diff --git a/src/api2/admin/datastore/backup.rs b/src/api2/admin/datastore/backup.rs
index 05332600..db9af1e0 100644
--- a/src/api2/admin/datastore/backup.rs
+++ b/src/api2/admin/datastore/backup.rs
@@ -261,11 +261,10 @@ fn dynamic_append (
 
     let env: &BackupEnvironment = rpcenv.as_ref();
 
-    let _size = 0;
-    let _digest = crate::tools::hex_to_digest(digest_str)?;
+    let digest = crate::tools::hex_to_digest(digest_str)?;
+    let size = env.lookup_chunk(&digest).ok_or_else(|| format_err!("no such chunk"))?;
 
-    // fixme: lookup digest and chunk size, then add
-    //env.dynamic_writer_append_chunk(wid, size, &digest)?;
+    env.dynamic_writer_append_chunk(wid, size, &digest)?;
 
     env.log(format!("sucessfully added chunk {} to dynamic index {}", digest_str, wid));
 
diff --git a/src/api2/admin/datastore/backup/environment.rs b/src/api2/admin/datastore/backup/environment.rs
index 652f23d3..744a27a0 100644
--- a/src/api2/admin/datastore/backup/environment.rs
+++ b/src/api2/admin/datastore/backup/environment.rs
@@ -14,6 +14,7 @@ struct SharedBackupState {
     finished: bool,
     uid_counter: usize,
     dynamic_writers: HashMap<usize, (u64 /* offset */, DynamicIndexWriter)>,
+    known_chunks: HashMap<[u8;32], u32>,
 }
 
 impl SharedBackupState {
@@ -61,6 +62,7 @@ impl BackupEnvironment {
             finished: false,
             uid_counter: 0,
             dynamic_writers: HashMap::new(),
+            known_chunks: HashMap::new(),
         };
 
         Self {
@@ -76,6 +78,27 @@ impl BackupEnvironment {
         }
     }
 
+    // Register a Chunk with associated length. A client may only use registered
+    // chunks (we do not trust clients that far ...)
+    pub fn register_chunk(&self, digest: [u8; 32], length: u32) -> Result<(), Error> {
+        let mut state = self.state.lock().unwrap();
+
+        state.ensure_unfinished()?;
+
+        state.known_chunks.insert(digest, length);
+
+        Ok(())
+    }
+
+    pub fn lookup_chunk(&self, digest: &[u8; 32]) -> Option<u32> {
+        let state = self.state.lock().unwrap();
+
+        match state.known_chunks.get(digest) {
+            Some(len) => Some(*len),
+            None => None,
+        }
+    }
+
     /// Store the writer with an unique ID
     pub fn register_dynamic_writer(&self, writer: DynamicIndexWriter) -> Result<usize, Error> {
         let mut state = self.state.lock().unwrap();
@@ -90,7 +113,7 @@ impl BackupEnvironment {
     }
 
     /// Append chunk to dynamic writer
-    pub fn dynamic_writer_append_chunk(&self, wid: usize, size: u64, digest: &[u8; 32]) -> Result<(), Error> {
+    pub fn dynamic_writer_append_chunk(&self, wid: usize, size: u32, digest: &[u8; 32]) -> Result<(), Error> {
         let mut state = self.state.lock().unwrap();
 
         state.ensure_unfinished()?;
@@ -100,7 +123,7 @@ impl BackupEnvironment {
             None => bail!("dynamic writer '{}' not registered", wid),
         };
 
-        data.0 += size;
+        data.0 += size as u64;
 
         data.1.add_chunk(data.0, digest)?;
 
diff --git a/src/api2/admin/datastore/backup/upload_chunk.rs b/src/api2/admin/datastore/backup/upload_chunk.rs
index 075623e3..8e7577fb 100644
--- a/src/api2/admin/datastore/backup/upload_chunk.rs
+++ b/src/api2/admin/datastore/backup/upload_chunk.rs
@@ -16,22 +16,22 @@ use super::environment::*;
 pub struct UploadChunk {
     stream: Body,
     store: Arc<DataStore>,
-    size: u64,
+    size: u32,
     chunk: Vec<u8>,
 }
 
 impl UploadChunk {
 
-    pub fn new(stream: Body,  store: Arc<DataStore>, size: u64) -> Self {
+    pub fn new(stream: Body,  store: Arc<DataStore>, size: u32) -> Self {
         Self { stream, store, size, chunk: vec![] }
     }
 }
 
 impl Future for UploadChunk {
-    type Item = ([u8; 32], u64);
+    type Item = ([u8; 32], u32);
     type Error = failure::Error;
 
-    fn poll(&mut self) -> Poll<([u8; 32], u64), failure::Error> {
+    fn poll(&mut self) -> Poll<([u8; 32], u32), failure::Error> {
         loop {
             match try_ready!(self.stream.poll()) {
                 Some(chunk) => {
@@ -77,19 +77,20 @@ fn upload_dynamic_chunk(
     rpcenv: Box<RpcEnvironment>,
 ) -> Result<BoxFut, Error> {
 
-    let size = tools::required_integer_param(&param, "size")?;
+    let size = tools::required_integer_param(&param, "size")? as u32;
     let wid = tools::required_integer_param(&param, "wid")? as usize;
 
 
     let env: &BackupEnvironment = rpcenv.as_ref();
 
-    let upload = UploadChunk::new(req_body, env.datastore.clone(), size as u64);
+    let upload = UploadChunk::new(req_body, env.datastore.clone(), size);
 
     let resp = upload
         .then(move |result| {
             let env: &BackupEnvironment = rpcenv.as_ref();
 
             let result = result.and_then(|(digest, size)| {
+                env.register_chunk(digest, size)?;
                 env.dynamic_writer_append_chunk(wid, size, &digest)?;
                 Ok(json!(tools::digest_to_hex(&digest)))
             });