Assuming you want to do the en/de-cryption within SSIS, the simplest method will probably to explore the command line version of whichever encryption software you are using, and execute it using an Execute Process
task.
If you haven't yet selected encryption software, obvious choices include PGP (commercial) and GnuPG (open source).
The other option would be to write .Net code to carry out en/de-cryption in a Script Task
- this would require a .dll from PGP or GnuPG. Various generic .Net examples (not specific to SSIS) are available - like this one - but I can't comment on their completeness or quality.
This SQLServerCentral thread may have some useful pointers.