Logic of Trezor.sign_with_pubkey and Trezor.ecdh_with_pubkey

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The trezor Device subclass contains two methods, sign_with_pubkey and ecdh_with_pubkey respectively. They are only implemented for trezor, and not for any of the other devices. They are necessary for age and signify to work.

What I want to know is if it is possible to add a default implementation to Device as so:

    def sign_with_pubkey(self, identity, blob):
        """Sign given blob and return the signature (as bytes)."""
        return self.sign(identity, blob), self.pubkey(identity, ecdh=False)

    def ecdh_with_pubkey(self, identity, pubkey):
        """Get shared session key using Elliptic Curve Diffie-Hellman."""
        return self.ecdh(identity, pubkey), self.pubkey(identity, ecdh=True)

Would this do essentially the same thing, but less efficiently? Would making this addition allow other devices to support age and signify? Or is the public key returned by sign_with_pubkey and ecdh_with_pubkey something different?

If the answers to the above questions are yes yes no, this change may appear in my upcoming PR.

[SlugFiller]

From a little bit of testing, they give different results. But it's unclear in what way. As in, why there are two different public keys, and how the second one is generated. And I kind of need that information for my next PR.

[SlugFiller]

Oh wait, it just has a single null byte prepended to it, and it's discarded in the ecdh version. So it would just be:

    def sign_with_pubkey(self, identity, blob):
        """Sign given blob and return the signature (as bytes)."""
        return self.sign(identity, blob), b'\x00' + bytes(self.pubkey(identity, ecdh=False))

Why isn't it pre-discarded in the sign case too, if it's known to always be 0?