diff --git a/spec.bs b/spec.bs
index 218b27a..bb7b119 100644
--- a/spec.bs
+++ b/spec.bs
@@ -131,6 +131,7 @@ interface PrivateAggregation {
 dictionary PAHistogramContribution {
   required bigint bucket;
   required long value;
+  bigint filteringId = 0;
 };
 
 dictionary PADebugModeOptions {
@@ -168,12 +169,22 @@ are:
 1. If |contribution|["{{PAHistogramContribution/value}}"] is negative,
     [=exception/throw=] a {{RangeError}}.
 1. Let |scopingDetails| be [=this=]'s [=PrivateAggregation/scoping details=].
+1. Let |batchingScope| be the result of running |scopingDetails|' [=scoping
+    details/get batching scope steps=].
+1. Let |filteringIdMaxBytes| be the [=default filtering ID max bytes=].
+1. If [=pre-specified report parameters map=][|batchingScope|] [=map/exists=]:
+    1. Set |filteringIdMaxBytes| to [=pre-specified report parameters
+        map=][|batchingScope|]'s [=pre-specified report parameters/filtering ID
+        max bytes=].
+1. If |contribution|["{{PAHistogramContribution/filteringId}}"] is not [=set/
+    contained=] in [=the exclusive range|the range=] 0 to
+    256<sup>|filteringIdMaxBytes|</sup>, exclusive, [=exception/throw=] a
+    {{RangeError}}.
 1. Let |entry| be a new [=contribution cache entry=] with the items:
     : [=contribution cache entry/contribution=]
     :: |contribution|
     : [=contribution cache entry/batching scope=]
-    :: The result of running |scopingDetails|' [=scoping details/get batching
-        scope steps=].
+    :: |batchingScope|
     : [=contribution cache entry/debug scope=]
     :: The result of running |scopingDetails|' [=scoping details/get debug scope
         steps=].
@@ -349,6 +360,8 @@ An aggregatable report is a [=struct=] with the following items:
 :: An [=aggregation coordinator=]
 : <dfn>context ID</dfn>
 :: A [=string=] or null
+: <dfn>filtering ID max bytes</dfn>
+:: A positive integer
 : <dfn>queued</dfn>
 :: A [=boolean=]
 
@@ -379,6 +392,8 @@ items:
 <dl dfn-for="pre-specified report parameters">
 : <dfn>context ID</dfn> (default: null)
 :: A [=string=] or null
+: <dfn>filtering ID max bytes</dfn> (default: [=default filtering ID max bytes=])
+:: A positive integer
 
 </dl>
 
@@ -413,6 +428,18 @@ The user agent may expose controls that allow the user to delete data from the
 [=contribution cache=], the [=debug scope map=] and the [=pre-specified report
 parameters map=].
 
+Constants {#constants}
+======================
+
+<dfn>Default filtering ID max bytes</dfn> is a positive integer controlling the
+max bytes used if none is explicitly chosen. Its value is 1.
+
+<dfn>Valid filtering ID max bytes range</dfn> is a [=set=] of positive integers
+controlling the allowable values of max bytes. Its value is [=the inclusive
+range|the range=] 1 to 8, inclusive.
+
+Issue: Consider adding more constants.
+
 [=Implementation-defined=] values {#implementation-defined-values}
 ==================================================================
 
@@ -510,12 +537,14 @@ To <dfn>determine if a report should be sent deterministically</dfn> given a
 steps. They return a [=boolean=]:
 1. If |preSpecifiedParams|' [=pre-specified report parameters/context ID=] is
     not null, return true.
+1. If |preSpecifiedParams|' [=pre-specified report parameters/filtering ID max
+    bytes=] is not the [=default filtering ID max bytes=], return true.
 1. Return false.
 
-Note: If a context ID was specified, a report is sent, even if there are no
-    contributions or there is insufficent budget for the requested
-    contributions. See [Protecting against leaks via the number of
-    reports](#protecting-against-leaks-via-the-number-of-reports).
+Note: If a context ID or non-default filtering ID max bytes was specified, a
+    report is sent, even if there are no contributions or there is insufficent
+    budget for the requested contributions. See [Protecting against leaks via
+    the number of reports](#protecting-against-leaks-via-the-number-of-reports).
 
 To <dfn algorithm export>process contributions for a batching scope</dfn> given
 a [=batching scope=] |batchingScope|, an [=origin=] |reportingOrigin|, a
@@ -598,6 +627,10 @@ scope</dfn> given a [=pre-specified report parameters=] |params| and a
 1. Let |contextId| be |params|' [=pre-specified report parameters/context ID=].
 1. [=Assert=]: |contextId| is null or |contextId|'s [=string/length=] is not
     larger than 64.
+1. Let |filteringIdMaxBytes| be |params|' [=pre-specified report parameters/
+    filtering ID max bytes=].
+1. [=Assert=]: |filteringIdMaxBytes| is [=set/contained=] in the [=valid
+    filtering ID max bytes range=]
 1. [=map/Set=] [=pre-specified report parameters map=][|batchingScope|] to
     |params|.
 
