DSIG - Digital Signature Table

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:


Type Name Description
uint32 version Version number of the DSIG table (0x00000001)
uint16 numSignatures Number of signatures in the table
uint16 flags permission flags
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.


SignatureRecord

Type Name Description
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

Type Name Description
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.
uint8[] signature PKCS#7 packet

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:

  1. If there is an existing DSIG table in the font,
    1. Remove DSIG table from font.
    2. Remove DSIG table entry from sfnt Table Directory.
    3. Adjust table offsets as necessary.
    4. Zero out the file checksum in the head table.
    5. Add the usFlag (reserved, set at 1 for now) to the stream of bytes
  2. Hash the full stream of bytes using a secure one-way hash (such as MD5) to create the content digest.
  3. Create the PKCS#7 signature block using the content digest.
  4. Create a new DSIG table containing the signature block.
  5. Add the DSIG table to the font, adjusting table offsets as necessary.
  6. Add a DSIG table entry to the sfnt Table Directory.
  7. Recalculate the checksum in the head table.

Prior to signing a font file, ensure that all the following attributes are true.

Signatures for TrueType Collections

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.

© 2017 Microsoft Corporation. All rights reserved. Terms of use.

Comments to the MST group: how to contact us.