[OpenType] Re: factual error in the DSIG description in the OT spec.

[Levantovsky, Vladimir]

On Thursday, April 21, 2016 7:19 AM Werner LEMBERG wrote:

Message from OpenType list:

> I'd like to ask all interested parties to express their views on the 
> DSIG table functionality and share their experiences (either negative 
> or positive) with using it.

Just wondering: In 

  https://www.microsoft.com/typography/otspec/head.htm

we can read the following for bit 11 of the `flags' field in the `head' table.

  Font data is ‘lossless’ as a results of having been subjected to
  optimizing transformation and/or compression (such as
  e.g. compression mechanisms defined by ISO/IEC 14496-18, MicroType
  Express, WOFF 2.0 or similar) where the original font functionality
  and features are retained but the binary compatibility between input
  and output font files is not guaranteed.  As a result of the applied
  transform, the ‘DSIG’ Table may also be invalidated.

Do I interpret this correctly that the DSIG data gets completely ignored if I set this bit?  Even some original MS fonts like `symbol.ttf' (e.g., version 5.00) have this bit set...

[VL]
In practice, any changes applied to a font file (even if it is simply a table reordering for example) would change the bitstream content (in my example the reordering would change the table offsets) and, therefore, invalidate the signature if it's present. The checksum can be recalculated after a change is made but the original vendor is the only authority who could re-sign the font so yes - my understanding that the DSIG will be made invalid and should be ignored.

However, I am not sure why the original font would ever have 'head' table flag bit 11 set - regardless of the tools used for the creation of the original data the font in question is released "as is", it can still be signed and hasn’t been subjected yet to any modifying transform.

Thank you,
Vlad

    Werner