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<div class="moz-cite-prefix">On 11/27/2017 10:09 AM, Michael StJohns
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:edf90ac5-4a48-28b1-505f-622511c4481c@comcast.net">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<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>
</blockquote>
I thought that we had ditched setParameter in favor of putting these
parameters in getInstance. IIRC we were headed toward an algorithm
naming convention of <KDF>/<PRF>, plus APS in the
getInstance (which may be null (and might be for most KDFs that we
start with: HKDF and possibly TLS-PRF).<br>
<br>
For those I could see naming conventions:<br>
HKDF would need a PRF specifier, so HKDF/HmacSHA256,
HKDF/HmacSHA384. Basically for that PRF field I want to see values
that line up with Mac algorthms in the standard names document.<br>
TLS-PRF would probably allow a default "TLS-PRF" would be TLS-PRF
used in 1.1 and earlier. "TLS-PRF/SHA256" would be P_SHA256 from
RFC 5246. Or we could make it also follow the Mac standard name, so
"TLS-PRF/HmacSHA256". I'm fine with that too. Basically each
implementation<br>
<br>
<blockquote type="cite"
cite="mid:edf90ac5-4a48-28b1-505f-622511c4481c@comcast.net"> 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>
</blockquote>
Okay. I think you've made a pretty strong case for the
DerivationParameterSpec objects up-front.<br>
<blockquote type="cite"
cite="mid:edf90ac5-4a48-28b1-505f-622511c4481c@comcast.net"> 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>
</blockquote>
There's a couple ways we could do this:<br>
byte[] deriveData(int length);<br>
int deriveData(byte[] buf, int offset, int length);<br>
<br>
I don't think we'll add these for this release of the KDF API. It's
easier to add these types of calls later if we need to than it is to
have these extra forms for a KDF use-case that is "mostly not a good
idea".<br>
<blockquote type="cite"
cite="mid:edf90ac5-4a48-28b1-505f-622511c4481c@comcast.net"> <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</blockquote>
<br>
See above with respect to set/getParameter. But hopefully you'll be
happy with the API after this next round. I have one other change I
will be making. I'm removing deriveObject. I'm uncomfortable right
now with the idea of the API executing an arbitrary class'
constructor. This is something I'm definitely willing to examine in
the future once the most pressing tasks both with this API, and
projects that are immediately depending on it are take care of. It
is easier to add calls to the API than it is to
remove/modify/deprecate them if there's a problem. I will file an
RFE so that we can track this enhancement.<br>
<br>
Modifications to the KeyAgreement API are beyond the scope of this
JEP. We can certainly discuss ideas you have, but this KDF JEP
isn't going to be dependent on those discussions.<br>
<br>
--Jamil<br>
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