Thanks for the feedback. See below for my responses.
On 8/7/2017 5:52 PM, Michael StJohns wrote:
On 8/7/2017 4:37 PM, Adam Petcher wrote:
I'm working on the Java implementation of RFC 7748 (Diffie-Hellman
with X25519 and X448). I know some of you have been anxious to talk
about how this would fit into JCA, and I appreciate your patience
while I learned enough about JCA and existing crypto implementations
to develop this API proposal. This API/design proposal is for RFC
7748 only, and it does not include the API for RFC 8032 (EdDSA).
So you're expecting yet another set of APIs to cover those as well?
Rather than one API to cover all of the new curves? Seems to be a bit
short sighted.
I'm expecting that we don't need to expose curve parameters/points in
the API, so we won't need any new API for EdDSA, other than the
algorithm name. If we decide we need to expose curve parameters, then we
may want to back up and consider how EdDSA fits into this.
Of course, I expect many of the decisions that we make for RFC 7748
will also impact RFC 8032.
First off, I think it is important to separate RFC 7748 from the
existing ECDH API and implementation. RFC 7748 is a different standard,
It's still an elliptic curve. Note that there is already a worked
example here - F2m vs Fp curves.
it uses different encodings and algorithms,
From a JCA point of view, the public key gets encoded as an
SubjectPublicKeyInfo and the private key gets encoded as a PKCS8 -
that's for lots and lots of compatibility reasons. The public point
of the public key might be encoded (inside the SPKI) as little endian
OCTET STRING array vs a big endian ASN1 INTEGER, but its still just
an integer internally.
The algorithms are Key Agreement and Signature - those are at least
what JCA will see them as. The actual
KeyAgreement.getInstance("name") is of course going to be different
than KeyAgreement.getInstance("ECDH") for example.
and it has different properties.
Details please? Or do you mean that you can't use a given type of key
for both key agreement and signature?
Specifically, RFC 7748 resists (some) side-channel attacks and
invalid-point attacks. The "ECDH" algorithm in JCA (PKCS#3) does not
have these properties, so I want to make sure programmers don't get them
confused. This difference in security properties partially motivates the
use of a new algorithm name, rather than reusing "ECDH" for RFC 7748.
Further, this separation will reduce the probability of programming
errors (e.g. accidentally interpreting a Weierstrass point as an RFC
7748 point).
Um. What? It actually won't.
This is the sort of problem I want to avoid:
KeyPairGenerator kpg = KeyPairGenerator.getInstance("ECDH");
KeyPair kp = kpg.generateKeyPair();
KeyFactory eckf = KeyFactory.getInstance("ECDH");
ECPrivateKeySpec priSpec = eckf.getKeySpec(kf.getPrivate(),
ECPrivateKeySpec.class);
KeyFactory xdhkf = KeyFactory.getInstance("XDH");
PrivateKey xdhPrivate = xdhkf.generatePrivate(priSpec);
// Now use xdhPrivate for key agreement, which uses the wrong algorithm
and curve, and may leak information about the private key
This sort of thing can happen if we use the existing EC spec classes for
RFC 7748 (e.g. redefining what "a" and "b" mean in EllipticCurve when
used with a Montgomery curve, leaving "y" null in ECPoint). Of course,
we can prevent it by tagging these objects and checking the tags to make
sure they are used with the correct algorithms, but I would prefer to
use separate classes (if necessary) and let the type checker do this.
My intention is that errors like the one above are impossible in my
proposed design. We should be able to accomplish this by only using
encoded key specs (which already have checking based on OID), or by
using new classes, if this is necessary (and I hope it is not).
So I propose that we use distinct algorithm names for RFC 7748,
Yes.
and that we don't use any of the existing EC classes like
EllipticCurve and ECPoint with RFC 7748.
No. (My opinion but...) It's *hard* to add new meta classes for
keys. Just considering the EC stuff you have ECKey, ECPublicKey,
ECPrivateKey, EllipticCurve, ECPublicKeySpec, ECPrivateKeySpec,
ECPoint, ECParameterSpec, ECGenParameterSpec, EllipticCurve and
ECField (with ECFieldF2M and ECFieldF2P being the differentiator for
all of the various keys within this space).
We can achieve this separation without duplicating a lot of code if
we start with some simplifying assumptions. My goal is to remove
functionality that nobody needs in order to simplify the design and
API. If I am simplifying away something that you think you will need,
please let me know.
There's a difference with what you do with the public API vs what you
do with the plugin provider. Throwing away all of the
"functionality that nobody needs" will probably come back to bite
those who come later with something that looks *almost* like what you
did, but needs just one more parameter than you were kind enough to
leave behind.
I agree, but we have to draw the line somewhere, and I think that line
should be determined by the most common use cases. I would prefer to put
in too little, and add more to the API later when people ask for it.
Adding things to the API isn't trivial, but it is much easier than
removing things that we don't need that are complicating the API and
causing problems.
A) We don't need to expose actual curve parameters over the API.
Curves can be specified using names (e.g. "X25519") or OIDs. The
underlying implementation will likely support arbitrary Montgomery
curves, but the JCA application will only be able to use the
supported named curves.
Strangely, this hasn't turned out all that well. There needs to be a
name, OID in the public space (primarily for the encodings and PKIX
stuff) and to be honest - you really want the parameters in public
space as well (the ECParameterSpec and its ilk) so that a given key
can be used with different providers or even to play around internally
with new curves before giving them a name.
I don't understand why we need public curve parameters to allow keys to
be used with different providers. It seems like this should work as long
as the providers all understand the OIDs or curve names. Can you explain
this part a bit more?
Related to tinkering with new curves that don't have a name: I don't
think that this is a feature that JCA needs to have. In the common use
case, the programmer wants to only use standard algorithms and curves,
and I think we should focus on that use case.
B) We don't need direct interoperability between different providers
using opaque key representations. We can communicate with other
providers using X509/PKCS8 encoding, or by using KeyFactory and key
specs.
I don't actually understand that statement. Keys of different
providers generally don't interoperate anyways, but you can mostly
take an "encoded" one and create a new one in a new provider via the
Keyfactory. KeySpecs provide you with a way of manually building a
key - and that turns out to be VERY necessary, especially when you're
dealing with adapting hardware modules to the JCA.
I'll admit that it is a strange statement. The only reason why I stated
this assumption is that ECPublicKey and ECPrivateKey expose a lot of
information that allows direct interoperability with opaque key objects
(that implement these interfaces). I don't know if this is
necessary/valuable, and I don't have any particular desire to allow the
same thing with RFC 7748 keys.
These two assumptions greatly simplify the API. We won't need classes
that mirror ECParameterSpec, EllipticCurve, ECPoint, ECField,
ECPublicKey, etc. for X25519/X448.
That assumption holds only if your various other assumptions hold. My
opinion is that they probably don't. (BTW - I'm pretty sure, given
that every single asymmetric JCA crypto api takes a PublicKey or
PrivateKey you're going to need to mirror those classes at least;
you'll also need a ParameterSpec and a GenParameterSpec class with
whatever underlying supporting classes are required to deal with
KeyFactory's)
I agree with the second part of your parenthetical statement, but I need
more information about the first. It sounds like what you are saying is
that I will need something like XDHPublicKey and XDHPrivateKey in
java.security.interfaces. Can you tell me why? What is it that we can't
do without these interfaces?
Now that the motivation and assumptions are out of the way, here is a
description of the proposed JCA API:
I'd suggest getting agreement on the above before proceeding.
Later, Mike
