diff --git a/backend/all/all.go b/backend/all/all.go
index cad76f6ad..f278e4ed7 100644
--- a/backend/all/all.go
+++ b/backend/all/all.go
@@ -18,6 +18,7 @@ import (
 	_ "github.com/rclone/rclone/backend/ftp"
 	_ "github.com/rclone/rclone/backend/googlecloudstorage"
 	_ "github.com/rclone/rclone/backend/googlephotos"
+	_ "github.com/rclone/rclone/backend/hasher"
 	_ "github.com/rclone/rclone/backend/hdfs"
 	_ "github.com/rclone/rclone/backend/http"
 	_ "github.com/rclone/rclone/backend/hubic"
diff --git a/backend/hasher/commands.go b/backend/hasher/commands.go
new file mode 100644
index 000000000..99101daba
--- /dev/null
+++ b/backend/hasher/commands.go
@@ -0,0 +1,179 @@
+package hasher
+
+import (
+	"context"
+	"path"
+
+	"github.com/pkg/errors"
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/accounting"
+	"github.com/rclone/rclone/fs/cache"
+	"github.com/rclone/rclone/fs/fspath"
+	"github.com/rclone/rclone/fs/hash"
+	"github.com/rclone/rclone/fs/operations"
+	"github.com/rclone/rclone/lib/kv"
+)
+
+// Command the backend to run a named command
+//
+// The command run is name
+// args may be used to read arguments from
+// opts may be used to read optional arguments from
+//
+// The result should be capable of being JSON encoded
+// If it is a string or a []string it will be shown to the user
+// otherwise it will be JSON encoded and shown to the user like that
+func (f *Fs) Command(ctx context.Context, name string, arg []string, opt map[string]string) (out interface{}, err error) {
+	switch name {
+	case "drop":
+		return nil, f.db.Stop(true)
+	case "dump", "fulldump":
+		return nil, f.dbDump(ctx, name == "fulldump", "")
+	case "import", "stickyimport":
+		sticky := name == "stickyimport"
+		if len(arg) != 2 {
+			return nil, errors.New("please provide checksum type and path to sum file")
+		}
+		return nil, f.dbImport(ctx, arg[0], arg[1], sticky)
+	default:
+		return nil, fs.ErrorCommandNotFound
+	}
+}
+
+var commandHelp = []fs.CommandHelp{{
+	Name:  "drop",
+	Short: "Drop cache",
+	Long: `Completely drop checksum cache.
+Usage Example:
+    rclone backend drop hasher:
+`,
+}, {
+	Name:  "dump",
+	Short: "Dump the database",
+	Long:  "Dump cache records covered by the current remote",
+}, {
+	Name:  "fulldump",
+	Short: "Full dump of the database",
+	Long:  "Dump all cache records in the database",
+}, {
+	Name:  "import",
+	Short: "Import a SUM file",
+	Long: `Amend hash cache from a SUM file and bind checksums to files by size/time.
+Usage Example:
+    rclone backend import hasher:subdir md5 /path/to/sum.md5
+`,
+}, {
+	Name:  "stickyimport",
+	Short: "Perform fast import of a SUM file",
+	Long: `Fill hash cache from a SUM file without verifying file fingerprints.
+Usage Example:
+    rclone backend stickyimport hasher:subdir md5 remote:path/to/sum.md5
+`,
+}}
+
+func (f *Fs) dbDump(ctx context.Context, full bool, root string) error {
+	if root == "" {
+		remoteFs, err := cache.Get(ctx, f.opt.Remote)
+		if err != nil {
+			return err
+		}
+		root = fspath.JoinRootPath(remoteFs.Root(), f.Root())
+	}
+	op := &kvDump{
+		full: full,
+		root: root,
+		path: f.db.Path(),
+		fs:   f,
+	}
+	err := f.db.Do(false, op)
+	if err == kv.ErrEmpty {
+		fs.Infof(op.path, "empty")
+		err = nil
+	}
+	return err
+}
+
+func (f *Fs) dbImport(ctx context.Context, hashName, sumRemote string, sticky bool) error {
+	var hashType hash.Type
+	if err := hashType.Set(hashName); err != nil {
+		return err
+	}
+	if hashType == hash.None {
+		return errors.New("please provide a valid hash type")
+	}
+	if !f.suppHashes.Contains(hashType) {
+		return errors.New("unsupported hash type")
+	}
+	if !f.keepHashes.Contains(hashType) {
+		fs.Infof(nil, "Need not import hashes of this type")
+		return nil
+	}
+
+	_, sumPath, err := fspath.SplitFs(sumRemote)
+	if err != nil {
+		return err
+	}
+	sumFs, err := cache.Get(ctx, sumRemote)
+	switch err {
+	case fs.ErrorIsFile:
+		// ok
+	case nil:
+		return errors.Errorf("not a file: %s", sumRemote)
+	default:
+		return err
+	}
+
+	sumObj, err := sumFs.NewObject(ctx, path.Base(sumPath))
+	if err != nil {
+		return errors.Wrap(err, "cannot open sum file")
+	}
+	hashes, err := operations.ParseSumFile(ctx, sumObj)
+	if err != nil {
+		return errors.Wrap(err, "failed to parse sum file")
+	}
+
+	if sticky {
+		rootPath := f.Fs.Root()
+		for remote, hashVal := range hashes {
+			key := path.Join(rootPath, remote)
+			hashSums := operations.HashSums{hashName: hashVal}
+			if err := f.putRawHashes(ctx, key, anyFingerprint, hashSums); err != nil {
+				fs.Errorf(nil, "%s: failed to import: %v", remote, err)
+			}
+		}
+		fs.Infof(nil, "Summary: %d checksum(s) imported", len(hashes))
+		return nil
+	}
+
+	const longImportThreshold = 100
+	if len(hashes) > longImportThreshold {
+		fs.Infof(nil, "Importing %d checksums. Please wait...", len(hashes))
+	}
+
+	doneCount := 0
+	err = operations.ListFn(ctx, f, func(obj fs.Object) {
+		remote := obj.Remote()
+		hash := hashes[remote]
+		hashes[remote] = "" // mark as handled
+		o, ok := obj.(*Object)
+		if ok && hash != "" {
+			if err := o.putHashes(ctx, hashMap{hashType: hash}); err != nil {
+				fs.Errorf(nil, "%s: failed to import: %v", remote, err)
+			}
+			accounting.Stats(ctx).NewCheckingTransfer(obj).Done(ctx, err)
+			doneCount++
+		}
+	})
+	if err != nil {
+		fs.Errorf(nil, "Import failed: %v", err)
+	}
+	skipCount := 0
