5 files changed, 568 insertions, 0 deletions
diff --git a/vendor/go.mau.fi/whatsmeow/appstate/decode.go b/vendor/go.mau.fi/whatsmeow/appstate/decode.go
new file mode 100644
index 00000000..5c895470
--- /dev/null
+++ b/vendor/go.mau.fi/whatsmeow/appstate/decode.go
@@ -0,0 +1,310 @@
+// Copyright (c) 2021 Tulir Asokan
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package appstate
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/json"
+	"fmt"
+
+	"google.golang.org/protobuf/proto"
+
+	waBinary "go.mau.fi/whatsmeow/binary"
+	waProto "go.mau.fi/whatsmeow/binary/proto"
+	"go.mau.fi/whatsmeow/store"
+	"go.mau.fi/whatsmeow/util/cbcutil"
+)
+
+// PatchList represents a decoded response to getting app state patches from the WhatsApp servers.
+type PatchList struct {
+	Name           WAPatchName
+	HasMorePatches bool
+	Patches        []*waProto.SyncdPatch
+	Snapshot       *waProto.SyncdSnapshot
+}
+
+// DownloadExternalFunc is a function that can download a blob of external app state patches.
+type DownloadExternalFunc func(*waProto.ExternalBlobReference) ([]byte, error)
+
+func parseSnapshotInternal(collection *waBinary.Node, downloadExternal DownloadExternalFunc) (*waProto.SyncdSnapshot, error) {
+	snapshotNode := collection.GetChildByTag("snapshot")
+	rawSnapshot, ok := snapshotNode.Content.([]byte)
+	if snapshotNode.Tag != "snapshot" || !ok {
+		return nil, nil
+	}
+	var snapshot waProto.ExternalBlobReference
+	err := proto.Unmarshal(rawSnapshot, &snapshot)
+	if err != nil {
+		return nil, fmt.Errorf("failed to unmarshal snapshot: %w", err)
+	}
+	var rawData []byte
+	rawData, err = downloadExternal(&snapshot)
+	if err != nil {
+		return nil, fmt.Errorf("failed to download external mutations: %w", err)
+	}
+	var downloaded waProto.SyncdSnapshot
+	err = proto.Unmarshal(rawData, &downloaded)
+	if err != nil {
+		return nil, fmt.Errorf("failed to unmarshal mutation list: %w", err)
+	}
+	return &downloaded, nil
+}
+
+func parsePatchListInternal(collection *waBinary.Node, downloadExternal DownloadExternalFunc) ([]*waProto.SyncdPatch, error) {
+	patchesNode := collection.GetChildByTag("patches")
+	patchNodes := patchesNode.GetChildren()
+	patches := make([]*waProto.SyncdPatch, 0, len(patchNodes))
+	for i, patchNode := range patchNodes {
+		rawPatch, ok := patchNode.Content.([]byte)
+		if patchNode.Tag != "patch" || !ok {
+			continue
+		}
+		var patch waProto.SyncdPatch
+		err := proto.Unmarshal(rawPatch, &patch)
+		if err != nil {
+			return nil, fmt.Errorf("failed to unmarshal patch #%d: %w", i+1, err)
+		}
+		if patch.GetExternalMutations() != nil && downloadExternal != nil {
+			var rawData []byte
+			rawData, err = downloadExternal(patch.GetExternalMutations())
+			if err != nil {
+				return nil, fmt.Errorf("failed to download external mutations: %w", err)
+			}
+			var downloaded waProto.SyncdMutations
+			err = proto.Unmarshal(rawData, &downloaded)
+			if err != nil {
+				return nil, fmt.Errorf("failed to unmarshal mutation list: %w", err)
+			} else if len(downloaded.GetMutations()) == 0 {
+				return nil, fmt.Errorf("didn't get any mutations from download")
+			}
+			patch.Mutations = downloaded.Mutations
+		}
+		patches = append(patches, &patch)
+	}
+	return patches, nil
+}
+
+// ParsePatchList will decode an XML node containing app state patches, including downloading any external blobs.
