Follow GPG implementation for subpacket prefix encoding.

Conflicts:
	trezor_agent/gpg/protocol.py

trezor_agent/gpg/protocol.py

@@ -47,17 +47,17 @@ def subpacket_byte(subpacket_type, value):
     return subpacket(subpacket_type, '>B', value)
 
 
-def subpacket_prefix_len(blob):
-    """Prefix GPG subpacket with the encoding of it's length."""
-    if len(blob) < 2**8:
-        length_type = 0
-    elif len(blob) < 2**16:
-        length_type = 1
+def subpacket_prefix_len(item):
+    """Prefix subpacket length according to RFC 4880 section-5.2.3.1."""
+    n = len(item)
+    if n >= 8384:
+        prefix = b'\xFF' + struct.pack('>L', n)
+    elif n >= 192:
+        n = n - 192
+        prefix = struct.pack('BB', (n // 256) + 192, n % 256)
     else:
-        length_type = 2
-    fmt = ['>B', '>H', '>L'][length_type]
-    prefix = [b'', b'', struct.pack('>B', 255)][length_type]
-    return prefix + struct.pack(fmt, len(blob)) + blob
+        prefix = struct.pack('B', n)
+    return prefix + item
 
 
 def subpackets(*items):

trezor_agent/gpg/tests/test_decode.py

@@ -5,7 +5,7 @@ import os
 
 import pytest
 
-from .. import decode
+from .. import decode, protocol
 from ... import util
 
 
@@ -14,6 +14,15 @@ def test_subpackets():
     assert decode.parse_subpackets(util.Reader(s)) == [b'\xAB\xCD', b'\xEF']
 
 
+def test_subpackets_prefix():
+    for n in [0, 1, 2, 4, 5, 10, 191, 192, 193,
+              255, 256, 257, 8383, 8384, 65530]:
+        item = b'?' * n  # create dummy subpacket
+        prefixed = protocol.subpackets(item)
+        result = decode.parse_subpackets(util.Reader(io.BytesIO(prefixed)))
+        assert [item] == result
+
+
 def test_mpi():
     s = io.BytesIO(b'\x00\x09\x01\x23')
     assert decode.parse_mpi(util.Reader(s)) == 0x123