I don't understand how FTP is insisting for encryption.
But you need to use gpg. It comes with most Linux distros and it is available on sunfreeware for Solaris.
First you need some keys of your own. You can do without, but you will be constantly fighting the program. So do:
pgp --gen-key
It will ask questions...take the defaults where provided and answer the others.
Next import that key you have:
pgp --import somefilecontainingkey
Now you should be able to list that key you imported...
pgp --list-keys
Sign the key you imported to assert that you trust it. This is why you needed to generate your own keys.
pgp --sign-key keyid
There should be an email address associated with that key. Let's say it is joe.blow@sample.com. Also let's say you have a file called secret.txt to send. So do:
gpg --recipient joe.blow@sample.com --encrypt secret.txt
This creates a file called secret.txt.pgp. Now you ftp secret.txt.pgp to the other end. When Joe Blow wants to decrypt the file, he will use:
gpg secret.txt.gpg
And a file called secret.txt will appear in his directory.
I have tried all of these steps between a Linux and a Solaris system and they worked for me. Your mileage may vary depending on your setup.