| Network Working Group | C. Huitema |
| Internet-Draft | Private Octopus Inc. |
| Intended status: Informational | September 30, 2018 |
| Expires: April 3, 2019 |
DNS-SD Privacy Scaling Tradeoffs
draft-ietf-dnssd-privacyscaling-00
DNS-SD (DNS Service Discovery) normally discloses information about both the devices offering services and the devices requesting services. This information includes host names, network parameters, and possibly a further description of the corresponding service instance. Especially when mobile devices engage in DNS Service Discovery over Multicast DNS at a public hotspot, a serious privacy problem arises.
The draft currently progressing in the DNS-SD Working Group assumes peer-to-peer pairing between the service to be discovered and each of its clients. This has good security properties, but creates scaling issues, because each server needs to publish as many announcements as it has paired clients. This leads to large number of operations when servers are paired with many clients.
Different designs are possible. For example, if there was only one server "discovery key" known by each authorized client, each server would only have to announce a single record, and clients would only have to process one response for each server that is present on the network. Yet, these designs will present different privacy profiles, and pose different management challenges. This draft analyses the tradeoffs between privacy and scaling in a set of different designs, using either shared secrets or public keys.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 3, 2019.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
DNS-SD [RFC6763] over mDNS [RFC6762] enables configurationless service discovery in local networks. It is very convenient for users, but it requires the public exposure of the offering and requesting identities along with information about the offered and requested services. Parts of the published information can seriously breach the users' privacy. These privacy issues and potential solutions are discussed in [KW14a] and [KW14b].
A recent draft [I-D.ietf-dnssd-privacy] proposes to solve this problem by relying on device pairing. Only clients that have paired with a device would be able to discover that device, and the discovery would not be observable by third parties. This design has a number of good privacy and security properties, but it has a cost, because each server must provide separate annoucements for each client. In this draft, we compare scaling and privacy properties of three different designs:
After presenting briefly these three solutions, the draft presents the scaling and privacy properties of each of them.
Private discovery tries to ensure that clients and servers can discover each other in a potentially hostile network context, while maintaining privacy. Unauthorized third parties must not be able to discover that a specific server or device is currently present on the network, and they must not be able to discover that a particular client is trying to discover a particular service. This cannot be achieved without some kind of shared secret between client and servers. We review here three particular designs for sharing these secrets.
The solution proposed in [I-D.ietf-dnssd-privacy] relies on pairing secrets. Each client obtains a pairing secret from each server that they are authorized to use. The servers publish announcements of the form "nonce|proof", in which the proof is the hash of the nonce and the pairing secret. The proof is of course different for each client, because the secrets are different. For better scaling, the nonce is common to all clients, and defined as a coarse function of time, such as the current 30 minutes interval.
Clients discover the required server by issuing queries containing the current nonce and proof. Servers respond to these queries if the nonce matches the current time interval, and if the proof matches the hash of the nonce with one of the pairing key of an authorized client.
In contrast to pair-wise shared secrets, applications may associate public and private key pairs with groups of equally authorized clients. This is identical to the pairwise sharing case if each client is given a unique key pair. However, this option permits multiple users to belong to the same group associated with a public key, depending on the type of public key and cryptographic scheme used. For example, broadcast encryption is a scheme where many users, each with their own private key, can access content encrypted under a single broadcast key. The scaling properties of this variant depend not only on how private keys are managed, but also on the associated cryptographic algorithm(s) by which those keys are used.
Instead of using a different secret for each client as in Section 2.1, another design is to have a single secret per server, shared by all authorized clients of that server. As in the previous solution, the servers publish announcements of the form "nonce|proof", but this time they only need to publish a single announcement per server, because each server maintains a single discovery secret. Again, the nonce can be common to all clients, and defined as a coarse function of time.
Clients discover the required server by issuing queries containing the current nonce and proof. Servers respond to these queries if the nonce matches the current time interval, and if the proof matches the hash of the nonce with one of the discovery secrets.
