diff --git a/src/borg/repoobj.py b/src/borg/repoobj.py
index ad8e52d6f..f2a57f95a 100644
--- a/src/borg/repoobj.py
+++ b/src/borg/repoobj.py
@@ -13,11 +13,14 @@ AUTHENTICATED_NO_KEY = "authenticated_no_key" in workarounds
 OBJ_MAGIC = b"BORG_OBJ"
 OBJ_VERSION = 0x01
 
+# Fixed header size per blob: OBJ_MAGIC(8) + version(1) + chunk_id(32) + meta_size(4) + data_size(4)
+REPOOBJ_HEADER_SIZE = 49
+
 
 class RepoObj:
-    # Object header: magic (8b), format version (1b), meta size (4b), data size (4b).
-    obj_header = Struct("<8sBII")
-    ObjHeader = namedtuple("ObjHeader", "magic version meta_size data_size")
+    # Object header: magic (8b), format version (1b), chunk_id (32b), meta size (4b), data size (4b).
+    obj_header = Struct("<8sB32sII")
+    ObjHeader = namedtuple("ObjHeader", "magic version chunk_id meta_size data_size")
 
     @classmethod
     def extract_crypted_data(cls, data: bytes) -> bytes:
@@ -72,7 +75,7 @@ class RepoObj:
         data_encrypted = self.key.encrypt(id, data_compressed)
         meta_packed = msgpack.packb(meta)
         meta_encrypted = self.key.encrypt(id, meta_packed)
-        hdr = self.ObjHeader(OBJ_MAGIC, OBJ_VERSION, len(meta_encrypted), len(data_encrypted))
+        hdr = self.ObjHeader(OBJ_MAGIC, OBJ_VERSION, id, len(meta_encrypted), len(data_encrypted))
         hdr_packed = self.obj_header.pack(*hdr)
         return hdr_packed + meta_encrypted + data_encrypted
 
diff --git a/src/borg/testsuite/repository_test.py b/src/borg/testsuite/repository_test.py
index 5e15ec794..becdb3635 100644
--- a/src/borg/testsuite/repository_test.py
+++ b/src/borg/testsuite/repository_test.py
@@ -53,9 +53,9 @@ def reopen(repository, exclusive: bool | None = True, create=False):
     )
 
 
-def fchunk(data, meta=b""):
+def fchunk(data, meta=b"", chunk_id=b"\x00" * 32):
     # Format chunk: create a raw chunk that has a valid RepoObj layout, but does not use encryption or compression.
-    hdr = RepoObj.obj_header.pack(OBJ_MAGIC, OBJ_VERSION, len(meta), len(data))
+    hdr = RepoObj.obj_header.pack(OBJ_MAGIC, OBJ_VERSION, chunk_id, len(meta), len(data))
     assert isinstance(data, bytes)
     chunk = hdr + meta + data
     return chunk
@@ -65,7 +65,7 @@ def pchunk(chunk):
     # Parse chunk: extract data and metadata from a raw chunk made by fchunk.
     hdr_size = RepoObj.obj_header.size
     hdr = chunk[:hdr_size]
-    meta_size, data_size = RepoObj.obj_header.unpack(hdr)[2:4]
+    meta_size, data_size = RepoObj.obj_header.unpack(hdr)[3:5]
     meta = chunk[hdr_size : hdr_size + meta_size]
     data = chunk[hdr_size + meta_size : hdr_size + meta_size + data_size]
     return data, meta
@@ -97,7 +97,7 @@ def test_basic_operations(repo_fixtures, request):
 def test_read_data(repo_fixtures, request):
     with get_repository_from_fixture(repo_fixtures, request) as repository:
         meta, data = b"meta", b"data"
-        hdr = RepoObj.obj_header.pack(OBJ_MAGIC, OBJ_VERSION, len(meta), len(data))
+        hdr = RepoObj.obj_header.pack(OBJ_MAGIC, OBJ_VERSION, H(0), len(meta), len(data))
         chunk_complete = hdr + meta + data
         chunk_short = hdr + meta
         repository.put(H(0), chunk_complete)