Re: [WEB SECURITY] quick question on password reset 'best practices'

[Sebastian Schinzel <Sebastian.Schinzel@xxxxxxxxxxxxxxx>]

Hi Jeremiah,

Jeremiah Grossman wrote:
>  However, you have to get really exact with the
> delays, hard to do, and delays with change dynamically with load on the 
> system. I think its possible to detect timing resolution down to 2-digit 
> ms. The other thing that's possible is implement random timing delays in 
> the flow. This would seem to me to be the most viable, but have not 
> tested it personally.

The delays shouldn't be too random. Given a large amount of probes, the 
random values would be visible as noise around the actual delay while 
processing.

true_delay = MIN(all_measured_delays_per_username)

I suggest that delays should be constant per input value, but not 
predictable. That way, there is no noise around the actual delay while 
processing. It is a fixed but random value. Thus, an attacker does not 
gain any value by comparing the overall delays of different user names.

Let's assume you want to prevent harvesting of user names:

<pseudo_code tested="false">
if authentication_successful == False:
	
	# Get the last three characters (hex) of the combined hash of
	# the username and a server-side secret
	delay_str = md5(username + secret_string)[-3:]
	
	# Cast the random string from hex to float.
	# The delay should be at least 100 ms.
	delay = (float(int(delay_str, 16)) / 1000) + 0.1
	
	# Sleep between 0 and 4 seconds (should probably be lower)
	time.sleep(delay)
</pseudo_code>

Using this methodology: It would be interesting to know whether an 
attacker can gain any value by measuring the delay deviation per user 
name. I.e.: Does an existing user name have different delay deviations 
resulting from server-side processing compared to the delays of non 
existing user name?

Comments?

Cheers,
Sebastian

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