Re: [Hipsec] Benjamin Kaduk's Discuss on draft-ietf-hip-dex-13: (with DISCUSS and COMMENT)

[Benjamin Kaduk <[e-mail]>]

Hi Bob,

Sorry this dropped off my radar for so long -- I got really swamped.
Just a few notes inline, as I'll focus on reading the -18.

On Mon, Mar 09, 2020 at 04:00:33PM -0400, Robert Moskowitz wrote:
> 
> 
> On 3/4/20 12:44 PM, Benjamin Kaduk via Datatracker wrote:
> > Benjamin Kaduk has entered the following ballot position for
> > draft-ietf-hip-dex-13: Discuss
> >
> > When responding, please keep the subject line intact and reply to all
> > email addresses included in the To and CC lines. (Feel free to cut this
> > introductory paragraph, however.)
> >
> >
> > Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> > for more information about IESG DISCUSS and COMMENT positions.
> >
> >
> > The document, along with other ballot positions, can be found here:
> > https://datatracker.ietf.org/doc/draft-ietf-hip-dex/
> >
> >
> >
> > ----------------------------------------------------------------------
> > DISCUSS:
> > ----------------------------------------------------------------------
> >
> > This is a placeholder discuss, intended to illustrate several key
> > omissions from the current document and as an indication that it is not
> > yet ready for full IESG Evaluation.  In that vein, I will defer the
> > evaluation shortly, to attempt to short-circuit the current round of
> > evaluation while the draft improves.  In particular, this is not
> > intended to be a complete review of the document.
> >
> > The FOLD scheme for compressing full host identities into ORCHIDs/HITs
> > is pretty problematic.  The current text acknowledges that collisions
> > are possible and attempts to justify the scheme by pointing out that no
> > collision-free scheme is possible absent a cryptographic hash, which is
> > an appeal to authority ("we can't use a cryptographic hash on
> > constrained systems") that does not attempt to answer the question of
> > whether it is actually reasonable to use a mechanism that allows
> > collisions for this purpose (vs. just not being able to do anything).
> > Additionally, there is not any discussion of second-preimage resistance,
> > which is the more important property here, in terms of an attacker being
> > able to construct a collision with an existing HIT of an honest node.
> 
> In my humble opinion, second-preimage attack defense will be the same as 
> any attack against the HI -> HIT mapping function.

Fair enough.  We should still use the words "second-preimage attack" in
some fashion, though, I think.  (Maybe I will have thoughts about where as
I review the -18.)

> The only place HITs are used in HIP unauthenticated is in the initial I1 
> - I2 part of the exchange.  By the R2, everything is authenticated.  All 
> other HIP messages containing HITs are authenticated.
> 
> So the attack is slipping in a HI-HIT mapping that is malicious. Per 
> Roman's comments, I will be adding to the I2 and R2 processing to 
> validate this mapping.
> 
> HIP has always had to handle probabilistic collisions.  DEX now requires 
> checking for collisions as critical (via ACLs or other mechanisms).  I 
> will see to adding text.
> 
> Operationally, the challenge is in those low level sensors that have no 
> way to have an ACL set up for the servers/gateways that they are 
> connected to.  But this is true even for BEX.  So inclusion of the 
> password authentication is part of the critical behavior is ACL or 

(I think I maybe hadn't made it to the password authentication part when I
stopped reading the -13.)

> similar HI-HIT mappings are not possible (sensors with no out-of-band 
> update mechanism).  We are always twisting ourselves in the 
> chicken-and-egg problem with these devices.
> 
> > In a related vein, Section 3.2.1 claims that the above concerns can be
> > remediated by deployment of a collision detection scheme, "achieved here
> > through either an ACL or some other lookup process".  This process is
> > vital to the security of the system as a whole, and it would be
> > irresponsible to publish this document without a precise specification
> > of what properties are needed in order to perform this process, as well
> > as a worked example that can be used absent other considerations.
> 
> I will be adding this per Roman's comments.  Most will be in the I2 and 
> R2 processing.
> 
> 
> > Given that the applicability statement ("in communicating with such
> > constrained devices") implies that there is intent to have full-featured
> > nodes that implement both HIP DEX and HIP BEX, I think we need
> > significantly more discussion of how such nodes avoid using DEX in
> > situations where it was not appropriate.  That is, how is it known that
> > the peer should be using DEX vs. BEX?  Yes, the HIT includes an
> > indication of whether the identity is for use with DEX vs. BEX, but that
> > does not seem like quite the relevant property.  Do we envision
> > scenarios where a node is positioned somewhat like a gateway, using DEX
> > on one interface and BEX to the broader internet?
> 
> 
> Yes to the gateway situation.  Or the sensor has E2E DEX connection to 
> the central server somewhere on the greater Internet.
> 
> Perhaps text that limits DEX on non-constrained nodes for use with peers 
> in the DEX ACL (or other equivalent control mechanism).

