SMSs have never been secret, but are still private
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Secrecy doesn't imply privacy, you can do things secretly, like for example bring weed with you in a secret pocket, that doesn't mean police can't stop you and do a search on you if a dog smells it.
They can't ask you to reveal who you're casually talking on the phone though without a warrant, even if the conversation is not secret.
You can use encryption to enforce secrecy, that's true, but not privacy.
If you are constantly messaging with a known drug dealer or someone wiretapped, even if the conversation is secret, your right to privacy goes out the window.
Privacy means you close the door when you go to the bathroom, secrecy means you don't tell anybody where are you going and why.
It’s a little moot if she doesn’t.