In all recent version of screen, including 3.9.4 in the 6.1 Errata, and
3.9.5 from the 6.2 beta, if the system is set to use MD5 for passwords,
when a user locks the screen session with Ctrl-a x, screen prompts for a
key instead of using the user's system password.  The following patch
fixes:

--- screen.c.orig       Wed Aug 11 13:13:43 1999
+++ screen.c    Wed Dec  8 09:29:50 1999
@@ -281,7 +281,7 @@
   for (; n < 13; n++)
     {
       char c = ppp->pw_passwd[n];
-      if (!(c == '.' || c == '/' ||
+      if (!(c == '.' || c == '/' || c == '$' ||
            (c >= '0' && c <= '9') ||
            (c >= 'a' && c <= 'z') ||
            (c >= 'A' && c <= 'Z')))