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provide details on the delays enforced by Weaver

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Daniel Micay 2020-12-28 22:49:07 -05:00
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static/faq.html
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@ -354,13 +354,30 @@
The Weaver token is stored alongside a Weaver key derived by the OS from the
password token. In order to retrieve the Weaver token, the secure element
requires the correct Weaver key. A secure internal timer is used to implement
hardware-based exponentially increasing delays for each attempt at key
derivation and quickly ramps up to 1 day per attempt. Weaver also provides
reliable wiping of data since the secure element can reliably wipe a Weaver
slot. Deleting a profile will wipe the corresponding Weaver slot and a factory
reset of the device wipes all of the Weaver slots. The secure element also
provides insider attack resistance preventing firmware updates before
authenticating with the owner profile.</p>
hardware-based delays for each attempt at key derivation. It quickly ramps up
to 1 day delays before the next attempt. Weaver also provides reliable wiping
of data since the secure element can reliably wipe a Weaver slot. Deleting a
profile will wipe the corresponding Weaver slot and a factory reset of the
device wipes all of the Weaver slots. The secure element also provides insider
attack resistance preventing firmware updates before authenticating with the
owner profile.</p>
<p>Standard delays for encryption key derivation enforced by the secure
element:</p>
<ul>
<li>0 to 4 failed attempts: no delay</li>
<li>5 failed attempts: 30 second delay</li>
<li>6 to 9 failed attempts: no delay</li>
<li>10 to 29 failed attempts: 30 second delay</li>
<li>30 to 139 failed attempts: 30 × 2<sup>⌊(<var>n</var> - 30) ÷ 10⌋</sup>
where <var>n</var> is the number of failed attempts. This means the delay
doubles after every 10 attempts. There's a 30 second delay after 30 failed
attempts, 60s after 40, 120s after 50, 240s after 60, 480s after 70, 960s
after 80, 1920s after 90, 3840s after 100, 7680s after 110, 15360s after
120 and 30720s after 130</li>
<li>140 or more failed attempts: 86400 second delay (1 day)</li>
</ul>
<p>GrapheneOS only officially supports devices with Weaver. The fallback
implementation for devices without it is out-of-scope for this FAQ.</p>