That could work, I think.

> > Using AES-CTR with the long-term static-static master key requires
> > careful tracking of counter (sequence) number to nonvolatile storage.  I
> > did not see discussion of the security consequences of inadvertent
> > counter reuse.
> 
> I will look at this and see what I can add.
> 
> > I appreciate the design to limit use of the long-term static-static
> > master key to essentially just key-wrap operations, but this seems to
> > require the presence of a CSPRNG in order to obtain secure session keys.
> > Expecting a strong CSPRNG on a node so constrained that DEX is necessary
> > seems to be a questionable assumption, and I see no discussion of the
> > need for a good RNG.  (Relying on the full-featured peer to contribute
> > good entropy to the key derivation is not an option, since DEX is
> > allowed to be used between two nodes that are both constrained.)
> 
> The current text is:
> 
>     o  The strength of the keys for the Pair-wise Key SA is based on the
>        quality of the random keying material generated by the Initiator
>        and the Responder.  As either peer may be a sensor or an actuator
>        device, there is a natural concern about the quality of its random
>        number generator.
> 
> Changed to:
> 
>     o  The strength of the keys for both the Master and Pair-wise Key SAs
>        is based on the quality of the random keying material generated by
>        the Initiator and the Responder.  As either peer may be a sensor
>        or an actuator device, there is a natural concern about the
>        quality of its random number generator.  Thus at least a CSPRNG
>        SHOULD be used.
> 
> > The default KEYMAT algorithm uses the "CKDF" (CMAC-based KDF)
> > construction, analogous to HKDF (RFC 5869).  However, the paper
> > motivating 5869's design choices does not seem to justify the usage of
> > CMAC instead of HMAC, since the proof requires a PRF* but CMAC (with
> > AES) is only a PRP.  Absent some detailed justification or prior art it
> > does not seem prudent to use such a novel construction for
> > security-critical functionality.
> 
> The CKDF design comes from NIST SP800-108.  I had extensive discussions 
> with NIST and the 5869 authors at the DEX design time. These points were 
> discussed and considered that CKDF is a prudent design.

We should cite SP800-108 for the derivation, then.

> 
> > ----------------------------------------------------------------------
> > COMMENT:
> > ----------------------------------------------------------------------
> >
> > Some additional comments (also incomplete), since they were already written.
> > It would be reasonable to ignore for now any that don't make sense or
> > are on parts of the text likely to change as a result of the higher-level discussion.
> >
> > Abstract
> >
> > My preference is to just use "forward secrecy" rather than "perfect
> > forward secrecy", as perfection is hard to attain.
> 
> I am all for that!  My Jewish Orthodox background makes me cringe at the 
> use of PFS.  No such thing in this world (btw, I also cringe at the 
> common use of "awesome")...
> 
> If there is consensus to drop PFS from all IETF standards, I will 
> replace "perfect forward secrecy" with "forward secrecy" and PFS with 
> just the full verbiage as FS does not seem to be meaningful.
> 
> 
> > Section 1.1
> >
> >     HIP DEX operationally is very similar to HIP BEX.  Moreover, the
> >     employed model is also fairly equivalent to 802.11-2007
> >     [IEEE.802-11.2007] Master Key and Pair-wise Transient Key, but
> >     handled in a single exchange.
> >
> > 802.11 security does not exactly have a shiny track record...
> 
> You want to see the "smoking gun" document on WEP design from Nov '94?  
> I have it.
> 
> The point is the Master key and Pair-wise key design.  Not necessarily 
> how they were constructed.  I also published the initial paper on the 
> attack on WPA-PSK.

I guess I'm still not entirely sure what value the reference is adding for
us.  Is it just an academic-style "this is not novel; here is some prior
related work" (as opposed to "group X used a similar thing, so our thing is
secure")?

