refactor: remove the unused GroupIndex.serialize chain

serialize/deserialize had no production caller, and took with them the
per-chunk signature, the ChaCha20 cipher variant and the zstandard
dependency. Key derivations are unchanged. Docs corrected, including
design §4.3's claim that chunks are compressed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 10 insertions, 17 deletions

diff --git a/packages/meshbay-common/src/meshbay_common/webcrypto.py b/packages/meshbay-common/src/meshbay_common/webcrypto.py
index 3bd5ae6..7119e2e 100644
--- a/packages/meshbay-common/src/meshbay_common/webcrypto.py
+++ b/packages/meshbay-common/src/meshbay_common/webcrypto.py
@@ -1,28 +1,21 @@
 """
-MeshBay — AES-256-GCM cipher variant for browser-accessible groups.
+MeshBay — the content cipher: AES-256-GCM, per-chunk keys derived from the GEK.
 
-The ChaCha20-Poly1305 GEK used in MNP (TCP+TLS and QUIC transport)
-is NOT available in the WebCrypto API. For groups whose content must
-be decryptable by a web browser (using SubtleCrypto), an AES-256-GCM
-variant is used instead.
-
-The GEK wrapping (X25519 + HKDF) is identical — only the content
-cipher changes. The hub stores and distributes GEK bundles the same way.
-
-Cipher selection is declared per-group in the hub registry:
-  "cipher": "chacha20-poly1305"  (default, native clients)
-  "cipher": "aes-256-gcm"        (browser-compatible groups)
+AES-GCM because it is what WebCrypto offers, and one cipher serves every client:
+the browser, the desktop client (the same engine) and the Python side here.
 
 Python side (this module):
-  encrypt_chunk_aes / decrypt_chunk_aes
+  chunk_key_aes / encrypt_chunk_aes / decrypt_chunk_aes
 
 JavaScript side (in static/crypto.js):
-  Uses SubtleCrypto.importKey + SubtleCrypto.decrypt with AES-GCM.
+  SubtleCrypto.importKey + SubtleCrypto.decrypt with AES-GCM.
 
-Key derivation for AES variant — same HKDF info string with suffix:
+Key derivation:
   info = b"file:" + file_hash + b":chunk:" + chunk_index + b":aes"
 
-This ensures AES and ChaCha20 keys are always distinct even from the same GEK.
+The `:aes` suffix dates from a ChaCha20-Poly1305 variant derived from the same
+GEK without it, which nothing used and which is gone. It stays: it is part of
+every chunk key in existence, and changing it would change them all.
 """
 
 import os
@@ -33,7 +26,7 @@ from cryptography.hazmat.primitives.kdf.hkdf import HKDF
 
 
 def chunk_key_aes(gek: bytes, file_hash: bytes, chunk_index: int) -> bytes:
-    """Derive a per-chunk AES-256 key. Distinct from ChaCha20 key."""
+    """Derive a per-chunk AES-256 key from the GEK."""
     return HKDF(
         algorithm=hashes.SHA256(), length=32, salt=None,
         info=b"file:" + file_hash + b":chunk:" + chunk_index.to_bytes(4, "big") + b":aes",