SDN (SDN Project)
An .SDN file is a compressed archive file associated with the SDN Project organization, and the SDNet/Works! software distribution network. SDN stands for Shareware Distribution Network. SDN files are cryptographically authenticated.
An SDN file might be accompanied by, or might contain, an .SDA "abstract" file. This is apparently just a text file describing the software in the SDN file.
There are at least two or three different SDN formats.
The documentation for the SDNV utility says:
SDN FILES BEFORE JUNE 15TH 1990......................................... Files with a .SDN extension that were packaged before June 15th 1990 were compressed using NoGate Consulting's PAK v2.10 utility. [...] SDN FILES AFTER JUNE 15TH 1990.......................................... All files and programs, released for distribution on SDNet/Works! after June 15th 1990 will be secured and compressed in SDNV format. Other archivers will not be able to read or uncompress these .SDN files. You must use a the SDNV utility SDNV.EXE. [...]
More research is needed, but it seems possible that the plan to switch to SDNV format was abandoned.
ARJ v2.39c (early 1993) simultaneously changed ARJ's native "security envelope" format, and claimed to introduce "SDN support". This is confusing, because these two similar features are not really related (though SDN files might well use both).
And ARJ doesn't actually fully validate SDN-secured files. The claimed "SDN support" appears to be referring to some convenience features related to identifying SDN files as such, and telling the user what to do.
- SDNV v1.00 (DOS binary)
(Wanted: ARJ-SDN samples.)