-\S{config-ssh-encryption} \ii{Encryption} algorithm selection
-
-\cfg{winhelp-topic}{ssh.ciphers}
-
-PuTTY supports a variety of different \i{encryption algorithm}s, and
-allows you to choose which one you prefer to use. You can do this by
-dragging the algorithms up and down in the list box (or moving them
-using the Up and Down buttons) to specify a preference order. When
-you make an SSH connection, PuTTY will search down the list from the
-top until it finds an algorithm supported by the server, and then
-use that.
-
-PuTTY currently supports the following algorithms:
-
-\b \i{AES} (Rijndael) - 256, 192, or 128-bit SDCTR or CBC (SSH-2 only)
-
-\b \i{Arcfour} (RC4) - 256 or 128-bit stream cipher (SSH-2 only)
-
-\b \i{Blowfish} - 256-bit SDCTR (SSH-2 only) or 128-bit CBC
-
-\b \ii{Triple-DES} - 168-bit SDCTR (SSH-2 only) or CBC
-
-\b \ii{Single-DES} - 56-bit CBC (see below for SSH-2)
-
-If the algorithm PuTTY finds is below the \q{warn below here} line,
-you will see a warning box when you make the connection:
-
-\c The first cipher supported by the server
-\c is single-DES, which is below the configured
-\c warning threshold.
-\c Do you want to continue with this connection?
-
-This warns you that the first available encryption is not a very
-secure one. Typically you would put the \q{warn below here} line
-between the encryptions you consider secure and the ones you
-consider substandard. By default, PuTTY supplies a preference order
-intended to reflect a reasonable preference in terms of security and
-speed.
-
-In SSH-2, the encryption algorithm is negotiated independently for
-each direction of the connection, although PuTTY does not support
-separate configuration of the preference orders. As a result you may
-get two warnings similar to the one above, possibly with different
-encryptions.
-
-Single-DES is not recommended in the SSH-2 protocol
-standards, but one or two server implementations do support it.
-PuTTY can use single-DES to interoperate with
-these servers if you enable the \q{Enable legacy use of single-DES in
-SSH-2} option; by default this is disabled and PuTTY will stick to
-recommended ciphers.
+You should normally leave this at the default, \q{2 only}. The older
+SSH-1 protocol is no longer developed, has many known cryptographic
+weaknesses, and is generally not considered to be secure. If you
+permit use of SSH-1 by selecting \q{2} instead of \q{2 only}, an
+active attacker can force downgrade to SSH-1 even if the server
+you're connecting to supports SSH-2.
+
+PuTTY's protocol 1 implementation is provided mainly for
+compatibility, and is no longer being enhanced.
+
+\S{config-ssh-sharing} Sharing an SSH connection between PuTTY tools
+
+\cfg{winhelp-topic}{ssh.sharing}
+
+The controls in this box allow you to configure PuTTY to reuse an
+existing SSH connection, where possible.
+
+The SSH-2 protocol permits you to run multiple data channels over the
+same SSH connection, so that you can log in just once (and do the
+expensive encryption setup just once) and then have more than one
+terminal window open.
+
+Each instance of PuTTY can still run at most one terminal session, but
+using the controls in this box, you can configure PuTTY to check if
+another instance of itself has already connected to the target host,
+and if so, share that instance's SSH connection instead of starting a
+separate new one.
+
+To enable this feature, just tick the box \q{Share SSH connections if
+possible}. Then, whenever you start up a PuTTY session connecting to a
+particular host, it will try to reuse an existing SSH connection if
+one is available. For example, selecting \q{Duplicate Session} from
+the system menu will launch another session on the same host, and if
+sharing is enabled then it will reuse the existing SSH connection.
+
+When this mode is in use, the first PuTTY that connected to a given
+server becomes the \q{upstream}, which means that it is the one
+managing the real SSH connection. All subsequent PuTTYs which reuse
+the connection are referred to as \q{downstreams}: they do not connect
+to the real server at all, but instead connect to the upstream PuTTY
+via local inter-process communication methods.
+
+For this system to be activated, \e{both} the upstream and downstream
+instances of PuTTY must have the sharing option enabled.
+
+The upstream PuTTY can therefore not terminate until all its
+downstreams have closed. This is similar to the effect you get with
+port forwarding or X11 forwarding, in which a PuTTY whose terminal
+session has already finished will still remain open so as to keep
+serving forwarded connections.
+
+In case you need to configure this system in more detail, there are
+two additional checkboxes which allow you to specify whether a
+particular PuTTY can act as an upstream or a downstream or both.
+(These boxes only take effect if the main \q{Share SSH connections if
+possible} box is also ticked.) By default both of these boxes are
+ticked, so that multiple PuTTYs started from the same configuration
+will designate one of themselves as the upstream and share a single
+connection; but if for some reason you need a particular PuTTY
+configuration \e{not} to be an upstream (e.g. because you definitely
+need it to close promptly) or not to be a downstream (e.g. because it
+needs to do its own authentication using a special private key) then
+you can untick one or the other of these boxes.
+
+I have referred to \q{PuTTY} throughout the above discussion, but all
+the other PuTTY tools which make SSH connections can use this
+mechanism too. For example, if PSCP or PSFTP loads a configuration
+with sharing enabled, then it can act as a downstream and use an
+existing SSH connection set up by an instance of GUI PuTTY. The one
+special case is that PSCP and PSFTP will \e{never} act as upstreams.
+
+It is possible to test programmatically for the existence of a live
+upstream using Plink. See \k{plink-option-shareexists}.