RE: WebFonts WG discussions

Hi Erik,

A short answer is - it would not affect this ability at all.
The long answer: the proposed checksum will be calculated based on the WOFF data including Header, Table Directory and extended and private metadata fields, and will be set to equal the difference between the calculated value and the checkSumAdjustment of the original font encapsulated in the WOFF as a payload. In order for UA to verify the integrity of WOFF data it will need to calculate the checksum for WOFF data and match its value against the 'checkSumAdjustment' field of downloaded 'head' table (this is the only table UA would have to download, but it would have to do it anyway, and the table size is small). Once UA downloaded the 'head' table and verified the integrity of WOFF data, the UA can proceed and download any desired selection of specific tables, the same way that you could do it now.

Hope this explanation helps.

Regards,
Vlad


From: Erik van Blokland [mailto:letterror@gmail.com] On Behalf Of Erik van Blokland
Sent: Tuesday, May 04, 2010 4:01 AM
To: Levantovsky, Vladimir
Cc: www-font@w3.org
Subject: Re: WebFonts WG discussions

Vlad,

On 3 mei 2010, at 17:40, Levantovsky, Vladimir wrote:


This approach would allow user agents to verify the integrity of WOFF data fields and match it against the font 'checkSumAdjustment' encoded as part of the font 'head' table [3]. The suggested approach would *not* require decompressing the whole font to calculate checksum, and  would preserve the ability of UA to selectively decompress font tables to only retrieve portions of font data as needed.

How would this checksum affect the ability of the UA to download only specific tables?

Erik