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Define the ECDH length limit for supports("deriveBits") #74

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Define the ECDH length limit for supports("deriveBits") #74

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Expand Up @@ -1120,6 +1120,59 @@ <h3>Checking support for an algorithm</h3>
(e.g. known nonconformance to the specification), return false.
</p>
</li>
<li>
<dl class="switch">
<dt>
If |op| is "`deriveBits`", the {{Algorithm/name}} member of
|normalizedAlgorithm| is a case-sensitive string match for "`ECDH`",
and |length| is not null:
</dt>
<dd>
<ol>
<li>
<p>
Let |publicKey| be the {{EcdhKeyDeriveParams/public}}
member of |normalizedAlgorithm|.
</p>
</li>
<li>
<p>
Let |publicKeyAlgorithm| be the
<a data-cite="webcrypto#dfn-CryptoKey-slot-algorithm">`[[algorithm]]`</a>
internal slot of |publicKey|.
</p>
</li>
<li>
<p>
Let |maximumLength| be the following value, depending on the
{{EcKeyAlgorithm/namedCurve}} attribute of |publicKeyAlgorithm|:
</p>
<dl class="switch">
<dt>If it is "`P-256`":</dt>
<dd>256.</dd>
<dt>If it is "`P-384`":</dt>
<dd>384.</dd>
<dt>If it is "`P-521`":</dt>
<dd>528.</dd>
<dt>Otherwise:</dt>
<dd>Return false.</dd>
</dl>
<p class="note">
This limit matches the length in bits of the byte sequence
returned by the ECDH derive bits operation. For P-521, the
x-coordinate is encoded as 66 octets, so the maximum length
is 528 bits, not 521 bits.
</p>
</li>
<li>
<p>
If |length| is greater than |maximumLength|, return false.
</p>
</li>
</ol>
</dd>
</dl>
</li>
<li>
<p>
If |op| is "`generateKey`" or "`importKey`", let |usages| be the empty list.
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