Include null terminator in prefix for payload encryption spec

The spec does not currently mention the null terminator in the prefix.
However, in the implementation, this null character survives into the
associated data used for encryption.

Change-Id: Ifa645371aabf9894444b89872376b9c8537a4732
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3582637
Reviewed-by: John Delaney <johnidel@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#991174}

content/browser/aggregation_service/payload_encryption.md

@@ -27,10 +27,10 @@ This map is serialized to binary and used as the plaintext input.
 
 The associated data is a string encoded as UTF-8. It consists of a prefix and a
 variable body. The prefix is a constant and is used for domain separation[^2];
-its value is "`aggregation_service`". The body is exactly the value of the
-`shared_info` string provided in the report plaintext. This is generated by the
-browser and encodes information both needed by the aggregation service and
-available for use by the reporting origin.
+its value is "`aggregation_service\0`" (where `\0` is a null terminator). The
+body is exactly the value of the `shared_info` string provided in the report
+plaintext. This is generated by the browser and encodes information both needed
+by the aggregation service and available for use by the reporting origin.
 
 An example shared\_info field is:
 
@@ -40,10 +40,11 @@ seconds]\",\"privacy_budget_key\":\"[string]\",\"version\":\"[api
 version]\",\"report_id\":\"[UUID]\",\"reporting_origin\":\"https://reporter.example\"}",
 ```
 
-The corresponding associated data would then be the following (encoded as UTF-8):
+The corresponding associated data would then be the following (encoded as UTF-8,
+again using `\0` to indicate a null character):
 
 ```jsonc
-aggregation_service{"scheduled_report_time":"[timestamp in
+aggregation_service\0{"scheduled_report_time":"[timestamp in
 seconds]","privacy_budget_key":"[string]","version":"[api
 version]","report_id":"[UUID]","reporting_origin":"https://reporter.example"}
 ```