@@ -681,6 +714,9 @@ perform the following steps. They return an [=aggregatable report=].
     :: |aggregationCoordinator|
     : [=aggregatable report/context ID=]
     :: |preSpecifiedParams|' [=pre-specified report parameters/context ID=]
+    : [=aggregatable report/filtering ID max bytes=]
+    :: |preSpecifiedParams|' [=pre-specified report parameters/filtering ID max
+        bytes=]
     : [=aggregatable report/queued=]
     :: false
 1. Return |report|.
@@ -909,6 +945,8 @@ To <dfn>obtain the plaintext payload</dfn> given an [=aggregatable report=]
         :: 0
         : {{PAHistogramContribution/value}}
         :: 0
+        : {{PAHistogramContribution/filteringId}}
+        :: 0
     1. [=list/Append=] |nullContribution| to |contributions|.
 
     Note: This padding protects against the number of contributions being leaked
@@ -916,14 +954,23 @@ To <dfn>obtain the plaintext payload</dfn> given an [=aggregatable report=]
         [below](#protecting-against-leaks-via-payload-size).
 1. [=list/iterate|For each=] |contribution| of |report|'s [=aggregatable report/
     contributions=]:
+    1. Let |filteringIdMaxBytes| be |report|'s [=aggregatable report/filtering
+        id max bytes=].
+    1. [=Assert=]: |contribution|["{{PAHistogramContribution/filteringId}}"]
+        is [=set/contained=] in [=the exclusive range|the range=] 0 to
+        256<sup>|filteringIdMaxBytes|</sup>, exclusive.
     1. Let |contributionData| be an [=ordered map=] of the following key/value
         pairs:
         : "`bucket`"
         :: The result of [=encoding an integer for the payload=] given
-            |contribution|["{{PAHistogramContribution/bucket}}"] and 128.
+            |contribution|["{{PAHistogramContribution/bucket}}"] and 16.
         : "`value`"
         :: The result of [=encoding an integer for the payload=] given
-            |contribution|["{{PAHistogramContribution/value}}"] and 32.
+            |contribution|["{{PAHistogramContribution/value}}"] and 4.
+        : "`id`"
+        :: The result of [=encoding an integer for the payload=] given
+            |contribution|[="{{PAHistogramContribution/filteringId}}"] and
+            |filteringIdMaxBytes|.
     1. [=list/Append=] |contributionData| to |payloadData|.
 1. Let |payload| be an [=ordered map=] of the following key/value pairs:
     : "`data`"
@@ -953,9 +1000,9 @@ They return a [=byte sequence=] or an error.
     with |hpkeContext| and |aad|.
 
 To <dfn>encode an integer for the payload</dfn> given an integer |intToEncode|
-and an integer |bitLength|, return the representation of |intToEncode| as a
-big-endian [=byte sequence=] of length |bitLength| / 8, left padding with zeroes
-as necessary.
+and an integer |byteLength|, return the representation of |intToEncode| as a
+big-endian [=byte sequence=] of length |byteLength|, left padding with zeroes as
+necessary.
 
 To <dfn>obtain a report's shared info</dfn> given an [=aggregatable report=]
 |report|, perform the following steps. They return a [=string=].
@@ -973,7 +1020,7 @@ To <dfn>obtain a report's shared info</dfn> given an [=aggregatable report=]
     :: The number of seconds in |scheduledReportTime|, rounded down to the
         nearest number of whole seconds and [=serialize an integer|serialized=]
     : "`version`"
-    :: "`0.1`"
+    :: "`1.0`"
 1. Return the result of [=serializing an infra value to a json string=] given
     |sharedInfo|.
 
@@ -1038,6 +1085,7 @@ partial interface SharedStorageWorkletGlobalScope {
 dictionary SharedStoragePrivateAggregationConfig {
   USVString aggregationCoordinatorOrigin;
   USVString contextId;
+  [EnforceRange] unsigned long long filteringIdMaxBytes;
 };
 
