Fix false positive in supports (#14)

Fix the case when e.g. `supports('deriveKey', 'PBKDF2', 'HKDF')` returns
true because the length parameter from the "get key length" operation of
HKDF (which returns null, which causes the "deriveKey" operation of
PBKDF2 to throw) is not available.
(The same is true for other combinations of PBKDF2 and HKDF.)

Author: twiss

index.html

@@ -772,16 +772,12 @@ <h4>The supports method</h4>
           </li>
           <li>
             <dl class="switch">
-              <dt>If |operation| is "`deriveKey`":</dt>
+              <dt>If |operation| is "`deriveKey`", "`unwrapKey`", "`encapsulateKey`" or "`decapsulateKey`":</dt>
               <dd>
                 <p>
                   If the result of [= check support for an algorithm | checking support for an algorithm =]
                   with `op` set to "`importKey`"
                   and `alg` set to |additionalAlgorithm|
-                  is false, or
-                  the result of [= check support for an algorithm | checking support for an algorithm =]
-                  with `op` set to "`get key length`"
-                  and `alg` set to |additionalAlgorithm|
                   is false,
                   return false.
                 </p>
@@ -796,32 +792,56 @@ <h4>The supports method</h4>
                   return false.
                 </p>
               </dd>
-              <dt>If |operation| is "`unwrapKey`":</dt>
-              <dd>
-                <p>
-                  If the result of [= check support for an algorithm | checking support for an algorithm =]
-                  with `op` set to "`importKey`"
-                  and `alg` set to |additionalAlgorithm|
-                  is false,
-                  return false.
-                </p>
-              </dd>
-              <dt>If |operation| is "`encapsulateKey`" or "`decapsulateKey`":</dt>
+            </dl>
+          </li>
+          <li>
+            <p>
+              Let |length| be null.
+            </p>
+          </li>
+          <li>
+            <dl class="switch">
+              <dt>If |operation| is "`deriveKey`":</dt>
               <dd>
-                <p>
-                  If the result of [= check support for an algorithm | checking support for an algorithm =]
-                  with `op` set to "`importKey`"
-                  and `alg` set to |additionalAlgorithm|
-                  is false,
-                  return false.
-                </p>
+                <ol>
+                  <li>
+                    <p>
+                      If the result of [= check support for an algorithm | checking support for an algorithm =]
+                      with `op` set to "`get key length`"
+                      and `alg` set to |additionalAlgorithm|
+                      is false,
+                      return false.
+                    </p>
+                  </li>
+                  <li>
+                    <p>
+                      Let |normalizedAdditionalAlgorithm| be the result of
+                      <a data-cite="webcrypto#dfn-normalize-an-algorithm">normalizing an algorithm</a>, with
+                      `alg` set to |additionalAlgorithm| and `op` set to
+                      "`get key length`".
+                    </p>
+                  </li>
+                  <li>
+                    <p>
+                      Let |length| be the result of performing the get key length
+                      algorithm specified by |additionalAlgorithm| using
+                      |normalizedAdditionalAlgorithm|.
+                    </p>
+                  </li>
+                  <li>
+                    <p>
+                      Set |operation| to "`deriveBits`".
+                    </p>
+                  </li>
+                </ol>
               </dd>
             </dl>
           </li>
           <li>
             <p>
               Return the result of [= check support for an algorithm | checking support for an algorithm =],
-              with `op` set to |operation| and `alg` set to |algorithm|.
+              with `op` set to |operation|, `alg` set to |algorithm|,
+              and `length` set to |length|.
             </p>
           </li>
         </ol>
@@ -834,15 +854,10 @@ <h3>Checking support for an algorithm</h3>
         The <dfn id="dfn-check-support-for-algorithm">check support for an algorithm</dfn> algorithm
         defines a process for checking whether the given algorithm is supported for the given operation.
         Its input is an operation name |op|, an {{AlgorithmIdentifier}} |alg|,
-        and an optional |length| parameter. Its output is a boolean.
+        and a |length| parameter. Its output is a boolean.
         It behaves as follows:
       </p>
       <ol>
-        <li>
-          <p>
-            If |op| is "`deriveKey`", set |op| to "`deriveBits`".
-          </p>
-        </li>
         <li>
           <p>
             If |op| is "`encapsulateKey`" or "`encapsulateBits`", set |op| is "`encapsulate`".