diff --git a/main.go b/main.go
index f19164c..d749517 100644
--- a/main.go
+++ b/main.go
@@ -1,12 +1,15 @@
+//3456789112345676892123456789312345678941234567895123456789612345678971234567898
+
 package main
 
-//import "crypto/sha256"
+import "crypto/sha256"
 import "fmt"
 import "github.com/sirupsen/logrus"
-
-//import "github.com/pkg/xattr"
-import "hash"
+import "github.com/multiformats/go-multihash"
+import "github.com/mr-tron/base58"
+import "github.com/pkg/xattr"
 import "os"
+import "io"
 import "flag"
 import "time"
 
@@ -16,6 +19,16 @@ var Builduser string
 var Buildarch string
 var log *logrus.Logger
 
+const namespacePrefix = "berlin.sneak.xsum"
+
+//FIXME(sneak) make this parallelize to NUM_CPUS when processing multiple
+//args
+
+//FIXME(sneak) add a -r (recursive) flag for directories
+
+//FIXME(sneak) make checking support reading hash algo type from multihash
+//instead of assumming sha256
+
 func main() {
 	os.Exit(xsum())
 }
@@ -40,11 +53,9 @@ func xsum() int {
 	)
 
 	paths := flag.Args()
-	fmt.Printf("%+v\n", paths)
 	if len(paths) > 1 {
 		paths = paths[1:]
 	}
-	fmt.Printf("%+v\n", paths)
 	switch flag.Arg(0) {
 	case "cron":
 		return xsfCheckAndUpdate(paths)
@@ -67,20 +78,21 @@ func usage() {
 
 func xsfCheck(paths []string) int {
 	log.Debugf("check")
+	log.Fatalf("not implemented")
 	return 0
 }
 
-func showError(e *error) {
+func showError(e error) {
 	fmt.Fprintf(os.Stderr, "error: %s\n", e)
 }
 
 func xsfUpdate(paths []string) int {
 	log.Debugf("update")
 	for _, path := range paths {
-		x := NewXsf(path)
+		x := newXsf(path)
 		err := x.Update()
 		if err != nil {
-			showError(&err)
+			showError(err)
 			return -1
 		}
 	}
@@ -96,17 +108,89 @@ func xsfCheckAndUpdate(paths []string) int {
 	return xsfUpdate(paths)
 }
 
+func HashFile(fp *os.File) (string, error) {
+	h := sha256.New()
+
+	if _, err := io.Copy(h, fp); err != nil {
+		return "", err
+	}
+
+	mHashBuf, err := multihash.EncodeName(h.Sum(nil), "sha1")
+
+	if err != nil {
+		return "", err
+	}
+
+	return base58.Encode(mHashBuf), nil
+}
+
+/////////////////////////////////////////////////////////////////////////////////
+// type xsf
+/////////////////////////////////////////////////////////////////////////////////
+
 type xsf struct {
-	path  string
-	mtime *time.Time
-	size  int64
-	sum   *hash.Hash
 	fi    *os.FileInfo
+	fp    *os.File
+	hash  string
+	mtime string
+	path  string
+	size  int64
+}
+
+/////////////////////////////////////////////////////////////////////////////////
+// constructor
+/////////////////////////////////////////////////////////////////////////////////
+
+type xsf struct {
+	fi    *os.FileInfo
+	fp    *os.File
+	hash  string
+	mtime string
+	path  string
+	size  int64
+}
+
+func newXsf(path string) *xsf {
+	x := xsf{}
+	x.path = path
+	return &x
+}
+
+func (x *xsf) writeXattrs(fp *os.File) error {
+	log.Infof("writing xattrs")
+
+	var xn string
+	var err error
+
+	xn = fmt.Sprintf("%s.%s", namespacePrefix, "mtime")
+	log.Infof("writing xattr %s=%s", xn, x.mtime)
+	err = xattr.FSet(fp, xn, []byte(x.mtime))
+	if err != nil {
+		return err
+	}
+
+	xn = fmt.Sprintf("%s.%s", namespacePrefix, "size")
+	log.Infof("writing xattr %s=%s", xn, fmt.Sprintf("%d", x.size))
+	err = xattr.FSet(fp, xn, []byte(fmt.Sprintf("%d", x.size)))
+	if err != nil {
+		return err
+	}
+
+	xn = fmt.Sprintf("%s.%s", namespacePrefix, "multihash")
+	log.Infof("writing xattr %s=%s", xn, x.hash)
+	err = xattr.FSet(fp, xn, []byte(x.hash))
+	if err != nil {
+		return err
+	}
+
+	return nil
 }
 
 func (x *xsf) Update() error {
 	fp, e1 := os.Open(x.path)
 	defer fp.Close()
+	log.Infof("updating file")
+	log.Debugf("path: %s", x.path)
 	if e1 != nil {
 		return e1
 	}
@@ -116,13 +200,24 @@ func (x *xsf) Update() error {
 		return e2
 	}
 	x.size = fi.Size()
-	t := fi.ModTime()
-	x.mtime = &t
+	log.Debugf("size: %d", x.size)
+	t := fi.ModTime().UTC().Format(time.RFC3339)
+	log.Debugf("modtime: %s", t)
+	x.mtime = t
+
+	log.Debugf("hashing...")
+	h, e3 := HashFile(fp)
+	if e3 != nil {
+		return e3
+	}
+	x.hash = h
+	log.Debugf("hash: %s", h)
+
+	e4 := x.writeXattrs(fp)
+
+	if e4 != nil {
+		return e4
+	}
+
 	return nil
 }
-
-func NewXsf(path string) *xsf {
-	x := xsf{}
-	x.path = path
-	return &x
-}