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<div class="moz-cite-prefix">On 11/27/2017 1:03 AM, Jamil Nimeh
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:7f0599a1-4aab-8c77-defe-d86120f7bbf9@oracle.com">
<br>
<blockquote type="cite">
<br>
<blockquote type="cite">
<br>
One additional topic for discussion: Late in the week we
talked about the current state of the API internally and one
item to revisit is where the DerivationParameterSpec objects
are passed. It was brought up by a couple people that it would
be better to provide the DPS objects pertaining to keys at the
time they are called for through deriveKey() and deriveKeys()
(and possibly deriveData).
<br>
<br>
Originally we had them all grouped in a List in the init
method. One reason for needing it up there was to know the
total length of material to generate. If we can provide the
total length through the AlgorithmParameterSpec passed in via
init() then things like:
<br>
<br>
Key deriveKey(DerivationParameterSpec param);
<br>
List<Key> deriveKeys(List<DerivationParameterSpec>
params);
<br>
<br>
become possible. To my eyes at least it does make it more
clear what DPS you're processing since they're provided at
derive time, rather than the caller having to keep track in
their heads where in the DPS list they might be with each
successive deriveKey or deriveKeys calls. And I think we
could do away with deriveKeys(int), too.
<br>
</blockquote>
<br>
See above - the key stream is logically produced in its entirety
before any assignment of that stream is made to any
cryptographic objects because the mixins (except for the round
differentiator) are the same for each key stream production
round. Simply passing in the total length may not give you the
right result if the KDF requires a per component length (and it
should to defeat (5) or it should only produce a single key).
<br>
</blockquote>
From looking at 800-108, I don't see any place where the KDF needs
a per-component length. It looks like it takes L (total length)
as an input and that is applied to each round of the PRF. HKDF
takes L up-front as an input too, though it doesn't use it as an
input to the HMAC function itself. For TLS 1.3 that component
length becomes part of the context info (HkdfLabel) through the
HKDF-Expand-Label function...and it's only doing one key for a
given label which is also part of that context specific info,
necessitating an init() call. Seems like the length can go into
the APS provided via init (for those KDFs that need it at least)
and you shouldn't need a DPS list up-front.
<br>
<br>
</blockquote>
<br>
HKDF and SP800-108 only deal with the creation of the key stream and
ignore the issues with assigning the key stream to cryptographic
objects. In the TLS version of HDKF, the L value is mandatory and
only a single object is assigned per init/call to the KDF. An HSM
can look at the HKDF label information and set the appropriate
policies for the assigned cryptographic object (because if any of
the label data changes, the entire key stream changes). That's not
the case for the raw HKDF nor for any KDF that allows for multiple
objects to be extracted out of a single key stream. Hence the
per-component length values. <br>
<br>
Ideally, there should be a complete object spec for each object to
be generated that is part of the mixins (label and context) for any
KDF. That allows an HSM to rely upon the object spec when setting
policy controls for each generated object - and incidentally allows
for a KDF to generate both public and non-public data in a secure
way.<br>
<br>
So as long as you allow for the specification of all of the
production objects as part of the .init() I'm good. A given KDF
might not require this - but I can't see any way of fixing the
current KDFs to work in HSMs without something like this.<br>
<br>
<blockquote type="cite"
cite="mid:7f0599a1-4aab-8c77-defe-d86120f7bbf9@oracle.com">As far
as your (5) scenario goes, I can see how you can twiddle the
lengths to get the keystream output with zero-length keys and
large IV buffers. But that scenario really glosses over what
should be a big hurdle and a major access control issue that
stands outside the KDF API: That the attacker shouldn't have
access to the input keying material in the first place. Protect
the input keying material properly and their attack cannot be
done.
