bugfix: attribute error on exception object + more 7z header tests

diff --git a/releases/Next-ChangeLog.md b/releases/Next-ChangeLog.md
index 6d53202..e1de2b3 100644
--- a/releases/Next-ChangeLog.md
+++ b/releases/Next-ChangeLog.md
@@ -11,6 +11,7 @@ This lists the new changes that have not yet been published in a normal release.
 - Bugfix: "Send Password" menu item inside Notes & Passwords visibility reversed
 - Bugfix: Yikes when using "Send Password" on entry with password None field
 - Bugfix: Do not show "Saving..." UX after failed Notes & Passwords import
+- Bugfix: Incorrect error message caused by error in Verify/Decrypt Backup
 
 # Mk Specific Changes
 
diff --git a/shared/compat7z.py b/shared/compat7z.py
index 55b6b88..4512405 100644
--- a/shared/compat7z.py
+++ b/shared/compat7z.py
@@ -51,9 +51,7 @@ def decode_utf_16_le(s):
 '''
 
 def read_var64(f):
-    '''
-        Decode their silly 64-bit encoding.
-    '''
+    # Decode their silly 64-bit encoding.
     first = ord(f.read(1))
     if first < 128:
         return first
@@ -113,22 +111,21 @@ def check_file_headers(f):
     if sh.size > 10000:
         raise ValueError("Second header too big")
 
-    # capture this spot
-    # TODO 'data_start' unused
-    data_start = f.tell()       # expect 0x20
+    # FileHeader.read() always reads exactly calcsize('<6sBBL') = 12 bytes
+    # SectionHeader.read() always reads exactly calcsize('<QQL') = 20 bytes
+    # after those two calls, f.tell() is always start_pos + 32
+    # assert f.tell() == 0x20       # expect 0x20
 
     try:
         f.seek(sh.offset, 1)
         th = f.read(sh.size)
-        if len(th) != sh.size:
-            raise IndexError("Truncated file?")
+        assert len(th) == sh.size, "Truncated file?"
 
         # Look for properties about compression. this could be 
         # faked-out but good enough for now
-        if b'\x24\x06\xf1\x07\x01' not in th:
-            raise RuntimeError("Not marked as AES+SHA encrypted?")
+        assert b'\x24\x06\xf1\x07\x01' in th, "Not marked as AES+SHA encrypted?"
     except Exception as e:
-        raise ValueError("Confused file? %s" % e.message)
+        raise ValueError("Confused file? %s" % e)
 
     if masked_crc(th) != sh.crc:
         raise ValueError("Trailing header has wrong CRC")
@@ -174,7 +171,6 @@ class FileHeader(object):
 
     def actual_crc(self):
         return masked_crc(self.bits)
-
         
 
 class SectionHeader(namedtuple('SectionHeader', ['offset', 'size', 'crc' ])):
@@ -213,6 +209,7 @@ class SectionHeader(namedtuple('SectionHeader', ['offset', 'size', 'crc' ])):
     def actual_crc(self):
         return masked_crc(self.bits)
 
+
 class Builder(object):
     def __init__(self, password=None, salt_len=16, iv_len=16, rounds_pow=13, progress_fcn=None):
         self.rounds_pow = rounds_pow            # standard is 19, 16 and 17 work fine
diff --git a/testing/test_backup.py b/testing/test_backup.py
index 21062b2..98e7992 100644
--- a/testing/test_backup.py
+++ b/testing/test_backup.py
@@ -892,4 +892,91 @@ def test_header_magic_check(microsd_path, src_root_dir, verify_backup_file, cap_
     title, story = cap_story()
     assert "Bad magic bytes" in story
 
+
+def test_confused_file_check(microsd_path, src_root_dir, verify_backup_file, cap_story):
+    fname = "backup.7z"
+    fn = microsd_path(fname)
+
+    with open(f'{src_root_dir}/docs/backup.7z', "rb") as f:
+        conts = f.read()
+
+    # truncate last bytes so trailing header read returns fewer bytes than sh.size
+    with open(fn, "wb") as f:
+        f.write(conts[:-10])
+
+    with pytest.raises(AssertionError):
+        verify_backup_file(fname)
+
+    title, story = cap_story()
+    assert "Confused file?" in story
+    assert "Truncated file?" in story
+
+    # remove AES+SHA marker so "not marked" assertion fires
+    marker = b'\x24\x06\xf1\x07\x01'
+    assert marker in conts
+    corrupted = conts.replace(marker, b'\x00\x00\x00\x00\x00', 1)
+    with open(fn, "wb") as f:
+        f.write(corrupted)
+
+    with pytest.raises(AssertionError):
+        verify_backup_file(fname)
+
+    title, story = cap_story()
+    assert "Confused file?" in story
+    assert "Not marked as AES+SHA encrypted?" in story
+
+
+def test_check_file_headers_errors(microsd_path, src_root_dir, verify_backup_file, cap_story):
+    from binascii import crc32 as host_crc32
+
+    fname = "backup.7z"
+    fn = microsd_path(fname)
+
+    with open(f'{src_root_dir}/docs/backup.7z', "rb") as f:
+        conts = f.read()
+
+    # Flip a byte in SectionHeader (file bytes 12-31); fh.crc stays the same
+    # but sh.actual_crc() changes --> mismatch --> error
+    corrupted = bytearray(conts)
+    corrupted[12] ^= 0xFF
+    with open(fn, "wb") as f:
+        f.write(bytes(corrupted))
+
+    with pytest.raises(AssertionError):
+        verify_backup_file(fname)
+
+    title, story = cap_story()
+    assert "Second header has wrong CRC" in story
+
+    # Set sh.size (offset 8 in SectionHeader = file bytes 20-27) to > 10000,
+    # then update fh.crc (file bytes 8-11) so the CRC check passes first.
+    fh_bytes = bytearray(conts[:12])
+    sh_bytes = bytearray(conts[12:32])
+    struct.pack_into('<Q', sh_bytes, 8, 99999)          # size field at offset 8 in SectionHeader
+    new_crc = host_crc32(bytes(sh_bytes)) & 0xFFFFFFFF
+    struct.pack_into('<L', fh_bytes, 8, new_crc)        # fh.crc at offset 8 in FileHeader
+    with open(fn, "wb") as f:
+        f.write(bytes(fh_bytes) + bytes(sh_bytes) + conts[32:])
+
+    with pytest.raises(AssertionError):
+        verify_backup_file(fname)
+
+    title, story = cap_story()
+    assert "Second header too big" in story
+
+    # Flip the last byte of the trailing header data
+    sh_offset_val, sh_size_val = struct.unpack_from('<QQ', conts, 12)
+    th_start = 0x20 + sh_offset_val
+    th_end = th_start + sh_size_val
+    corrupted = bytearray(conts)
+    corrupted[th_end - 1] ^= 0xFF
+    with open(fn, "wb") as f:
+        f.write(bytes(corrupted))
+
+    with pytest.raises(AssertionError):
+        verify_backup_file(fname)
+
+    title, story = cap_story()
+    assert "Trailing header has wrong CRC" in story
+
 # EOF