+	for remote, emptyOrDone := range hashes {
+		if emptyOrDone != "" {
+			fs.Infof(nil, "Skip vanished object: %s", remote)
+			skipCount++
+		}
+	}
+	fs.Infof(nil, "Summary: %d imported, %d skipped", doneCount, skipCount)
+	return err
+}
diff --git a/backend/hasher/hasher.go b/backend/hasher/hasher.go
new file mode 100644
index 000000000..1a7df38fc
--- /dev/null
+++ b/backend/hasher/hasher.go
@@ -0,0 +1,508 @@
+// Package hasher implements a checksum handling overlay backend
+package hasher
+
+import (
+	"context"
+	"encoding/gob"
+	"fmt"
+	"io"
+	"path"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/pkg/errors"
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/cache"
+	"github.com/rclone/rclone/fs/config/configmap"
+	"github.com/rclone/rclone/fs/config/configstruct"
+	"github.com/rclone/rclone/fs/fspath"
+	"github.com/rclone/rclone/fs/hash"
+	"github.com/rclone/rclone/lib/kv"
+)
+
+// Register with Fs
+func init() {
+	fs.Register(&fs.RegInfo{
+		Name:        "hasher",
+		Description: "Better checksums for other remotes",
+		NewFs:       NewFs,
+		CommandHelp: commandHelp,
+		Options: []fs.Option{{
+			Name:     "remote",
+			Required: true,
+			Help:     "Remote to cache checksums for (e.g. myRemote:path).",
+		}, {
+			Name:     "hashes",
+			Default:  fs.CommaSepList{"md5", "sha1"},
+			Advanced: false,
+			Help:     "Comma separated list of supported checksum types.",
+		}, {
+			Name:     "max_age",
+			Advanced: false,
+			Default:  fs.DurationOff,
+			Help:     "Maximum time to keep checksums in cache (0 = no cache, off = cache forever).",
+		}, {
+			Name:     "auto_size",
+			Advanced: true,
+			Default:  fs.SizeSuffix(0),
+			Help:     "Auto-update checksum for files smaller than this size (disabled by default).",
+		}},
+	})
+}
+
+// Options defines the configuration for this backend
+type Options struct {
+	Remote   string          `config:"remote"`
+	Hashes   fs.CommaSepList `config:"hashes"`
+	AutoSize fs.SizeSuffix   `config:"auto_size"`
+	MaxAge   fs.Duration     `config:"max_age"`
+}
+
+// Fs represents a wrapped fs.Fs
+type Fs struct {
+	fs.Fs
+	name     string
+	root     string
+	wrapper  fs.Fs
+	features *fs.Features
+	opt      *Options
+	db       *kv.DB
+	// fingerprinting
+	fpTime bool      // true if using time in fingerprints
+	fpHash hash.Type // hash type to use in fingerprints or None
+	// hash types triaged by groups
+	suppHashes hash.Set // all supported checksum types
+	passHashes hash.Set // passed directly to the base without caching
+	slowHashes hash.Set // passed to the base and then cached
+	autoHashes hash.Set // calculated in-house and cached
+	keepHashes hash.Set // checksums to keep in cache (slow + auto)
+}
+
+var warnExperimental sync.Once
+
+// NewFs constructs an Fs from the remote:path string
+func NewFs(ctx context.Context, fsname, rpath string, cmap configmap.Mapper) (fs.Fs, error) {
+	if !kv.Supported() {
+		return nil, errors.New("hasher is not supported on this OS")
+	}
+	warnExperimental.Do(func() {
+		fs.Infof(nil, "Hasher is EXPERIMENTAL!")
+	})
+
+	opt := &Options{}
+	err := configstruct.Set(cmap, opt)
+	if err != nil {
+		return nil, err
+	}
+
+	if strings.HasPrefix(opt.Remote, fsname+":") {
+		return nil, errors.New("can't point remote at itself")
+	}
+	remotePath := fspath.JoinRootPath(opt.Remote, rpath)
+	baseFs, err := cache.Get(ctx, remotePath)
+	if err != nil && err != fs.ErrorIsFile {
+		return nil, errors.Wrapf(err, "failed to derive base remote %q", opt.Remote)
+	}
+
+	f := &Fs{
+		Fs:   baseFs,
+		name: fsname,
+		root: rpath,
+		opt:  opt,
+	}
+	baseFeatures := baseFs.Features()
+	f.fpTime = baseFs.Precision() != fs.ModTimeNotSupported
+
+	if baseFeatures.SlowHash {
+		f.slowHashes = f.Fs.Hashes()
+	} else {
+		f.passHashes = f.Fs.Hashes()
+		f.fpHash = f.passHashes.GetOne()
+	}
+
+	f.suppHashes = f.passHashes
+	f.suppHashes.Add(f.slowHashes.Array()...)
+
+	for _, hashName := range opt.Hashes {
+		var ht hash.Type
+		if err := ht.Set(hashName); err != nil {
+			return nil, errors.Errorf("invalid token %q in hash string %q", hashName, opt.Hashes.String())
+		}
+		if !f.slowHashes.Contains(ht) {
+			f.autoHashes.Add(ht)
+		}
+		f.keepHashes.Add(ht)
+		f.suppHashes.Add(ht)
+	}
+
+	fs.Debugf(f, "Groups by usage: cached %s, passed %s, auto %s, slow %s, supported %s",
+		f.keepHashes, f.passHashes, f.autoHashes, f.slowHashes, f.suppHashes)
+
+	var nilSet hash.Set
+	if f.keepHashes == nilSet {
+		return nil, errors.New("configured hash_names have nothing to keep in cache")
+	}
+
+	if f.opt.MaxAge > 0 {
+		gob.Register(hashRecord{})
+		db, err := kv.Start(ctx, "hasher", f.Fs)
+		if err != nil {
+			return nil, err
+		}
+		f.db = db
+	}
+
+	stubFeatures := &fs.Features{
+		CanHaveEmptyDirectories: true,
+		IsLocal:                 true,
+		ReadMimeType:            true,
+		WriteMimeType:           true,
+	}
+	f.features = stubFeatures.Fill(ctx, f).Mask(ctx, f.Fs).WrapsFs(f, f.Fs)
+
+	cache.PinUntilFinalized(f.Fs, f)
+	return f, err
+}
+
+//
+// Filesystem
+//
+
+// Name of the remote (as passed into NewFs)
+func (f *Fs) Name() string { return f.name }
+
+// Root of the remote (as passed into NewFs)
+func (f *Fs) Root() string { return f.root }
+
+// Features returns the optional features of this Fs
+func (f *Fs) Features() *fs.Features { return f.features }
+
+// Hashes returns the supported hash sets.
