Re: [Bitcoin-development] BIP 32.5

[Mike Hearn <mike@plan99.net>]

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I filed a bug in the bitcoinj tracker for this a few days ago referencing
rfc 6967, but that RFC is very complicated and I'm not sure it's really
necessary to go that far. H(sighash||key) is easy to implement and I feel I
understand it better.

In our case it wouldn't have helped anyway - if anything it would just
delayed discovery of the underlying weakness. The same RNG is typically
used to generate both keys and signatures today. However in future it may
be the case that people put more effort into generating a really random key
because they only have to do it once, and then the signing RNG would be
different.

Your concern about hardware devices leaking private key bits via a side
channel is also well made, although I think you have to find some way to
establish trust in these devices anyway as sniffing all their IO traffic
and analysing it is really hard (plus it inverts the threat model - if you
trust your computer and not your hardware wallet, why do you have a
hardware wallet?)

The other advantage is that deterministic keys and signatures together mean
two instances of the same wallet generate identical transactions given an
identical sequence of commands. This could help keep wallets in sync. For
example we had a few users who got confused because they had cloned their
Android wallets across devices (NOT SUPPORTED!) and then one device updated
first, did key rotation, and then the other device showed a transaction
that sent all their money to a new address it knew nothing about. If they
didn't realise the other device had updated this looked identical to theft!

I don't think fractional BIP numbers are the way to go :) but a new BIP
that standardised a way to select K would, if reviewed, be something I'd
implement.


On Fri, Aug 16, 2013 at 4:26 AM, Gregory Maxwell <gmaxwell@gmail.com> wrote:

> I am wondering if we shouldn't have a BIP32 addendum which makes the
> following signing related recommendations:
>
> (1) Recommend a specific deterministic DSA derandomization procedure
> (a deterministic way to generate the DSA nonce), presumably one based
> on HMAC-SHA512 (since BIP32 uses that construct) or SHA256 in the
> style of RFC 6979.
>
> DSA systems being compromised due to poor randomness at runtime is not
> new. It effected other systems before it effected Bitcoin systems,
> it's not a new problem and it's not going away.  It's difficult to
> tell if an implementation is correct or not.
>
> Use of a fully deterministic signature  would allow for complete test
> vectors in signing and complete confidence that there is no random
> number related weakness in a signing implementation.
>
> In particular, with relevance to our ecosystem a maliciously modified
> difficult to audit hardware wallet could be leaking its keys material
> via its signatures. Even without producing insecure K values it could
> use the choice of K to leak a couple bits of an encrypted root key
> with every signature, and allow the malicious party to recover the
> keys by simply observing the network. Making the signatures
> deterministic would make this kind of misbehavior practically
> discoverable.
>
> We wouldn't be alone in making this change, in general industry is
> moving in this direction because it has become clear that DSA is a
> hazard otherwise.
>
> The primary arguments in most spaces against derandomizing DSA are
> FIPS conformance (irrelevant for us) and reasonable concerns about the
> risks of using a (less) reviewed cryptographic construct. With
> widespread motion towards derandomized DSA this latter concern is less
> of an issue.
>
> Libcrypt has also implemented derandomized DSA in git. The ed25519
> signature system of DJB, et. al. also uses a similar derandomization.
>
> An alternative is implementing a still random construct where K is
> some H(message||key||random) which should remain secure even where the
> randomness is poor, but this loses the advantage of being able to
> externally verify that an implementation is not leaking information.
> OpenSSL development has implemented a form of this recently.
>
> See also: http://tools.ietf.org/rfc/rfc6979.txt
>
> (2) Recommends a procedure for using only even S values in signatures,
> eliminating this source of mutability in transactions.
>
> This can be accomplished via post-processing of existing signatures,
> but since it requires bignum math it is usually preferable to
> implement it along with signing.  I believe someday this will become a
> network requirement for Bitcoin, but regardless it makes sense to
> implement it as a best practice sooner rather than later.
>
> Thoughts?
>
>
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