Simple TPM PK11

A simple library for using the TPM chip to secure SSH keys.

Copyright 2013-2016 Google Inc. All Rights Reserved. Apache 2.0 license.

This is NOT a Google product.

Contact: thomas@habets.se / habets@google.com
https://github.com/ThomasHabets/

Install dependencies

Debian

apt-get install tpm-tools libtspi-dev libopencryptoki-dev libssl-dev

Fedora

tpm-tools
opencryptoki-devel
trousers-devel
openssl-devel

FreeBSD

pkg install tpm-tools trousers-tddl opencryptoki openssl

Build simple-tpm-pk11

./configure && make && sudo make install

Init TPM chip

1. If you have not taken ownership, do so.

tpm_takeownership -z
Enter owner password: [enter something secret here]
Confirm password: [enter something secret here]

2. SRK password is usually the Well Known Secret (all nulls). You can specify a password but it's easier it you don't. The SRK password is only used to allow crypto operations. You still need blobs and key passwords to use other peoples keys.

   The "SRK password" is needed to be able to do operations with the "SRK", which is the actual cryptographic key. The user has no access to the SRK directly. The same goes for other keys protected by the TPM chip.

tpm_changeownerauth -s -r

If you get any error messages, see read TPM-TROUBLESHOOTING.

User setup

1. Create key

mkdir ~/.simple-tpm-pk11/
stpm-keygen -o ~/.simple-tpm-pk11/my.key

(use -p if you want to set a password on the key)

Try out the key:

dd if=/dev/urandom of=to-sign bs=1 count=35
stpm-sign -k ~/.simple-tpm-pk11/my.key -f to-sign
stpm-sign -k ~/.simple-tpm-pk11/my.key -f to-sign -r > to-sign.sig
stpm-verify -f to-sign -k ~/.simple-tpm-pk11/my.key -s to-sign.sig

2. Create config

echo "key my.key" > ~/.simple-tpm-pk11/config

Optionally add "log foo.log" in there too.

3. Extract the public key in SSH format

ssh-keygen -D libsimple-tpm-pk11.so

Install it where you want to log in, in the usual authorized_keys way.

Try logging in using your new fancy key:

ssh -I libsimple-tpm-pk11.so shell.example.com

4. Configure SSH to always use this module

Add this to ~/.ssh/config:

Host *
      PKCS11Provider libsimple-tpm-pk11.so

then try:

ssh shell.example.com

4a. Alternatively, add the TPM to your ssh-agent

This has to be the OpenSSH ssh-agent, since gnome-keyring doesn't support PKCS#11. A sign that you run gnome-keyring (or your OpenSSH is compiled without PKCS#11 support) is that you see this error message when you try:

$ ssh-add -s /…/libsimple-tpm-pk11.so
Enter passphrase for PKCS#11: 
Could not add card "/…/libsimple-tpm-pk11.so": agent refused operation

Tested with

Hardware

• Dell Precision T3500 / WEC TPM 1.2.2.81
• HP Z440 / IFX TPM 1.2.4.40
• Lenovo T410 / STM TPM 1.2.8.16
• Lenovo T440s / STM TPM 1.2.13.12
• Lenovo T500 / INTC STM 1.2.4.1
• Lenono X200s / INTC TPM 1.2.4.1
• Lenovo X240 / STM TPM 1.2.13.12

Software

• OpenSSH 5.9
• OpenSSH 6.0p1 on Debian 7.2
• OpenSSH 6.4p1 on CentOS 7.0
• OpenSSH 6.6.1p1 on FreeBSD 11-CURRENT
• OpenSSH 6.8p1 on Arch Linux
• OpenSSH 7.1p2 on Debian

TODO

• Clean up code.
• config option: log to stdout and/or stderr in addition to logfile.
• Install in the correct place.
• Add PKCS11 support to ssh server.
• Global config in /etc.
• Optionally stir with /dev/random when generating keys.
• Script to automate setting up, including verifying TPM state and fixing it.
• Auto-generate keys on demand? Or should this only be part of script to set up?
• Make it work with gpg, and document.
• Make it work with SSH certs, and document.
• Make it work with openssl, and document.
• Make it work with Firefox, and document.
• Make it work with Chrome, and document.
• Make it work with encrypted home directories, and document.

Reference links

• http://secgroup.ext.dsi.unive.it/projects/security-apis/tookan/
• http://secgroup.ext.dsi.unive.it/projects/security-apis/cryptokix/
• http://trousers.sourceforge.net/pkcs11.html
• http://www.trustedcomputinggroup.org/resources/tcg_software_stack_tss_specification
• http://www.infineon.com/dgdl/TPM+Key+Backup+and+Recovery.pdf
• http://www.engadget.com/2010/02/12/christopher-tarnovsky-hacks-infineons-unhackable-chip-we-pre/
• http://trousers.sourceforge.net/dev_faq.html
• http://resources.infosecinstitute.com/linux-tpm-encryption-initializing-and-using-the-tpm/
• http://p11-glue.freedesktop.org/p11-kit.html
• http://trousers.sourceforge.net/dev_faq.html

Make new release

• Update configure.ac with new version, commit.
• git tag -a -s 0.0x
• git push --tags

Some random notes, not instructions

openssl genrsa -out rsa-key 2048
openssl rsa -in rsa-key -modulus
exponent is always 65537.
ssh-keygen -f rsa-key -y > rsa-key.pub