changed public id format to be 128 bit length.

shared/util.go

@@ -1,8 +1,17 @@
 package shared
 
-import "os"
+import (
+	"crypto/subtle"
+	"encoding/base64"
+	"errors"
+	"github.com/dchest/siphash"
+	"gopkg.in/mgo.v2/bson"
+	"os"
+)
 
-func Exists(name string) bool {
+var idHashKey []byte = []byte("semi-private-key")
+
+func exists(name string) bool {
 	if _, err := os.Stat(name); err != nil {
 		if os.IsNotExist(err) {
 			return false
@@ -10,3 +19,38 @@ func Exists(name string) bool {
 	}
 	return true
 }
+
+func validateId(id []byte) error {
+	if len(id) != 16 {
+		return errors.New("valid ids are 16 bytes (126 bit) long.")
+	}
+	h := siphash.New(idHashKey)
+	h.Write(id[4:16])
+	if subtle.ConstantTimeCompare(h.Sum(nil)[0:4], id[0:4]) != 1 {
+		return errors.New("invalid id checksum")
+	}
+	return nil
+}
+
+func ObjectIdToPublicId(id bson.ObjectId) string {
+	if id == "" {
+		return ""
+	}
+	h := siphash.New(idHashKey)
+	h.Write([]byte(id))
+	return base64.URLEncoding.EncodeToString(append(h.Sum(nil)[0:4], []byte(id)...))
+}
+
+func PublicIdToObjectId(id string) (bson.ObjectId, error) {
+	if id == "" {
+		return "", nil
+	}
+	cypher, err := base64.URLEncoding.DecodeString(id)
+	if err != nil {
+		return "", err
+	}
+	if err := validateId(cypher); err != nil {
+		return "", err
+	}
+	return bson.ObjectId(cypher[4:16]), nil
+}