+func (f *Fs) Hashes() hash.Set { return f.suppHashes }
+
+// String returns a description of the FS
+// The "hasher::" prefix is a distinctive feature.
+func (f *Fs) String() string {
+	return fmt.Sprintf("hasher::%s:%s", f.name, f.root)
+}
+
+// UnWrap returns the Fs that this Fs is wrapping
+func (f *Fs) UnWrap() fs.Fs { return f.Fs }
+
+// WrapFs returns the Fs that is wrapping this Fs
+func (f *Fs) WrapFs() fs.Fs { return f.wrapper }
+
+// SetWrapper sets the Fs that is wrapping this Fs
+func (f *Fs) SetWrapper(wrapper fs.Fs) { f.wrapper = wrapper }
+
+// Wrap base entries into hasher entries.
+func (f *Fs) wrapEntries(baseEntries fs.DirEntries) (hashEntries fs.DirEntries, err error) {
+	hashEntries = baseEntries[:0] // work inplace
+	for _, entry := range baseEntries {
+		switch x := entry.(type) {
+		case fs.Object:
+			hashEntries = append(hashEntries, f.wrapObject(x, nil))
+		default:
+			hashEntries = append(hashEntries, entry) // trash in - trash out
+		}
+	}
+	return hashEntries, nil
+}
+
+// List the objects and directories in dir into entries.
+func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err error) {
+	if entries, err = f.Fs.List(ctx, dir); err != nil {
+		return nil, err
+	}
+	return f.wrapEntries(entries)
+}
+
+// ListR lists the objects and directories recursively into out.
+func (f *Fs) ListR(ctx context.Context, dir string, callback fs.ListRCallback) (err error) {
+	return f.Fs.Features().ListR(ctx, dir, func(baseEntries fs.DirEntries) error {
+		hashEntries, err := f.wrapEntries(baseEntries)
+		if err != nil {
+			return err
+		}
+		return callback(hashEntries)
+	})
+}
+
+// Purge a directory
+func (f *Fs) Purge(ctx context.Context, dir string) error {
+	if do := f.Fs.Features().Purge; do != nil {
+		if err := do(ctx, dir); err != nil {
+			return err
+		}
+		err := f.db.Do(true, &kvPurge{
+			dir: path.Join(f.Fs.Root(), dir),
+		})
+		if err != nil {
+			fs.Errorf(f, "Failed to purge some hashes: %v", err)
+		}
+		return nil
+	}
+	return fs.ErrorCantPurge
+}
+
+// PutStream uploads to the remote path with undeterminate size.
+func (f *Fs) PutStream(ctx context.Context, in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) (fs.Object, error) {
+	if do := f.Fs.Features().PutStream; do != nil {
+		_ = f.pruneHash(src.Remote())
+		oResult, err := do(ctx, in, src, options...)
+		return f.wrapObject(oResult, err), err
+	}
+	return nil, errors.New("PutStream not supported")
+}
+
+// PutUnchecked uploads the object, allowing duplicates.
+func (f *Fs) PutUnchecked(ctx context.Context, in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) (fs.Object, error) {
+	if do := f.Fs.Features().PutUnchecked; do != nil {
+		_ = f.pruneHash(src.Remote())
+		oResult, err := do(ctx, in, src, options...)
+		return f.wrapObject(oResult, err), err
+	}
+	return nil, errors.New("PutUnchecked not supported")
+}
+
+// pruneHash deletes hash for a path
+func (f *Fs) pruneHash(remote string) error {
+	return f.db.Do(true, &kvPrune{
+		key: path.Join(f.Fs.Root(), remote),
+	})
+}
+
+// CleanUp the trash in the Fs
+func (f *Fs) CleanUp(ctx context.Context) error {
+	if do := f.Fs.Features().CleanUp; do != nil {
+		return do(ctx)
+	}
+	return errors.New("CleanUp not supported")
+}
+
+// About gets quota information from the Fs
+func (f *Fs) About(ctx context.Context) (*fs.Usage, error) {
+	if do := f.Fs.Features().About; do != nil {
+		return do(ctx)
+	}
+	return nil, errors.New("About not supported")
+}
+
+// ChangeNotify calls the passed function with a path that has had changes.
+func (f *Fs) ChangeNotify(ctx context.Context, notifyFunc func(string, fs.EntryType), pollIntervalChan <-chan time.Duration) {
+	if do := f.Fs.Features().ChangeNotify; do != nil {
+		do(ctx, notifyFunc, pollIntervalChan)
+	}
+}
+
+// UserInfo returns info about the connected user
+func (f *Fs) UserInfo(ctx context.Context) (map[string]string, error) {
+	if do := f.Fs.Features().UserInfo; do != nil {
+		return do(ctx)
+	}
+	return nil, fs.ErrorNotImplemented
+}
+
+// Disconnect the current user
+func (f *Fs) Disconnect(ctx context.Context) error {
+	if do := f.Fs.Features().Disconnect; do != nil {
+		return do(ctx)
+	}
+	return fs.ErrorNotImplemented
+}
+
+// MergeDirs merges the contents of all the directories passed
+// in into the first one and rmdirs the other directories.