 partial dictionary SharedStorageRunOperationMethodOptions {
@@ -1078,15 +1126,25 @@ steps. They return a [=pre-specified report parameters=], null, or a
 {{DOMException}}:
 1. If |options|["{{SharedStorageRunOperationMethodOptions/privateAggregationConfig}}"]
     does not [=map/exist=], return null.
+1. Let |privateAggregationConfig| be
+    |options|["{{SharedStorageRunOperationMethodOptions/privateAggregationConfig}}"].
 1. Let |contextId| be null.
-1. If |options|["{{SharedStorageRunOperationMethodOptions/privateAggregationConfig}}"]["{{SharedStoragePrivateAggregationConfig/contextId}}"]
+1. If |privateAggregationConfig|["{{SharedStoragePrivateAggregationConfig/contextId}}"]
     [=map/exists=], set |contextId| to
-    |options|["{{SharedStorageRunOperationMethodOptions/privateAggregationConfig}}"]["{{SharedStoragePrivateAggregationConfig/contextId}}"].
+    |privateAggregationConfig|["{{SharedStoragePrivateAggregationConfig/contextId}}"].
 1. If |contextId|'s [=string/length=] is greater than 64, return a new
     {{DOMException}} with name "`DataError`".
+1. Let |filteringIdMaxBytes| be the [=default filtering ID max bytes=].
+1. If |privateAggregationConfig|["{{SharedStoragePrivateAggregationConfig/filteringIdMaxBytes}}"]
+    [=map/exists=], set |filteringIdMaxBytes| to
+    |privateAggregationConfig|["{{SharedStoragePrivateAggregationConfig/filteringIdMaxBytes}}"].
+1. If |filteringIdMaxBytes| is not [=set/contained=] in the [=valid filtering ID
+    max bytes range=], return a new {{DOMException}} with name "`DataError`".
 1. Return a new [=pre-specified report parameters=] with the items:
     : [=pre-specified report parameters/context ID=]
     :: |contextId|
+    : [=pre-specified report parameters/filtering ID max bytes=]
+    :: |filteringIdMaxBytes|
 
 The {{WindowSharedStorage}}'s {{WindowSharedStorage/run()}} method steps are
 modified in four ways. First, add the following steps just after step 2 ("If
@@ -1282,6 +1340,7 @@ dictionary PASignalValue {
 dictionary PAExtendedHistogramContribution {
   required (PASignalValue or bigint) bucket;
   required (PASignalValue or long) value;
+  bigint filteringId = 0;
 };
 
 [Exposed=InterestGroupScriptRunnerGlobalScope, SecureContext]
@@ -1349,9 +1408,16 @@ event, PAExtendedHistogramContribution contribution)</dfn> method steps are:
         throw=] a {{TypeError}}.
 1. Otherwise, if |contribution|["{{PAHistogramContribution/value}}"] is
     negative, [=exception/throw=] a {{TypeError}}.
+1. If |contribution|["{{PAExtendedHistogramContribution/filteringId}}"] is
+    not [=set/contained=] in [=the exclusive range|the range=] 0 to
+    256<sup>[=default filtering ID max bytes=]</sup>, exclusive, [=exception/
+    throw=] a {{TypeError}}.
 
     Issue: Make the error types on validation issues here and above consistent
         with {{PrivateAggregation/contributeToHistogram(contribution)}}.
+
+    Note: It is not currently possible to set a non-default filtering ID max
+        bytes for Protected Audience.
 1. Let |batchingScope| be null.
 1. If |event| [=string/starts with=] "`reserved.`", set |batchingScope| to the
     result of running |scopingDetails|' [=scoping details/get batching scope
@@ -1974,7 +2040,10 @@ an <a spec="turtledove">auction config</a> |auctionConfig| and a
             :: |bucket|
             : {{PAHistogramContribution/value}}
             :: |value|
+            : {{PAHistogramContribution/filteringId}}
+            :: 0
 
+              Issue: Consider allowing the filtering ID to be set here.
         1. [=map/For each=] |ig| of the [=user agent=]'s <a spec="turtledove">
             interest group set</a> whose
             <a spec="turtledove" for="interest group">owner</a> is
@@ -2029,11 +2098,15 @@ following steps. They return a {{PAHistogramContribution}}.
 1. Let |value| be |contribution|["{{PAExtendedHistogramContribution/value}}"].
 1. If |value| is a {{PASignalValue}}, set |value| to the result of [=filling in
     the signal value=] given |value|, 2<sup>31</sup>−1 and |leadingBidInfo|.
-1. Return a new {{PAHistogramContribution}} with the items:
+1. Let |filledInContribution| be a new {{PAHistogramContribution}} with the
+    items:
     : {{PAHistogramContribution/bucket}}
     :: |bucket|
     : {{PAHistogramContribution/value}}
     :: |value|
+    : {{PAHistogramContribution/filteringId}}
+    :: |contribution|["{{PAExtendedHistogramContribution/filteringId}}"]
+1. Return |filledInContribution|.
 
 To <dfn>fill in the signal value</dfn> given a {{PASignalValue}} |value|, an
 integer |maxAllowed| and a <a spec="turtledove">leading bid info</a>
@@ -2218,7 +2291,8 @@ However, the number of reports with the given metadata could expose some
 cross-site information. To protect against this, the API delays sending reports
 by a randomized amount of time to make it difficult to determine whether a
 report was sent or not from any particular event. In the case that a
-[=aggregatable report/context ID=] is supplied, the API makes the number of
+[=aggregatable report/context ID=] is supplied or a non-default [=aggregatable
+report/filtering ID max bytes=] is specified, the API makes the number of
 reports sent deterministic (sending 'null reports' if necessary -- each
 containing only a contribution with a value of 0 in the payload). Additional
 mitigations may also be possible in the future, e.g. adding noise to the report