+func ParsePatchList(node *waBinary.Node, downloadExternal DownloadExternalFunc) (*PatchList, error) {
+	collection := node.GetChildByTag("sync", "collection")
+	ag := collection.AttrGetter()
+	snapshot, err := parseSnapshotInternal(&collection, downloadExternal)
+	if err != nil {
+		return nil, err
+	}
+	patches, err := parsePatchListInternal(&collection, downloadExternal)
+	if err != nil {
+		return nil, err
+	}
+	list := &PatchList{
+		Name:           WAPatchName(ag.String("name")),
+		HasMorePatches: ag.OptionalBool("has_more_patches"),
+		Patches:        patches,
+		Snapshot:       snapshot,
+	}
+	return list, ag.Error()
+}
+
+type patchOutput struct {
+	RemovedMACs [][]byte
+	AddedMACs   []store.AppStateMutationMAC
+	Mutations   []Mutation
+}
+
+func (proc *Processor) decodeMutations(mutations []*waProto.SyncdMutation, out *patchOutput, validateMACs bool) error {
+	for i, mutation := range mutations {
+		keyID := mutation.GetRecord().GetKeyId().GetId()
+		keys, err := proc.getAppStateKey(keyID)
+		if err != nil {
+			return fmt.Errorf("failed to get key %X to decode mutation: %w", keyID, err)
+		}
+		content := mutation.GetRecord().GetValue().GetBlob()
+		content, valueMAC := content[:len(content)-32], content[len(content)-32:]
+		if validateMACs {
+			expectedValueMAC := generateContentMAC(mutation.GetOperation(), content, keyID, keys.ValueMAC)
+			if !bytes.Equal(expectedValueMAC, valueMAC) {
+				return fmt.Errorf("failed to verify mutation #%d: %w", i+1, ErrMismatchingContentMAC)
+			}
+		}
+		iv, content := content[:16], content[16:]
+		plaintext, err := cbcutil.Decrypt(keys.ValueEncryption, iv, content)
+		if err != nil {
+			return fmt.Errorf("failed to decrypt mutation #%d: %w", i+1, err)
+		}
+		var syncAction waProto.SyncActionData
+		err = proto.Unmarshal(plaintext, &syncAction)
+		if err != nil {
+			return fmt.Errorf("failed to unmarshal mutation #%d: %w", i+1, err)
+		}
+		indexMAC := mutation.GetRecord().GetIndex().GetBlob()
+		if validateMACs {
+			expectedIndexMAC := concatAndHMAC(sha256.New, keys.Index, syncAction.Index)
+			if !bytes.Equal(expectedIndexMAC, indexMAC) {
+				return fmt.Errorf("failed to verify mutation #%d: %w", i+1, ErrMismatchingIndexMAC)
+			}
+		}
+		var index []string
+		err = json.Unmarshal(syncAction.GetIndex(), &index)
+		if err != nil {
+			return fmt.Errorf("failed to unmarshal index of mutation #%d: %w", i+1, err)
+		}
+		if mutation.GetOperation() == waProto.SyncdMutation_REMOVE {
+			out.RemovedMACs = append(out.RemovedMACs, indexMAC)
+		} else if mutation.GetOperation() == waProto.SyncdMutation_SET {
+			out.AddedMACs = append(out.AddedMACs, store.AppStateMutationMAC{
+				IndexMAC: indexMAC,
+				ValueMAC: valueMAC,
+			})
+		}
+		out.Mutations = append(out.Mutations, Mutation{
+			Operation: mutation.GetOperation(),
+			Action:    syncAction.GetValue(),
+			Index:     index,
+			IndexMAC:  indexMAC,
+			ValueMAC:  valueMAC,
+		})
+	}
+	return nil
+}
+
+func (proc *Processor) storeMACs(name WAPatchName, currentState HashState, out *patchOutput) {
+	err := proc.Store.AppState.PutAppStateVersion(string(name), currentState.Version, currentState.Hash)
+	if err != nil {
+		proc.Log.Errorf("Failed to update app state version in the database: %v", err)
+	}
+	err = proc.Store.AppState.DeleteAppStateMutationMACs(string(name), out.RemovedMACs)
+	if err != nil {
+		proc.Log.Errorf("Failed to remove deleted mutation MACs from the database: %v", err)
+	}
+	err = proc.Store.AppState.PutAppStateMutationMACs(string(name), currentState.Version, out.AddedMACs)
+	if err != nil {
