Problem - Kevin Tafuro

See Also
• NIST Cryptographic Module Validation Program home page: http://csrc.nist.gov/
cryptval/
• Recipe 11.2
11.19 Performing Entropy Estimation and
Management
Problem
You are collecting your own entropy, and you need to determine when you have collected
enough data to use the entropy.
Solution
At the highest level, the solution is to be incredibly conservative in entropy estimation.
In the discussion, we will examine general practices and guidelines for particular
sources.
Discussion
Fundamentally, the practical way to look at entropy is as a measurement of how
much information in a piece of “random” data an attacker can glean about your randomness
infrastructure. For example, if you have a trusted channel where you get
128 bits of data, the question we are really asking is this: how much of that data is
provided to an attacker through whatever data channels are available to him? The
complexity of an attack is based on how much data an attacker has to guess.
Clearly, in the practical sense, a single piece of data can have different amounts of
entropy for different people. For example, suppose that we use the machine boot
time to the nearest second as a source of entropy. An attacker who has information
about the system startup time narrowing it down to the nearest week still has a much
harder problem than an attacker who can narrow it down to a 10-second period. The
second attacker can try all 10 possible starting values and see if he gets the correct
value. The first has far, far more values to try before finding the original value.
In practice, it turns out that boot time is often an even more horrible source of
entropy than we have already suggested. The nmap tool can often give the system
uptime of a remote host with little effort, although this depends on the operating system
and the firewall configuration of the host being targeted.
The basic lesson here is that, before you decide how to estimate entropy, you should
figure out what your threat model is. That is, what kinds of attacks are you worried
Performing Entropy Estimation and Management | 621
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