> >     HIP DEX does not have the option to encrypt the Host Identity of the
> >     Initiator in the I2 packet.  The Responder's Host Identity also is
> >     not protected.  Thus, contrary to HIPv2, HIP DEX does not provide for
> >     end-point anonymity and any signaling (i.e., HOST_ID parameter
> >     contained with an ENCRYPTED parameter) that indicates such anonymity
> >     should be ignored.
> >
> > What would you do if you didn't ignore such signalling?  Drop the
> > connection as being with a misbehaving peer?
> 
> Probably more like a ill thought-out implementation.  Right now I am of 
> the opinion of leaving this as is.  But I can be convinced to add a drop 
> connection.
> 
> >     As in [RFC7401], data packets start to flow after the R2 packet.  The
> >     I2 and R2 packets may carry a data payload in the future.  The
> >     details of this may be defined later.
> >
> > I'm not sure what value is added by mentioning the possibility of data
> > payload in I2/R2.
> 
> This is carried over from 5201.  There were ideas pointing how the 3-way 
> TCP setup can become a 5-way HIP - TCPinESP setup. A few other were 
> discussed in HIPRG, but no one has proposed to actually use this 
> feature.  It stays for some future thinker to tinker with.
> 
> 
> >     An existing HIP association can be updated with the update mechanism
> >     defined in [RFC7401].  Likewise, the association can be torn down
> >     with the defined closing mechanism for HIPv2 if it is no longer
> >     needed.  In doing so, HIP DEX omits the HIP_SIGNATURE parameters of
> >     the original HIPv2 specification.
> >
> > I think the intent here is more along the lines of "HIP DEX does so even
> > in the absence of the HIP_SIGNATURE that is used in standard HIPv2".
> > (I also note that there's some subtle semantic mismatch between DEX as
> > "diet exchange" and its used to indicate continuing lack of security
> > functionality throughout the extent of the association, after the
> > exchange is completed.)
> 
> Changed to:
> 
>     An existing HIP association can be updated with the update mechanism
>     defined in [RFC7401].  Likewise, the association can be torn down
>     with the defined closing mechanism for HIPv2 if it is no longer
>     needed.  In doing so, HIP DEX does so even in the absence of the
>     HIP_SIGNATURE that is used in standard HIPv2.

Maybe swap out the last sentence for "Standard HIPv2 uses a HIP_SIGNATURE
to authenticate the close [ed. is there a better word?], but since DEX does
not provide for signatures, the usual per-message MAC suffices to
authenticate the close."?