+		proc.Log.Errorf("Failed to insert added mutation MACs to the database: %v", err)
+	}
+}
+
+func (proc *Processor) validateSnapshotMAC(name WAPatchName, currentState HashState, keyID, expectedSnapshotMAC []byte) (keys ExpandedAppStateKeys, err error) {
+	keys, err = proc.getAppStateKey(keyID)
+	if err != nil {
+		err = fmt.Errorf("failed to get key %X to verify patch v%d MACs: %w", keyID, currentState.Version, err)
+		return
+	}
+	snapshotMAC := currentState.generateSnapshotMAC(name, keys.SnapshotMAC)
+	if !bytes.Equal(snapshotMAC, expectedSnapshotMAC) {
+		err = fmt.Errorf("failed to verify patch v%d: %w", currentState.Version, ErrMismatchingLTHash)
+	}
+	return
+}
+
+func (proc *Processor) decodeSnapshot(name WAPatchName, ss *waProto.SyncdSnapshot, initialState HashState, validateMACs bool, newMutationsInput []Mutation) (newMutations []Mutation, currentState HashState, err error) {
+	currentState = initialState
+	currentState.Version = ss.GetVersion().GetVersion()
+
+	encryptedMutations := make([]*waProto.SyncdMutation, len(ss.GetRecords()))
+	for i, record := range ss.GetRecords() {
+		encryptedMutations[i] = &waProto.SyncdMutation{
+			Operation: waProto.SyncdMutation_SET.Enum(),
+			Record:    record,
+		}
+	}
+
+	var warn []error
+	warn, err = currentState.updateHash(encryptedMutations, func(indexMAC []byte, maxIndex int) ([]byte, error) {
+		return nil, nil
+	})
+	if len(warn) > 0 {
+		proc.Log.Warnf("Warnings while updating hash for %s: %+v", name, warn)
+	}
+	if err != nil {
+		err = fmt.Errorf("failed to update state hash: %w", err)
+		return
+	}
+
+	if validateMACs {
+		_, err = proc.validateSnapshotMAC(name, currentState, ss.GetKeyId().GetId(), ss.GetMac())
+		if err != nil {
+			return
+		}
+	}
+
+	var out patchOutput
+	out.Mutations = newMutationsInput
+	err = proc.decodeMutations(encryptedMutations, &out, validateMACs)
+	if err != nil {
+		err = fmt.Errorf("failed to decode snapshot of v%d: %w", currentState.Version, err)
+		return
+	}
+	proc.storeMACs(name, currentState, &out)
+	newMutations = out.Mutations
+	return
+}
+
+// DecodePatches will decode all the patches in a PatchList into a list of app state mutations.
+func (proc *Processor) DecodePatches(list *PatchList, initialState HashState, validateMACs bool) (newMutations []Mutation, currentState HashState, err error) {
+	currentState = initialState
+	var expectedLength int
+	if list.Snapshot != nil {
+		expectedLength = len(list.Snapshot.GetRecords())
+	}
+	for _, patch := range list.Patches {
+		expectedLength += len(patch.GetMutations())
+	}
+	newMutations = make([]Mutation, 0, expectedLength)
+
+	if list.Snapshot != nil {
+		newMutations, currentState, err = proc.decodeSnapshot(list.Name, list.Snapshot, currentState, validateMACs, newMutations)
+		if err != nil {
+			return
+		}
+	}
+
+	for _, patch := range list.Patches {
+		version := patch.GetVersion().GetVersion()
+		currentState.Version = version
+		var warn []error
+		warn, err = currentState.updateHash(patch.GetMutations(), func(indexMAC []byte, maxIndex int) ([]byte, error) {
+			for i := maxIndex - 1; i >= 0; i-- {
+				if bytes.Equal(patch.Mutations[i].GetRecord().GetIndex().GetBlob(), indexMAC) {
+					value := patch.Mutations[i].GetRecord().GetValue().GetBlob()
+					return value[len(value)-32:], nil
+				}
+			}
+			// Previous value not found in current patch, look in the database
+			return proc.Store.AppState.GetAppStateMutationMAC(string(list.Name), indexMAC)
+		})
+		if len(warn) > 0 {
+			proc.Log.Warnf("Warnings while updating hash for %s: %+v", list.Name, warn)
