TimingAttackAgainstHeaderValue.qhelp

<!DOCTYPE qhelp PUBLIC "-//Semmle//qhelp//EN" "qhelp.dtd">
<qhelp>

<overview>
<p>
  A constant-time algorithm should be used for checking the value of sensitive headers.
In other words, the comparison time should not depend on the content of the input. 
Otherwise timing information could be used to infer the header's expected, secret value.
</p>
</overview>


<recommendation>
<p>
 Two types of countermeasures can be applied against timing attacks. The first one consists 
in eliminating timing variations whereas the second renders these variations useless for an attacker.
The only absolute way to prevent timing attacks is to make the computation strictly constant time,
independent of the input.

 Use <code>hmac.compare_digest()</code> method to securely check the secret value.
If this method is used, then the calculation time depends only on the length of input byte arrays,
and does not depend on the contents of the arrays. 
 Unlike <code>==</code> is a fail fast check, If the first byte is not equal, it will return immediately.
</p>
</recommendation>
<example>
<p>
  The following example uses <code>==</code> which is a fail fast check for validating the value of sensitive headers.
</p>
<sample src="UnsafeComparisonOfHeaderValue.py" />

<p> 
  The next example use a safe constant-time algorithm for validating the value of sensitive headers:
</p>
<sample src="SafeComparisonOfHeaderValue.py" />
</example>

<references>
<li>
  Wikipedia:
  <a href="https://en.wikipedia.org/wiki/Timing_attack">Timing attack</a>.
</li>

<li>
  <a href="https://docs.python.org/3/library/hmac.html#hmac.compare_digest">hmac.compare_digest() method</a>
</li>

</references>

</qhelp>