On 11/17/2017 1:07 PM, Adam Petcher wrote:
On 11/17/2017 10:04 AM, Michael StJohns wrote:
On 11/16/2017 2:15 PM, Adam Petcher wrote:
So it seems like they could all be supplied to init. Alternatively,
algorithm names could specify more concrete algorithms that include
the mode/PRF/etc. Can you provide more information to explain why
these existing patterns won't work in this case?
What I need to do is provide a lifecycle diagram, but its hard to do
in text. But basically, the .getInstance() followed by
.setParameters() builds a concrete engine while the .init()
initializes that engine with a key and the derivation parameters.
Think about a TLS 1.2 instance - the PRF is selected once, but the
KDF may be used multiple times.
This is the information I was missing. There are two sets of
parameters, and the first set should be fixed, but the second set
should be changed on each init.
I considered the mode/PRF/etc stuff but that works for things like
Cipher and Signature because most of those have exactly the same
pattern. For the KDF pattern we;ve got fully specified KDFs (e.g.
TLS 1.1 and before, IPSEC), almost fully specified KDFs (TLS 1.2 and
HDKF needs a PRF) and then the SP800 style KDFs which are defined to
be *very* flexible. So translating that into a naming convention is
going to be restrictive and may not cover all of the possible
approaches. I'd rather do it as an algorithmparameter instead. With
a given KDF implementation having a default if nothing is specified
during instantiation.
I agree that this is challenging because there is so much variety in
KDFs. But I don't think that SP 800-108 is a good example of something
that should be exposed as an algorithm in JCA, because it is too
broad. SP 800-108 is more of a toolbox that can be used to construct
KDFs. Particular specializations of SP 800-108 are widely used, and
they will get names that can be used in getInstance. For example,
HKDF-Expand is a particular specialization of SP 800-108.
So I think the existing pattern of using algorithm names to specify
concrete algorithms should work just as well in this API as it does in
the rest of JCA. Of course, more flexibility in the API is a nice
feature, but supporting this level of generality may be out of scope
for this effort.
The more I think about it the more I think you're mostly right. But
let's split this slightly as almost every KDF allows for the
specification of the PRF. So
<kdfname>/<prf> as the standard naming convention.
Or TLS13/HMAC-SHA256 and HKDF/HMAC-SHA256 (which are different because
of the mandatory inclusion of "L" in the derivation parameters and each
component object for TLS13)
Still - let's include the .setParameters() call as a failsafe as looking
forward I can see the need for flexibility rearing its ugly head (e.g.
adding PSS parameters to RSA signatures way late in the game.....) and
it does match the pattern for Signature so its not a new concept. A
given provider need not support the call, but its there if needed.
Mike
Mike
