Re: DRAFT: eXIf 2017-03-09

[Therefore, it begins with a standard TIFF header (starting with
either "II" or "MM", depending upon the byte order used), and contains
a 0th IFD (Image File Directory) and optionally a 1st IFD. The 0th IFD
may contain pointers to an Exif IFD and/or a GPS IFD, and the 1st IFD
(if any) contains a thumbnail image. Any gap preceding an IFD is
filled with bytes of unspecified content.]

This seems unhelpful.   It begs the (security) question of whether the
gap can be detected; so far as I can see tags in the IFDs can point
anywhere and, so far as I can see, unrecognized tags which might point
anywhere (perhaps indirectly) are permitted.

I think this is raising a whole collection of TIFF security issues
which should not be part of a PNG specification.

John Bowler

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