+func (f *Fs) MergeDirs(ctx context.Context, dirs []fs.Directory) error {
+	if do := f.Fs.Features().MergeDirs; do != nil {
+		return do(ctx, dirs)
+	}
+	return errors.New("MergeDirs not supported")
+}
+
+// DirCacheFlush resets the directory cache - used in testing
+// as an optional interface
+func (f *Fs) DirCacheFlush() {
+	if do := f.Fs.Features().DirCacheFlush; do != nil {
+		do()
+	}
+}
+
+// PublicLink generates a public link to the remote path (usually readable by anyone)
+func (f *Fs) PublicLink(ctx context.Context, remote string, expire fs.Duration, unlink bool) (string, error) {
+	if do := f.Fs.Features().PublicLink; do != nil {
+		return do(ctx, remote, expire, unlink)
+	}
+	return "", errors.New("PublicLink not supported")
+}
+
+// Copy src to this remote using server-side copy operations.
+func (f *Fs) Copy(ctx context.Context, src fs.Object, remote string) (fs.Object, error) {
+	do := f.Fs.Features().Copy
+	if do == nil {
+		return nil, fs.ErrorCantCopy
+	}
+	o, ok := src.(*Object)
+	if !ok {
+		return nil, fs.ErrorCantCopy
+	}
+	oResult, err := do(ctx, o.Object, remote)
+	return f.wrapObject(oResult, err), err
+}
+
+// Move src to this remote using server-side move operations.
+func (f *Fs) Move(ctx context.Context, src fs.Object, remote string) (fs.Object, error) {
+	do := f.Fs.Features().Move
+	if do == nil {
+		return nil, fs.ErrorCantMove
+	}
+	o, ok := src.(*Object)
+	if !ok {
+		return nil, fs.ErrorCantMove
+	}
+	oResult, err := do(ctx, o.Object, remote)
+	if err != nil {
+		return nil, err
+	}
+	_ = f.db.Do(true, &kvMove{
+		src: path.Join(f.Fs.Root(), src.Remote()),
+		dst: path.Join(f.Fs.Root(), remote),
+		dir: false,
+		fs:  f,
+	})
+	return f.wrapObject(oResult, nil), nil
+}
+
+// DirMove moves src, srcRemote to this remote at dstRemote using server-side move operations.
+func (f *Fs) DirMove(ctx context.Context, src fs.Fs, srcRemote, dstRemote string) error {
+	do := f.Fs.Features().DirMove
+	if do == nil {
+		return fs.ErrorCantDirMove
+	}
+	srcFs, ok := src.(*Fs)
+	if !ok {
+		return fs.ErrorCantDirMove
+	}
+	err := do(ctx, srcFs.Fs, srcRemote, dstRemote)
+	if err == nil {
+		_ = f.db.Do(true, &kvMove{
+			src: path.Join(srcFs.Fs.Root(), srcRemote),
+			dst: path.Join(f.Fs.Root(), dstRemote),
+			dir: true,
+			fs:  f,
+		})
+	}
+	return err
+}
+
+// Shutdown the backend, closing any background tasks and any cached connections.
+func (f *Fs) Shutdown(ctx context.Context) (err error) {
+	err = f.db.Stop(false)
+	if do := f.Fs.Features().Shutdown; do != nil {
+		if err2 := do(ctx); err2 != nil {
+			err = err2
+		}
+	}
+	return
+}
+
+// NewObject finds the Object at remote.
+func (f *Fs) NewObject(ctx context.Context, remote string) (fs.Object, error) {
+	o, err := f.Fs.NewObject(ctx, remote)
+	return f.wrapObject(o, err), err
+}
+
+//
+// Object
+//
+
+// Object represents a composite file wrapping one or more data chunks
+type Object struct {
+	fs.Object
+	f *Fs
+}
+
+// Wrap base object into hasher object
+func (f *Fs) wrapObject(o fs.Object, err error) *Object {
+	if err != nil || o == nil {
+		return nil
+	}
+	return &Object{Object: o, f: f}
+}
+
+// Fs returns read only access to the Fs that this object is part of
+func (o *Object) Fs() fs.Info { return o.f }
+
+// UnWrap returns the wrapped Object
+func (o *Object) UnWrap() fs.Object { return o.Object }
+
+// Return a string version
+func (o *Object) String() string {
+	if o == nil {
+		return "<nil>"
+	}
+	return o.Object.String()
+}
+
+// ID returns the ID of the Object if possible
+func (o *Object) ID() string {
+	if doer, ok := o.Object.(fs.IDer); ok {
+		return doer.ID()
+	}
+	return ""
+}
+
+// GetTier returns the Tier of the Object if possible
+func (o *Object) GetTier() string {
+	if doer, ok := o.Object.(fs.GetTierer); ok {
+		return doer.GetTier()
+	}
+	return ""
+}
+
+// SetTier set the Tier of the Object if possible
+func (o *Object) SetTier(tier string) error {
+	if doer, ok := o.Object.(fs.SetTierer); ok {
+		return doer.SetTier(tier)
+	}
+	return errors.New("SetTier not supported")
+}
+
+// MimeType of an Object if known, "" otherwise
+func (o *Object) MimeType(ctx context.Context) string {
+	if doer, ok := o.Object.(fs.MimeTyper); ok {
+		return doer.MimeType(ctx)
+	}
+	return ""
+}
+
+// Check the interfaces are satisfied
+var (
+	_ fs.Fs              = (*Fs)(nil)
+	_ fs.Purger          = (*Fs)(nil)
+	_ fs.Copier          = (*Fs)(nil)
+	_ fs.Mover           = (*Fs)(nil)
+	_ fs.DirMover        = (*Fs)(nil)
+	_ fs.Commander       = (*Fs)(nil)
+	_ fs.PutUncheckeder  = (*Fs)(nil)
+	_ fs.PutStreamer     = (*Fs)(nil)
