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.

Discussion
There are at least two or three different SDN formats.

PAK-based format
The original format is a PAK file with a special extended record. PAK itself is a variant of ARC.

SDNV format
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-based format
In 1993, it was announced that SDN files would switch to a format based on ARJ, and FSecure by PaceSoft.

ARJ v2.39c (early 1993) simultaneously changed ARJ's native "security envelope" format, and claimed to add "SDN support". This is confusing, because there are two different security features involved here: ARJ security envelopes, and the FSecure security seals associated with SDN. It is not clear what feature the claimed "SDN support" is referring to. ARJ does not do anything with FSecure security seals, though its documentation discusses them.

Software
PAK format:
 * (DOS binary)
 * See also PAK (ARC extension).

SDNV format:
 * SDNV v1.00 (DOS binary)

ARJ format:
 * For extraction, see ARJ.
 * For validation, see FSecure (PaceSoft).

Sample files
PAK format:
 * SDN1 Plus (CD-ROM)
 * SDN1 Plus (CD-ROM)
 * SDN1 Plus (CD-ROM)

ARJ format:
 * MediaShare 8 → misc/storyv22.sdn
 * AOL File Library - 7500 to 7599 series → 7500/Bible Study Programs/Bible Slide Show/ISRAEL20.sdn
 * aol file protocol 4400 5001-to-5044 → AOLDLs/Religion Graphics Library/Kids Color Book/SCOLOR33.sdn