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I appreciate the encryption key is almost impossible to break.
But if server 1 is connecting to server 2. Presumably once the connection is initiated from server 1 to server 2 - a public key must be passed between the two servers thus if one were to intercept the key (is this possible - I don't know) - then the SFTP connection is not secure.
3https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange – gronostaj – 2019-05-15T10:19:30.290
https://enterprisedt.com/security/how-secure-is-sftp/ as well – CaldeiraG – 2019-05-15T10:22:47.347
Intercepting a public key doesn't break security -- that's why it's called a "public" key. Intercepting the corresponding private key would break security, but that's never sent (even in encrypted form). – Gordon Davisson – 2019-05-15T15:39:03.373