How to interpret unprintable characters in the timestamp sent by the Lotus Notes POP3 server?

Question

I wrote a program for retrieving mail from POP3 servers. One of its users encounters the following problem...

His mail server when my program connects to it issues a greeting:

+OK Lotus Notes POP3 server version X2.0 ready <PrintableCharacters.UnprintableCharacters>

The part in angle brackets is a so-called challenge for the APOP authorization. The challenge (and all other strings in POP3 protocol) typically only contains ASCII characters. But this time the challenge is composed of two parts - the second one contains non-printable characters. The pattern is the following: first 0x5, then some non-ASCII character. This looks like special encoding introduced by Lotus Notes developers.

When I extract the challenge as-is and use it for APOP authentication the server declines.

After carefully reading all I could find on LMBSC I deducted the following. The 0x5 is a prefix to indicate so-called "Group 5" characters. Each character is in fact a Win-1251 character but is prepended by 0x5.

With that I just stripped away all 0x5's only leaving legal ASCII and non-printable Win-1251 and passed those into usual APOP authentication algorithm. However the server declines.

Is that the right way to decode LMBSC with Win-1251 characters or is that a problem on the server side?

Answer 1

I checked some domino servers (7.x, 8.x), and didn't find a APOP timestamp in the server greeting. Then I Googled and found this: POP3 authentication failure using APOP commands

It seems like the Domino POP server doesn't support APOP, but versions prior to 6.5.5 presents a timestamp in the greeting anyway.