Hi Ioana,
I apology for the delay. It is still unclear to me what the Key Server is
doing with Finished message. My understanding is that the Key Server checks
the Finished message. If that is the case, I believe the handshake messages
needs also to be provided by the edge_server to the Key Server, in which
case the exchange would become:
+++ mention the addition.
TLS12 Request Header
TLS12MasterRSARequestPayload
key_id
client_random
S
pfs
EncryptedPremasterSecret
++++ client Finished
++++ client Handshake Context
-------->
server_random = pfs( S )
master_secret = PRF(\
pre_master_secret + \
"master secret" +\
client_random +\
server_random)[0..47];
++++ check server_randoms in the Handshake
Context
++++ check Finished versus Handshake Context
TLS12 Response Header
TLS12MasterResponsePayload
master
<--------
Is that correct ?
Yours,
Daniel
On Tue, Jun 12, 2018 at 4:41 AM, <i.boureanu=40surrey.ac...@dmarc.ietf.org>
wrote:
> Hi,
>
>
>
>> <daniel>
> I agree that finding a newly found S' that achieves pfs( S' ) = pfs( S )
> would make this attack feasible. My understanding is that this corresponds
> to a pre-image attack, and cryptographic hash function such as the
> currently specified sha256 are resistant to these attacks.
>
>
> This attack is to find a collision
>
> I agree it would take time and computation power to find a collision.
>
> I also agree that we have not YET exposed any collision for sha256.
>
> BUT you offered this as a computationally-cheaper solution to something
> else I proposed, so I looked at it overall.
> I was looking at this from the perspective of your *pfs* being *ANY* hash
> function and the perfect forward secrecy and therefore channel security
> being potentially ever broken.
> *Conceptually*, you have this collision problem with an offline attack
> which can happen over long time.
>
> Now, sending the Client Finished to the key-server (along with all that
> you send already) helps counteract this attack.
>
>
>
>
>
>
>
>> *@my solution, still*
>> *3. *One way in which I see that we can protect again these is the
>> following: the key-sever produces the “*server-random*” to be send to
>> the client and the key-server accepts a query on an "*encrypted pmk” *only
>> in some time-window from when it generated that “*server-random*”. The
>> key-server is sent the *Client Finished message *too on a query on a
>> given "*encrypted pmk”**, *and the key-server only processes the query
>> if it sees that it is on the *server-random* that it sent some
>> milliseconds before.
>>
>> In other words, this point * 3 *of mine, just above equates still what I
>> said on the 24th in terms of solutions to the pb.
>> You can revisit that as well, further down in this email.
>>
>> <daniel>
> The inconvenient I found with this alternative are that:
> * it requires an additional round trip to retrieve the server_random value
> which adds latency.
> * it requires additional data to be sent to the key server which adds
> latency.
> * when multiple Key servers are used for load balancing, it may be hard to
> keep them synchronized unless we have a wide time window.
>
> The main advantage I see for using the Finished message is that we provide
> the ability to move to other hash functions as the output is not fixed and
> depends on the hash length. Such properties seems to me interesting when
> sha256 will not be considered safe. I am wondering how much the security of
> the Finished message relies on the security of the hash function.
> </daniel>
>
>
> Forget this solution; I agree that this is an overkill.
> If you engage the server at that early stage of the “hello”, it’d be for
> other reasons (like cross-protocol attacks) not to protect against this
> potential collision problem.
>
>
>
>> *@also (in any case, and almost irrespective of these attacks):*
>> *4.* At the step where * S*, *encrypted pmk* etc are sent from the
>> edge-server to the key-server, we should have the *edge-server send the
>> *Client
>> Finished message* to the key-server* too, so that the key-server can
>> verify the *Client Finished message* against the *client_random* too.
>>
>
> Sending the *Client Finished message* to the key-server does help though,
> as explained in the phrase above.
>
> Best,
> Ioana
>
>
>
>>
>> Chat soon.
>>
>> Best,
>> Ioana
>>
>>
>> On 26 May 2018, at 01:08, Daniel Migault <daniel.miga...@ericsson.com>
>> wrote:
>>
>> Hi Ioana,
>>
>> Thanks for the feed back. I agree with you that the document should be
>> focused on TLS 1.2. This is especially true as the designation of the
>> extension is "tls12". We are also planning to design an extension for TLS
>> 1.3 by next IETF in Montreal.
>>
>> My understanding of step 1 and 2 is to prevent a passive attacker to send
>> a request to the key server. The key server is bound to a specific TLS
>> handshake, by providing an random nonce involved in the handshake (and
>> checking that random nonce has effectively been used in the exchange). More
>> specifically, a recorded TLS handshake can only be replayed if the nonce
>> provided by the key server matches the recorded one.
>>
>> The two drawback I see with this proposed mechanism are that it requires
>> two interactions with the key server ( one to send the ServerHello, and one
>> to retrieve the keys). Then, the handshake_message as well as the Finished
>> message needs to be send to the key server. The advantage is on the other
>> hand an explicit binding to a TLS handshake.
>>
>> The current protocol prevents an exchange to be replayed by generating
>> the server_random as the output of a hash function. server_random = hash( M
>> ) and the operations performed by the edge server as well as by the key
>> server. A passive attacker would observed server_random but could not
>> reverse M. In addition, the server_random carry the time and the key server
>> does not respond when the server_random is out of a windows. Note that this
>> latest time control may also be performed in your case.)
>>
>> I believe that the two mechanisms achieve the same goal with a different
>> perspective. Explicit biding, versus unability to replay a query based on
>> cryptographic hash function. While the mechanism is not described in the
>> appendix, I am wondering if you see any reason to change the mechanism.