+		}
+		if err != nil {
+			err = fmt.Errorf("failed to update state hash: %w", err)
+			return
+		}
+
+		if validateMACs {
+			var keys ExpandedAppStateKeys
+			keys, err = proc.validateSnapshotMAC(list.Name, currentState, patch.GetKeyId().GetId(), patch.GetSnapshotMac())
+			if err != nil {
+				return
+			}
+			patchMAC := generatePatchMAC(patch, list.Name, keys.PatchMAC)
+			if !bytes.Equal(patchMAC, patch.GetPatchMac()) {
+				err = fmt.Errorf("failed to verify patch v%d: %w", version, ErrMismatchingPatchMAC)
+				return
+			}
+		}
+
+		var out patchOutput
+		out.Mutations = newMutations
+		err = proc.decodeMutations(patch.GetMutations(), &out, validateMACs)
+		if err != nil {
+			return
+		}
+		proc.storeMACs(list.Name, currentState, &out)
+		newMutations = out.Mutations
+	}
+	return
+}
diff --git a/vendor/go.mau.fi/whatsmeow/appstate/errors.go b/vendor/go.mau.fi/whatsmeow/appstate/errors.go
new file mode 100644
index 00000000..0668d86a
--- /dev/null
+++ b/vendor/go.mau.fi/whatsmeow/appstate/errors.go
@@ -0,0 +1,19 @@
+// Copyright (c) 2021 Tulir Asokan
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package appstate
+
+import "errors"
+
+// Errors that this package can return.
+var (
+	ErrMissingPreviousSetValueOperation = errors.New("missing value MAC of previous SET operation")
+	ErrMismatchingLTHash                = errors.New("mismatching LTHash")
+	ErrMismatchingPatchMAC              = errors.New("mismatching patch MAC")
+	ErrMismatchingContentMAC            = errors.New("mismatching content MAC")
+	ErrMismatchingIndexMAC              = errors.New("mismatching index MAC")
+	ErrKeyNotFound                      = errors.New("didn't find app state key")
+)
diff --git a/vendor/go.mau.fi/whatsmeow/appstate/hash.go b/vendor/go.mau.fi/whatsmeow/appstate/hash.go
new file mode 100644
index 00000000..4fdbabf6
--- /dev/null
+++ b/vendor/go.mau.fi/whatsmeow/appstate/hash.go
@@ -0,0 +1,96 @@
+// Copyright (c) 2021 Tulir Asokan
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package appstate
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"crypto/sha512"
+	"encoding/binary"
+	"fmt"
+	"hash"
+
+	"go.mau.fi/whatsmeow/appstate/lthash"
+	waProto "go.mau.fi/whatsmeow/binary/proto"
+)
+
+type Mutation struct {
+	Operation waProto.SyncdMutation_SyncdMutationSyncdOperation
+	Action    *waProto.SyncActionValue
+	Index     []string
+	IndexMAC  []byte
+	ValueMAC  []byte
+}
+
+type HashState struct {
+	Version uint64
+	Hash    [128]byte
+}
+
+func (hs *HashState) updateHash(mutations []*waProto.SyncdMutation, getPrevSetValueMAC func(indexMAC []byte, maxIndex int) ([]byte, error)) ([]error, error) {
+	var added, removed [][]byte
+	var warnings []error
+
+	for i, mutation := range mutations {
+		if mutation.GetOperation() == waProto.SyncdMutation_SET {
+			value := mutation.GetRecord().GetValue().GetBlob()
+			added = append(added, value[len(value)-32:])
+		}
+		indexMAC := mutation.GetRecord().GetIndex().GetBlob()
+		removal, err := getPrevSetValueMAC(indexMAC, i)
+		if err != nil {
+			return warnings, fmt.Errorf("failed to get value MAC of previous SET operation: %w", err)
+		} else if removal != nil {
+			removed = append(removed, removal)
+		} else if mutation.GetOperation() == waProto.SyncdMutation_REMOVE {
+			// TODO figure out if there are certain cases that are safe to ignore and others that aren't
+			// At least removing contact access from WhatsApp seems to create a REMOVE op for your own JID
+			// that points to a non-existent index and is safe to ignore here. Other keys might not be safe to ignore.
