One clarification below.

e

On 02/02/2015 02:54 PM, Eric Voskuil wrote:
> On Feb 2, 2015, at 11:53 AM, Mike Hearn wrote:
>>
>> In sending the first-signed transaction to another for second
>> signature, how does the first signer authenticate to the second
>> without compromising the  independence of the two factors?
>>
>> Not sure what you mean. The idea is the second factor displays the
>> transaction and the user confirms it matches what they input to the
>> first factor. Ideally, using BIP70, but I don't know if BA actually
>> uses that currently.
>>
>> It's the same model as the TREZOR, except with a desktop app instead
>> of myTREZOR and a phone instead of a dedicated hardware device. 
> 
> Sorry for the slow reply, traveling.
> 
> My comments were made in reference to this proposal:
> 
>>> On Feb 2, 2015, at 10:40 AM, Brian Erdelyi <brian.erdelyi@gmail.com
>>> <mailto:brian.erdelyi@gmail.com>> wrote:
>>>
>>> Another concept...
>>>
>>> It should be possible to use multisig wallets to protect against
>>> malware.  For example, a user could generate a wallet with 3 keys and
>>> require a transaction that has been signed by 2 of those keys.  One
>>> key is placed in cold storage and anther sent to a third-party.
>>>
>>> It is now possible to generate and sign transactions on the users
>>> computer and send this signed transaction to the third-party for the
>>> second signature.  This now permits the use of out of band transaction
>>> verification techniques before the third party signs the transaction
>>> and sends to the blockchain.
>>>
>>> If the third-party is malicious or becomes compromised they would not
>>> have the ability to complete transactions as they only have one
>>> private key.  If the third-party disappeared, the user could use the
>>> key in cold storage to sign transactions and send funds to a new wallet.
>>>
>>> Thoughts?

My comments below start out with the presumption of user platform
compromise, but the same analysis holds for the case where the user
platform is clean but a web wallet is compromised. Obviously the idea is
that either or both may be compromised, but integrity is retained as
long as both are not compromised and in collusion.

> In the multisig scenario the presumption is of a user platform
> compromised by malware. It envisions a user signing a 2 of 3 output with
> a first signature. The precondition that the platform is compromised
> implies that this process results in a loss of integrity of the private
> key, and as such if it were not for the second signature requirement,
> the malware would be able to spend the output. This may be extended to
> all of the keys in the wallet.
> 
> The scenario envisions sending the signed transaction to an another
> ("third") party. The objective is for the third party to provide the
> second signature, thereby spending the output as intended by the user,
> who is not necessarily the first signer. The send must be authenticated
> to the user. Otherwise the third party would have to sign anything it
> received, obviously rendering the second signature pointless. This
> implies that the compromised platform must transmit a secret, or proof
> of a secret, to the third party.
> 
> The problem is that the two secrets are not independent if the first
> platform is compromised. So of course the malware has the ability to
> sign, impersonate the user and send to the third party. So the third
> party *must* send the transaction to an *independent* platform for
> verification by the user, and obtain consent before adding the second
> signature. The user, upon receiving the transaction details, must be
> able to verify, on the independent platform, that the details match
> those of the transaction that user presumably signed. Even for simple
> transactions this must include amount, address and fees.
> 
> The central assumptions are that, while the second user platform may be
> compromised, the attack against the second platform is not coordinated
> with that of the first, nor is the third party in collusion with the
> first platform.
> 
> Upon these assumptions rests the actual security benefit (increased
> difficulty of the coordinated attack). The strength of these assumptions
> is an interesting question, since it is hard to quantify. But without
> independence the entire security model is destroyed and there is thus no
> protection whatsoever against malware.
> 
> So for example a web-based or other third-party-provisioned
> implementation of the first platform breaks the anti-collusion
> assumption. Also, weak comsec allows an attack against the second
> platform to be carried out against its network. So for example a simple
> SMS-based confirmation could be executed by the first platform alone and
> thereby also break the the anti-collusion assumption. This is why I
> asked how independence is maintained.
> 
> The assumption of a hardware wallet scenario is that the device itself
> is not compromised. So the scenario is not the same. If the user signs
> with a hardware wallet, nothing can collude with that process, with one
> caveat.
> 
> While a hardware wallet is not subject to onboard malware, it is not
> inconceivable that its keys could be extracted through probing or other
> direct attack against the hardware. It's nevertheless an assumption of
> hardware wallets that these attacks require loss of the hardware.
> Physical possession constitutes compromise. So the collusion model with
> a hardware wallet does exist, it just requires device possession.
> Depending on the implementation the extraction may require a non-trivial
> amount of time and money.
> 
> In a scenario where the user signs with HW, then sends the transaction
> to a third party for a second of three signatures, and finally to a
> second platform for user verification, a HW thief needs to collude with
> the third party or the second platform before the owner becomes aware of
> the theft (notifying the third party). This of course implies that
> keeping both the fist and second platforms in close proximity
> constitutes collusion from a physical security standpoint. This is
> probably sufficient justification for not implementing such a model,
> especially given the cost and complexity of stealing and cracking a
> well-designed device. A device backup would provide comparable time to
> recover with far less complexity (and loss of privacy).
> 
> Incidentally the hardware wallet idea breaks down once any aspect of the
> platform or network to which it connects must be trusted, so for these
> purposes I do not consider certain hybrid models as hardware wallets at
> all. For example one such device trusts that the compromised computer
> does not carry out a MITM attack between the signing device and a shared
> secret entered in parts over time by the user. This reduces to a single
> factor with no protection against a compromised platform.
> 
> Of course these questions address integrity, not privacy. Use of a third
> party implies loss of privacy to that party, and with weak comsec to the
> network. Similarly, use of hardware signing devices implies loss of
> privacy to the compromised platforms with which they exchange transactions.
> 
> e