+	_ fs.CleanUpper      = (*Fs)(nil)
+	_ fs.UnWrapper       = (*Fs)(nil)
+	_ fs.ListRer         = (*Fs)(nil)
+	_ fs.Abouter         = (*Fs)(nil)
+	_ fs.Wrapper         = (*Fs)(nil)
+	_ fs.MergeDirser     = (*Fs)(nil)
+	_ fs.DirCacheFlusher = (*Fs)(nil)
+	_ fs.ChangeNotifier  = (*Fs)(nil)
+	_ fs.PublicLinker    = (*Fs)(nil)
+	_ fs.UserInfoer      = (*Fs)(nil)
+	_ fs.Disconnecter    = (*Fs)(nil)
+	_ fs.Shutdowner      = (*Fs)(nil)
+	_ fs.Object          = (*Object)(nil)
+	_ fs.ObjectUnWrapper = (*Object)(nil)
+	_ fs.IDer            = (*Object)(nil)
+	_ fs.SetTierer       = (*Object)(nil)
+	_ fs.GetTierer       = (*Object)(nil)
+	_ fs.MimeTyper       = (*Object)(nil)
+)
diff --git a/backend/hasher/hasher_internal_test.go b/backend/hasher/hasher_internal_test.go
new file mode 100644
index 000000000..3ec7a11f9
--- /dev/null
+++ b/backend/hasher/hasher_internal_test.go
@@ -0,0 +1,78 @@
+package hasher
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"testing"
+
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/config/obscure"
+	"github.com/rclone/rclone/fs/operations"
+	"github.com/rclone/rclone/fstest"
+	"github.com/rclone/rclone/fstest/fstests"
+	"github.com/rclone/rclone/lib/kv"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func putFile(ctx context.Context, t *testing.T, f fs.Fs, name, data string) fs.Object {
+	mtime1 := fstest.Time("2001-02-03T04:05:06.499999999Z")
+	item := fstest.Item{Path: name, ModTime: mtime1}
+	_, o := fstests.PutTestContents(ctx, t, f, &item, data, true)
+	require.NotNil(t, o)
+	return o
+}
+
+func (f *Fs) testUploadFromCrypt(t *testing.T) {
+	// make a temporary local remote
+	tempRoot, err := fstest.LocalRemote()
+	require.NoError(t, err)
+	defer func() {
+		_ = os.RemoveAll(tempRoot)
+	}()
+
+	// make a temporary crypt remote
+	ctx := context.Background()
+	pass := obscure.MustObscure("crypt")
+	remote := fmt.Sprintf(":crypt,remote=%s,password=%s:", tempRoot, pass)
+	cryptFs, err := fs.NewFs(ctx, remote)
+	require.NoError(t, err)
+
+	// make a test file on the crypt remote
+	const dirName = "from_crypt_1"
+	const fileName = dirName + "/file_from_crypt_1"
+	const longTime = fs.ModTimeNotSupported
+	src := putFile(ctx, t, cryptFs, fileName, "doggy froggy")
+
+	// ensure that hash does not exist yet
+	_ = f.pruneHash(fileName)
+	hashType := f.keepHashes.GetOne()
+	hash, err := f.getRawHash(ctx, hashType, fileName, anyFingerprint, longTime)
+	assert.Error(t, err)
+	assert.Empty(t, hash)
+
+	// upload file to hasher
+	in, err := src.Open(ctx)
+	require.NoError(t, err)
+	dst, err := f.Put(ctx, in, src)
+	require.NoError(t, err)
+	assert.NotNil(t, dst)
+
+	// check that hash was created
+	hash, err = f.getRawHash(ctx, hashType, fileName, anyFingerprint, longTime)
+	assert.NoError(t, err)
+	assert.NotEmpty(t, hash)
+	//t.Logf("hash is %q", hash)
+	_ = operations.Purge(ctx, f, dirName)
+}
+
+// InternalTest dispatches all internal tests
+func (f *Fs) InternalTest(t *testing.T) {
+	if !kv.Supported() {
+		t.Skip("hasher is not supported on this OS")
+	}
+	t.Run("UploadFromCrypt", f.testUploadFromCrypt)
+}
+
+var _ fstests.InternalTester = (*Fs)(nil)
diff --git a/backend/hasher/hasher_test.go b/backend/hasher/hasher_test.go
new file mode 100644
index 000000000..09d60f33d
--- /dev/null
+++ b/backend/hasher/hasher_test.go
@@ -0,0 +1,38 @@
+package hasher_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/rclone/rclone/backend/hasher"
+	"github.com/rclone/rclone/fstest"
+	"github.com/rclone/rclone/fstest/fstests"
+	"github.com/rclone/rclone/lib/kv"
+
+	_ "github.com/rclone/rclone/backend/all" // for integration tests
+)
+
+// TestIntegration runs integration tests against the remote
+func TestIntegration(t *testing.T) {
+	if !kv.Supported() {
+		t.Skip("hasher is not supported on this OS")
+	}
+	opt := fstests.Opt{
+		RemoteName: *fstest.RemoteName,
+		NilObject:  (*hasher.Object)(nil),
+		UnimplementableFsMethods: []string{
+			"OpenWriterAt",
+		},
+		UnimplementableObjectMethods: []string{},
+	}
+	if *fstest.RemoteName == "" {
+		tempDir := filepath.Join(os.TempDir(), "rclone-hasher-test")
+		opt.ExtraConfig = []fstests.ExtraConfigItem{
+			{Name: "TestHasher", Key: "type", Value: "hasher"},
+			{Name: "TestHasher", Key: "remote", Value: tempDir},
+		}
+		opt.RemoteName = "TestHasher:"
+	}
+	fstests.Run(t, &opt)
+}
diff --git a/backend/hasher/kv.go b/backend/hasher/kv.go
new file mode 100644
index 000000000..881661b4f
--- /dev/null
+++ b/backend/hasher/kv.go
@@ -0,0 +1,315 @@
+package hasher
+
+import (
+	"bytes"
+	"context"
+	"encoding/gob"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/pkg/errors"