> 
> >     Finally, HIP DEX is designed as an end-to-end authentication and key
> >     establishment protocol.  As such, it can be used in combination with
> >
> > Don't we have a LAKE WG now?  How does DEX compare to what they are
> > working on?
> 
> I looked some more at LAKE.  They are proposing to use ephemeral DH as 
> part of the exchange.  That goes counter to sec 1.2 of HIP-DEX. If they 
> come up with an approach that performs "acceptably", then I will be 
> looking at it.
> 
> > Section 1.2
> >
> > In lieu of detailed comments, allow me to propose a rewrite of the whole
> > section:
> >
> > % HIP DEX achieves its lightweight nature in large part due to the
> > % intentional removal of Forward Secrecy (FS) from the key exchange.  Current
> > % mechanisms to achieve FS use an authenticated ephemeral Diffie-Hellman
> > % exchange (e.g., SIGMA or PAKE).  HIP DEX targets usage on devices where
> > % even the most lightweight ECDH exchange is prohibitively expensive for
> > % recurring (ephemeral) use.  For example, experience with the 8-bit
> > % 8051-based ZWAWVE ZW0500 microprocessor has shown that EC25519 keypair
> > % generation exceeds 10 seconds and consumes significant energy (i.e.,
> > % battery resources).  Even the ECDH multiplication for the HIP DEX
> > % static-static key exchange takes 8-9 seconds, again with measurable
> > % energy consumption.  This resource consumption is tolerable as a
> > % one-time event during provisioning, but would render the protocol
> > % unsuitable for use on these devices if it was required to be a
> > % recurring part of the protocol.  For devices constrained in this
> > % manner, a FS-enabled protocol will likely provide little gain.  The
> > % resulting "FS" key, likely produced during device provisioning, would
> > % typically end up being used for the remainder of the device's
> > % lifetime.  With such a usage pattern, the inherent benefit of
> > % ephemeral keys is not realized.  The security properties of such usage
> > % are very similar to those of using a statically provisioned symmetric
> > % pre-shared key, in that there remains a single PSK in static storage
> > % that is susceptible to exfiltration/compromise, and compromise of that
> > % key in effect compromises the entire protocol for that node.  HIP DEX
> > % achieves marginally better security properties by computing the
> > % effective long-term PSK from a DH exchange, so that the provisioning
> > % service is not required to be part of the risk surface due to also
> > % possessing the PSK.
> > %
> > % Due to the substantially reduced security guarantees of HIP DEX
> > % compared to HIP BEX, HIP DEX MUST only be used when at least one of
> > % the two endpoints is a class 0 or 1 constrained device defined in
> > % Section 3 of [RFC7228]).  HIP DEX MUST NOT be used when both endpoints
> > % are class 2 devices or unconstrained.
> 
> I have accepted your text with one typo and some formatting.  Of course 
> this text uses FS rather than PFS so that is a mis-match for now.
> 
> 1.2.  Applicability
> 
>     HIP DEX achieves its lightweight nature in large part due to the
>     intentional removal of Forward Secrecy (FS) from the key exchange.
>     Current mechanisms to achieve FS use an authenticated ephemeral
>     Diffie-Hellman exchange (e.g., SIGMA or PAKE).  HIP DEX targets usage
>     on devices where even the most lightweight ECDH exchange is
>     prohibitively expensive for recurring (ephemeral) use.  For example,
>     experience with the 8-bit 8051-based ZWAVE ZW0500 microprocessor has
>     shown that EC25519 keypair generation exceeds 10 seconds and consumes
>     significant energy (i.e., battery resources).  Even the ECDH
>     multiplication for the HIP DEX static-static key exchange takes 8-9
>     seconds, again with measurable energy consumption.  This resource
>     consumption is tolerable as a one-time event during provisioning, but
>     would render the protocol unsuitable for use on these devices if it
>     was required to be a recurring part of the protocol.  For devices
>     constrained in this manner, a FS-enabled protocol will likely provide
>     little gain.  The resulting "FS" key, likely produced during device
>     provisioning, would typically end up being used for the remainder of
>     the device's lifetime.
> 
>     With such a usage pattern, the inherent benefit of ephemeral keys is
>     not realized.  The security properties of such usage are very similar
>     to those of using a statically provisioned symmetric pre-shared key,
>     in that there remains a single PSK in static storage that is
>     susceptible to exfiltration/compromise, and compromise of that key in
>     effect compromises the entire protocol for that node.  HIP DEX
>     achieves marginally better security properties by computing the
>     effective long-term PSK from a DH exchange, so that the provisioning
>     service is not required to be part of the risk surface due to also
>     possessing the PSK.
> 
>     Due to the substantially reduced security guarantees of HIP DEX
>     compared to HIP BEX, HIP DEX MUST only be used when at least one of
>     the two endpoints is a class 0 or 1 constrained device defined in
>     Section 3 of [RFC7228]).  HIP DEX MUST NOT be used when both
>     endpoints are class 2 devices or unconstrained.
> 
> 
> > Section 2.2
> >
> >     Ltrunc (M(x), K)   denotes the lowest order K bits of the result of
> >        the MAC function M on the input x.
> >
> > I'm not sure I'm going to interpret the "lowest order K bits" the same
> > way that everyone else will.  I think "leftmost" or "first" are more
> > common terms for describing this sort of truncation.
> 
> This text goes back to 5201.  Implementors of 5201 did not have a 
> problem with this, in fact probably one of them supplied the text. But I 
> am open to change based on consensus.
> 
> > Section 2.3
> >
> >     CMAC:  The Cipher-based Message Authentication Code with the 128-bit
> >        Advanced Encryption Standard (AES) defined in RFC 4493 [RFC4493].
> >
> > I would suggest just using CMAC as the acronym and not trying to
> > overload it to also be AES-specific.
> 
> Do you recommend I just reference SP800-38B?

That would work, but my recommendation would be to use this definition text
as the definition of "AES-CMAC" ... though the RFC Editor might want to
make us change all the usages of CMAC() in formulae as well.

