Its a few years old now but this should work: https://danielorodriguez.com/TW5-EncryptTiddlerPlugin/
This https://dynalist.io/d/zUP-nIWu2FFoXH-oM7L7d9DM is a useful list of bit and pieces for Tiddlywiki that you may find useful Ben -- You received this message because you are subscribed to the Google Groups "TiddlyWiki" group. To unsubscribe from this group and stop receiving emails from it, send an email to tiddlywiki+unsubscr...@googlegroups.com. To post to this group, send email to email@example.com. Visit this group at https://groups.google.com/group/tiddlywiki. To view this discussion on the web visit https://groups.google.com/d/msgid/tiddlywiki/51e3c6a9-43d0-4061-8bc6-52897b0cf4e1%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.