Jim, All,
please see a proposal for the Applicability statement that can be used as a starting point for the Webex. Abhinav 5.1 Applicability statement This document describes two architectures based on symmetric group keys in Section 3 and asymmetric keys in Section 4. The symmetric key solution is based on a group key that is shared between all group members including senders and receivers. As all members of the group posses the same key, it is only possible to authenticate group membership for the source of a message. In particular, it is not possible to authenticate the unique source of a message and consequently it is not possible to authorize a single node to control a group. This implies in particular that any hacked receiver in a group could then be used to control all the receivers in the group. Moreover, because the group key is shared across multiple nodes, it may be easier for an attacker to determine the group key by attacking any member of the group (note that this group key is dynamically generated and is usually stored in volatile memory which offers some additional protection). The probability of a stolen key increases with the number of nodes that are in possession of the key. Moreover, subsequent to such an attack, it is also difficult to determine which of the group members was compromised and this makes it difficult to return the system to normal operation after an attack. The asymmetric key solution distinguishes between a sender in the group and the receivers. In particular, the sender is in possession of a private key and the receivers are in possession of the corresponding public key. This allows the unique source of any group message to be authenticated. Moreover, an attacker cannot compromise the system by breaking into any of the receiving nodes. However, for constrained devices, the asymmetric key solution comes at a processing cost with cryptographic computations taking rather long. Therefore, it is recommended that whenever possible, the architecture with source authentication SHOULD be used to secure all multicast communication. However, in less sensitive applications where low-latency group communication is important (e.g. controlling luminaires in non-emergency applications), the architecture without source authentication MAY be used. In sensitive applications such as health and safety, building security and emergency applications the symmetric key based solution SHOULD not be used. When using the symmetric key solution two mitigating factors could improve system security. It is possible to achieve source authentication of messages at lower layers by requiring unique MAC layer keys for all devices within the network. The symmetric group keys are dynamically generated and therefore SHOULD be stored in volatile memory. ________________________________ From: Jim Schaad <[email protected]> Sent: Friday, February 3, 2017 7:02:42 PM To: Somaraju Abhinav; [email protected] Cc: 'ace' Subject: RE: [Ace] draft-somaraju-ace-multicast See comments inline From: Ace [mailto:[email protected]] On Behalf Of Somaraju Abhinav Sent: 02 February 2017 03:48 To: Jim Schaad <[email protected]>; [email protected] Cc: 'ace' <[email protected]> Subject: Re: [Ace] draft-somaraju-ace-multicast Hi Jim, thank you for the review and I apologise for the delayed response - I was on sick leave due to a surgery. Please see comments inline from the authors. Why restriction on reading messages? It is not like an external observer is not going to be able to see the lights go on or off. [AS] There are several situations where lights are not visible but (multicast) network data is accessible. Moreover, sensors (e.g. presence detectors) are continuously talking to actuators and controllers without necessarily having a visible effect on the lights. For several customers privacy is a very important concern and is almost a given. The statement "anybody can listen to the traffic and tell when sensors detect presence in a building without even being in the building" is a very difficult sell. Having said that, it is true that simply encrypting the multicast traffic at the application layer is only a prerequisite to provide the privacy needed and additional work is required (e.g. generating random messages at different times). In that sense the symmetric solution is probably not much better than the asymmetric solution. But the demand for privacy from customers is very clear and the perception among them is that unencrypted data implies poor security. [JLS] I am sensing a problem here. You have stated that there is a requirement that encryption is a requirement that people are going to say must be me. However, below you have stated that if authentication is a requirement then encryption suddenly becomes a non-requirement? You appear to be stating that there are circumstances where it is fine not to have the data encrypted if one needs to know where it came from. Consider the following case I have a sensor in a room. When the sensor sees movement, it broadcasts a lights one command. The command is picked up by both the lightbulbs and by the security system. The security system must know which sensor provided the command and therefore no encryption is going be needed here? That just seems wrong. Additionally, the situation where things are “continuously” talking would seem to be a good place where one would want to install a controller and not have the sensor directly talking to the actuator. You don’t want to flood the actuators with trying to constantly turn on the lights. Also the use of actuators in this sense makes one think that this is a solution for things other than lighting systems which is what people are complaining about. The solution in section 4 does not seem to meet the following requirement "Only authorized members of the application group must be able to read and process messages." [AS] You are right, we cannot satisfy the privacy requirement in Section 4. We could extend the current solution to include a group wide encryption key to meet this requirement. However, this will add additional latency to the asymmetric solution. This document needs to have a solution for dealing with nonce space allocation for the cases where more than one sender is going be able to use the same key. This is going to be part of the problems with replay detection as well as security considerations. [AS] Okay. Will add some text in the next version of the draft for better clarification. The idea as written in 4.3 (Nonce value) is to use the Client ID along with the sender’s sequence number to create the complete nonce for replay and CCM processing. Should the algorithms be using high water detection of sequence numbers rather than the case of not yet used? Or is that an application specific type thing? [SK] This is tricky since it can create all kind of new issues. One way to handle if the sequence number of a sender is about to roll over is that the sender requests a new key issued for the group by the KDC. Tricky part is if there are multiple senders who are not reaching the roll over of their sequence number then have to be forced to use a new key or there needs to be some overlap between the old key and new key before every sender in the group starts using the new key. [JLS] Lots of spinning in graves from the idea of having a sequence number roll over given the harsh requirements that a nonce (built from the sequence number) must never be re-used twice for many of the algorithms that are going to be used here. I do not think that the current security requirements is sufficiently strident to reflect both the threat of breakage, cross-breakage and restrictions on where it should be used to pass muster. [AS] I thing this will be the main discussion item in the webex. We will make a proposal for the security guidelines section after the interim webex. [JLS] A proposal before the call is better because then we have a starting point for discussions as well as allowing people who will not make the call be able to have some initial input on where discussions points should be directed. _______________________________________________ Ace mailing list [email protected]<mailto:[email protected]> https://www.ietf.org/mailman/listinfo/ace ________________________________________________________ The contents of this e-mail and any attachments are confidential to the intended recipient. They may not be disclosed to or used by or copied in any way by anyone other than the intended recipient. If this e-mail is received in error, please immediately notify the sender and delete the e-mail and attached documents. 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