TPM

TPM (Trusted Platform Module) is a secure microprocessor commonly embedded in modern computers. It can be used for boot chain audit, key storage and random number generation.

NixOS configuration

A minimal NixOS configuration to be able to use the TPM from userspace can be:

security.tpm2.enable = true;
security.tpm2.pkcs11.enable = true;  # expose /run/current-system/sw/lib/libtpm2_pkcs11.so
security.tpm2.tctiEnvironment.enable = true;  # TPM2TOOLS_TCTI and TPM2_PKCS11_TCTI env variables
users.users.YOUR_USER.extraGroups = [ "tss" ];  # tss group has access to TPM devices

After rebooting with this configuration, TPM2TOOLS_TCTI and TPM2_PKCS11_TCTI should point to device:/dev/tpmrm0 and your user should be able to read and write to /dev/tpmrm0.

Using a TPM2 with OpenSSH

For example, the following commands create a new token associated with PIN-code YOUR_PIN (Personal Identification Number) and a recovery SOPIN-code YOUR_SOPIN (Security Officer Personal Identification Number) and then a new secp256r1 key:

tpm2_ptool init
tpm2_ptool addtoken --pid=1 --label=ssh --userpin=YOUR_PIN --sopin=YOUR_SOPIN
tpm2_ptool addkey --label=ssh --userpin=YOUR_PIN --algorithm=ecc256

Now you may show your public key:

ssh-keygen -D /run/current-system/sw/lib/libtpm2_pkcs11.so

To tell OpenSSH to use the TPM2 during login, you may add the following line to your ~/.ssh/config:

PKCS11Provider /run/current-system/sw/lib/libtpm2_pkcs11.so

Frequently Asked Questions

How does the PIN-code bruteforce protection work?

You may run the following command to query the variable properties of your TPM:

nix-shell -p tpm2-tools --run "tpm2_getcap properties-variable"

TPM2_PT_LOCKOUT_COUNTER is the current number of failed attempts,
TPM2_PT_MAX_AUTH_FAIL is the maximum number of failed attempts before lockdown,
TPM2_PT_LOCKOUT_INTERVAL and TPM2_PT_LOCKOUT_RECOVERY are durations in seconds for recovery.

References