Letters can be sealed (put in an envelope) then unsealed.
How:
Existing letters and new letters can remain as-is, people can choose to seal or not.
Verb: >seal letter
"You slide the letter into a thin paper envelope and lick the adhesive edge, sealing it shut."
When the letter is sealed, the contents cannot be read.
A letter can be unsealed:
Verb: >unseal letter
"You tear open the envelope, destroying the seal and extracting the letter."
When the letter is unsealed, the contents can be read. If you look at the letter when it has been opened you'd be able to easily tell it was tampered.
"The envelope is already torn open.".
