PKCS#11 Implementation for Linux
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README.md

Chaps: A PKCS #11 Implementation For Linux

This repository is a framework for building and packaging the Chaps PKCS #11 implementation from ChromiumOS on Linux. Chaps provides an alternative to OpenCryptoKi that has been designed to be faster and more maintainable.

This repo does not hold the source code for Chaps; instead, the build files here retrieve the source code from the ChromiumOS open source project, together with the other required source code from the Chromium open source project.

The Linux build of Chaps is still under development, and should be considered to be alpha status.

This is NOT an official Google product.

Installing Chaps

To build and install Chaps on a Debian-based Linux system:

  • Build the Chaps packages:
    • Install the prerequisites: sudo apt-get install debhelper scons protobuf-compiler libdbus-1-dev libdbus-c++-dev libprotobuf-dev libsnappy-dev libleveldb-dev libglib2.0-dev libctemplate-dev libssl-dev libtspi-dev libpam0g-dev
    • Ensure that the TPM is initialized and accessible (see below).
    • Build a source tree (under chaps-<version>) with make src_generate
    • Build the packages with make package
  • Install the Chaps packages:
    • Install the packages with sudo dpkg -i chaps_*_amd64.deb libchaps0_*_amd64.deb
      • This needs some pre-requisite packages to be installed, notably TrouSerS (the key library that allows software access to the hardware TPM).
    • Log out and in again, so that Chaps can create a per-user token.

Package Configuration

The behavior of the Chaps package is influenced by two configuration values:

  • If the CHAPS_SYSTEM_TOKEN variable in /etc/default/chaps is set (the default), the Chaps daemon will load a system-wide token on startup.
  • If the Chaps PAM module is enabled (via pam-auth-update) then per-user tokens will be created when a user logs in.

PKCS#11 and TPMs

PKCS #11 is a standard C API for accessing cryptographic hardware. The API allows encryption/decryption with both symmetric and asymmetric keys, together with signature generation/verification and random number generation.

In Chaps, the majority of these cryptographic operations are performed in software (using the OpenSSL libraries); however, the presence of a trusted platform module (TPM) on the system allows for a few key operations to be performed in hardware:

  • Random number generation.
  • Generation of RSA public/private keypairs.
  • Wrapping/unwrapping of other keys (i.e. encrypting/decrypting them).

This enables a key use case for PKCS#11: storing cryptographic material so that it can only be used on the local machine. A key can be stored in encrypted form, where decrypting the key (eventually) requires the use of a private key that is held in the TPM, and which cannot be extracted from the TPM. This means that an attacker with that gets full access to the local machine's disk still can't access the key. (If a TPM is not available, this protection is obviously not available, but Chaps will continue to work.)

PKCS#11 Slots and Users

In the PKCS#11 API, an item of cryptographic hardware is known as a token, and tokens are accessed via a particular slot.

% pkcs11-tool --module /usr/lib/libchaps.so.0 --list-slots
Available slots:
Slot 0 (0x0): TPM Slot
  token label        : System TPM Token
  token manufacturer : Chromium OS
  token model        :
  token flags        : rng, PIN initialized, PIN pad present, token initialized
  hardware version   : 1.0
  firmware version   : 1.0
  serial num         : Not Available

The PKCS#11 API also includes authentication to allow token contents to be secured, where the API user needs to log-in to access private objects on the token. However, there is only a single user PIN/password for the whole API; the PKCS#11 API has no mechanism to allow different users to store cryptographic material and be isolated from each other. (The API does include another role with a distinct PIN/password, the security officer, but this role is intended for administrative operations – such as resetting the token or the user's PIN – rather than cryptographic operations.)

Therefore, to allow multi-user use, Chaps does not make use of the PKCS#11 login mechanism (C_Login). Instead, Chaps creates a token (held under /var/lib/chaps/tokens/<user>/) for each user when it first sees that user log in to the system, together with a blob of authentication data that is only visible to that user (in /var/lib/chaps/isolates/<user>). When that user subsequently makes PKCS#11 API calls, only their own slot/token contents are visible to them. (To ease compatibility with other PKCS#11 implementations, Chaps will accept a C_Login operation with a PIN of '111111', although this is a no-op; Chaps does not allow C_Login for the security office role.)

The contents of the per-user token are encrypted using a hash of the user's password, which means that the user's token is only available when they are properly logged in (i.e. su <user> as root does not give access). However, this does impose a requirement for the user to re-login after installation (and after anything that restarts the Chaps system daemon).

Chaps also optionally provides a system token, which is visible to users that do not have their own per-user token, because no PAM login event has occurred for them (e.g. for privileged users like root, or because PAM notifications are administratively disabled on the system). This option is configured via CHAPS_SYSTEM_TOKEN variable in /etc/default/chaps, which controls the --auto_load_system_token option for the chapsd system daemon.

