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AS2 is an electronic document interchange(EDI) standard, using HTTPS secure transport, the successor of AS1 which used email ( SMTP). Still widely used as the major business to business secure document exchange method, unlikely the newer AS3 standard ( sync only, FTP or SFTP based transport) or AS4 (claimed payload format independent, but basically a form of secure XML/SOAP exchange, least popular of all).
CrushFTP 11 supports RFC compliant AS2 as both sender and recipient, and AS3 as recipient only.

External links

Wikipedia

AS2 RFC document


The AS2 protocol itself is very simple, client (sending partner) , signs and encrypts the payload, does basic HTTP authentication against the server ( receiving partner), sends the encrypted payload. The server decrypts the payload if it can, validates the digital signature, validates message integrity(MIC), generates a success response (MDN), sends that back to the client over the same HTTPS connection (sync MDN) or a separate HTTPS connection (async MDN). In this case latter, the receiving partner authenticates back to the sender using HTTP basic authentication.
The payload related cryptography uses x509 digital certificates, basically the same as the ones used in SSL, the certificate extensions must include key and data encipherment and digatal signature ( the usual cert for web servers will do, can be even a self signed cert).

Setting up CrushFTP as the _sending partner_, it's an Enterprise licenses-only feature.
Acting as an AS2 server (receiving partner) is a core feature, available with all license types, only requires a _properly configured user account_ in User Manager and a publicly available HTTPS port.