anomy-list

Re: The QP encoding issue - again!

From: Bjarni R. Einarsson (102013@xyz.molar.is)
Date: Mon 17 Nov 2003 - 17:51:16 GMT

  • Next message: Dustin Puryear: "Re: The QP encoding issue - again!"

    Hi all,

    I'm sorry I haven't been able to participate in the QP thread going
    on recently - I had a business trip abroad last week and had very
    limited access to email during the trip - and for about a month
    before it I was swamped with preperations.

    I wish I had spoken up sooner though, since I probably could have
    saved y'all some time and effort - because right before getting
    swamped with other work I did quite a bit of hacking on this very
    problem.

    First of all - because of ambiguities in the QP spec, there is no
    algorithm for decoding/encoding which will always result in the
    output being identical to the input, short of simply storing the
    entire input and using it verbatim. No matter how you tweak the
    newline handling of Anomy, you're never going to get it right!

    My strategy was basically to emulate the Windows CRLF conventions
    during rewriting, since all the broken clients I know of which are
    encoding binaries using QP are on Windows. (For non-binaries what
    CRLF convention Anomy uses internally doesn't matter). I have
    working code which does this, and it does seem to help.

    This would still result in corruption of binary files QP encoded
    using Mac or Unix newline conventions, but such files should be far
    rarer and this behavior should cause much fewer problems. The
    current Anomy release uses unix-newlines internally, because that's
    what the CPAN perl module does. This not only causes binary
    corruption, but is has security implications as well, which is why
    I'm working on changing the behavior.

    Hopefully I should be able to release a new version of Anomy
    sometime this week for you to test.

    Regarding Dave Cridland's earlier message, that Anomy is buggy and
    shouldn't be decoding/recoding attachments which it isn't going to
    modify... well, that would be true if Anomy knew ahead of time if it
    was going to modify a file. But the fact is, that it doesn't always
    know this ahead of time and since the program is expected to process
    it's input as a stream (not a file it can seek back and forth in)
    there's not much which can be done about this.

    For those of us who are virus-scanning all attachments, then we
    don't know whether files are clean or not until they've been decoded
    and scanned - and in order to avoid the QP problem in such cases, it
    would be necessary to store the encoded attachment on disk somewhere
    as well, and re-inject that into the processing stream instead of
    re-encoding.

    Considering the additional I/O involved in doing that, I've decided
    that isn't worth the effort - it costs about the same in
    performance, is far simpler and at nearly as effective to simply
    store the entire undecoded message and resend it completely
    unmodified if Anomy doesn't make any "critical" changes.

    This strategy however is best done at the next level up (by the
    tools which invoke Anomy), not within the Anomy code itself.

    So... the problem is probably never going to be solved completely
    within Anomy. However, I really do expect the fixes I mentioned
    above to make a big difference, so please help me test them as soon
    as I have time to get them released. :-)

    -- 
    Bjarni R. Einarsson                           PGP: 02764305, B7A3AB89
     102013@xyz.molar.is                -><-              http://bre.klaki.net/
    

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