Instead of a discovery secret used in Section 2.3, clients could obtain the public keys of the servers that they are authorized to use.
Many public key systems assume that the public key of the server is, well, not secret. But if adversaries know the public key of a server, they can use that public key as a unique identifier to track the server. Moreover, they could use variations of the padding oracle to observe discovery protocol messages and attribute them to a specific public key, thus breaking server privacy. For these reasons, we assume here that the discovery public key is kept secret, only known to authorized clients.
As in the previous solution, the servers publish announcements of the form "nonce|proof", but this time they only need to publish a single announcement per server, because each server maintains a single discovery secret. The proof is obtained by either hashing the nonce with the public key, or using the public key to encrypt the nonce -- the point being that both clients and server can construct the proof. Again, the nonce can be common to all clients, and defined as a coarse function of time.
The advantage of public key based solutions is that the clients can easily verify the identity of the server, for example if the service is accessed over TLS. On the other hand, just using standard TLS would disclose the certificate of the server to any client that attempts a connection, not just to authorized clients. The server should thus only accept connections from clients that demonstrate knowledge of its public key.
To analyze scaling issues we will use the following variables:
The big difference between the three proposals is the number of records that need to be published by a server when using DNS-SD in server mode, or the number of broadcast messages that needs to be announced per server in mDNS mode:
There are other elements of scaling, linked to the mapping of the privacy discovery service to DNS-SD. DNS-SD identifies services by a combination of a service type and an instance name. In classic mapping behavior, clients send a query for a service type, and will receive responses from each server instance supporting that type:
The DNS-SD Privacy draft suggests an optimization that considerably reduces the considerations about scaling of responses -- see section 4.6 of [I-D.ietf-dnssd-privacy]. In that case, clients compose the list of instance names that they are looking for, and specifically query for these instance names:
Finally, another element of scaling is cacheability. Responses to DNS queries can be cached by DNS resolvers, and mDNS responses can be cached by mDNS resolvers. If several clients send the same queries, and if previous responses could be cached, the client can be served immediately. There are of course differences between the solutions:
The analysis of scaling issues in Section 3 shows that the solutions base on a common discovery secret or discovery public key scale much better than the solutions based on pairing secret. All these solutions protect against tracking of clients or servers by third parties, as long as the secret on which they rely are kept secret. There are however significant differences in privacy properties, which become visible when one of the clients becomes compromised.
If a client is compromised, an adversary will take possession of the secrets owned by that client. The effects will be the following:
Assume an administrator discovers that a client has been compromised. As seen in Section 4.1, compromising a client entails a loss of privacy for all the servers that the client was authorized to use, and also to all other users of these servers. The worse situation happens in the solutions based on "discovery secrets", but no solution provides a great defense. The administrator will have to remedy the problem, which means different actions based on the different solutions:
The revocation of public keys might benefit from some kind of centralized revocation list, and thus may actually be easier to organize than simple scaling considerations would dictate.
If a server is compromised, an adversary will take possession of the secrets owned by that server. The effects are pretty much the same in all configurations. With a set of valid credentials, the adversary can impersonate the server. It can track all of the server's clients. There are no differences between the various solutions.
As remedy, once the compromise is discovered, the administrator will have to revoke the credentials of O(N) clients, or O(G) groups, connected to that server. In all cases, this could be done by notifying all potential clients to not trust this particular server anymore.
In the preceding sections, we have reviewed the scaling and privacy properties of three possible secret sharing solutions for privacy discovery. The comparison can be summed up as follow:
| Solution | Scaling | Resistance | Remediation |
|---|---|---|---|
| Pairing secret | Poor | Bad | Good |
| Group public key | Medium | Bad | Maybe |
| Shared symmetric secret | Good | Really bad | Poor |
| Shared public secret | Good | Bad | Maybe |
All four types of solutions provide reasonable privacy when the secrets are not compromised. They all have poor resistance to the compromise of a client, as explained in Section 4.1, but sharing a symmetric secret is much worse because it does not prevent server impersonation. The pairing secret solution scales worse than the discovery secret and discovery public key solutions. The group public key scales as the number of groups for the total set of clients; this depends on group assignment and will be intermediate between the pairing secret and shared secret solutions. The pairing secret solution can recover from a compromise with a smaller number of updates, but the public key solutions may benefit from a simple recovery solution using some form of "revocation list".
