KarpelesLab / authenticode / 25972372437

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Compute the PE optional-header CheckSum

EmbedSignature now writes Microsoft's CheckSumMappedFile value into
the optional-header CheckSum field after appending the WIN_CERTIFICATE
table, so osslsigncode no longer prints "Warning: invalid PE checksum"
on our signed output.

The algorithm is fold-add of every 16-bit LE word (with the four
checksum bytes treated as zero) plus the file length, matching the
value already stored in testdata/hello.exe to the bit. Two unit tests
pin the algorithm: one against the fixture's stored checksum, one for
self-consistency after EmbedSignature. The osslsigncode integration
tests now fail if the warning ever reappears.

README no longer carries the "left as-is" caveat.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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