The DSIG table contains the digital signature of the OpenType™ font. Signature formats are widely documented and rely on a key pair architecture. Software developers, or publishers posting material on the Internet, create signatures using a private key. Operating systems or applications authenticate the signature using a public key.
The W3C and major software and operating system developers have specified security standards that describe signature formats, specify secure collections of web objects, and recommend authentication architecture. OpenType fonts with signatures will support these standards.
OpenType fonts offer many security features:
The enforcement of signatures is an administrative policy, enabled by the operating system. Windows will soon require installed software components, including fonts, to be signed. Internet browsers will also give users and administrators the ability to screen out unsigned objects obtained on-line, including web pages, fonts, graphics, and software components.
Anyone can obtain identity certificates and encryption keys from a certifying agency, such as Verisign or GTE's Cybertrust, free or at a very low cost.
The DSIG table is organized as follows. The first portion of the table is the header:
|uint32||version||Version number of the DSIG table (0x00000001)|
|uint16||numSignatures||Number of signatures in the table|
Bit 0: cannot be resigned
Bits 1-7: Reserved (Set to 0)
|SignatureRecord||signatureRecords[numSignatures]||Array of signature records|
The version of the DSIG table is expressed as a uint32, beginning at 0. The version of the DSIG table currently used is version 1 (0x00000001).
Permission bit 0 allows a party signing the font to prevent any other parties from also signing the font (counter-signatures). If this bit is set to zero (0) the font may have a signature applied over the existing digital signature(s). A party who wants to ensure that their signature is the last signature can set this bit.
The DSIG header has an array of signature records, which specifying the format and offset of signature blocks.
|uint32||format||Format of the signature|
|uint32||length||Length of signature in bytes|
|Offset32||offset||Offset to the signature block from the beginning of the table|
Signatures are contained in one or more signature blocks. Signature blocks may have various formats; currently one format is defined. The format identifier specifies both the format of the signature block, as well as the hashing algorithm used to create and authenticate the signature.
Signature Block Format 1
|uint16||reserved1||Reserved for future use; set to zero.|
|uint16||reserved2||Reserved for future use; set to zero.|
|uint32||signatureLength||Length (in bytes) of the PKCS#7 packet in the signature field.|
For more information about PKCS#7 signatures, see ftp://ftp.rsa.com/pub/pkcs/ascii/pkcs-7.asc
For more information about counter-signatures, see ftp://ftp.rsa.com/pub/pkcs/ascii/pkcs-9.asc
Format 1: For whole fonts, with either TrueType outlines and/or CFF data
PKCS#7 or PKCS#9. The signed content digest is created as follows:
Prior to signing a font file, ensure that all the following attributes are true.
The DSIG table for a TrueType Collection (TTC) must be the last table in the TTC file. The offset and checksum to the table is put in the TTCHeader (version 2). Signatures of TTC files are expected to be Format 1 signatures.
The signature of a TTC file applies to the entire file, not to the individual fonts contained within the TTC. Signing the TTC file ensures that other contents are not added to the TTC.
Individual fonts included in a TrueType collection should not be individually signed as the process of making the TTC could invalidate the signature on the font.
This page was last updated 8 July 2017.
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