Apply emlun's changes from code review

Co-authored-by: Emil Lundberg <emil@yubico.com>

index.bs

@@ -7109,7 +7109,7 @@ A usage example is thus:
 
 Another example of supplemental keys:
 
-> Say that a sign-in request appears at a website along with some geolocation signal that has not been seen for this [=user account=] before, and is outside of the typical usage hours observed for the account. The risk may be deemed high enough not to allow the request, even with an assertion by a [=multi-device credential=] on its own. But if a signature from a supplimental key that is device-bound, and that is <i>well established</i> for this user can also be presented, then that may tip the balance.
+> Say that a sign-in request appears at a website along with some geolocation signal that has not been seen for this [=user account=] before, and is outside of the typical usage hours observed for the account. The risk may be deemed high enough not to allow the request, even with an assertion by a [=multi-device credential=] on its own. But if a signature from a supplemental key that is device-bound, and that is <i>well established</i> for this user can also be presented, then that may tip the balance.
 
 The weight that [=[RPS]=] give to the presence of a signature from a supplemental key may be based on information learned from its optional attestation. An attestation can indicate the level of protection enjoyed by a hardware-bound key, or the policies for other types of supplemental key.
 
@@ -7135,7 +7135,7 @@ The weight that [=[RPS]=] give to the presence of a signature from a supplementa
     </xmp>
     <div dfn-type="dict-member" dfn-for="AuthenticationExtensionsSupplementalPublicKeysInputs">
         :   <dfn>scopes</dfn>
-        ::  This required member specifies the scopes of supplemental public keys that the [=[RP]=] requests. Values are taken from the `scope` group in the CDDL below. (I.e. currently valid values are `provider` and `device`.) Specifying the scopes that a [=[RP]=] can use allows an [=authenticator=] to avoid the work of generating superfluous supplemental keys.
+        ::  This required member specifies the scopes of supplemental public keys that the [=[RP]=] requests. Values are taken from the `scope` group in the CDDL below (i.e., currently defined values are `provider` and `device`); authenticators silently ignore unrecognized values. Specifying the scopes that a [=[RP]=] can use allows an [=authenticator=] to avoid the work of generating superfluous supplemental keys.
 
         :   <dfn>attestation</dfn>
         ::  The [=[RP]=] MAY use this OPTIONAL member to specify a preference regarding [=attestation conveyance=].
@@ -7157,7 +7157,7 @@ The weight that [=[RPS]=] give to the presence of a signature from a supplementa
 :: If {{AuthenticationExtensionsClientInputs/supplementalPubKeys}} is present, the client creates the authenticator extension input from the client extension input.
 
 : Client extension output
-:: An ArrayBuffer containing the signature returned as the [=unsigned extension output=].
+:: A sequence of {{ArrayBuffer}}s containing the signatures returned as the [=unsigned extension output=].
     <xmp class="idl">
     dictionary AuthenticationExtensionsSupplementalPublicKeysOutputs {
         sequence<ArrayBuffer> signatures;
@@ -7174,9 +7174,9 @@ The weight that [=[RPS]=] give to the presence of a signature from a supplementa
 
     ```
     supplementalPublicKeyInputs = {
-      scopes: [tstr],
+      scopes: [+ tstr],
       attestation: tstr,
-      attestationFormats: [tstr],
+      attestationFormats: [* tstr],
     }
     $$extensionInput //= (
       supplementalPubKeys: supplementalPublicKeyInputs,
@@ -7192,7 +7192,7 @@ The weight that [=[RPS]=] give to the presence of a signature from a supplementa
         ; lexicographically by scope and MUST NOT include more than one element
         ; with a given scope.
 
-        supplementalPubKeys: [attObjForSupplementalPublicKey],
+        supplementalPubKeys: [+ attObjForSupplementalPublicKey],
     )
 
     scope = (
@@ -7268,13 +7268,13 @@ The weight that [=[RPS]=] give to the presence of a signature from a supplementa
     ```
 
 : Unsigned extension output
-:: A CBOR array of byte strings containing the signatures generated with the supplemental private keys, in the same order as in the authenticator extension output.
+:: A non-empty CBOR array of byte strings containing the signatures generated with the supplemental private keys, in the same order as in the authenticator extension output.
 
 : Authenticator extension processing
 :: For both [=authenticatorMakeCredential=] and [=authenticatorGetAssertion=] operations:
     1. Create or select the [=public key credential source=] as usual (see [[#sctn-op-make-cred]], or [[#sctn-op-get-assertion]] as appropriate).
 
-    1. Let |scopes| be the [=set=] of all supplemental public key scopes that the [=authenticator=] supports. Updates |scopes| to be the [=set/intersection=] of itself and {{AuthenticationExtensionsSupplementalPublicKeysInputs/scopes}}. If |scopes| is empty, terminate these processing steps with no extension output.
+    1. Let |scopes| be the [=set=] of all supplemental public key scopes that the [=authenticator=] supports. Update |scopes| to be the [=set/intersection=] of itself and {{AuthenticationExtensionsSupplementalPublicKeysInputs/scopes}}. If |scopes| is empty, terminate these processing steps with no extension output.
 
     1. Let |spks| and |spkSigs| be empty arrays.
 
@@ -7341,7 +7341,7 @@ The weight that [=[RPS]=] give to the presence of a signature from a supplementa
 
 ##### AAGUIDs ##### {#sctn-supplemental-public-keys-attestation-aaguid}
 
-The [=AAGUID=] included in the <code>[=supplementalPubKeys=]</code> extension output, if non-zero, aids a [=[RP]=] in validating the [=attestation statement=] of the supplemental public key. Its interpretation depends on the scope of the key. It may differ from the [=AAGUID=] in the [=attested credential data=] of a [=multi-device credential=]. Thus the AAGUID of [=supplemental public key=] MAY be different in a single response and either, or both, may be zero depending on the options requested and authenticator behaviour.
+Any non-zero [=/AAGUID=]s included in the <code>[=supplementalPubKeys=]</code> extension output aid a [=[RP]=] in validating the [=attestation statement=] of the supplemental public key. The interpretation of each AAGUID depends on the scope of the corresponding key. Some or all may differ from the [=authData/attestedCredentialData/aaguid=] in the [=attested credential data=] of a [=multi-device credential=]. Thus the AAGUID of a [=supplemental public key=] MAY be different in a single response and either, or both, may be zero depending on the options requested and authenticator behaviour.
 
 ##### Attestation calculations ##### {#sctn-supplemental-public-keys-attestation-calculations}