class: lettersdigits
        character_sequences:
          - sequence: "*$"

  one might reasonably expect to find an average of 3 '*' or '$'
  characters in the output, but in fact there would be an average of 33.

  Since users mostly make use of this feature to generate default
  passwords for services they are deploying, this would result in the
  generated passwords having slightly less entropy than expected.
  Pathological cases where the entropy is massively reduced (like the
  one above - where it is only 229.5 bits vs. the expected 391 bits) are
  possible, although it's probably unlikely that users would encounter
  them by accident.

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