TPM Initialization and Configuration

A system with a hardware TPM may need configuration and initialization before the TPM is available for use by Chaps. Working upwards from the hardware, the things to configure and check are as follows; note that re-initializing the TPM will make any pre-existing TPM-backed cryptographic material inaccessible:

  • Check the TPM is visible to the system with the tpm_version command (from the tpm-tools package). If not:
    • Confirm that the TPM is enabled. This is usually a BIOS setup option, although note that accessing all of the relevant BIOS TPM options may require a cold boot.
      • If the TPM has been configured previously and the owner password is not known, it will need to be cleared via the cold-boot BIOS options or tpm_clear --force. Obviously this will destroy any previous cryptographic material.
      • The TPM needs to be enabled and ownable.
    • Confirm that the local kernel has been configured with TPM support
      • either compiled-in to the kernel: grep -i tpm /boot/config-$(uname -r)
      • or available as a module: lsmod | grep -i tpm
    • Confirm that Trousers is installed and running (ps -ef | grep tcsd).
  • The system also needs to have taken ownership of the TPM, which generates the storage root key (SRK) that will encrypt sensitive material. This operation also involves setting two passwords:
    • The SRK password governs use of the storage root key, and so is needed for most TPM operations.
    • The owner password governs authentication of the TPM itself; in particular, it is needed to change the SRK password.
  • The tpm_takeownership command performs the take-ownership operation, if required. To use the TPM with Chaps, specify an empty SRK password, and whatever you like for the owner password. (Note that an empty password is different than the --well-known option to this tool, which uses a 20-bytes-of-zero password.)
    • If the SRK already has a non-empty password, Chaps can be configured to use this password with the --srk_password or --srk_zeros options to chapsd. However, these options are not currently exposed externally (i.e. they cannot be configured in /etc/default/chaps), and a non-empty SRK password would prevent current Debian versions of OpenCryptoKi from using the TPM.

Troubleshooting

Chaps emits log messages to the system log (e.g. /var/log/syslog); the verbosity of these logs can be altered with chaps_client --set_log_level=<level>.

  • If any operation fails with a "No EK" error, ensure that the TPM has generated an endorsement key (EK) that identifies the TPM (which is normally only created once at first use of the TPM). The tpm_getpubek command displays this (and requires the owner password); if no EK is available, generate one with tpm_createek.
  • Check for the following errors from chapsd in the system log:
    • TPM_E_NOSRK indicates that the TPM has not been taken ownership of. Use tpm_takeownership to take ownership, which generates an SRK and setting the owner and SRK passwords.
    • TPM_E_AUTHFAIL indicates that the Chaps and the TPM have different ideas of what the SRK password is. Use tpm_changeownerauth (which requires the TPM owner password) to set an empty SRK password.
    • TPM_E_DEFEND_LOCK_RUNNING indicates that the TPM is defending against dictionary attacks after multiple failed password attempts. Wait for the timer to expire, or use tpm_resetdalock (which requires the TPM owner password) to reset the lock.
  • If no slots are visible (with pkcs11-tool --module /usr/lib/libchaps.so.0 --list-slots):
    • Try logging out and in again.
    • Check that PAM authentication is enabled.
    • For users that never log in, check whether the system token option is enabled.

Detailed Build Instructions

First ensure the prerequisites are available. The master list is given in the Build-Depends section of the debian/control file, but specifically includes:

  • The <leveldb/memenv.h> header file, available in the liblevedb-dev Debian package.
  • The SCons build tool (typically from the scons Debian package).
  • Development headers for GLib 2.0 (libglib2.0-dev package).
  • Development headers for the DBus C++ library (libdbus-c++-dev package).
  • Development headers for protocol buffers (libprotobuf-dev package).
  • Development headers for OpenSSL (libssl-dev package).
  • Development headers for PAM modules (libpam0g-dev package).
  • Development headers for TSS (libtspi-dev package).

At the top level of this repository, run make. This will:

  • Create a source tree under chaps-<version>/ via the src_generate target:
    • Download the relevant ChromiumOS and Chromium code under chaps-<version>/.
    • Copy additional files needed for the Linux build.
  • Build the library code that Chaps needs:
    • chaps-<version>/libchrome-$(BASE_VER).a: Chromium utility code
    • chaps-<version>/libchromeos-$(BASE_VER).a: ChromiumOS utility code
  • Build the Chaps code into binaries in chaps-<version>/out/.

To build Debian packages, run make package. This will generate two packages:

  • chaps_<version>_<platform>.deb: Chaps system daemon
  • libchaps0_<version>_<platform>.deb: PKCS#11 client library

A corresponding source package can be generated with make src-package.

Repository Layout

This small repo contains the following:

  • README.md: this file
  • makefile: master makefile
  • extrasrc/: additional source files needed for the Linux build
  • patches/: source code changes needed for the Linux build
  • debian/: Debian packaging files
  • man/: vestigial man pages

Source Code Layout

Executing the src_generate target of the master makefile will retrieve additional source code from various upstream locations, and will place it under chaps-<version>/.

Versioning

The Chaps source code tree is primarily built from two upstream repositories, ChromiumOS's platform2 repo together with the base repo from Chromium.

The platform2 code has branches named like release-R42-6812.B that correspond to CrOS releases, and the equivalent Debian package for Chaps will have version 0.42-6812-<debian_revision>.

To update Chaps so that it corresponds to a new CrOS release:

  • Change the CROS_VERSION variable in Makefile to match the numeric part of the CrOS release ID; for example, release-R42-6812.B gives CROS_VERSION=42-6812
  • Find the value of BASE_VER in platform2/chaps/Makefile (as of the relevant CrOS branch for the release, e.g. origin/release-R42-6812.B). This indicates the revision of the Chromium base repo that is expected.
  • Update the CHROMEBASE_VER value in Makefile to match that value.
  • Follow the instructions in Makefile to determine a commit ID for the Chromium base repo that matches that BASE_VER revision.
  • Update the CHROMEBASE_COMMIT value in Makefile to hold that commit ID.
  • Update the debian/changelog file to include a stanza for the new version, and describe the changes therein.
  • Reset the DEB_REVISION value in Makefile to 1.
  • Force a re-generation of the source tree with make distclean.

If the upstream source remains the same, but there are local packaging changes or patches, then just the DEB_REVISION value in Makefile needs to be incremented.