Re: [Cfrg] Internet-Draft: Collective Edwards-Curve Digital Signature Algorithm

[Daniel Slamanig <[e-mail]>]

Hi Philipp,

looks interesting!

After looking at the protocol I also have some security concerns 
(related to what has been raised by Oleg in a previous mail).

Not committing to Z as well as the actual sum of signers public keys A' 
in the computation of the hash c for the signature seems to be problematic.

It seems to me that committing Z in the signature in plain as well as A' 
does not work given the later application of the signing procedure 
(where till the end of the protocol it is not clear who actually 
participates in the signing but c must be known at the beginning).

Committing to neither of both however seems problematic as it does not 
fix the set of actual signers.

This could be circumvented by allowing an honest leader to adapt the 
commitment to the final Z (and A') after it knows the signers without 
modifying the initially computed c using a chameleon hash (trapdoor 
commitment). I use P as a generator below, omit the brackets [a]P 
notation for scalar multiplication and write it from the perspective of 
the leader, i.e., the signer that initiates the protocol.

The leader could choose chameleon hash parameters sk_ch=y for random y 
and pk_ch=(P,Y=yP) and add pk_ch and chameleon hash CH(Z'):=h(Z')P+u'Y 
for randomly chosen u' to the computation of c, i.e., set 
c=H(R||A||pk_ch||CH(Z')||S), where h() is a collision resistant hash 
function (this is just the standard way of constructing a chameleon hash 
from DL for arbitrary message spaces as propsed in [1]). If Z' is not 
yet known one simply sets Z' to some fixed string at the beginning, 
e.g., Z':=0. At the end of the protocol, when the leader knows the exact 
signers, it can update Z' to the final value Z and use sk_ch to compute 
a collision in the chameleon hash, i.e., such that CH(Z)=CH(Z') by 
solving h(Z')+u'y = h(Z)+uy for u. He then adds u and Z as well as pk_ch 
to the signature, i.e., sets it as (R,s,u,Z,pk_ch) and discards sk_ch 
and u'. If Z is known beforehand, one simply computes the chameleon hash 
once and does not need to compute a collision. Clearly, the use of a 
chameleon hash allows to simply adapt to the actual set of signers 
representing Z without modifying c (with the knowledge of sk_ch - but it 
does not work without the knowledge of sk_ch). This would resolve the 
issue with the unknown Z at the time of signing.

Also, A is committed in the signature, but not the concrete A'=\sum A_i 
of keys used for computing the signature (I guess also due to the reason 
that one does not know the correct set of signers when computing the 
hash c). That can be problematic in a rogue key scenario in the current 
protocol, i.e., when not all signers need to prove knowledge of the 
secret key when they register their public keys but their public keys 
could be functions of other public keys (guess this could be a problem 
here). For instance, a malicious A^* could just take some key from A, 
say A_j, and compute and publish A^*=a^*A_j. And then given a collective 
signature (R,s,Z) where A_j participated, the signer can update s to s^* 
= s+a^*c as well as Z^* to exclude A_j and include A^* (as Z is not 
committed in the signature generation) and the signature (R,s^*,Z^*) 
will be a valid signature that certifies that A^* participated in 
signing - although he didn't.

If Z is committed as proposed above and it is checked during the 
verification, then this should no longer work. One could also adapt the 
hash c=H(R||pk_ch||CH(Z'||A)||S) to include a chameleon hash to 
CH(Z||A'), where A' is the real set of signers that participated in the 
signature generation and is adapted before finalizing the signature when 
known as above.

I hope that what I proposed is not just complete nonsense :) Also it 
produces quite a bit of an overhead. There may also be easier ways to 
avoid the issues.

Some editorial issues: the message to be signed is sometimes called a 
satement S and later then message msg. S could be confusing as part of 
the signature is s and your semantic is that scalars are lowercase and 
points are uppercase. Also in 4.2 "Signature Generation" steps 2 and 3 
are confusing as I guess that in the protocol all the signer compute 
their own [r_i]B values and R=\sum R_i instead of R=[r]B which would 
mean that they would send their r_i's (hope they will not do so).

Also I find it somewhat confusing that the bitmask Z identifies the 
non-signers with a bit set to 1. Why not identifying the signers with a 
bit set to 1?

Cheers,
Daniel


[1] Hugo Krawczyk, Tal Rabin: Chameleon Signatures. NDSS 2000

On 01.07.2017 23:58, Philipp Jovanovic wrote:
> Hi CFRG,
> 
> Here’s a first version of an Internet-Draft on “Collective Edwards-Curve Digital Signature Algorithms” based on Ed25519 and Ed448: https://datatracker.ietf.org/doc/draft-ford-cfrg-cosi/
> 
> We plan to give a short presentation on that topic at the next CFRG meeting in Prague.
> 
> Any feedback is more than welcome. Thanks!
> 
> All the best,
> Philipp
> 
> 
> 
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--
Dr. Daniel Slamanig
Institute for Applied Information Processing and Communications
Graz University of Technology
Inffeldgasse 16a, 8010 Graz, Austria.
Phone: [phone]
http://www.iaik.tugraz.at