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/hash"
+	"github.com/rclone/rclone/fs/operations"
+	"github.com/rclone/rclone/lib/kv"
+)
+
+const (
+	timeFormat     = "2006-01-02T15:04:05.000000000-0700"
+	anyFingerprint = "*"
+)
+
+type hashMap map[hash.Type]string
+
+type hashRecord struct {
+	Fp      string // fingerprint
+	Hashes  operations.HashSums
+	Created time.Time
+}
+
+func (r *hashRecord) encode(key string) ([]byte, error) {
+	var buf bytes.Buffer
+	if err := gob.NewEncoder(&buf).Encode(r); err != nil {
+		fs.Debugf(key, "hasher encoding %v: %v", r, err)
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+func (r *hashRecord) decode(key string, data []byte) error {
+	if err := gob.NewDecoder(bytes.NewBuffer(data)).Decode(r); err != nil {
+		fs.Debugf(key, "hasher decoding %q failed: %v", data, err)
+		return err
+	}
+	return nil
+}
+
+// kvPrune: prune a single hash
+type kvPrune struct {
+	key string
+}
+
+func (op *kvPrune) Do(ctx context.Context, b kv.Bucket) error {
+	return b.Delete([]byte(op.key))
+}
+
+// kvPurge: delete a subtree
+type kvPurge struct {
+	dir string
+}
+
+func (op *kvPurge) Do(ctx context.Context, b kv.Bucket) error {
+	dir := op.dir
+	if !strings.HasSuffix(dir, "/") {
+		dir += "/"
+	}
+	var items []string
+	cur := b.Cursor()
+	bkey, _ := cur.Seek([]byte(dir))
+	for bkey != nil {
+		key := string(bkey)
+		if !strings.HasPrefix(key, dir) {
+			break
+		}
+		items = append(items, key[len(dir):])
+		bkey, _ = cur.Next()
+	}
+	nerr := 0
+	for _, sub := range items {
+		if err := b.Delete([]byte(dir + sub)); err != nil {
+			nerr++
+		}
+	}
+	fs.Debugf(dir, "%d hashes purged, %d failed", len(items)-nerr, nerr)
+	return nil
+}
+
+// kvMove: assign hashes to new path
+type kvMove struct {
+	src string
+	dst string
+	dir bool
+	fs  *Fs
+}
+
+func (op *kvMove) Do(ctx context.Context, b kv.Bucket) error {
+	src, dst := op.src, op.dst
+	if !op.dir {
+		err := moveHash(b, src, dst)
+		fs.Debugf(op.fs, "moving cached hash %s to %s (err: %v)", src, dst, err)
+		return err
+	}
+
+	if !strings.HasSuffix(src, "/") {
+		src += "/"
+	}
+	if !strings.HasSuffix(dst, "/") {
+		dst += "/"
+	}
+
+	var items []string
+	cur := b.Cursor()
+	bkey, _ := cur.Seek([]byte(src))
+	for bkey != nil {
+		key := string(bkey)
+		if !strings.HasPrefix(key, src) {
+			break
+		}
+		items = append(items, key[len(src):])
+		bkey, _ = cur.Next()
+	}
+
+	nerr := 0
+	for _, suffix := range items {
+		srcKey, dstKey := src+suffix, dst+suffix
+		err := moveHash(b, srcKey, dstKey)
+		fs.Debugf(op.fs, "Rename cache record %s -> %s (err: %v)", srcKey, dstKey, err)
+		if err != nil {
+			nerr++
+		}
+	}
+	fs.Debugf(op.fs, "%d hashes moved, %d failed", len(items)-nerr, nerr)
+	return nil
+}
+
+func moveHash(b kv.Bucket, src, dst string) error {
+	data := b.Get([]byte(src))
+	err := b.Delete([]byte(src))
+	if err != nil || len(data) == 0 {
+		return err
+	}
+	return b.Put([]byte(dst), data)
+}
+
+// kvGet: get single hash from database
+type kvGet struct {
+	key  string
+	fp   string
+	hash string
+	val  string
+	age  time.Duration
+}
+
+func (op *kvGet) Do(ctx context.Context, b kv.Bucket) error {
+	data := b.Get([]byte(op.key))
+	if len(data) == 0 {
+		return errors.New("no record")
+	}
+	var r hashRecord
+	if err := r.decode(op.key, data); err != nil {
+		return errors.New("invalid record")
+	}
+	if !(r.Fp == anyFingerprint || op.fp == anyFingerprint || r.Fp == op.fp) {
+		return errors.New("fingerprint changed")
+	}
+	if time.Since(r.Created) > op.age {
+		return errors.New("record timed out")
+	}
+	if r.Hashes != nil {
+		op.val = r.Hashes[op.hash]
+	}
+	return nil
+}
+
+// kvPut: set hashes for an object by key
+type kvPut struct {
+	key    string
+	fp     string
+	hashes operations.HashSums
+	age    time.Duration
+}
+
+func (op *kvPut) Do(ctx context.Context, b kv.Bucket) (err error) {
+	data := b.Get([]byte(op.key))
+	var r hashRecord
+	if len(data) > 0 {
+		err = r.decode(op.key, data)
+		if err != nil || r.Fp != op.fp || time.Since(r.Created) > op.age {
+			r.Hashes = nil
+		}
+	}
+	if len(r.Hashes) == 0 {
+		r.Created = time.Now()
+		r.Hashes = operations.HashSums{}
+		r.Fp = op.fp
+	}
+
+	for hashType, hashVal := range op.hashes {
+		r.Hashes[hashType] = hashVal
+	}
+	if data, err = r.encode(op.key); err != nil {
+		return errors.Wrap(err, "marshal failed")
+	}
+	if err = b.Put([]byte(op.key), data); err != nil {
+		return errors.Wrap(err, "put failed")
+	}
+	return err
+}
+
+// kvDump: dump the database.
+// Note: long dump can cause concurrent operations to fail.
