Make topOrigin verification backwards compatible with L2

index.bs

@@ -3833,6 +3833,14 @@ Verifiers may use the following algorithm to verify an encoded {{CollectedClient
     1. A string, |origin|, that contains the expected {{CollectedClientData/origin}} that issued the request to the user agent.
     1. A boolean, |crossOrigin|, that is true if, and only if, the request should have been performed within a cross-origin <{iframe}>.
     1. An optional string, |topOrigin|, that contains the expected {{CollectedClientData/topOrigin}} that issued the request to the user agent if |crossOrigin| is true and if available.
+    1. A boolean, |requireTopOrigin|, that is true if, and only if, the validation should fail
+        if |topOrigin| is defined and the {{CollectedClientData/topOrigin}} attribute is not present in |clientDataJSON|.
+
+        This means that the verification algorithm is backwards compatible
+        with the [[webauthn-2-20210408#clientdatajson-serialization|JSON-compatible serialization algorithm]]
+        in Web Authentication Level 2 [[webauthn-2-20210408]]
+        if, and only if, |requireTopOrigin| is [FALSE].
+
 1. Let |expected| be an empty byte string.
 1. Append 0x7b2274797065223a (`{"type":`) to |expected|.
 1. Append [=CCDToString=](|type|) to |expected|.
@@ -3845,8 +3853,11 @@ Verifiers may use the following algorithm to verify an encoded {{CollectedClient
 1. If |crossOrigin| is true:
     1. Append 0x74727565 (`true`) to |expected|.
     1. If |topOrigin| is defined:
-        1. Append 0x2c22746f704f726967696e223a (`,"topOrigin":`) to |expected|.
-        1. Append [=CCDToString=](|topOrigin|) to |expected|.
+        1. If |requireTopOrigin| is true
+            or if 0x2c22746f704f726967696e223a (`,"topOrigin":`) is a prefix
+            of the substring of |clientDataJSON| beginning at the offset equal to the length of |expected|:
+            1. Append 0x2c22746f704f726967696e223a (`,"topOrigin":`) to |expected|.
+            1. Append [=CCDToString=](|topOrigin|) to |expected|.
 1. Otherwise, i.e. |crossOrigin| is false:
     1. Append 0x66616c7365 (`false`) to |expected|.
 1. If |expected| is not a prefix of |clientDataJSON| then the verification has failed.