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}.
+
\H{config-ssh-kex} The Kex panel
The Kex panel (short for \q{\i{key exchange}}) allows you to configure
PuTTY currently supports the following algorithms:
+\b \i{ChaCha20-Poly1305}, a combined cipher and \i{MAC} (SSH-2 only)
+
\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)