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<div class="moz-cite-prefix">On 11/28/2017 9:34 AM, Michael StJohns
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:e29daa1d-1706-3608-1b6c-ca98ccb9810a@comcast.net">
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<div class="moz-cite-prefix">On 11/28/2017 1:04 AM, Jamil Nimeh
wrote:<br>
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<blockquote type="cite"
cite="mid:1e69c4c2-005a-7587-77ce-2dda634fe902@oracle.com">
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Hi Mike, I know I said you made arguments in favor of specifying
the keys up front in init, but I'm still really uncomfortable
with this. It's been bothering me all day. Comments below:<br>
</blockquote>
<br>
Before I get to those:<br>
<br>
1) Do you know of any protocol using a KDF where the key
production information is not known before you'd need to call the
.init()?<br>
</blockquote>
I honestly don't. I think it's safe to say you probably don't need
a KDF instance until you know at least the first object you want out
of it. But for the protocols I know of all the objects are known
once a cipher suite or proposal is agreed upon.<br>
<blockquote type="cite"
cite="mid:e29daa1d-1706-3608-1b6c-ca98ccb9810a@comcast.net"> 2) If
you do, couldn't you simply provide an empty or null list of key
derivation spec's to .init()?<br>
</blockquote>
You could, but that would end up necessitating two models of
operation. One where we give a list up front of all objects and
call derive actions with no parameters, and a second model where you
specify nothing and then provide object specs one-by-one. Each one
has pros and cons, but trying to support both models I think would
make the API even more confusing.<br>
<blockquote type="cite"
cite="mid:e29daa1d-1706-3608-1b6c-ca98ccb9810a@comcast.net"> 3) If
you're doing a multiobject production from a single call to
.init() do you expect in all cases to NOT include the production
data as mixins?<br>
</blockquote>
In all cases? I can't honestly say that. For the protocols I know
of, the individual object attributes (like length) are not mixins.
But you later go on to say that you know of a couple protocols where
they do. If we have real-world scenarios where individual object
lengths or other attributes really affect the keystream then I guess
we need to take that into account.<br>
<blockquote type="cite"
cite="mid:e29daa1d-1706-3608-1b6c-ca98ccb9810a@comcast.net"> <br>
My problem is that I have use cases where ALL of my key production
information is used as mixins to the key stream. Now I could
provide a List<DerivationParameterSpec> as part of the KDF
init algorithm parameter spec (kdfParams), but that means that I
have to provide a different APS for each different key schedule
(consider TLS1.3s various calls). If you take out the
List<DerivationParameterSpec> out of the .init() I'll end up
having to do that and probably having to accept null values for
the deriveKey calls.<br>
<br>
</blockquote>
<blockquote type="cite"
cite="mid:e29daa1d-1706-3608-1b6c-ca98ccb9810a@comcast.net"> More
in line.<br>
<br>
<br>
<blockquote type="cite"
cite="mid:1e69c4c2-005a-7587-77ce-2dda634fe902@oracle.com"> <br>
<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">
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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>
<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.</blockquote>
So enforce a no-zero-length key policy in your provider code.
You probably can't affect the internals of the HSM, but you
should be able to prevent it in the provider code. I can't get
away from the feeling that this could be dealt with in other
ways besides specifying all this up-front.<br>
</blockquote>
<br>
The best way to understand this is to look at the PKCS11 TLS1.2
and before KDF stuff. The key production schedule was for an
encryption key, an integrity key and two IVs, all from the same
key stream. It turns out that NOTHING the HSM could do could
prevent the extraction of key material because changing the
boundaries between each object did not change the key stream. In
the TLS case (and IPSec for that matter), it's a simple matter to
move confidential key material into non-confidential IVs.
However, even if you limit the production to only confidential
items, you still have a problem in that using the same key
material for different algorithms (e.g. using part of an AES key
as a single DES key) can lead to vulnerabilities.<br>
<br>
TLS 1.3 fixed this problem by only doing single key productions
for each call to the KDF (and by adding the length of the
production to the mixins). Because of this, an HSM can look at
the mixin data and "do the right thing" with respect to policy.
If TLS1.3 had kept the multiple object production model, they
would have included the per-object lengths in the KDF mixin data.<br>
<br>
The HSM can do the right thing because the bits it can depend upon
(in the TLS 1.3 case the label and the length) are included in the
mixin and not simply as part of the added on key creation stuff.