>> That said agree that the appendix should be clarified and updated.
>>
>> Regarding 3) we effectively prove the master secret. This provides
>> session resumption for efficiency reasons.
>>
>>
>> Regarding extended master, the current design does not prevent anti
>> replay mechanism as the edge server provides the hash of the session. In
>> this case, there is probably a trade off between perfect forward secrecy
>> versus efficiency. I would be happy to know which direction we should
>> take. pfs would require sending the handshake messages to the key server so
>> the key server can generate the server_random and the session hash.
>>
>> Thanks you for your feed backs!
>>
>> Yours,
>> Daniel
>>
>>
>> On Thu, May 24, 2018 at 10:39 AM, <i.boureanu=40surrey.ac.uk@dma
>> rc.ietf.org> wrote:
>>
>>> Dear all,
>>>
>>> I’ve had a look at a draft of Lurk that Daniel Migault sent me a while
>>> back; it was dated February 2018.
>>> Here come a mix of comments:
>>>
>>>
>>> 1. I like the aspect of termination of TLS be split into different
>>> services (e.g., network + crypto); I think we should expand on this side..
>>> We should expand both because it’s a nice idea and because I’m a bit
>>> worried of weird DoS attacks where one service is left in limbo.
>>>
>>> 2. I would do away with TLS 1.1.
>>>
>>> 3. I would introduce a version for TLS 1.3.
>>>
>>> 4. Let us focus on annex A1 (Lurk/TLS 1.2. RSA mode)
>>>
>>> As you know there is this work: , "Content delivery over TLS: a
>>> cryptographic analysis of keyless SSL,” by K. Bhargavan, I. Boureanu, P. A.
>>> Fouque, C. Onete and B. Richard at *2017 IEEE European Symposium on
>>> Security and Privacy (EuroS&P)*, Paris, 2017, pp. 1-16.
>>> And an attack was shown on Cloudflare’s "Keyless SSL” when run in RSA
>>> mode.
>>>
>>> The attack rests on the fact that the client sends the “encrypted
>>> premaster secret” to the edge server who forwards this to the key server
>>> and the **answer the key-server gives back to the edge-server**.
>>> As per Annex A1,* it is not clear to me what does the key-server reply
>>> with, to the edge-server, in this step*. This we need to make clear.
>>>
>>> However, in this last step of the LURK handshake in TLS1.2. RSA-mode,
>>> the key-server should not *reply to the edge server with the premaster
>>> secret* .
>>> Such a reply would be an issue. Namely, if one edge server E1 becomes
>>> corrupt this is what it can be used to do.
>>> The attacker collects handshake data (which is over insecure channel)
>>> from one session —called session1-- between a client and an edge server E2;
>>> this collected data includes the “encrypted premaster secret” of this
>>> session 1.
>>> Then, the attacker used the corrupted edge server E2 to query the
>>> key-server on this “encrypted premaster secret” of session 1. The key
>>> server would reply back to the corrupted E2 with the premaster secret from
>>> session1. The attacker who controls E2 and has the handshake data from
>>> session1 can now obtain the channel key for session1 and therefore decrypt
>>> the record-layer of session1.
>>>
>>> In the work I mentioned above, there are *several solutions* to this
>>> and we can discuss them.
>>> *One solution I would suggest, and is very pertinent as a tightening of
>>> the design in LURK, is like so:*
>>>
>>> 1. the key-server is involved in the handshake at the beginning and
>>> generates a nonce N_S which is sent to the edge server who sends it further
>>> to the client, as to is essentially used in the ServerHello from the
>>> edge-server to the client.
>>>
>>> 2. the edge-server sends to the key server (in the step attacked above)
>>> not just the “encrypted premaster secret” but also the nonce of the client
>>> and the encrypted Finished message by the client. (In this way the
>>> key-server can find his nonce N_S inside the finished message and the
>>> attacker above is counteracted).
>>>
>>> 3. the key-server answers with *X, *where depending on what we wish for
>>> then we make *X* be different things. My top preference would be that *
>>> X *be the "channel keys + the Server-Finished message”. In this way,
>>> the edge-server cannot do session-resumption. This is therefore inefficient
>>> in practice. So, if we want session resumption, then we can make* X* be
>>> *pmk* or *msk*. Of course, we can link this to the options of the
>>> handshake..
>>>
>>>
>>> (Also, there is the question as to whether we want RSA mode, but this
>>> is orthogonal to the above).
>>>
>>>
>>> 5. I did not look at the description TLS 1.2 DHE-mode.
>>> But there we need to be able to describe well the beginning of the
>>> handshake as the work I mentioned above also exposes some weird
>>> cross-protocol attacks.
>>> I.e., the edge-server is corrupted and makes the key-server sign a QUIC
>>> hash (with a long TTL inside) and then this edge-server can run for quite
>>> some time.
>>> So, we need to pay some attention to this.
>>>
>>> Speak soon.
>>>
>>> Best,
>>> Ioana Boureanu
>>>
>>>
>>> Dr. Ioana Boureanu, FHEA
>>>
>>> Lecturer in Secure Systems
>>> Department of Computer Science
>>> Surrey Centre for Cyber Security
>>> University of Surrey, Guildford, GU2 7XH
>>> Web: people.itcarlson.com/ioana
>>> Linkedin: goo.gl/540OHa
>>> T.: +44 1483 683425
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>> https://www.ietf.org/mailman/listinfo/lurk
>>>
>>>
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