[Python-Dev] [Python-checkins] cpython: Issue #12049: Add RAND_bytes() and RAND_pseudo_bytes() functions to the ssl (original) (raw)

Victor Stinner victor.stinner at haypocalc.com
Tue May 24 18:06:15 CEST 2011

Le mardi 24 mai 2011 à 11:27 -0400, Terry Reedy a écrit :

> > +.. function:: RANDbytes(num) > + > + Returns num cryptographically strong pseudo-random bytes. > + > + .. versionadded:: 3.3 > + > +.. function:: RANDpseudobytes(num) > + > + Returns (bytes, iscryptographic): bytes are num pseudo-random bytes, > + iscryptographic is True if the bytes generated are cryptographically > + strong. > + > + .. versionadded:: 3.3

I am curious what 'cryptographically strong' means, what the real difference is between the above two functions, and how these do not duplicate what is in random.random.

An important feature of a CPRNG (cryptographic pseudo-random number generator) is that even if you know all of its output, you cannot rebuild its internal state to guess next (or maybe previous number). The CPRNG can for example hash its output using SHA-1: you will have to "break" the SHA-1 hash (maybe using "salt").

Another important feature is that even if you know the internal state, you will not be able to guess all previous and next numbers, because the internal state is regulary updated using an external source of entropy. Use RAND_add() to do that explicitly.

We may add a link to Wikipedia: http://en.wikipedia.org/wiki/CPRNG

Read the "Requirements" section, it's maybe more correct than my explanation: http://en.wikipedia.org/wiki/CPRNG#Requirements

About the random module, it must not be used to generate passwords or certificates, because it is easy to rebuild the internal state of a Mersenne Twister generator if you know the previous 624 numbers. Since you know the state, it's also easy to generate all next numbers. Seed a Mersenne Twister PRNG doesn't help. See my Hasard project if you would like to learn more about PRNG ;-)

We may also add a link from random to SSL.RAND_bytes() and SSL.RAND_pseudo_bytes().

https://bitbucket.org/haypo/hasard/

Victor

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