Without this, there is nothing an HSM can do for enforcement
because changing these inputs wouldn't change the key stream.<br>
<br>
<br>
<blockquote type="cite"
cite="mid:1e69c4c2-005a-7587-77ce-2dda634fe902@oracle.com">
<blockquote type="cite"
cite="mid:edf90ac5-4a48-28b1-505f-622511c4481c@comcast.net">
<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>
</blockquote>
Between different generations of keystreams do you expect to
have different sets of policy controls? The KDF API has no way
for you to set those things so I would assume those would be
pretty static, or at least controlled outside the KDF API. If
so, why is the KDF API concerning itself with how some HSM sets
its policy on objects it makes?<br>
</blockquote>
<br>
If I call a KDF with the same key but with different key
productions, I *want* the key stream to be different. If I call
it with the same key but with same key productions, I *want* it to
be the same. Say I call the KDF to produce two objects - an AES
key of length 16 bytes and a HMAC-SHA256 key of also length 16
bytes. If I then call the same kdf with the same key to produce
two AES keys of length 16 bytes (same overall length of the key
stream, but different objects), I would *really* like it if the
second object did not have the same key bytes as the HMAC-SHA256
key of the first call. The only way I can ensure this is to
provide mixins that cause the entire key stream to change if
anything changes in the key production data.<br>
</blockquote>
With the KDFs I know of I don't see how you're going to pull that
off. If you call HKDF with the same key, same salt, same info,
you're going to create the same keystream, no matter how you choose
to segment it or what kinds of objects you wish to assign them to.
I guess in your implementation of a KDF you can choose to go through
the DPS objects and mix their attributes in.<br>
<br>
I had been working on the model that kdfParams provides the mixins
(salt, context info, iteration count, whatever the KDF needs to make
a keystream). That was based on how the KDFs I know of function.
Even TLS 1.3 keys can be done via HDKF in this manner by just adding
those label and length properties to the context info field. But if
you want your implementation to draw it from the DPS, I guess you
could do that. It just seems like two providers providing the same
algorithm would come to different answers.<br>
<br>
<blockquote type="cite"
cite="mid:e29daa1d-1706-3608-1b6c-ca98ccb9810a@comcast.net"> <br>
If the mixins include policy hints (key type, key length, label,
etc) then the HSM can rely upon those and set policy accordingly
for the objects. <br>
</blockquote>
I think I alluded to that up above with TLS 1.3 key derivation using
HKDF. The kdfParams APS for an HKDF-Expand operation would provide
context specific info in the form of an HkdfLabel. You'd have the
key-specific info you're talking about already as part of the
mixin. You don't need to get it from the DPS directly.<br>
<blockquote type="cite"
cite="mid:e29daa1d-1706-3608-1b6c-ca98ccb9810a@comcast.net"> <br>
<blockquote type="cite"
cite="mid:1e69c4c2-005a-7587-77ce-2dda634fe902@oracle.com">
<blockquote type="cite"
cite="mid:edf90ac5-4a48-28b1-505f-622511c4481c@comcast.net"> <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>
</blockquote>
"If the attacker compromises a TLS server" is the part that gets
me...we're using external software bugs/security holes as a
justification to make the KDF API in ways that I think are less
clear to the consumer, to cover one class of providers (HSMs).<br>
</blockquote>
<br>
This isn't a bug in the HSM - its a bug in thinking about how KDFs
work/should work. There are three parts to a KDF - extraction
of entropy from the master secret, expansion of that entropy into
a key stream and finally, assignment of that key stream to
cryptographic objects. HKDF and SP800-108 talk about the first
two, but don't consider the implications of the third. Because
of this, neither TLS1.2 nor IPSec provide a KDF with secure key
production.<br>
</blockquote>
When I referred to "bug" I wasn't talking about the HSM, I was
referring to the server that could be compromised, but no matter.