This document does not specify a solution, but discusses future choices when providing privacy for discovery protocols.
This draft does not require any IANA action.
This draft results from initial feedback in the DNS SD working group on [I-D.ietf-dnssd-privacy]. The text on Group public keys is based on Chris Wood's contributions.
This section surveys several private service discovery designs in the context of the threat model detailed above.
Huitema and Kaiser [I-D.ietf-dnssd-privacy] decompose private service discovery into two stages: (1) identify specific peers offering private services, and (2) issue unicast DNS-SD queries to those hosts after connecting over TLS using a previously agreed upon pre-shared key (PSK), or pairing key. Any out-of-band pairing mechanism will suffice for PSK establishment, though the authors specifically mention [I-D.ietf-dnssd-pairing] as the pairing mechanism. Step (1) is done by broadcasting "private instance names" to local peers, using service-specific pairing keys. A private instance name N' for some service with name N is composed of a unique nonce r and commitment to r using N_k. Commitments are constructed by hashing N_k with the nonce. Only owners of N_k may verify its correctness and, upon doing so, answer as needed. The draft recommends randomizing hostnames in SRV responses along with other identifiers, such as MAC addresses, to minimize likability to specific hosts. Note that this alone does not prevent fingerprinting and tracking using that hostname. However, when done in conjunction with steps (1) and (2) above, this mitigates fingerprinting and tracking since different hostnames are used across venues and real discovered services remain hidden behind private instance names.
After discovering its peers, a node will directly connect to each device using TLS, authenticated with a PSK derived from each associated pairing key, and issue DNS-SD queries per usual. DNS messages are formulated as per [RFC7858].
As an optimization, the authors recommend that each nonce be deterministically derived based on time so that commitment proofs may be precomputed asynchronously. This avoids O(N*M) computation, where N is the number of nodes in a local network and M is the number of per-node pairings.
This system has the following properties:
Boneh et al. [Wu16] developed an approach for private service discovery that reduces to private mutual authentication. Moreover, it should be infeasible for any adversary to forge advertisements or impersonate anyone else on the network. Specifically, service discoverers only wish to reveal their identity to services they trust, and vice versa. Existing protocols such as TLS, IKE, and SIGMA require that one side reveal its identity first. Their approach first allocates, via some policy manager, key pairs associated with human-readable policy names. For example, user Alice might have a key pair associated with the names /Alice, /Alice/Family, and /Alice/Device. Her key is bound to each of these names. Authentication policies (and trust models) are then expressed as policy prefix patterns, e.g., /Alice/*. Broadcast messages are encrypted to policies. For example, Alice might encrypt a message m to the policy /Bob/*. Only Bob, who owns a private key bound to, e.g., /Bob/Devices, can decrypt m. (This procedure uses a form of identity-based encryption called prefix-based encryption. Readers are referred to [Wu16] for a thorough description.)
Using prefix- and policy-based encryption, service discovery is decomposed into two steps: (1) service announcement and (2) key exchange, similar to [I-D.ietf-dnssd-privacy]. Announcements carry service identities, ephemeral key shares, and a signature, all encrypted under the service’s desired policy prefix, e.g., /Alice/Family/*. Upon receipt of an announcement, clients with matching policy private keys can decrypt the announcement and use the ephemeral key share to perform an Authenticated Diffie Hellman key exchange with the service. Upon completion, the derived shared secret may be used for any further communication, e.g., DNS-SD queries, if needed.
This system has the following properties: