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Public key cryptography means that the traffic is secured and that if you verify keys correctly, it isn’t vulnerable to man-in-themiddle attacks. Exploits are occasionally found, but they are quickly fixed. However, your system could still be vulnerable to brute-force attacks. If you have only a small number of user accounts, if your usernames are unusual, and if your passwords are carefully crafted, this might not be an issue. But if you have multiple users on a system, it becomes harder to ensure that all passwords really are secure. One solution is to run a passwordguessing system such as John the Ripper, which can discover a poor password before anyone with nefarious intentions has time to exploit it.

Also, you can create firewall rules that establish a maximum number of connection attempts from the same IP address, either indefinitely or for the next few minutes. (The latter option is preferable to avoid problems for real users, who do occasionally mistype their password several times.)

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