diff --git a/Cargo.toml b/Cargo.toml
index 74529f0..a37f332 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -5,6 +5,7 @@ authors = ["Cyryl Płotnicki <cyplo@cyplo.net>"]
 
 [dependencies]
 walkdir = "2.2"
+rust-crypto = "0.2"
 
 [dev-dependencies]
 tempfile = "3.0"
diff --git a/src/backup.rs b/src/backup.rs
index e110a2f..ae4fedc 100644
--- a/src/backup.rs
+++ b/src/backup.rs
@@ -1,6 +1,7 @@
 use std::fs;
 use std::io;
 use std::path::Path;
+use storage::Version;
 use walkdir::DirEntry;
 use walkdir::WalkDir;
 
@@ -28,11 +29,15 @@ impl<'a> Engine<'a> {
         Ok(())
     }
 
-    pub fn file_version(&self, path: &Path) -> u64 {
-        0
+    pub fn file_version(&self, path: &Path) -> Version {
+        Version(0)
     }
 
     fn process_entry(&self, entry: &DirEntry) -> Result<(), io::Error> {
+        // TODO: remember entry in index
+
+        // TODO: store file data
+
         if entry.file_type().is_dir() {
             fs::create_dir(self.repository_path.join(entry.file_name()))?;
         }
diff --git a/src/lib.rs b/src/lib.rs
index b212788..d26f206 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,4 +1,5 @@
 extern crate core;
+extern crate crypto;
 #[cfg(test)]
 extern crate dir_diff;
 #[cfg(test)]
@@ -7,3 +8,5 @@ extern crate walkdir;
 
 pub mod backup;
 pub mod restore;
+
+mod storage;
diff --git a/src/restore.rs b/src/restore.rs
index 89c1517..77b3df1 100644
--- a/src/restore.rs
+++ b/src/restore.rs
@@ -1,6 +1,7 @@
 use std::fs;
 use std::io;
 use std::path::Path;
+use storage::Version;
 use walkdir::DirEntry;
 use walkdir::WalkDir;
 
@@ -27,10 +28,10 @@ impl<'a> Engine<'a> {
     }
 
     fn restore(&self, what: WhatToRestore) -> Result<(), io::Error> {
-        self.restore_as_of_version(what, 0)
+        self.restore_as_of_version(what, Version(0))
     }
 
-    pub fn restore_as_of_version(&self, what: WhatToRestore, version: u64) -> Result<(), io::Error> {
+    pub fn restore_as_of_version(&self, what: WhatToRestore, version: Version) -> Result<(), io::Error> {
         let walker = WalkDir::new(self.repository_path);
         for maybe_entry in walker {
             match maybe_entry {
diff --git a/src/storage.rs b/src/storage.rs
new file mode 100644
index 0000000..cb59102
--- /dev/null
+++ b/src/storage.rs
@@ -0,0 +1,42 @@
+use std::cmp::Ordering;
+use std::path::Path;
+
+#[derive(Copy, Clone, PartialOrd, PartialEq)]
+pub struct Version(pub u64);
+
+struct Index;
+
+impl Index {
+    fn new() -> Self {
+        Self {}
+    }
+
+    fn store(&mut self, path: &Path, hash: &[u8]) -> Version {
+        Version(0)
+    }
+}
+
+#[cfg(test)]
+mod should {
+
+    use super::*;
+
+    #[test]
+    fn support_file_versions() {
+        // put path and hash into index -> v0
+        // put same path different hash -> v1
+        // query for v0, v1
+        let mut index = Index::new();
+        let v1 = index.store(Path::new("/some/path"), "some hash".as_bytes());
+        let v2 = index.store(Path::new("/some/path"), "some other hash".as_bytes());
+
+        assert!(v2 > v1);
+    }
+
+    #[test]
+    fn support_deduplication() {
+        // put path and hash into index
+        // put same hash, different path
+        // should get same storage paths
+    }
+}