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.\\
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External links\\
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[https://en.wikipedia.org/wiki/AS2|Wikipedia]\\
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[https://www.ietf.org/rfc/rfc4130.txt|AS2 RFC document]\\
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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.\\