Is that safe to use ramdom.random() for key to encrypt?

[Steven D'Aprano]

On Mon, 18 Jun 2012 08:41:57 +1000, Chris Angelico wrote:

> On Mon, Jun 18, 2012 at 3:06 AM, Rafael Durán Castañeda
> <rafadurancastaneda at gmail.com> wrote:
>> The language Python includes a SystemRandom class that obtains
>> cryptographic grade random bits from /dev/urandom on a Unix-like
>> system, including Linux and Mac OS X, while on Windows it uses
>> CryptGenRandom.
> 
> /dev/urandom isn't actually cryptographically secure; it promises not to
> block, even if it has insufficient entropy. But in your instance...

Correct. /dev/random is meant to be used for long-lasting 
cryptographically-significant uses, such as keys. urandom is not.

http://en.wikipedia.org/wiki//dev/random

 
>> Do you think is secure enough for token generation? (40 chars long
>> tokens are used for password reset links in a website, there isn't any
>> special security concern for the web).
> 
> ... it probably is fine, since password reset tokens don't need to be as
> secure as encryption keys (if anyone _does_ figure out how to predict
> your password resets, all they'll be able to do is lock people out of
> their accounts one by one, not snoop on them all unbeknownst, and you'll
> be able to see log entries showing the resets - you DO log them,
> right?). In fact, you could probably get away with something pretty
> trivial there, like a SHA1 of the current timestamp, the user name, and
> the user's current password hash. The chances that anybody would be able
> to exploit that are fairly low, given that you're not a bank or other
> high-profile target.

If I were an identity thief, I would *love* low-profile targets. Even 
though the payoff would be reduced, the cost would be reduced even more:

- they tend to be complacent, even more so than high-profile targets;

- they tend to be smaller, with fewer resources for security;

- mandatory disclosure laws tend not to apply to them;

- they don't tend to have the resources to look for anomalous usage
  patterns, if they even cared enough to want to.


If there was a Facebook-like website that wasn't Facebook[1], but still 
with multiple tens of thousands of users, I reckon a cracker who didn't 
vandalise people's accounts could steal private data from it for *years* 
before anyone noticed, and months or years more before they did something 
about it.



[1] And very likely a Facebook-like website that *was* Facebook. I reckon 
the odds are about 50:50 that FB would prefer to keep a breach secret 
than risk the bad publicity by fixing it.


-- 
Steven