Hi,
If you want SSL based authentication, you can go for FTPS as explained in
FTPS on PI for Sender & Receiver
SAP Library - Configuring the File/FTP Adapter in Integration Directory/PCK
If you want SSH based authentication, you should go for SFTP which is provided by SAP now and also recommended
How To Configure SFTP Adapter in SAP PI
SSH is more secured than SSL and you may add PGP encryption with this to add more security
PGP encryption guide: PGPEncryption Module: A Simple How to Guide
AS2 adapter basically for EDI scenarios and you need to install B2B addon to use.
Suman