<br>
</blockquote>
<br>
Let me give you an example. I'm running an embedded HSM - to
protect TLS keys and to do all of the crypto. An attacker
compromises the TLS server and now has access to the HSM. No
problem - I'm going to notice if the attacker starts extraditing
large amounts of data from the server (e.g. copies of the TLS in the
clear but possibly reencrypted data stream) so this isn't a threat
or is it? Smart attacker does an extraction attack on the TLS 1.2
and before KDF and turns all of the key stream material into IV
material and exports it from the HSM. The attacker now has the much
smaller key material so he can send a few messages with those keys
and allow for the passive external interception of the traffic and
decryption thereof without the risk of detection of all that traffic
being sent. Alternately, I can place the key material in a picture
via steganography and publish it as part of the server data.<br>
<br>
The idea is to protect extraction of the key material from an HSM <u><b>even
from authorized users of that key material</b></u>. <br>
<br>
KDFs don't currently do this well. Adding the overall length and
per component length stuff as well as a per component spec to the
data used to derive the key stream means that 1) changes to any of
those change the entire key stream, 2) the per component spec data
may be used by the security module policy engine to enforce
restrictions and 3) because of (1) and (2) calling the KDF a second
time gets me exactly the same objects rather than just the same key
stream. The last isn't very important in a software based security
domain, but turns out to have real implications for policy enforcing
security modules.<br>
<br>
This gets worse when you realize that the KDF key is under it all
either a HASH HMAC or CMAC key and all of those algorithms produce
public data. Ideally you need a way of preventing a KDF key from
calling the raw HASH/HMAC/CMAC functions directly (and vice versa).<br>
<br>
<blockquote type="cite"
cite="mid:7f0599a1-4aab-8c77-defe-d86120f7bbf9@oracle.com">
<br>
I would rather see the DPS provided in the deriveKey. It couples
what you want out with the call that makes the object and it makes
a lot more sense to keep those two together than try to remember
where in the submitted list of DPS objects you are.
<br>
<blockquote type="cite">
<br>
95% of the time this will be a call to produce a single key. 4%
of the time it will be a call to produce multiple keys. Only 1%
of the time will it need to intermix key, data and object
productions. Anybody who is doing that is going to write a
wrapper around this class to make sure they get the key and data
production order correct for each call. So I'm not all that
bothered by keeping the complexity as a price for keeping
flexibility.
<br>
<br>
You could have a Key deriveKey(Key k, DerivationParameterSpec
param) for some things like TLS1.3 (where you can only make a
single call to derive key between inits) , but then you'd also
need at least a byte[] deriveData (Key k,
DerivationParameterSpec param) and an Object deriveObject(Key k,
DerivationParameterSpec param).
<br>
</blockquote>
I don't think those are necessary. If you're just doing
HKDF-Expand (for the HKDF-Expand-Label TLS 1.3 key derivation)
then you can provide the input key, label and max length and any
other context info that goes into that HkdfLabel structure...all
of that would go into init(). Then provide the key alg and
desired length via the DPS at deriveKey time. Any subsequent keys
in the TLS 1.3 key schedule would need a new init call anyway
since the labels change and possibly the output length.
<br>
<br>
Over the next day or so I'm going to have to make some final
decisions on this API as there are internal projects that are
waiting on this API to proceed. I'm already past the cut-off date
I set, but I recognize these discussions are important to have and
I appreciate the input you and others have provided.
<br>
<br>
--Jamil
<br>
<br>
</blockquote>
<br>
Reading this last I think I've lost the context. Here's where I
think we are:<br>
<br>
1) Get instance gets the default configuration of a given KDF (and
that default will be attached to the instance name defintion)<br>
2) .setParameter() may be used to update the KDF configuration -
once.<br>
3) .init() takes at least the key, it may optionally take a set of
derivation parameters. The derivation parameters provided in
.init() are intended for use in forming the label and context mixins
for the KDF. They may provide - for example - the total length of
the key stream, the objects to be derived, the length of the
objects, protection parameters for each of the objects etc.<br>
4) A kdf generate a free-running or fixed length key stream
depending on the derivation parameters (e.g. if "L" is not a mixin
to the KDF then it is free-running and may produce as much key
stream as desired or if the production object specifications are not
part of the derivation mixins).<br>
<br>
Doing (4) is mostly not a good idea, but someone might want to do
this. In that case it may make the most sense to just allow them
to do deriveData(int length) calls as the only function (a keyed
PRNG basically).<br>
<br>
Re the last version of your api - if you add the .setParameter()
.getParameter() calls to both KeyDerivation and KeyDerivationSpi I
think I'm happy with this part of the API. I'm wondering if we
should talk about KeyAgreement though.<br>
<br>
<br>
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