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Re: [GNUnet-developers] EcDSA signature scheme

From: Bernd Fix
Subject: Re: [GNUnet-developers] EcDSA signature scheme
Date: Fri, 13 Jul 2018 16:39:41 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0

On 07/13/2018 12:31 PM, Christian Grothoff wrote:
> On 07/13/2018 12:33 AM, Bernd Fix wrote:
>> ECDHE: ------ Good, but it is used with curve Ed25519. The problem 
>> is that ECDHE requires scalar multiplication of a point on the 
>> curve (that is not the base point!) - and that functionality is
>> not available in many Ed25519 implementations - they often only
>> provide sign/verify operations.
>> A more natural choice for me would be ECDHE with Curve25519.
> EdDSA here does not refer to the actual EdDSA signature algorithm, 
> but to the point compression algorithm which should be used by 
> libgcrypt (see page 37, 

My point was that EdDSA (and the flag "eddsa" used with gcrypt) refers
to and enforces the Ed25519 curve. For implementations not using gcrypt
that can be a problem as ECDHE requires operations on the curve (scalar
multiplication of a arbitrary point) that a lot of libraries do not
support. But I understand why that is not an issue for GNUnet itself.

> Now, I can see why you might argue that the resulting point should
> be in X25519 format for Curve25519, that would seem to require the
> 0x40 byte to explicitly request the algorithm's native compression
> method to be used.  I don't mind changing the point format for the
> ECDHE case to X25519 (at a point where we break compatibility).

As you said: X25519 is based on Curve25519, not Ed25519 (although there
is a bijective mapping between them). And yes, I would "add/strip" the
compression indicator for points when retrieving/storing keys (so public
keys remain 256 bits in size and cause no alignment issues). But this
approach seems void given your next statement:

> Well, there is one tricky bit with that I forgot to point out in my 
> previous message: we do use (rarely) the same point/key for both 
> ECDHE and EdDSA (specifically, in Taler).  Hence, here it is
> relevant to have the same public key representation across both
> schemes.

This constraint of course make things trickier, because that means we
are stuck in using Ed25519 for ECDHE. A possible solution (again: not
for GNUnet itself, but for implementators in general) for using Ed25519
points with ECDHE is to use the bijective mapping between Ed25519 and
Curve25519 and to do the ECDHE on Curve25519. Not nice probably, but
that could work.

>> ECDSA: ------ I also have some problems with this one - and it is 
>> used everywhere in GNUnet for (user) identities (although not by 
>> the crypto used for secure message exchange in sessions).
>> AFAIK is the choice of curve for ECDSA limited (NIST curves 
>> basically). I think that is one of the reason we have EdDSA in the 
>> first place. But maybe I am wrong here...
> You are. You can use Curve25519 with ECDSA, and that is what we do in
> GNUnet. No NIST curves to be found.

Well, ECDSA in GNUnet is used with Ed25519 (and not Curve25519). Even
the gcrypt manual

"Here is an example on how to create a key using curve Ed25519 with the
ECDSA signature algorithm. Note that the use of ECDSA with that curve is
in general not recommended."

Yes, you *can* use ECDSA with Ed25519 (gcrypt implements it) or
Curve25519, but to my knowledge both curve types (Montgomery/twisted
Edwards) are not *recommended* for ECDSA, which was designed with
Weierstrass curves in mind (but I am not so deep into ECC that I can
recall the exact security considerations leading to that statement - or
do you consider that statement incorrect?)

>> Changing the code to really use Curve25519 would ease the pain 
>> significantly. But if someone asks me, I would replace ECDSA with 
>> EdDSA completely...
> Please note that this is not so easy: we use ECDSA specifically 
> because the following holds (needed for GNS):
> Suppose A := aG.  Let b := xa for some random x.  Then B := xaG.
> Now, suppose I give you my public key A and x. Now I can sign
> something with xa as the private key, and you can verify the
> signature using xA.
> EdDSA applies h(ax) to the private key, and this hashing destroys the
> (otherwise) linear relationship as with
> A := h(a)G, b := xa and B := h(xa)G
> no longer allows for xA = B (and also not h(x)A = B).
> Doing GNS-crypto (which is what we do with identities) thus would 
> require you to re-implement parts of EdDSA to get past the 'h' 
> application. Doable, but messy. So please make sure that if you do
> try to change this, that the GNS crypto is still 'happy'.
Let's assume ECDSA is used with Curve25519 (ignoring my previous
arguments against this). For me it then looks like all requirements for
GNS-crypto are also satisfied. So are there any other arguments to at
least switch ECDSA from Ed25519 to Curve25519?

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