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crypt_tpmhmac

Use a TPM based hmac for shadow passwords

These patches to libxcrypt and pam provide a shadow password hash that cannot be compromised in an offline attack. A local hardware TPM is used to HMAC the salt and passphrase to generate the hash. Since an attacker cannot obtain the TPM's HMAC key, there is no way to mount any offline attacks. Details are provided in tpmhmac.doc These patches were developed on Fedora 38. YMMV.

Installation:

  1. Apply the supplied patches to libxcrypt and pam source, compile and install. (On fedora I used the respective source rpms, adding the patches to rpmbuild/SOURCES, and spec files.)

  2. copy tpmhmac.conf to /etc

  3. Provision your TPM as with an SRK and optionally a DRSK as described in the TPM_KEYS repository. (https://github.com/safforddr/tpm_keys)

  4. create the HMAC keys. For example, with a DRSK at 0x81000004 "tpm2_create -C 0x81000004 -G hmac -a "sensitivedataorigin|userwithauth|sign" -u hmac.pub -r hmac.priv" and place them in /etc. (The parent key handle and hmac key filenames can be tailored in /etc/tpmhmac.conf.)

  5. in /etc/login.defs change ENCRYPT_METHOD to ENCRYPT_METHOD TPMHMAC

  6. update the selinux policy to allow passwd, unix_chkpwd, and su to access the TPM. The easiest way to do this is run each command, and when they fail, check journalctl, Which will have detailed instructions on how to add the needed rules to the current policy. This need only be done once for each command. This will look something like

    ausearch -c 'passwd' --raw | audit2allow -M my-passwd

    semodule -X 300 -i my-passwd.pp

All new password hashes will now use the TPM. Existing hashes continue to use their existing hash even when updated with passwd. To force en existing hash to change to tpmhmac, you have to delete the hash for that account in /etc/shadow (everything between the first and second ':'.)