diff --git a/check.go b/check.go
new file mode 100644
index 0000000..4e067e9
--- /dev/null
+++ b/check.go
@@ -0,0 +1,103 @@
+package main
+
+import (
+	"bufio"
+	"crypto/md5"
+	"crypto/sha1"
+	"crypto/sha256"
+	"crypto/sha512"
+	"errors"
+	"fmt"
+	"hash"
+	"io"
+	"log"
+	"os"
+	"strings"
+)
+
+type checksum struct {
+	filename string
+	hash     hash.Hash
+	checksum string
+}
+
+func parseCS(line string) (checksum, error) {
+	elems := strings.Fields(line)
+	if len(elems) != 2 {
+		return checksum{}, fmt.Errorf("unexpected content: %d != 2", len(elems))
+	}
+	cs, f := elems[0], elems[1]
+	var hsh hash.Hash
+	switch len(cs) {
+	case 32:
+		hsh = md5.New()
+	case 40:
+		hsh = sha1.New()
+	case 64:
+		hsh = sha256.New()
+	case 128:
+		hsh = sha512.New()
+	default:
+		return checksum{}, fmt.Errorf("unknown format: %q", line)
+	}
+	return checksum{filename: f, hash: hsh, checksum: cs}, nil
+}
+
+func check(files []string) error {
+	streams := []io.ReadCloser{}
+	defer func() {
+		for _, stream := range streams {
+			stream.Close()
+		}
+	}()
+
+	for _, name := range files {
+		f, err := os.Open(name)
+		if err != nil {
+			return err
+		}
+		streams = append(streams, f)
+	}
+	if len(files) == 0 {
+		streams = append(streams, os.Stdin)
+	}
+
+	jobs := []checksum{}
+	for _, stream := range streams {
+		s := bufio.NewScanner(stream)
+		for s.Scan() {
+			cs, err := parseCS(s.Text())
+			if err != nil {
+				return err
+			}
+			jobs = append(jobs, cs)
+		}
+		if s.Err() != nil {
+			return s.Err()
+		}
+	}
+
+	errs := 0
+	for _, job := range jobs {
+		f, err := os.Open(job.filename)
+		if err != nil {
+			return fmt.Errorf("open: %v", err)
+		}
+		if _, err := io.Copy(job.hash, f); err != nil {
+			log.Printf("%+v", err)
+		}
+		f.Close()
+		if fmt.Sprintf("%x", job.hash.Sum(nil)) == job.checksum {
+			fmt.Printf("%s: OK\n", job.filename)
+		} else {
+			errs++
+			fmt.Fprintf(os.Stderr, "%s: bad\n", job.filename)
+		}
+	}
+
+	var err error
+	if errs != 0 {
+		err = errors.New("bad files found")
+	}
+	return err
+}
diff --git a/main.go b/main.go
index 5a5b019..cd2de4d 100644
--- a/main.go
+++ b/main.go
@@ -12,10 +12,26 @@ import (
 )
 
 var algo = flag.String("a", "sha1", "algorithm to use")
+var mode = flag.Bool("c", false, "check")
 
 func main() {
 	flag.Parse()
 	files := flag.Args()
+	switch *mode {
+	case true:
+		if err := check(files); err != nil {
+			fmt.Fprintf(os.Stderr, "%v\n", err)
+			os.Exit(1)
+		}
+	case false:
+		if err := hsh(files); err != nil {
+			fmt.Fprintf(os.Stderr, "%v\n", err)
+			os.Exit(1)
+		}
+	}
+}
+
+func hsh(files []string) error {
 	h := sha256.New()
 	switch *algo {
 	case "sha1", "1":
@@ -27,8 +43,7 @@ func main() {
 	case "md5":
 		h = md5.New()
 	default:
-		fmt.Fprintf(os.Stderr, "unsupported algorithm: %v\n", *algo)
-		os.Exit(1)
+		return fmt.Errorf("unsupported algorithm: %v", *algo)
 	}
 
 	if len(files) == 0 {
@@ -55,4 +70,5 @@ func main() {
 			fmt.Printf("%x  %s\n", h.Sum(nil), name)
 		}
 	}
+	return nil
 }