I'm not sure there's any KDF API out there that talks about the
third class. Seems like they're all concerned with providing the
first two. I had envisioned our KDF API providing equivalent
functionality.<br>
<blockquote type="cite"
cite="mid:e29daa1d-1706-3608-1b6c-ca98ccb9810a@comcast.net"> <br>
<br>
<blockquote type="cite"
cite="mid:1e69c4c2-005a-7587-77ce-2dda634fe902@oracle.com">
<blockquote type="cite"
cite="mid:edf90ac5-4a48-28b1-505f-622511c4481c@comcast.net"> <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>
</blockquote>
That may well be a goal for the HSM, to be solved by the HSM or
the provider that front-ends it. I do not see that as something
to be solved by the KDF API.<br>
</blockquote>
<br>
It has to be solved by the KDF API because the only way this works
is if the mixin data for all the productions is included prior to
producing the first object.<br>
<br>
<blockquote type="cite"
cite="mid:1e69c4c2-005a-7587-77ce-2dda634fe902@oracle.com">
<blockquote type="cite"
cite="mid:edf90ac5-4a48-28b1-505f-622511c4481c@comcast.net"> <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>
</blockquote>
But there aren't KDFs that take individual component lengths as
inputs, so alterations to individual key component lengths don't
change the keystream (unless someone decides to write a KDF that
does, but none that I've seen do). With the way the KDF API is
taking shape, there's no enforcement that you get the same
objects - none of that is locked to the instance. It can change
between inits. If you reinitialize with the same key and KDF
parameters, whether you specify all objects up front or one at a
time in derive calls you can still ask for a different set of
output objects. And changing lengths on various objects won't
matter because HKDF, Counter-mode KDF, Feedback-mode KDF...none
of those care a whit about individual component lengths. All
they care about is the total length of the keystream (and HKDF
only cares about that to make sure it's not more than 255 * Hmac
length).<br>
</blockquote>
Yes but.<br>
<br>
TLS1.3 will be NOT be an HKDF KDF instantiation, it will be a
TLS1.3 KDF instantiation (which uses the HDKF function internally)
that will limit production to a single object per init and with a
known set of labels and using L as a mixin. Because that's how
TLS13 dealt with the problem.<br>
<br>
AND - there are KDFs that take individual components lengths as
inputs - in at least two proprietary protocols that I know of.
Mostly though, with the trend to AEAD algorithms most of the
protocols are tending to move to a single production per init.
(since they don't need both an integrity and confidentiality key
nor an IV per se)<br>
</blockquote>
<blockquote type="cite"
cite="mid:e29daa1d-1706-3608-1b6c-ca98ccb9810a@comcast.net"> <br>
<br>
<blockquote type="cite"
cite="mid:1e69c4c2-005a-7587-77ce-2dda634fe902@oracle.com">
<blockquote type="cite"
cite="mid:edf90ac5-4a48-28b1-505f-622511c4481c@comcast.net"> <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>
</blockquote>
I don't see how we'd prevent this in software. If I've got a
key as input to a KDF (a SecretKey) there's no way to prevent it
being used by anything else that takes a SecretKey. If you need
to prevent that in hardware then that seems like a concern for
your provider or the HSM itself.<br>
</blockquote>
<br>
If I tag a key as MasterSecret (where MasterSecret is not a
subinterface of SecretKey, but is of Key) and use MasterSecret
instead of Key in .init().....<br>
<br>
The HSM (and the JVM) would both identify functions that can be
used with that key and keep others away.<br>
<br>
This is what I was talking about with cryptographic type safety in
my last email - the idea that the Key objects be as strongly typed
as possible to prevent them from being used inappropriately or in
ways that mathematically bypass security. Take a KDF with a PRF
of CMAC-AES-128. The KDF is meant to produce secret data (a key
stream for the production of keys), but a CMAC-AES-128 is meant to
produce public data (an integrity tag over a set of data). Given
that KDF algorithm is simply a wrapper to the PRF to allow for the
production of multiple blocks of data, then its trivial - if you
have access to *use* the KDF key - to use it with the CMAC
function to extract the key stream.<br>
<br>
In the HSM I can *somewhat* combat this by (in PKCS11) attributing
the key, but how do get those attributes on the key in the first
place if I'm using a Java front end?<br>
<br>
In software this isn't a big thing as the confidential key
material and the public CMAC integrity tag are both in the same
software domain. But over the years we've tried to do the right
thing (see javax.security.auth.Destroyable for example) by
thinking about security past the limitations of what we can get in
software.<br>
<br>
For KDFs I'd add a jaxa.crypto.MasterSecret interface extending
Key,Destroyable (and pretty much a clone of SecretKey) a
javax.crypto.spec.MasterSecretSpec implementing KeySpec and
MasterSecret (and a clone of SecretKeySpec) to tag these secret
keys as for use only with a KDF.<br>
<br>
Mike<br>
<br>
<br>
<br>
<br>
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</blockquote>
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