How does age-plugin-piv-agent compare to age-plugin-yubikey?#

Firstly, age-plugin-yubikey is created by professional cryptographic engineers, and has a much larger user base. piv-agent has neither attribute, so you should probably just use age-plugin-yubikey.

That said, here are the technical differences I am aware of in the design of these two pieces of software.

age-plugin-yubikey:

• supports multiple age key types
• stores the ML-KEM seed in the metadata of the certificate stored in a slot. It is stored on the YubiKey hardware, but is publicly readable when the YubiKey is plugged in.
• is multi-platform.

piv-agent:

• only support hybrid PQ crypto for age.
• stores the ML-KEM seed as a systemd credential on your laptop.
• is strictly Linux / systemd only.

Considering only hybrid PQ cryptography, there are some interesting usability consequences to these two designs.

age-plugin-yubikey stores the two parts of the hybrid key entirely in the YubiKey hardware. So for every machine you plug your YubiKey into, you’ll have the same age identity. There is a 1:1 relationship between YubiKey slots and identities.

age-plugin-piv-agent also enforces a 1:1 relationship between YubiKey slots and identities, but the seed is kept on your laptop (sealed by the TPM). This means that to decrypt files on multiple laptops, you should provision a separate decrypting slot on the YubiKey for each laptop.

For the threat model of malware stealing your ML-KEM seed, I am not sure there is much difference between the two. For age-plugin-yubikey: the certificate can be read without requiring a touch on the YubiKey. For age-plugin-piv-agent: systemd credentials are protected by the TPM, but due to piv-agent running as a user service, any process running as your user can execute systemd-creds and decrypt the seed.