If you're getting this message when encrypting files with a GnuPG—the GNU Pretty Good Privacy (PGP) package—you can mark your key as trusted (if that's the case). This often happens when you copy a trusted key from one machine to another.
Here's what I was getting before 'trusting' my own key.
gpg: <KEY_ID>: There is no assurance this key belongs to the named user It is NOT certain that the key belongs to the person named in the user ID. If you *really* know what you are doing, you may answer the next question with yes. Use this key anyway? (y/N) y
Every time, I'd have to answer
y to complete the encryption process.
I learned how to skip this step from this StackOverflow post.
gpg --edit-key <KEY_ID> gpg › trust
Then you set the degree to which you trust the specified key.
1 = I don't know or won't say 2 = I do NOT trust 3 = I trust marginally 4 = I trust fully 5 = I trust ultimately m = back to the main menu
I selected 5 as this is a key I created for myself.
Your decision? 5 Do you really want to set this key to ultimate trust? (y/N) y
Confirm, then quit.
gpg › quit
Now you can use your GPG key without confirm every operation.