+type kvDump struct {
+	full  bool
+	root  string
+	path  string
+	fs    *Fs
+	num   int
+	total int
+}
+
+func (op *kvDump) Do(ctx context.Context, b kv.Bucket) error {
+	f, baseRoot, dbPath := op.fs, op.root, op.path
+
+	if op.full {
+		total := 0
+		num := 0
+		_ = b.ForEach(func(bkey, data []byte) error {
+			total++
+			key := string(bkey)
+			include := (baseRoot == "" || key == baseRoot || strings.HasPrefix(key, baseRoot+"/"))
+			var r hashRecord
+			if err := r.decode(key, data); err != nil {
+				fs.Errorf(nil, "%s: invalid record: %v", key, err)
+				return nil
+			}
+			fmt.Println(f.dumpLine(&r, key, include, nil))
+			if include {
+				num++
+			}
+			return nil
+		})
+		fs.Infof(dbPath, "%d records out of %d", num, total)
+		op.num, op.total = num, total // for unit tests
+		return nil
+	}
+
+	num := 0
+	cur := b.Cursor()
+	var bkey, data []byte
+	if baseRoot != "" {
+		bkey, data = cur.Seek([]byte(baseRoot))
+	} else {
+		bkey, data = cur.First()
+	}
+	for bkey != nil {
+		key := string(bkey)
+		if !(baseRoot == "" || key == baseRoot || strings.HasPrefix(key, baseRoot+"/")) {
+			break
+		}
+		var r hashRecord
+		if err := r.decode(key, data); err != nil {
+			fs.Errorf(nil, "%s: invalid record: %v", key, err)
+			continue
+		}
+		if key = strings.TrimPrefix(key[len(baseRoot):], "/"); key == "" {
+			key = "/"
+		}
+		fmt.Println(f.dumpLine(&r, key, true, nil))
+		num++
+		bkey, data = cur.Next()
+	}
+	fs.Infof(dbPath, "%d records", num)
+	op.num = num // for unit tests
+	return nil
+}
+
+func (f *Fs) dumpLine(r *hashRecord, path string, include bool, err error) string {
+	var status string
+	switch {
+	case !include:
+		status = "ext"
+	case err != nil:
+		status = "bad"
+	case r.Fp == anyFingerprint:
+		status = "stk"
+	default:
+		status = "ok "
+	}
+
+	var hashes []string
+	for _, hashType := range f.keepHashes.Array() {
+		hashName := hashType.String()
+		hashVal := r.Hashes[hashName]
+		if hashVal == "" || err != nil {
+			hashVal = "-"
+		}
+		hashVal = fmt.Sprintf("%-*s", hash.Width(hashType), hashVal)
+		hashes = append(hashes, hashName+":"+hashVal)
+	}
+	hashesStr := strings.Join(hashes, " ")
+
+	age := time.Since(r.Created).Round(time.Second)
+	if age > 24*time.Hour {
+		age = age.Round(time.Hour)
+	}
+	if err != nil {
+		age = 0
+	}
+	ageStr := age.String()
+	if strings.HasSuffix(ageStr, "h0m0s") {
+		ageStr = strings.TrimSuffix(ageStr, "0m0s")
+	}
+
+	return fmt.Sprintf("%s %s %9s %s", status, hashesStr, ageStr, path)
+}
diff --git a/backend/hasher/object.go b/backend/hasher/object.go
new file mode 100644
index 000000000..3c6e5af78
--- /dev/null
+++ b/backend/hasher/object.go
@@ -0,0 +1,305 @@
+package hasher
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"path"
+	"time"
+
+	"github.com/pkg/errors"
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/hash"
+	"github.com/rclone/rclone/fs/operations"
+)
+
+// obtain hash for an object
+func (o *Object) getHash(ctx context.Context, hashType hash.Type) (string, error) {
+	maxAge := time.Duration(o.f.opt.MaxAge)
+	if maxAge <= 0 {
+		return "", nil
+	}
+	fp := o.fingerprint(ctx)
+	if fp == "" {
+		return "", errors.New("fingerprint failed")
+	}
+	return o.f.getRawHash(ctx, hashType, o.Remote(), fp, maxAge)
+}
+
+// obtain hash for a path
+func (f *Fs) getRawHash(ctx context.Context, hashType hash.Type, remote, fp string, age time.Duration) (string, error) {
+	key := path.Join(f.Fs.Root(), remote)
+	op := &kvGet{
+		key:  key,
+		fp:   fp,
+		hash: hashType.String(),
+		age:  age,
+	}
+	err := f.db.Do(false, op)
+	return op.val, err
+}
+
+// put new hashes for an object
+func (o *Object) putHashes(ctx context.Context, rawHashes hashMap) error {
+	if o.f.opt.MaxAge <= 0 {
+		return nil
+	}
+	fp := o.fingerprint(ctx)
+	if fp == "" {
+		return nil
+	}
+	key := path.Join(o.f.Fs.Root(), o.Remote())
+	hashes := operations.HashSums{}
+	for hashType, hashVal := range rawHashes {
+		hashes[hashType.String()] = hashVal
+	}
+	return o.f.putRawHashes(ctx, key, fp, hashes)
+}
+
+// set hashes for a path without any validation
+func (f *Fs) putRawHashes(ctx context.Context, key, fp string, hashes operations.HashSums) error {
+	return f.db.Do(true, &kvPut{
+		key:    key,
+		fp:     fp,
+		hashes: hashes,
+		age:    time.Duration(f.opt.MaxAge),
+	})
+}
+
+// Hash returns the selected checksum of the file or "" if unavailable.
+func (o *Object) Hash(ctx context.Context, hashType hash.Type) (hashVal string, err error) {
+	f := o.f
+	if f.passHashes.Contains(hashType) {
+		fs.Debugf(o, "pass %s", hashType)
+		return o.Object.Hash(ctx, hashType)
+	}
+	if !f.suppHashes.Contains(hashType) {
+		fs.Debugf(o, "unsupp %s", hashType)
+		return "", hash.ErrUnsupported
+	}
+	if hashVal, err = o.getHash(ctx, hashType); err != nil {
+		fs.Debugf(o, "getHash: %v", err)
+		err = nil
+		hashVal = ""
+	}
+	if hashVal != "" {
+		fs.Debugf(o, "cached %s = %q", hashType, hashVal)
+		return hashVal, nil
+	}
+	if f.slowHashes.Contains(hashType) {
+		fs.Debugf(o, "slow %s", hashType)
+		hashVal, err = o.Object.Hash(ctx, hashType)
+		if err == nil && hashVal != "" && f.keepHashes.Contains(hashType) {
+			if err = o.putHashes(ctx, hashMap{hashType: hashVal}); err != nil {
+				fs.Debugf(o, "putHashes: %v", err)
+				err = nil
+			}
+		}
+		return hashVal, err
+	}
+	if f.autoHashes.Contains(hashType) && o.Size() < int64(f.opt.AutoSize) {
+		_ = o.updateHashes(ctx)
+		if hashVal, err = o.getHash(ctx, hashType); err != nil {
+			fs.Debugf(o, "auto %s = %q (%v)", hashType, hashVal, err)
+			err = nil
+		}
+	}
+	return hashVal, err
+}
+
+// updateHashes performs implicit "rclone hashsum --download" and updates cache.
