A Tale of Two File Names

May 17, 2026 · 3:29 AM UTC ·16 min read · 0 reactions · 0 comments · 25 views

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Tomgalvin

#windows #file-system #legacy-software #reverse-engineering #checksum

TL;DR · WeSearch summary

The article explains the legacy 8.3 filename system in Windows, a holdover from DOS and early Windows that shortens long filenames using tildes and checksums to maintain compatibility. It details how filenames are truncated and transformed, including the use of an undocumented checksum function when naming conflicts occur. The author reverse-engineers the Windows API using debugging tools to uncover how the system generates these short filenames.

Key facts

▪8.3 filenames are a legacy feature from DOS and early Windows, limiting filenames to 8 characters and extensions to 3.
▪Special characters are replaced, filenames are upper-cased, and tildes with numbers (e.g., ~1) are used to resolve naming conflicts.
▪When ~5 would be reached, a hexadecimal checksum replaces the number, such as in SOBC84~1.ASP.
▪The Windows API function GetShortPathName retrieves precomputed short filenames rather than generating them on the fly.
▪The checksum algorithm used for 8.3 filenames is undocumented and appears to be a custom Microsoft implementation.

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Tomgalvin

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Record

Original publisher	Tomgalvin
Canonical URL	https://tomgalvin.uk/blog/gen/2015/06/09/filenames/
Publication time	Sun, 17 May 2026 03:29:01 +0000
Retrieval time	2026-05-17T04:03:58.374Z
Last seen	2026-05-17T04:04:02.390Z
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Excerpt method	First ~120 words (~800 chars) of extracted publisher body, fair-use limited.
Summary	WeSearch · cerebras-chat (WeSearch summarizer)
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Publisher visit	Yes — open original
Substitutes article?	No — link-out required for full text

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Indexing	May the item be indexed (stored, ranked, made findable)?	Allowed
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Opening excerpt (first ~120 words) tap to expand

Users of DOS or older versions of Windows will have invariably stumbled upon a quirk of Windows’ handling of file names at some point. File names which are longer than 8 characters, have an extension other than 3 character long, or aren’t upper-case and alphanumeric, are (in some situations) truncated to an ugly shorter version which contains a tilde (~) in it somewhere. For example, 5+6 June Report.doc will be turned into 5_6JUN~1.DOC. This is relic of the limitations brought about by older versions of FAT used in DOS and older versions of pre-NT Windows. In case you aren’t aware of how 8.3 file names work, here’s a quick run-down. All periods other than the one separating the filename from the extension are dropped - a.testing.file.bat turns into atestingfile.bat.

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