How to connect IMAP using AUTHENTICATE PLAIN correctly?

Question

I'm using OpenSSL to connect to mail server.

POP3 works fine but I have problems with IMAP. Based on CAPABILITY command server supports PLAIN, NTLM and GSS-API authentication methods.

I want to use PLAIN because it's easier than others. I have read it's needed to use <NUL> for it.

I have run the next variations, but no success:

? login user pass
? login <nul>user<nul>pass
? <nul>login <nul>user<nul>pass

What am I doing wrong?

Answer 1

None of the previous answers actually said how to use PLAIN authentication, so I did some more digging. It turns out that authentication information is expected in base64. It's probably easiest to explain by example. Assume a username of "bob" and a password of "munchkin".

We'll first need to encode in base64. On a Linux-ish system, it goes likes this:

echo -en "\0bob\0munchkin" | base64

This incorporates the null characters as required, and also does the base64 encoding. We get this string out: AGJvYgBtdW5jaGtpbg==.

Now, we can do the actual authentication (S = Server, C = Client):

S: * OK The Microsoft Exchange IMAP4 service is ready.
C: D0 CAPABILITY
S: * CAPABILITY IMAP4 IMAP4rev1 AUTH=NTLM AUTH=GSSAPI AUTH=PLAIN CHILDREN IDLE NAMESPACE LITERAL+
S: D0 OK CAPABILITY completed.
C: D1 AUTHENTICATE PLAIN
S: +
C: AGJvYgBtdW5jaGtpbg==
S: D1 OK AUTHENTICATE completed

And you're done!