Microsoft Catalog Files and Digital Signatures decoded

TL;DR: Parse and print .cat files: parsemscat

Introduction

Günther Deschner and myself are looking into the new Microsoft Printing Protocol [MS-PAR]. Printing always means you have to deal with drivers. Microsoft package-aware v3 print drivers and v4 print drivers contain Microsoft Catalog files.

A Catalog file (.cat) is a digitally-signed file. To be more precise it is a PKCS7 certificate with embedded data. Before I started to look into the problem understanding them I’ve searched the web, if someone already decoded them. I found a post by Richard Hughes: Building a better catalog file. Richard described some of the things we already discovered and some new details. It looks like he gave up when it came down to understand the embedded data and write an ASN.1 description for it. I started to decode the myth of Catalog files the last two weeks and created a tool for parsing them and printing what they contain, in human readable form.

Details

The embedded data in the PKCS7 signature of a Microsoft Catalog is a Certificate Trust List (CTL). Nikos Mavrogiannopoulos taught me ASN.1 and helped to create an ASN.1 description for the CTL. With this description I was able to start parsing Catalog files.

CATALOG {}
DEFINITIONS IMPLICIT TAGS ::=

BEGIN

-- CATALOG_NAME_VALUE
CatalogNameValue ::= SEQUENCE {
    name       BMPString, -- UCS2-BE
    flags      INTEGER,
    value      OCTET STRING -- UCS2-LE
}

...

END

mscat.asn

The PKCS7 part of the .cat-file is the signature for the CTL. Nikos implemented support to get the embedded raw data from the PKCS7 Signature with GnuTLS. It is also possible to verify the signature using GnuTLS now!
The CTL includes members and attributes. A member holds information about file name included in the driver package, OS attributes and often a hash for the content of the file name, either SHA1 or SHA256. I’ve written abstracted function so it is possible to create a library and a simple command line tool called dumpmscat.

Here is an example of the output:

CATALOG MEMBER COUNT=1
CATALOG MEMBER
  CHECKSUM: E5221540DC4B974F54DB4E390BFF4132399C8037

  FILE: sambap1000.inf, FLAGS=0x10010001
  OSATTR: 2:6.0,2:6.1,2:6.4, FLAGS=0x10010001
  MAC: SHA1, DIGEST: E5221540DC4B974F54DB4E39BFF4132399C8037

In addition the CTL has normally a list of attributes. In those attributes are normally OS Flags, Version information and Hardware IDs.

CATALOG ATTRIBUTE COUNT=2
  NAME=OS, FLAGS=0x10010001, VALUE=VistaX86,7X86,10X86
  NAME=HWID1, FLAGS=0x10010001, VALUE=usb\\vid_0ff0&pid_ff00&mi_01

Currently the projects only has a command line tool called: dumpmscat. And it can only print the CTL for now. I plan to add options to verify the signature, dump only parts etc. When this is done I will create a library so it can easily be consumed by other software. If someone is interested and wants to contribute. Something like signtool.exe would be nice to have.

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