Hello Dennis,
As to my knowledge, SAP SFTP adapter does not support authentication using shared keys (symmetric), but only supports authentication using public key (asymmetric). Authentication using shared keys is generally considered less secure since in this case, the same key is used by client and server, and compromising any one of them means compromising entire authentication mechanism and access to SFTP server. In case of authentication using public key, a public key is used by an SFTP server, whereas SFTP adapter (which stands for an SFTP client), uses a private key in authentication process (keys are not shared between client and server).
Regards,
Vadim