be clear about what buzhash is used for, fixes #2390

and want it is not used for (deduplication).

also say already in the readme that we use a cryptohash
for dedupe, so people don't worry.

(cherry picked from commit bf69b049e9dca6d66e5a30b7addf70aea86510e8)

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.

docs/internals.rst

@@ -96,6 +96,8 @@ The id_hash function is:
 * sha256 (no encryption keys available)
 * hmac-sha256 (encryption keys available)
 
+As the id / key is used for deduplication, id_hash must be a cryptographically
+strong hash or MAC.
 
 Segments and archives
 ---------------------
@@ -233,6 +235,11 @@ 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: