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On 9/19/2018 1:37 PM, Bernd Eckenfels wrote:<br>
<blockquote type="cite"
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<div style="direction: ltr;">Hello,</div>
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<div style="direction: ltr;">I think I missed it, but where
is the conversion on BigInteger branching on key material?
Isn’t this only branching on effective constant values?</div>
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<div style="direction: ltr;">Or are you concerned about
Spectre-type problems?</div>
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</blockquote>
<br>
This is not for Spectre (etc.) issues, which effectively dump all
memory contents, so there is nothing we can do to prevent it at this
level. The proposed implementation defends against cache attacks
like Flush+Reload[1], as well as timing attacks.<br>
<br>
Imagine an EC private key (i.e. a scalar) is stored in a
fixed-length little-endian byte array in the key implementation
class. I can perform the required EC operations directly on this
byte array without branching on any part of its value. Now if I want
to convert this key to an ECPrivateKeySpec using the KeyFactory, I
need to convert this scalar to a BigInteger, which uses a
variable-length representation. This conversion necessarily branches
on the value of the private key, which we are trying to avoid.<br>
<br>
Branching on any non-secret value (including algorithm parameters
and constants) is fine, but in the example above, we would be
branching on the valud of the private key.<br>
<br>
[1] <a class="moz-txt-link-freetext" href="https://eprint.iacr.org/2013/448.pdf">https://eprint.iacr.org/2013/448.pdf</a><br>
<br>
<blockquote type="cite"
cite="mid:DB7PR08MB3307C27FB14917AAB27037C8FF1C0@DB7PR08MB3307.eurprd08.prod.outlook.com">
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<div style="direction: ltr;">Besides that I totally agree on
the idea of having a more secure implementation which can
be activated by simply switching provider priorities.</div>
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<div style="direction: ltr;">Gruss</div>
<div style="direction: ltr;">Bernd<br>
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</blockquote>
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