+func (o *Object) updateHashes(ctx context.Context) error {
+	r, err := o.Open(ctx)
+	if err != nil {
+		fs.Infof(o, "update failed (open): %v", err)
+		return err
+	}
+	defer func() {
+		_ = r.Close()
+	}()
+	if _, err = io.Copy(ioutil.Discard, r); err != nil {
+		fs.Infof(o, "update failed (copy): %v", err)
+		return err
+	}
+	return nil
+}
+
+// Update the object with the given data, time and size.
+func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) error {
+	_ = o.f.pruneHash(src.Remote())
+	return o.Object.Update(ctx, in, src, options...)
+}
+
+// Remove an object.
+func (o *Object) Remove(ctx context.Context) error {
+	_ = o.f.pruneHash(o.Remote())
+	return o.Object.Remove(ctx)
+}
+
+// SetModTime sets the modification time of the file.
+// Also prunes the cache entry when modtime changes so that
+// touching a file will trigger checksum recalculation even
+// on backends that don't provide modTime with fingerprint.
+func (o *Object) SetModTime(ctx context.Context, mtime time.Time) error {
+	if mtime != o.Object.ModTime(ctx) {
+		_ = o.f.pruneHash(o.Remote())
+	}
+	return o.Object.SetModTime(ctx, mtime)
+}
+
+// Open opens the file for read.
+// Full reads will also update object hashes.
+func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (r io.ReadCloser, err error) {
+	size := o.Size()
+	var offset, limit int64 = 0, -1
+	for _, option := range options {
+		switch opt := option.(type) {
+		case *fs.SeekOption:
+			offset = opt.Offset
+		case *fs.RangeOption:
+			offset, limit = opt.Decode(size)
+		}
+	}
+	if offset < 0 {
+		return nil, errors.New("invalid offset")
+	}
+	if limit < 0 {
+		limit = size - offset
+	}
+	if r, err = o.Object.Open(ctx, options...); err != nil {
+		return nil, err
+	}
+	if offset != 0 || limit < size {
+		// It's a partial read
+		return r, err
+	}
+	return o.f.newHashingReader(ctx, r, func(sums hashMap) {
+		if err := o.putHashes(ctx, sums); err != nil {
+			fs.Infof(o, "auto hashing error: %v", err)
+		}
+	})
+}
+
+// Put data into the remote path with given modTime and size
+func (f *Fs) Put(ctx context.Context, in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) (fs.Object, error) {
+	var (
+		o      *Object
+		common hash.Set
+		rehash bool
+		hashes hashMap
+	)
+	if fsrc := src.Fs(); fsrc != nil {
+		common = fsrc.Hashes().Overlap(f.keepHashes)
+		// Rehash if source does not have all required hashes or hashing is slow
+		rehash = fsrc.Features().SlowHash || common != f.keepHashes
+	}
+
+	wrapIn := in
+	if rehash {
+		r, err := f.newHashingReader(ctx, in, func(sums hashMap) {
+			hashes = sums
+		})
+		fs.Debugf(src, "Rehash in-fly due to incomplete or slow source set %v (err: %v)", common, err)
+		if err == nil {
+			wrapIn = r
+		} else {
+			rehash = false
+		}
+	}
+
+	_ = f.pruneHash(src.Remote())
+	oResult, err := f.Fs.Put(ctx, wrapIn, src, options...)
+	o = f.wrapObject(oResult, err)
+	if o == nil {
+		return nil, err
+	}
+
+	if !rehash {
+		hashes = hashMap{}
+		for _, ht := range common.Array() {
+			if h, e := src.Hash(ctx, ht); e == nil && h != "" {
+				hashes[ht] = h
+			}
+		}
+	}
+	if len(hashes) > 0 {
+		err := o.putHashes(ctx, hashes)
+		fs.Debugf(o, "Applied %d source hashes, err: %v", len(hashes), err)
+	}
+	return o, err
+}
+
+type hashingReader struct {
+	rd     io.Reader
+	hasher *hash.MultiHasher
+	fun    func(hashMap)
+}
+
+func (f *Fs) newHashingReader(ctx context.Context, rd io.Reader, fun func(hashMap)) (*hashingReader, error) {
+	hasher, err := hash.NewMultiHasherTypes(f.keepHashes)
+	if err != nil {
+		return nil, err
+	}
+	hr := &hashingReader{
+		rd:     rd,
+		hasher: hasher,
+		fun:    fun,
+	}
+	return hr, nil
+}
+
+func (r *hashingReader) Read(p []byte) (n int, err error) {
+	n, err = r.rd.Read(p)
+	if err != nil && err != io.EOF {
+		r.hasher = nil
+	}
+	if r.hasher != nil {
+		if _, errHash := r.hasher.Write(p[:n]); errHash != nil {
+			r.hasher = nil
+			err = errHash
+		}
+	}
+	if err == io.EOF && r.hasher != nil {
+		r.fun(r.hasher.Sums())
+		r.hasher = nil
+	}
+	return
+}
+
+func (r *hashingReader) Close() error {
+	if rc, ok := r.rd.(io.ReadCloser); ok {
+		return rc.Close()
+	}
+	return nil
+}
+
+// Return object fingerprint or empty string in case of errors
+//
+// Note that we can't use the generic `fs.Fingerprint` here because
+// this fingerprint is used to pick _derived hashes_ that are slow
+// to calculate or completely unsupported by the base remote.
+//
+// The hasher fingerprint must be based on `fsHash`, the first _fast_
+// hash supported _by the underlying remote_ (if there is one),
+// while `fs.Fingerprint` would select a hash _produced by hasher_
+// creating unresolvable fingerprint loop.
+func (o *Object) fingerprint(ctx context.Context) string {
+	size := o.Object.Size()
+	timeStr := "-"
+	if o.f.fpTime {
+		timeStr = o.Object.ModTime(ctx).UTC().Format(timeFormat)
+		if timeStr == "" {
+			return ""
+		}
+	}
+	hashStr := "-"
+	if o.f.fpHash != hash.None {
+		var err error
+		hashStr, err = o.Object.Hash(ctx, o.f.fpHash)
+		if hashStr == "" || err != nil {
+			return ""
+		}
+	}
+	return fmt.Sprintf("%d,%s,%s", size, timeStr, hashStr)
+}