How good is security via hashing

[geremy condra]

On Tue, Jun 7, 2011 at 3:18 AM, Robin Becker <robin at reportlab.com> wrote:
> A python web process is producing files that are given randomized names of
> the form
>
> hhhhhh-YYYYMMDDhhmmss-rrrrrrrr.pdf
>
> where rrr.. is a 128bit random number (encoded as base62). The intent of the
> random part is to prevent recipients of one file from being able to guess
> the names of others.
>
> The process was originally a cgi script which meant each random number was
> produced thusly
>
>
> pid is process id, dur is 4 bytes from /dev/urandom.
>
> random.seed(long(time.time()*someprimeint)|(pid<<64)|(dur<<32))
> rrr = random.getrandbits(128)
>
>
> is this algorithm safe? Is it safe if the process is switched to fastcgi and
> the initialization is only carried out once and then say 50 rrr values are
> generated.

The advice you got about just using urandom seems to be the best
you're likely to get. Given how few values you have to pull out of
random.random to reconstruct its state, the progress that's been made
in the last few years on similar hidden state problems, and the
limited amount of entropy you're feeding it in the first place, I'd
probably stay away from this method. And besides,

# adds random junk to the filename- should make it hard to guess
rrr = os.urandom(16)
fname += base64.b64encode(rrr)

has to be easier to read and reason about than the process above.

Geremy Condra