bugtraq - Re: base64

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Date: Thu, 25 Sep 2003 09:10:04 +0200
From: Christian Vogel <chris@...lix.hedonism.cx>
To: David Wilson <David.Wilson@...de.com>
Cc: Lothar Kimmeringer <bugtraq@...meringer.de>,
	"BugTraq@...urityfocus.com" <BugTraq@...urityfocus.com>
Subject: Re: base64

Hi David,

> RFC 2045 states (section 6.8):
>    data, characters other than those in Table 1, line breaks, and other
>    white space probably indicate a transmission error, about which a
>    warning message or even a message rejection might be appropriate
>    under some circumstances."

A user-agent has to assume that it's message might be dropped if it
creates base64 with junk in it. So it should not create these things
and it's perfectly resaonable for a MTA/virus-scanner to drop those
messages.

>    "Because it is used only for padding at the end of the data, the
>    occurrence of any "=" characters may be taken as evidence that the
>    end of the data has been reached (without truncation in transit).  No
>    such assurance is possible, however, when the number of octets
>    transmitted was a multiple of three and no "=" characters are
>    present."

Again, as the mail-client does not have a way to know how the generated
data is interpreted in those ambigous cases its reasonable to just
drop those messages.

> But there are too many common email user
> agents which generate non-conforming messages.

Is there already a list of broken MUAs? Do the vendors even know
(yes, they should have cought that during testing... ;-) )

> Or should we reject all these broken messages? ;-)

Either reject them or convert them to a canonical form. But that will
generate further problems, e.g. if you modify signed payload that way.

	Chris

-- 
Message passing as the fundamental operation of the OS is just an
excercise in computer science masturbation.  It may feel good, but you
don't actually get anything DONE. -- Linus Torvalds