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I just got a yubikey and found this guide today. It's quite good.

One thing that I still haven't found a good answer for that's not mentioned in the guide: what's KDF for?

The new yubikey firmware has release notes here: https://support.yubico.com/support/solutions/articles/150000...

This is the bit that has me lost:

> To remove the transmission and on-card storage of OpenPGP PINs in plain text, the YubiKey supports the Key Derived Function (KDF) functionality. With the KDF function enabled, the PIN is stored as a hash on the YubiKey. When entering the PIN to the OpenPGP Smart Card, the OpenPGP client will only pass the hashed value, never passing the PIN directly. KDF functionality is set on the card itself, and communicated to the client; it is transparent to the user. Should the KDF functionality not be enabled, the PIN function will work as previously. The KDF function is listed in section 4.3.2 of the OpenPGP Smart Card 3.4 spec.

Can someone explain to me how KDF matters at all here?

It seems like the keys are encrypted on the yubikey via pin, or at least protected in hardware via pin, and that the pin is stored on the device. KDF seems to take that plain text pin and replace it with a hashed pin. If you steal my yubikey, it looks like KDF would prevent you from... dumping the PIN? But if you could dump the pin, wouldn't you just dump the key instead? I can't seem to figure out the threat model for this feature.

discuss

LIV2|6 years ago

I'm guessing it's to protect against MITM of the USB interface

QualityReboot|6 years ago

How would that help though? If you have a compromised USB interface, and you're entering your pin on that machine, you could just capture the keyboard input anyway.

Leace|6 years ago

New firmware is quite interesting if one is using OpenPGP: it supports newer algorithms (25519) and key attestation (proofs that the key was generated in hardware, useful for enterprises). Still, PIV applet has some more advantages (like dozens of possible encryption subkeys) but this is mainly a limitation of OponPGP Card spec.