+			warnings = append(warnings, fmt.Errorf("%w for %X", ErrMissingPreviousSetValueOperation, indexMAC))
+			//return ErrMissingPreviousSetValueOperation
+		}
+	}
+
+	lthash.WAPatchIntegrity.SubtractThenAddInPlace(hs.Hash[:], removed, added)
+	return warnings, nil
+}
+
+func uint64ToBytes(val uint64) []byte {
+	data := make([]byte, 8)
+	binary.BigEndian.PutUint64(data, val)
+	return data
+}
+
+func concatAndHMAC(alg func() hash.Hash, key []byte, data ...[]byte) []byte {
+	h := hmac.New(alg, key)
+	for _, item := range data {
+		h.Write(item)
+	}
+	return h.Sum(nil)
+}
+
+func (hs *HashState) generateSnapshotMAC(name WAPatchName, key []byte) []byte {
+	return concatAndHMAC(sha256.New, key, hs.Hash[:], uint64ToBytes(hs.Version), []byte(name))
+}
+
+func generatePatchMAC(patch *waProto.SyncdPatch, name WAPatchName, key []byte) []byte {
+	dataToHash := make([][]byte, len(patch.GetMutations())+3)
+	dataToHash[0] = patch.GetSnapshotMac()
+	for i, mutation := range patch.Mutations {
+		val := mutation.GetRecord().GetValue().GetBlob()
+		dataToHash[i+1] = val[len(val)-32:]
+	}
+	dataToHash[len(dataToHash)-2] = uint64ToBytes(patch.GetVersion().GetVersion())
+	dataToHash[len(dataToHash)-1] = []byte(name)
+	return concatAndHMAC(sha256.New, key, dataToHash...)
+}
+
+func generateContentMAC(operation waProto.SyncdMutation_SyncdMutationSyncdOperation, data, keyID, key []byte) []byte {
+	operationBytes := []byte{byte(operation) + 1}
+	keyDataLength := uint64ToBytes(uint64(len(keyID) + 1))
+	return concatAndHMAC(sha512.New, key, operationBytes, keyID, data, keyDataLength)[:32]
+}
diff --git a/vendor/go.mau.fi/whatsmeow/appstate/keys.go b/vendor/go.mau.fi/whatsmeow/appstate/keys.go
new file mode 100644
index 00000000..30ecbcc4
--- /dev/null
+++ b/vendor/go.mau.fi/whatsmeow/appstate/keys.go
@@ -0,0 +1,85 @@
+// Copyright (c) 2021 Tulir Asokan
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// Package appstate implements encoding and decoding WhatsApp's app state patches.
+package appstate
+
+import (
+	"encoding/base64"
+	"sync"
+
+	"go.mau.fi/whatsmeow/store"
+	"go.mau.fi/whatsmeow/util/hkdfutil"
+	waLog "go.mau.fi/whatsmeow/util/log"
+)
+
+// WAPatchName represents a type of app state patch.
+type WAPatchName string
+
+const (
+	// WAPatchCriticalBlock contains the user's settings like push name and locale.
+	WAPatchCriticalBlock WAPatchName = "critical_block"
+	// WAPatchCriticalUnblockLow contains the user's contact list.
+	WAPatchCriticalUnblockLow WAPatchName = "critical_unblock_low"
+	// WAPatchRegularLow contains some local chat settings like pin, archive status, and the setting of whether to unarchive chats when messages come in.
+	WAPatchRegularLow WAPatchName = "regular_low"
+	// WAPatchRegularHigh contains more local chat settings like mute status and starred messages.
+	WAPatchRegularHigh WAPatchName = "regular_high"
+	// WAPatchRegular contains protocol info about app state patches like key expiration.
+	WAPatchRegular WAPatchName = "regular"
+)
+
+// AllPatchNames contains all currently known patch state names.
