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Is Telegram really an encrypted messaging app?
(blog.cryptographyengineering.com)
This is a most excellent place for technology news and articles.
No, it's not the rule itself. It's rather an advice not to do as rolling own crypto is very tricky and complicated thing. You have to be very aware of many possible attacks, how they do work, to create own crypto properly
More like "don't roll your own crypto unless you're ready to spend years getting it scrutinized and polished".