Add key fetch

spec.bs

@@ -3031,6 +3031,14 @@ The <dfn for=Navigator method>getInterestGroupAdAuctionData(|configIDL|)</dfn> m
       |configIDL|["{{AdAuctionDataConfig/perBuyerConfig}}"]
   1. If |config|'s [=auction data config/seller=] or [=auction data config/coordinator=]
      fields are failure, then [=exception/throw=] a {{TypeError}}.
+  1. If |config|'s [=auction data config/coordinator=] is not one of the [=implementation-defined=]
+     coordinators supported by this [=user agent=], then [=exception/throw=] a {{TypeError}}.
+  1. Set |config|'s [=auction data config/encryption key=] to one of the [[RFC9180|HPKE]] public
+     encryption keys that the coordinator corresponding to |config|'s [=auction data config/coordinator=]
+     associates with |config|'s [=auction data config/seller]. The actual method
+     for this lookup is [=implementation-defined=], and may consist of fetching
+     the keys from a known [=URL=]. If the public key is not available, then
+     [=exception/throw=] a {{TypeError}}.
   1. If |config|'s [=auction data config/per buyer config=] [=map/is not empty=]
      and |config|'s [=auction data config/request size=] is null:
      1. Let |request_size| equal 0.
@@ -7349,6 +7357,9 @@ An <dfn>auction data config</dfn> is a [=struct=] with the following [=struct/it
   :  <dfn>coordinator</dfn>
   :: The origin of the coordinator hosting public encryption keys for the server
     running the ad auction. The [=origin/scheme=] must be "`https`".
+  :  <dfn>encryption key</dfn>
+  :: A [=byte sequence=]. The public [[RFC9180|HPKE]] encryption key to be used
+     to construct the request.
   :  <dfn>request size</dfn>
   :: {{unsigned long}} or null. An optional field, containing the desired size
     for the returned encrypted request blob.