I'm posting this developer info here in response to a query on the xrdp-questions thread.

Should I put this on the wiki maybe?

SCP Protocol

V1 Current Status

As far as I can tell, the current state of affairs with SCP V1 is as follows:-

• SCP V1 appears to be the brainchild of a user ilsimo. This account no longer exists, in that the links to it from the commits below no longer work.
  -SCP V1 was added in commit 078b4d3f4127042b020e78bb9d9762196ff070c3 by ilsimo along with the sestest utility (Nov 2006). The idea seems to have been to add a more capable protocol allowing for password changes, etc. SCP V0 was left 'as is', as SCP V1 is new and needs some work.
• In commit 7c7929861246310d48789748cc150c9a4a492e09 (Sep 2008), a management sub-protocol was added to SCP V1 along with sesadmin, providing a list function and a kill function (not plumbed in).
• The last commit I can find to the project from ilsimo was 1cae42594b7d3958a6c92a201086afcbbe065fda. This was also on the same day as the previous commit but about half-an-hour later.
• There's one pretty serious problem I've found with SCP V1 (which I've shared with metalefty).

It will also be obvious to the reader that SCP V1 is pretty code-heavy and hence has a high maintenance overhead. For example, there's no need for the protocol to handle password changes. The protocol should simply be wrapping the PAM dialog and xrdp should be prompting the user as appropriate. This would also allow for proper 2FA to be introduced with barely any changes to our code.

Other badly needed system functions are reconnection and NLA. Both of these will require a dialog between xrdp and sesman. Having a simpler protocol will make it easier to get these implemented.

Next step

My current plan is to remove the unused SCP V1 code in the near future (i.e. when we get #1708 merged). This should make the existing libscp a little easier to navigate.

Future requirements

A replacement protocol needs to satisfy the following requirements:-

• Merge V0 and V1 functionality into one protocol
• Not do more than it needs to at the protocol level.
• Be simple to understand and maintain - this is critical for getting more developers involved in this area of XRDP.
• Support local authentication for management functions. An administrator should be able to disconnect or stop sessions without needing to reauthenticate.