filtermail: allow partial body length in OpenPGP payloads

CHANGELOG.md

@@ -8,6 +8,9 @@
 - Add config value after how many days large files are deleted
   ([#555](https://github.com/chatmail/relay/pull/555))
 
+- filtermail: allow partial body length in OpenPGP payloads
+  ([#570](https://github.com/chatmail/relay/pull/570))
+
 ## 1.6.0 2025-04-11
 
 - Handle Port-25 connect errors more gracefully (common with VPNs)

chatmaild/src/chatmaild/filtermail.py

@@ -38,6 +38,12 @@ def check_openpgp_payload(payload: bytes):
 
         packet_type_id = payload[i] & 0x3F
         i += 1
+
+        while payload[i] >= 224 and payload[i] < 255:
+            # Partial body length.
+            partial_length = 1 << (payload[i] & 0x1F)
+            i += 1 + partial_length
+
         if payload[i] < 192:
             # One-octet length.
             body_len = payload[i]
@@ -56,7 +62,7 @@ def check_openpgp_payload(payload: bytes):
             )
             i += 5
         else:
-            # Partial body length is not allowed.
+            # Impossible, partial body length was processed above.
             return False
 
         i += body_len

chatmaild/src/chatmaild/tests/test_filtermail.py

@@ -304,3 +304,45 @@ HELLOWORLD
 \r
 """
     assert check_armored_payload(payload) == False
+
+    # Test payload using partial body length
+    # as generated by GopenPGP.
+    payload = """-----BEGIN PGP MESSAGE-----\r
+\r
+wV4DdCVjRfOT3TQSAQdAY5+pjT6mlCxPGdR3be4w7oJJRUGIPI/Vnh+mJxGSm34w\r
+LNlVc89S1g22uQYFif2sUJsQWbpoHpNkuWpkSgOaHmNvrZiY/YU5iv+cZ3LbmtUG\r
+0uoBisSHh9O1c+5sYZSbrvYZ1NOwlD7Fv/U5/Mw4E5+CjxfdgNGp5o3DDddzPK78\r
+jseDhdSXxnaiIJC93hxNX6R1RPt3G2gukyzx69wciPQShcF8zf3W3o75Ed7B8etV\r
+QEeB16xzdFhKa9JxdjTu3osgCs21IO7wpcFkjc7nZzlW6jPnELJJaNmv4yOOCjMp\r
+6YAkaN/BkL+jHTznHDuDsT5ilnTXpwHDU1Cm9PIx/KFcNCQnIB+2DcdIHPHUH1ci\r
+jvqoeXAVWjKXEjS7PqPFuP/xGbrWG2ugs+toXJOKbgRkExvKs1dwPFKrgghvCVbW\r
+AcKejQKAPArLwpkA7aD875TZQShvGt74fNs45XBlGOYOnNOAJ1KAmzrXLIDViyyB\r
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+2nVgCi/QVnxZyEc9OYt/8AE3g4cdojE+vsSDifZLSWYIetpfrohHv3dT3StD1QRG\r
+0QE6qq6oKpg/IL0cjvuX4c7a7bslv2fXp8t75y37RU6253qdIebhxc/cRhPbc/yu\r
+p0YLyD4SrvKTLP2ZV95jT4IPEpqm4AN3QmiOzdtqR2gLyb62L8QfqI/FdwsIiRiM\r
+hqydwoqt/lfSqG1WKPh+6EkMkH+TDiCC1BQdbN1MNcyUtcjb35PR2c8Ld2TF3guA\r
+jLIqMt/Vb7hBoMb2FcsOYY25ka9oV62OwgKWLXnFzk+modMR5fzb4kxVVAYEqP+D\r
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++cEM3V8NFnSEpC12xA+RX/CezuJtwXZK5MpG76eYqMO6qyC+c25YcFecEufDZDxx\r
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+6x2tDK2BHeYl182gCbhlKISRFwCtbjqZSkiKWao/VtygHkw0fK34avJuyQ/X9YaN\r
+BRy+7Lf3VA53pnB5WJ1xwRXN8VDvmZeXzv2krHveCMemj0OjnRoCLu117xN0A5m9\r
+Fm/RoDix5PolDHtWTtr2m1n2hp2LHnj8at9lFEd0SKhAYHVL9KjzycwWODZRXt+x\r
+zGDDuooEeTvdY5NLyKcl4gETz1ZP4Ez5jGGjhPSwSpq1mU7UaJ9ZXXdr4KHyifW6\r
+ggNzNsGhXTap7IWZpTtqXABydfiBshmH2NjqtNDwBweJVSgP10+r0WhMWlaZs6xl\r
+V3o5yskJt6GlkwpJxZrTvN6Tiww/eW7HFV6NGf7IRSWY5tJc/iA7/92tOmkdvJ1q\r
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+/zHEkYZSTKpVSvAIGu4=\r
+=6iHb\r
+-----END PGP MESSAGE-----\r
+"""
+    assert check_armored_payload(payload) == True