gpg

curl https://keybase.io/aerth/key.asc | gpg --import
echo "hello world" >> yourmessage.txt
cat yourmessage.txt | gpg -er aerth@riseup.net > yourmessage.txt.out

# see this
gpg: 3C66B4C6: There is no assurance this key belongs to the named user

pub  4096R/3C66B4C6 2016-05-02 aerth <aerth@riseup.net>
 Primary key fingerprint: 8295 3CFA 6F4F 49D0 2B6A  082F EBC4 61F6 8638 5D5C
      Subkey fingerprint: C08F EA77 F70C 5B37 2D8C  12EF F76A 0E59 3C66 B4C6

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) 


# type y
mv yourmessage.txt.out yourmessage.txt