The past year I have been trying to learn more about the Trusted Platform Module (TPM). This is a small device found on most modern laptops that has several cool security features like key creation, sealing and attestation, however I have been struggling to find a small project where I can learn more about it.

To my surprised I learned a couple of months ago that nobody has written a TPM plugin for age!

age is a file encryption tool written by Filippo Valsorda to replace the file encryption feature people utilize from GnuPG¹. It has plugin system which allows you to encrypt to whatever key formate and storage you would need. Currently the most popular plugin seems to be age-plugin-yubikey which allows key storage on yubikeys.

Hardware tokens

So why do we want to store key material on hardware like Yubikey or a TPM?

I think the most obvious one is to prevent key extraction or the key leaking. We are all aware of the funny examples when people confuse a SSH private key with a public key and share the wrong file. If it’s stored as a resident key on the Yubikey, or stored inside the TPM, leaking this file would be harmless as it just contains some metadata.

It also prevents a compromised system from getting the key extracted. This means the active exploiter would need continuous presence on the system to abuse the key file. Disconnecting the machine from the internet would make the key material inaccessible to the attacker.

Another cool property of the TPM is that it supports defending against dictionary attacks. After 3 attempts to brute force the password of the key, the TPM will lock access to it until the TPM has been reset. Which makes brute forcing not possible.

This feature allows us to effectively move away from a password, or a passphrase, to a smaller PIN as we don’t need to rely on the entropy any more.

A TPM plugin for age would make the above security aspects more easily accessible for people. Instead of buying an expensive security key, we can just utilize the TPM we already have.

age-plugin-tpm

age-plugin-tpm is my first attempt at writing something useable that interfaces with the TPM. It is written in Golang and uses the google/go-tpm library which is a native API for interacting with the TPM. This is neat as it allows us to not rely on external C libraries.

https://github.com/Foxboron/age-plugin-tpm

The crypto primitives is a very close reimplementation of what age itself is doing with it’s native keys.

It uses NIST P256 keys with a Elliptic-curve Diffie–Hellman (ECDH) shared secret which is used inside a HMAC based key derivative function (hkdf) to protect the key used for file encryption and decryption. The ECDH shared secret is done inside the TPM, while rest is done in Go itself.

To use the plugin one can either install it with go install, or download the pre-compiled binaries and throw them into $PATH.

λ » age-plugin-tpm --generate -o - | tee age-identity.txt
# Created: 2023-07-17 17:51:01.821730119 +0200 CEST m=+0.473471400
# Recipient: age1tpm1q045hl84w9hhy2kjvs9tyugfqal083qw9nc60m5pk824u8q3manuuy96qpf

AGE-PLUGIN-TPM-1QYQQQKQQYVQQKQQZQPEQQQQQZQQPJQQTQQPSQYQQYR45HL84W9HHY2KJVS9TYUGFQAL083QW9NC60M5PK824U8Q3MANUUQPQ2NEEA5Q3DST4WUWFQLXH67F50DFQZD4WVA704NMM23HXE7Z2JMASQLSQYZ904GFUL3XLSX8HZ0RM6E852JDUAKZAP63ZAPWC9MSR6MP6NTDD5QQSWMLCU8UKWF4ZEP9GH77R6JHQA6H68WSJ3VCJSHALCCWT7NDNMUJNKTYTQ93KQWJ53J03J2W9Z4GXRK0ZERPR3NDAY5GL7NWCAM7YXA3UH7856J780P9R2ER6G57PU9HJRVQC7VAPDKN5N4YM7D5M6F

All identities we create with age-plugin-tpm are stored outside of the TPM as the TPM are small devices with limited memory. The private parts of the TPM object is wrapped by a parent key which never leaves the TPM and can’t be decrypted outside of the TPM.

We can also optionally include a PIN to the key by passing the --pin flag or setting the AGE_TPM_PIN environment variable.

To get the recipient from the identity, we can just pass the identity with the -y flag.

λ » age-plugin-tpm -y age-identity.txt
age1tpm1qfnxa6c5q007tt6pn9h4yz6krf04qusdf76ppxnax0na8h9ygwx7zp4dq3l

You can also pipe stuff to encrypt and decrypt things.

λ » echo "Hack the planet" | \
    age -r $(age-plugin-tpm -y age-identity.txt) -a | \
    age -d -i age-identity.txt
Hack the planet

If you don’t feel like trying to use your TPM when trying out the tool, I’ve implemented a --swtpm and a AGE_PLUGIN_TPM_SWTPM environment variable which is going to start a new instance of the software TPM swtpm and allow you to try out the plugin. This requires swtpm installed to work.

This project uses the new tpmdirect API which intends to replace the old tpm2 library inside go-tpm. This allows us to make use of session encryption when communicating with the TPM. This prevents a common attack vector where people with physical access to the machine is capable of sniffing secrets part of the TPM interaction.

In age-plugin-tpm the PIN passed to the TPM through an encrypted session, as well as the output when we generated the shared secret during the ECDH handshake.

Chris Fenner, who is the author of this library, was also very kind to look over the API usage :)

Generally I think this project should in a useable state for people. The key format and the crypto should not see any large revisions. However I would like to try and implement some PCR sealing, but this is a larger thing I need to learn more about.

The codebase needs some work in some places, and while go-tpm supports Windows I have only been writing this for Linux. Patches and feature requests are always welcome!

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