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PasswordAuthentication
        Specifies whether password authentication is allowed.  The de-
        fault is ``yes''. Note that this option applies to both protocol
        versions 1 and 2.

PermitEmptyPasswords
        When password authentication is allowed, it specifies whether the
        server allows login to accounts with empty password strings.  The
        default is ``no''.
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If PasswordAuthentication is yes, then you'll need to ensure that 
PermitEmptyPasswords is also yes, or you won't be allowed passphraseless 
authentication.
If you're not permitted to change that, just use a simple ssh passphrase.