You can use a Custom Function (if you have FileMaker Pro Advanced) to scramble an entry. See
But there is a free plug-in which can do encryption. ScriptMaster from 360Works. There is an Encryption example in the included FileMaker file.
It is very fast. I did about 3000 credit cards in about a second (new computer :-). So doing one at a time should be transparent. Be sure to have a backup however, while learning.