> >     HIT Suite:  A HIT Suite groups all algorithms that are required to
> >        generate and use an HI and its HIT.  In particular, these
> >        algorithms are: 1) ECDH and 2) FOLD.
> >
> > For DEX.  For normal HIPv2 we wouldn't touch FOLD with a long pole.
> 
> :)
> 
>     HIT Suite:  A HIT Suite groups all algorithms that are required to
>        generate and use an HI and its HIT.  In particular for HIP DEX,
>        these algorithms are: 1) ECDH and 2) FOLD.
> 
> BTW, I once DID use a 10' pole to chase a family of raccoons out of my 
> garage.  Really, it WAS 10' long, I had just gotten it from the lumber 
> yard.  Came home and there were a bunch of beady eyes in the garage..
> 
> >     HI (Host Identity):  The static ECDH public key that represents the
> >        identity of the host.  In HIP DEX, a host proves ownership of the
> >        private key belonging to its HI by creating a HIP_MAC with the
> >        derived ECDH key (see Section 3).
> >
> > This may sound pedantic, but this doesn't actually prove ownership of
> > the private key.  Someone who knows the private key of the other party
> > and the public key of the host in question would be able to produce the
> > same MAC from the corresponding derived ECDH key.  I think the most we
> > can say here is that a host authenticates itself as that host identity
> > [with that HIP_MAC].  There's the corresponding trust of the recipient
> > that its own private key remains secure and thus that no party other
> > than itself or the peer identity could have generated that message.
> 
> I will think on this one.  See what verbiage helps.
> 
> >     Initiator:  The host that initiates the HIP DEX handshake.  This role
> >        is typically forgotten once the handshake is completed.
> >
> > "typically"?  Perhaps it's best to say that the role is not used or
> > needed after the handshake is completed.
> 
> I the HIP state machine, either peer can be the Initiator.  Roles can be 
> reversed.  If one party looses state, it can then become the Initiator 
> regardless of what role it had in the original exchange.
> 
> This is the text used in 7401.
> 
> >     KEYMAT:  Keying material.  That is, the bit string(s) used as
> >        cryptographic keys.
> >
> > I'm surprised we need an abbreviation for this.
> 
> I got comments in early drafts of 5201-bis.  Put it in.  Take it out.  
> So for now, I leave it in.
> 
> >     Length of the Responder's HIT Hash Algorithm (RHASH_len):  The
> >        natural output length of RHASH in bits.
> >
> > [this doesn't really fit the pattern of "definition"s]
> 
> It is in 7401.  If the AD says pull it.  It goes.
> 
> Though perhaps the definition is of RHASH_len?
> 
> >     Responder:  The host that responds to the Initiator in the HIP DEX
> >        handshake.  This role is typically forgotten once the handshake is
> >        completed.
> >
> > [same thing re "typically"]
> 
> Same response.
> 
> > Section 3
> >
> >     HIP DEX implementations MUST support the Elliptic Curve Diffie-
> >     Hellman (ECDH) [RFC6090] key exchange for generating the HI as
> >     defined in Section 5.2.3.  No additional algorithms are supported at
> >     this time.
> >
> > It's kind of weird to see a "MUST" for "RFC6090 key exchange"; 6090
> > discusses the general class of things but is not a specific key exchange
> > algorithm (e.g., curve).
> > I'd also consider s/supported/defined/.
> 
> Good point.  Changed to:
> 
>     HIP DEX implementations use the Elliptic Curve Diffie-Hellman (ECDH)
>     [RFC6090] key exchange for generating the HI as defined in
>     Section 5.2.3.  No alternative algorithms are defined at this time.
> 
> >     Due to the latter property, an attacker may be able to find a
> >     collision with a HIT that is in use.  Hence, policy decisions such as
> >     access control MUST NOT be based solely on the HIT.  Instead, the HI
> >     of a host SHOULD be considered.
> >
> > I don't think this is correct or a strong enough statement.  In
> > particular, I don't think access control should be based on the HIT at
> > all, so strike "solely".  Also, the "SHOULD" seems too week.  I can
> > understand that "MUST use the HI" could be overly constraining, but
> > "access control decisions MUST be made on the actual identity of the
> > host, e.g., the full HI" should allow for sufficient flexibility.
> 
> I will see how this changes with the ACL additions.
> 
> >     Carrying HIs and HITs in the header of user data packets would
> >     increase the overhead of packets.  Thus, it is not expected that
> >
> > s/and/or/?
> 
> fixed.
> 
> >     association.  When other user data packet formats are used, the
> >     corresponding extensions need to define a replacement for the
> >     ESP_TRANSFORM [RFC7402] parameter along with associated semantics,
> >     but this procedure is outside the scope of this document.
> >
> > Why is ESP_TRANSFORM the most important parameter here, when we talk
> > about mapping a packet to the HIP association?  I thought ESP_TRANSFORM
> > was literally about the encryption mechanics, not metadata around it.
> 
> Again, this goes back to 5201.  We are talking about ~20 years of 
> discussions.
> 
> We are discussing HIs and HITs, but that SPIs are used in everyday 
> packets as the pointer to the HIs and HITs involved.  I will think on 
> this, but it is down the list on things to change that were inherited 
> from 5201.
> 
> > Section 3.2
> >
> > ORCHID claims to provide statistical uniqueness and routability at some
> > overlay layer, neither of which this FOLD procedure provides, due to
> > easily-generatable second preimages.
> >
> > Section 3.2.1
> >
> >     Since collision-resistance is not possible with the tools at hand,
> >     any reasonable function (e.g.  FOLD) that takes the full value of the
> >     HI into generating the HIT can be used, provided that collision
> >     detection is part of the HIP-DEX deployment design.  This is achieved
> >
> > This is not an argument that this is a reasonable thing to do; it's
> > merely an argument that it's a thing that can be done that has the same
> > claimed properties as the only type of thing that could be done.  It
> > might be a bad idea to do the only type of thing that can be done, and
> > you have not convinced me otherwise.  (See also the distinction between
> > collision-resistance and second-preimage-resistance alluded to in my
> > comment on the previous section.)
> 
> Other changes may help, or not.  We can rejoin this point after draft 14 
> (note I will be pushing out draft 13 today for the publish deadline for 
> changes done so far).
> 
> >     here through either an ACL or some other lookup process that
> >     externally binds the HIT and HI.
> >
> > Without at least one well-specified mechanism for actually doing this
> > and clear documentation of what precise properties such a mechanism
> > needs to provide, I think it's irresponsible to publish this document.
> 
> In the works.
> 
> > Section 4.1
> >
> >     By definition, the system initiating a HIP Diet EXchange is the
> >     Initiator, and the peer is the Responder.  This distinction is
> >     typically forgotten once the handshake completes, and either party
> >     can become the Initiator in future communications.
> >
> > ["typically" again]
> 
> same response.
> 
> >     Diffie-Hellman Group IDs supported by the Initiator.  Note that in
> >     some cases it may be possible to replace this trigger packet by some
> >     other form of a trigger, in which case the protocol starts with the
> >     Responder sending the R1 packet.  In such cases, another mechanism to
> >     convey the Initiator's supported DH Groups (e.g., by using a default
> >     group) must be specified.
> >
> > This seems under-specified for a proposed standard and is probably
> > better off omitted entirely.
> 
> This is carried over from 5201, which WAS experimental.  So I can see it 
> as reasonable to drop this as no one proposed another mechanism.
> 
>     The Initiator first sends a trigger packet, I1, to the Responder.
>     This packet contains the HIT of the Initiator and the HIT of the
>     Responder, if it is known.  Moreover, the I1 packet initializes the
>     negotiation of the Diffie-Hellman group that is used for generating
>     the Master Key SA.  Therefore, the I1 packet contains a list of
>     Diffie-Hellman Group IDs supported by the Initiator.

