I think the communication would have to go the other way around. Trezor
has to commit to a value First. Like this:

Trezor picks random s and sends S=s*G to computer, keeping s secret.
Computer picks random t and sends t to Trezor.  Trezor makes r := s+t
its internal master private key with corresponding master public key 
R := (s+t)*G. Since R = S+t*G, the computer can verify the master
public key. As you say, the computer can then store R and can later
verify for each derived pubkey that it was indeed derived from R, hence
from his own entropy t.

However, Trezor could not use straight bip32 out of the box. The
chaincode would have to be something like SHA(R). And the seed (that
gets translated to mnemonic) would be r itself, making it 256 bit
instead of only 128 bit.

If the longer seed is bearable then this is a good way to do it.

One question remains: if you only write down the mnemonic how can you be
sure that it is correct and corresponds to the secret in Trezor? You
cannot verify that on paper. You would have to restore it on some
device, eg another empty Trezor, and see if it brings up the same master
pubkey. Right? 


On Sun, Nov 03, 2013 at 08:03:54AM +0100, Thomas Voegtlin wrote:
> Le 03/11/2013 07:41, Timo Hanke a écrit :
> >No. You mean the computer would use B for this check? (k,K) could
> >be rigged by Trezor, who computes b as k-a. Timo
> I was just asking a question, in order to understand how this device
> works, and what are its requirements.
> if you think you can help, please explain.

Timo Hanke
PGP 1EFF 69BC 6FB7 8744 14DB  631D 1BB5 D6E3 AB96 7DA8

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