diff --git a/README.rst b/README.rst
index 41765b80f..ba7c735f0 100644
--- a/README.rst
+++ b/README.rst
@@ -27,6 +27,10 @@ Main features
   of bytes stored: each file is split into a number of variable length chunks
   and only chunks that have never been seen before are added to the repository.
 
+  A chunk is considered duplicate if its id_hash value is identical.
+  A cryptographically strong hash or MAC function is used as id_hash, e.g.
+  (hmac-)sha256.
+
   To deduplicate, all the chunks in the same repository are considered, no
   matter whether they come from different machines, from previous backups,
   from the same backup or even from the same single file.
diff --git a/docs/internals/data-structures.rst b/docs/internals/data-structures.rst
index a76f14e1b..339338a37 100644
--- a/docs/internals/data-structures.rst
+++ b/docs/internals/data-structures.rst
@@ -69,6 +69,9 @@ Normally the keys are computed like this::
 
 The id_hash function depends on the :ref:`encryption mode <borg_init>`.
 
+As the id / key is used for deduplication, id_hash must be a cryptographically
+strong hash or MAC.
+
 Segments
 ~~~~~~~~
 
@@ -243,6 +246,11 @@ The |project_name| chunker uses a rolling hash computed by the Buzhash_ algorith
 It triggers (chunks) when the last HASH_MASK_BITS bits of the hash are zero,
 producing chunks of 2^HASH_MASK_BITS Bytes on average.
 
+Buzhash is **only** used for cutting the chunks at places defined by the
+content, the buzhash value is **not** used as the deduplication criteria (we
+use a cryptographically strong hash/MAC over the chunk contents for this, the
+id_hash).
+
 ``borg create --chunker-params CHUNK_MIN_EXP,CHUNK_MAX_EXP,HASH_MASK_BITS,HASH_WINDOW_SIZE``
 can be used to tune the chunker parameters, the default is: