For a long time now, dallas has had a secure micro.
8051 variant, the code in flash is "encrypted", and the die isn't probeable
in any useful sense.

But I suspect it is subject to what I've seen in SRAM cells, where leaving
them in a given state for a few months gives them a semi-permanent "lean"
twoard the data pattern that was there.  We saw this storing encryption keys
in SRAM and came up with a way to store the keys in ring buffers and move
the contents every so often.

It's pretty embarrasing when you pull the power to a module, and it's
supposed to forget the encryption keys, but it dosen't.  :)
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