bugtraq - Re: base64

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Date: Wed, 24 Sep 2003 13:01:21 -0400 (EDT)
From: Seth Breidbart <sethb@...ix.com>
To: BugTraq@...urityfocus.com
Subject: Re: base64

> See the corresponding RFC. The number of characters in a base64-coded
> text must be a multiply of 4. So ='s are used if there aren't enough
> characters and are added at the end of the text.
>
> = is not a valid character inside Base64 and an encoder should stop
> with an error or stops decoding.

That depends on the purpose of the decoder.

If your anti-virus decoder stops there, but a virus writer knows that
the decoder used by some popular mail client keeps going, then your
program isn't going to detect his virus.

If you're looking for something that might be hidden, then you need to
decode in _every_ way that _any_ mail client (that you support) does,
lest you have false negatives.

Seth