EPIC for Hidden Paths
Author: Marc Wyss, Markus Legner
Last updated: 2020-10-14
Discussion at: #3884
This file introduces EPIC and documents the design rationales and best practices for EPIC-HP.
One important security property of SCION is that end hosts are not allowed to send packets along arbitrary paths, but only along paths that were advertised by all on-path ASes. This property is called path authorization. The ASes only advertise paths that serve their economic interests, and path authorization protects those routing decisions against malicious end hosts.
In SCION, this is implemented by having the ASes create authenticators during beaconing, which end hosts then have to include in the MAC field of their packets. Those MAC fields prove to the ASes, that the packets are allowed to traverse them. The MAC fields are static, meaning that they are the same for every packet on that path.
However, this implementation of path authorization is insufficient against adversaries that are able to observe the MAC fields in data plane packets of other hosts. Because the MACs are static, once observed MACs for some path can be reused by the adversary to send arbitrarily many other packets (until the authenticators expire).
The EPIC (Every Packet Is Checked) protocol  solves this problem by introducing per-packet MACs. Even if an adversary is able to discover the MACs for one packet, he cannot reuse the MACs to send any other traffic. The improved path authorization provided by EPIC is especially important for hidden paths . Hidden paths are paths which are not publicly announced, but only communicated to a group of authorized sources. If one of those sources sends traffic on the hidden path using SCION path type packets, an on-path adversary can observe the MACs and reuse them to send traffic on the hidden path himself. This allows the adversary to reach services that were meant to be hidden, or to launch DoS attacks directed towards them. EPIC precludes such attacks, making hidden paths more secure.
EPIC-HP (EPIC for Hidden Paths) provides the improved security properties of EPIC on the very last inter-AS link of a path. It is meant as a lightweight EPIC version and is specifically designed to better protect hidden paths. We will refer to ASes and hosts protected by a hidden path as “being behind a hidden link”.
With EPIC-HP, the last two ASes on a path extend the SCION authenticators to 16 bytes (instead of only 6 bytes). ASes and end hosts that want to send EPIC-HP traffic on such a path need to be in the possession of those extended authenticators. If they are, they can use the authenticators to calculate two per-packet MACs (for the last and penultimate ASes), which they add to the EPIC-HP packet header. The forwarding of EPIC-HP traffic at the on-path ASes is the same as for SCION path type traffic, which is possible because the EPIC-HP path type header contains the complete SCION path type header. In addition to the normal SCION forwarding operations, the last two ASes on the path recompute and validate the per-packet MACs. If the validation succeeds, the packet is forwarded, otherwise it is dropped.
Furthermore, the last two ASes on the path need to either filter out non-EPIC-HP traffic or prioritize EPIC-HP over other path type traffic. For this, the last AS on the path needs cooperation from its upstream provider.
EPIC-HP makes the following assumptions necessary to provide a meaningful level of security:
The AS protected by the hidden path is the last AS. By “last AS” we mean that the beacon which defined the hidden path ends in this AS. Or stated differently: the AS reachable through the hidden path does not forward the beacon defining the hidden path to further downstream/peering ASes.
On the interface-pairs (ingress/egress pair) that affect the hidden path, the last two ASes employ one of two different strategies in the data plane:
The last two ASes of the hidden path have a duplicate-suppression system in place . This prohibits DOS attacks based on replayed packets.
Here, AS 6 is the AS protected by the hidden path (blue lines). The hidden path terminates at AS 6 (represented by black dot), so AS 6 did not forward the beacon that defines the hidden path further down to AS 7. This is however still allowed for SCION path type traffic (green lines): there are SCION paths that enter AS 6 from AS 4. One of the two paths ends in AS 6, while the other one is extended further to AS 7.
In this example, the border routers of AS 6 and AS 5 (the last and penultimate ASes on the hidden path) further implement path type filtering (red dots). For example, AS 5 will block (red “X” in the figure) SCION path type traffic from AS 3 that is destined towards AS 6, as it would affect the hidden path. Instead of blocking non-EPIC-HP path type traffic, ASes could also prioritize EPIC-HP traffic, which would still satisfy the assumptions above.
Of course the ASes can always decide to be more restrictive, for example AS 6 could additionally disallow SCION path type traffic from AS 4, so that it is reachable through the hidden path only.
SCION Path Type Responses
EPIC-HP path type packets contain the full SCION path type header plus a timestamp and verification fields for the penultimate and last ASes on the path.
The included SCION path type header allows the destination behind a hidden link to directly respond with SCION path type packets. The destination only has to extract the SCION path type header from the EPIC-HP header and reverse the path. If the source is protected by a hidden path itself, the destination needs to respond with EPIC-HP traffic. This means that the destination is responsible to configure or fetch the necessary authenticators.
In the control plane, the ASes do not only append 6 bytes of the hop authenticators to the beacon, but also the remaining 10 bytes (the authenticator is the 16 byte long output of a MAC function).
The data plane operations for EPIC-HP path type packets are the same as for SCION path type packets, but the source additionally computes two per-packet validation fields for the penultimate and last ASes on the path. The last two ASes need to validate the fields accordingly. A more concise description can be found in the EPIC-HP path type specification.
Distributing the Authenticators
The last AS on the path needs to distribute the authenticators to the set of trusted sources that should be able to send EPIC-HP traffic over the hidden link. This can be done over any secure channel, for example using secure end-to-end protocols, out-of-band communication, or the dedicated hidden path infrastructure .
Path Type Filtering
Network operators should be able to clearly define which kind of traffic (SCION, EPIC-HP, COLIBRI, and other protocols) they want to allow. Therefore, for each AS and every interface pair, it should be configurable to allow only certain types of traffic. The path type filtering between two interfaces can be different depending on the direction of the traffic. We might for example only allow EPIC-HP traffic from some interface 1 to some interface 2, but not apply any filtering for traffic from interface 2 to interface 1.
There are two main applications for EPIC-HP:
Highly Secure Hidden Paths
The last and penultimate ASes on the hidden path only allow EPIC-HP traffic on the interface pairs that affect the hidden path. With such a setup it is not possible for unauthorized sources to reach the services in the last AS. Therefore, EPIC-HP effectively prevents adversaries from running attacks like denial of service, or attack preparations like scanning the services for vulnerabilities.
If some host H1 inside an AS with such a setup wants to communicate with a host inside another AS that is also behind a hidden link, both hosts need to have valid authenticators to send traffic over the corresponding hidden paths - the hosts can exclusively communicate using EPIC-HP.
Note that hosts behind a hidden link can send SCION path type packets towards hosts in other ASes, but that those hosts can not send a response back if they do not have the necessary authenticators.
DoS-Secure Hidden Paths
The last and penultimate ASes on the hidden path allow EPIC-HP and other path types simultaneously, but prioritize traffic using the EPIC-HP path type over the SCION path type. This has the advantage that the link can still be used as a normal SCION path type access link. DOS attacks are not possible in this case, because an adversary is limited to sending low-priority SCION path type packets. However, an adversary can still reach the services behind the hidden link.
In this scenario hosts behind a hidden link can send SCION path type packets towards hosts in other ASes, and those hosts can reply with SCION path type traffic when they do not have the necessary authenticators to send back EPIC-HP traffic.