The U.S. Army M-138 Strip Cipher

Whole-channel inclusion: Cryptography for Everybody is an honest teaching channel (title sample + 3 transcripts read: The transcripts show structured educational lectures on malware analysis and historical cryptanalysis, consistent with the channel's academic teaching focus.); this title passed a strict title check (sonnet) after a labelling pass.

A 23-minute video from Cryptography for Everybody: “The U.S. Army M-138 Strip Cipher”.

Video by Cryptography for Everybody, played through YouTube's embedded player.