[j_s]

I don't understand what option one has to do with any of this, if you can switch keys there's nothing any amount of extra hardware can do. In that case, just use the key you switched to.

It is true that hardware tokens without an integrated external I/O (not through the PC it is plugged into) are vulnerable to this type of attack during initial key setup. Maybe they should support using the indicator LED to morse code thumbprints or something, but if you can't trust I/O to the device during setup you're going to have a bad time. I will quote my original caveat here:

> Just plug it into a computer with a CPU you do trust first to get the official public key(s) for future verifications!

Your second point seems to be that the plaintext has to be exposed (either going in or coming out) at the USB/hardware interface level, which makes more sense to discuss. The sales page promises the following, but I didn't quickly find any details on how it works:

https://www.yubico.com/products/yubihsm/

Secure session between HSM and application

The integrity and privacy of commands and data in transit between the HSM and applications are protected using a mutually authenticated, integrity and confidentiality protected tunnel.

[Cyph0n]

> I don't understand what option one has to do with any of this, if you can switch keys there's nothing any amount of extra hardware can do. In that case, just use the key you switched to.

Take TLS/HTTPS as an example. The underlying assumption is that your browser's and/or OS certificate store are trusted. If an attacker compromises your machine and adds a new certificate to that "trusted" store, all secure communication is broken. That is the inherent weakness of public-key crypto.

Back to the Yubikey scenario. Let's say a remote server R wants to talk to the Yubikey Y on your PC. Y initially generates a key pair and then (somehow) securely delivers the public key to the remote server R. Clearly, whenever R sends something to Y, an attacker wouldn't be able to decrypt the message unless it has the private key which is securely stored on the HSM.

Scenario (1) above was looking at the case of an attacker who somehow "tricks" R into updating/replacing/revoking the original public key and inserting his own key into the remote key database. If successful, the attacker could then intercept all inbound communication from the remote server and decrypt it.

> The sales page promises the following, but I didn't quickly find any details on how it works:

It looks like marketing speak to me honestly. If the computer/server the Yubikey is talking to is compromised, there really is nothing stopping an attacker from using the Yubikey to perform arbitrary encryptions and decryptions.

[j_s]

Are we in agreement that no HSM can completely protect against scenario 1? (If I can't verify the key [varying degrees of "hard" when connecting across the internet], it can be replaced.)

[edited] That is why I don't see this as relevant, it is outside the threat model any HSM attempts to protect against. More mitigation is possible than the YubiHSM provides, using a display/inputs on the HSM itself to verify keys and choose/confirm operations.

I appreciate your patience in carefully explaining your perspective, and this issue is definitely something to keep in mind in general.

[Cyph0n]

> That is why I don't see why this could possibly be relevant, it is outside the threat model the HSM attempts to protect against.

I agree completely. It's just something to keep in mind.