+var AllPatchNames = [...]WAPatchName{WAPatchCriticalBlock, WAPatchCriticalUnblockLow, WAPatchRegularHigh, WAPatchRegular, WAPatchRegularLow}
+
+type Processor struct {
+	keyCache     map[string]ExpandedAppStateKeys
+	keyCacheLock sync.Mutex
+	Store        *store.Device
+	Log          waLog.Logger
+}
+
+func NewProcessor(store *store.Device, log waLog.Logger) *Processor {
+	return &Processor{
+		keyCache: make(map[string]ExpandedAppStateKeys),
+		Store:    store,
+		Log:      log,
+	}
+}
+
+type ExpandedAppStateKeys struct {
+	Index           []byte
+	ValueEncryption []byte
+	ValueMAC        []byte
+	SnapshotMAC     []byte
+	PatchMAC        []byte
+}
+
+func expandAppStateKeys(keyData []byte) (keys ExpandedAppStateKeys) {
+	appStateKeyExpanded := hkdfutil.SHA256(keyData, nil, []byte("WhatsApp Mutation Keys"), 160)
+	return ExpandedAppStateKeys{appStateKeyExpanded[0:32], appStateKeyExpanded[32:64], appStateKeyExpanded[64:96], appStateKeyExpanded[96:128], appStateKeyExpanded[128:160]}
+}
+
+func (proc *Processor) getAppStateKey(keyID []byte) (keys ExpandedAppStateKeys, err error) {
+	keyCacheID := base64.RawStdEncoding.EncodeToString(keyID)
+	var ok bool
+
+	proc.keyCacheLock.Lock()
+	defer proc.keyCacheLock.Unlock()
+
+	keys, ok = proc.keyCache[keyCacheID]
+	if !ok {
+		var keyData *store.AppStateSyncKey
+		keyData, err = proc.Store.AppStateKeys.GetAppStateSyncKey(keyID)
+		if keyData != nil {
+			keys = expandAppStateKeys(keyData.Data)
+			proc.keyCache[keyCacheID] = keys
+		} else if err == nil {
+			err = ErrKeyNotFound
+		}
+	}
+	return
+}
diff --git a/vendor/go.mau.fi/whatsmeow/appstate/lthash/lthash.go b/vendor/go.mau.fi/whatsmeow/appstate/lthash/lthash.go
new file mode 100644
index 00000000..8d3045d6
--- /dev/null
+++ b/vendor/go.mau.fi/whatsmeow/appstate/lthash/lthash.go
@@ -0,0 +1,58 @@
+// Copyright (c) 2021 Tulir Asokan
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// Package lthash implements a summation based hash algorithm that maintains the
+// integrity of a piece of data over a series of mutations. You can add/remove
+// mutations, and it'll return a hash equal to if the same series of mutations
+// was made sequentially.
+package lthash
+
+import (
+	"encoding/binary"
+
+	"go.mau.fi/whatsmeow/util/hkdfutil"
+)
+
+type LTHash struct {
+	HKDFInfo []byte
+	HKDFSize uint8
+}
+
+// WAPatchIntegrity is a LTHash instance initialized with the details used for verifying integrity of WhatsApp app state sync patches.
+var WAPatchIntegrity = LTHash{[]byte("WhatsApp Patch Integrity"), 128}
+
+func (lth LTHash) SubtractThenAdd(base []byte, subtract, add [][]byte) []byte {
+	output := make([]byte, len(base))
+	copy(output, base)
+	lth.SubtractThenAddInPlace(output, subtract, add)
+	return output
+}
+
+func (lth LTHash) SubtractThenAddInPlace(base []byte, subtract, add [][]byte) {
+	lth.multipleOp(base, subtract, true)
+	lth.multipleOp(base, add, false)
+}
+
+func (lth LTHash) multipleOp(base []byte, input [][]byte, subtract bool) {
+	for _, item := range input {
+		performPointwiseWithOverflow(base, hkdfutil.SHA256(item, nil, lth.HKDFInfo, lth.HKDFSize), subtract)
+	}
+}
+
+func performPointwiseWithOverflow(base, input []byte, subtract bool) []byte {
+	for i := 0; i < len(base); i += 2 {
+		x := binary.LittleEndian.Uint16(base[i : i+2])
+		y := binary.LittleEndian.Uint16(input[i : i+2])
+		var result uint16
+		if subtract {
+			result = x - y
+		} else {
+			result = x + y
+		}
+		binary.LittleEndian.PutUint16(base[i:i+2], result)
+	}
+	return base
+}