+Protocol 2 supports both RSA and DSA keys;
+protocol 1 only supports RSA keys.
+For both protocols,
+each host has a host-specific key,
+normally 2048 bits,
+used to identify the host.
+.Pp
+Forward security for protocol 1 is provided through
+an additional server key,
+normally 768 bits,
+generated when the server starts.
+This key is normally regenerated every hour if it has been used, and
+is never stored on disk.
+Whenever a client connects, the daemon responds with its public
+host and server keys.
+The client compares the
+RSA host key against its own database to verify that it has not changed.
+The client then generates a 256-bit random number.
+It encrypts this
+random number using both the host key and the server key, and sends
+the encrypted number to the server.
+Both sides then use this
+random number as a session key which is used to encrypt all further
+communications in the session.
+The rest of the session is encrypted
+using a conventional cipher, currently Blowfish or 3DES, with 3DES
+being used by default.
+The client selects the encryption algorithm
+to use from those offered by the server.
+.Pp
+For protocol 2,
+forward security is provided through a Diffie-Hellman key agreement.
+This key agreement results in a shared session key.
+The rest of the session is encrypted using a symmetric cipher, currently
+128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
+The client selects the encryption algorithm
+to use from those offered by the server.
+Additionally, session integrity is provided
+through a cryptographic message authentication code
+(hmac-sha1 or hmac-md5).
+.Pp
+Finally, the server and the client enter an authentication dialog.
+The client tries to authenticate itself using
+host-based authentication,
+public key authentication,
+challenge-response authentication,
+or password authentication.
+.Pp
+Regardless of the authentication type, the account is checked to
+ensure that it is accessible. An account is not accessible if it is
+locked, listed in
+.Cm DenyUsers
+or its group is listed in
+.Cm DenyGroups
+\&. The definition of a locked account is system dependant. Some platforms
+have their own account database (eg AIX) and some modify the passwd field (
+.Ql \&*LK\&*
+on Solaris and UnixWare,
+.Ql \&*
+on HP-UX, containing
+.Ql Nologin
+on Tru64,
+a leading
+.Ql \&*LOCKED\&*
+on FreeBSD and a leading
+.Ql \&!!
+on Linux). If there is a requirement to disable password authentication
+for the account while allowing still public-key, then the passwd field
+should be set to something other than these values (eg
+.Ql NP
+or
+.Ql \&*NP\&*
+).
+.Pp
+System security is not improved unless
+.Nm rshd ,
+.Nm rlogind ,
+and
+.Nm rexecd
+are disabled (thus completely disabling
+.Xr rlogin
+and
+.Xr rsh
+into the machine).
+.Sh COMMAND EXECUTION AND DATA FORWARDING
+If the client successfully authenticates itself, a dialog for
+preparing the session is entered.
+At this time the client may request
+things like allocating a pseudo-tty, forwarding X11 connections,
+forwarding TCP connections, or forwarding the authentication agent
+connection over the secure channel.
+.Pp
+Finally, the client either requests a shell or execution of a command.
+The sides then enter session mode.
+In this mode, either side may send
+data at any time, and such data is forwarded to/from the shell or
+command on the server side, and the user terminal in the client side.
+.Pp
+When the user program terminates and all forwarded X11 and other
+connections have been closed, the server sends command exit status to
+the client, and both sides exit.