KDF API review, round 2

Michael StJohns mstjohns at comcast.net
Tue Nov 28 17:34:52 UTC 2017

On 11/28/2017 1:04 AM, Jamil Nimeh wrote:
> 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:

Before I get to those:

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()?
2) If you do, couldn't you simply provide an empty or null list of key 
derivation spec's to .init()?
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 

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.

More in line.

> On 11/27/2017 10:09 AM, Michael StJohns wrote:
>> On 11/27/2017 1:03 AM, Jamil Nimeh wrote:
>> 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.
> 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.

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.

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.

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.

>> 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.
> 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?

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.

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 

>> 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.
>>> 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.
>> 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.
> "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).

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.

>> The idea is to protect extraction of the key material from an HSM 
>> _*even from authorized users of that key material*_.
> 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.

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.

>>  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.
> 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).
Yes but.

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.

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)

>> 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).
> 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.

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 

The HSM (and the JVM) would both identify functions that can be used 
with that key and keep others away.

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.

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?

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.

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.


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