("Therefore" feels a bit out of place, but this helps, thanks.)
> 
> >     The second packet, R1, starts the actual authenticated Diffie-Hellman
> >     key exchange.  It contains a puzzle - a cryptographic challenge that
> >     the Initiator must solve before continuing the exchange.  The level
> >     of difficulty of the puzzle can be adjusted based on level of trust
> >     with the Initiator, current load, or other factors.  In addition, the
> >
> > The Initiator is unauthenticated at this point, so "level of trust"
> > seems to not really be defined...
> 
> Changed to "knowledge of the".  If the Responder "knows" that the 
> Initiator is a sensor, using a smaller puzzle may be preferred. there is 
> discussion about large puzzles being an attack on sensors.
> 
> > Section 4.1.1
> >
> > If an unconstrained (DoSing) attacker is competing with a constrained
> > honest initiator to solve puzzles during an attack, it seems like the
> > honest initiator is going to lose out pretty badly.
> 
> You do what you can that makes some degree of sense.  You just don't 
> walk away from the problem.
> 
> > Section 4.1.4
> >
> > There are security considerations for serializing the HIP state to
> > nonvolatile storage!
> 
> Do you want text about this in the Securities Considerations?

Yes, please!

I see there's new text about having the ENCRYPTED_KEY counter be part of
the state stored to nonvolatile storage, which is good.  We may also want
discussion of whether having any crypto state (e.g., private key) on
nonvolatile storage is a security risk when there is physical device
access, and how bad ("very!") the consequences are when the data non
nonvolatile storage is not properly sync'd or lost entirely.

Thanks for the updates,

Ben