IRIF, University of Paris-Diderot

75205 Paris Cedex 13 France clarado_perso@yahoo.fr

IRIF, University of Paris-Diderot

75205 Paris Cedex 13 France weronika.kolodziejak@gmail.com

IRIF, University of Paris-Diderot

Case 7014 75205 Paris Cedex 13 France jch@irif.fr

This document describes a cryptographic authentication mechanism for the Babel routing protocol that has provisions for replay avoidance. This document updates RFC 6126bis and obsoletes RFC 7298.

By default, the Babel routing protocol trusts the information contained in every UDP packet it receives on the Babel port. An attacker can redirect traffic to itself or to a different node in the network, causing a variety of potential issues. In particular, an attacker might: spoof a Babel packet, and redirect traffic by announcing a smaller metric, a larger seqno, or a longer prefix; spoof a malformed packet, which could cause an insufficiently robust implementation to crash or interfere with the rest of the network; replay a previously captured Babel packet, which could cause traffic to be redirected or otherwise interfere with the network. Protecting a Babel network is challenging due to the fact that the Babel protocol uses both unicast and multicast communication. One possible approach, used notably by the Babel over DTLS protocol, is to require a secured version of Babel to use unicast communication for all semantically significant communication, and then use a standard unicast security protocol to protect the Babel traffic. In this document, we take the opposite approach: we define a cryptographic extension to the Babel protocol that is able to protect both unicast and multicast traffic, and thus requires very few changes to the core protocol.

The protocol defined in this document assumes that all interfaces on a given link are equally trusted and share a small set of symmetric keys (usually just one, two during key rotation). The protocol is inapplicable in situations where asymmetric keying is required, where the trust relationship is partial, or where large numbers of trusted keys are provisioned on a single link at the same time. This protocol supports incremental deployment (where an insecure Babel network is made secure with no service interruption), and it supports graceful key rotation (where the set of keys is changed with no service interruption). This protocol does not require synchronised clocks, it does not require persistently monotonic clocks, and it does not require any form of persistent storage.

The correctness of the protocol relies on the following assumptions: that the HMAC being used is invulnerable to spoofing, i.e. that an attacker is unable to generate a packet with a correct HMAC; that a node never generates the same index or nonce twice over the lifetime of a key. The first assumption is a property of the HMAC being used, and is therefore out-of-scope for this document. The second assumption can be met either by using a robust random number generator and sufficiently large indices and nonces, by using a reliable hardware clock, or by rekeying whenever a collision becomes likely. If the assumptions above are met, the protocol described in this document has the following properties: it is invulnerable to spoofing: any packet accepted as authentic is the exact copy of a packet originally sent by an authorised node; locally to a single node, it is invulnerable to replay: if a node has previously accepted a given packet, then it will never again accept a copy of this packet or an earlier packet from the same sender; among different nodes, it is only vulnerable to immediate replay: if a node A has accepted a packet from C as valid, then a node B will only accept a copy of that packet as authentic if B has accepted an older packet from C and B has received no later packet from C. While this protocol makes serious efforts to mitigate the effects of a denial of service attack, it does not fully protect against such attacks.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

When a node B sends out a Babel packet through an interface that is configured for cryptographic protection, it computes one or more HMACs which it appends to the packet. When a node A receives a packet over an interface that requires cryptographic protection, it independently computes a set of HMACs and compares them to the HMACs appended to the packet; if there is no match, the packet is discarded. In order to protect against replay B maintains a per-interface 32-bit integer known as the "packet counter" (PC). Whenever B sends a packet through the interface, it embeds the current value of the PC within the region of the packet that is protected by the HMACs and increases the PC by at least one. When A receives the packet, it compares the value of the PC with the one contained in the previous packet received from B, and unless it is strictly greater, the packet is discarded. By itself, the PC mechanism is not sufficient to protect against replay. Consider a peer A that has no information about a pair B (e.g., because it has recently rebooted). Suppose that A receives a packet ostensibly from B carrying a given PC; since A has no information about B, it has no way to determine whether the packet is freshly generated or a replay of a previously sent packet. In this situation, A discards the packet and challenges B to prove that it knows the HMAC key. It sends a "challenge request", a TLV containing a unique nonce, a value that has never been used before and will never be used again. B replies to the challenge request with a "challenge reply", a TLV containing a copy of the nonce chosen by A, in a packet protected by HMAC and containing the new value of B's PC. Since the nonce has never been used before, B's reply proves B's knowledge of the HMAC key and the freshness of the PC. By itself, this mechanism is safe against replay if B never resets its PC. In practice, however, this is difficult to ensure, as persistent storage is prone to failure, and hardware clocks, even when available, are occasionally reset. Suppose that B resets its PC to an earlier value, and sends a packet with a previously used PC n. A challenges B, B successfully responds to the challenge, and A accepts the PC equal to n + 1. At this point, an attacker C may send a replayed packet with PC equal to n + 2, which will be accepted by A. Another mechanism is needed to protect against this attack. In this protocol, every PC is tagged with an "index", an arbitrary string of octets. Whenever B resets its PC, or whenever B doesn't know whether its PC has been reset, it picks an index that it has never used before (either by drawing it randomly or by using a reliable hardware clock) and starts sending PCs with that index. Whenever A detects that B has changed its index, it challenges B again. With this additional mechanism, this protocol is provably invulnerable to replay attacks (see above).

Every Babel node maintains an interface table, as described in Section 3.2.3. This protocol extends the entries in this table with a set of HMAC keys, and a pair (Index, PC), where Index is an arbitrary string of bytes and PC is a 32-bit integer. The Index is initialised to a value that has never been used before (e.g., by choosing a random string of sufficient length).

Every Babel node maintains a neighbour table, as described in Section 3.2.4. This protocol extends the entries in this table with a pair (Index, PC), as well as a nonce (an arbitrary string of bytes) and a challenge expiry timer. The Index and PC are initially undefined, and are managed as described in . The Nonce and expiry timer are initially undefined and used as described in .

A Babel node computes an HMAC as follows. First, the node builds a pseudo-header that will participate in HMAC computation but will not be sent. The pseudo-header has the following format:

Fields : The source IP address of the packet. The source UDP port number of the packet. The destination IP address of the packet. The destination UDP port number of the packet. The node takes the concatenation of the pseudo-header and the packet including the packet header but excluding the packet trailer (from octet 0 inclusive up to Body Length + 4 exclusive) and computes an HMAC as defined in Section 2 of with one of the implemented hash algorithms. Every implementation MUST implement HMAC-SHA256 , and MAY implement other HMAC algorithms.

A Babel node may delay actually sending TLVs by a small amount, in order to aggregate multiple TLVs in a single packet up to the interface MTU (Section 4 of ). For an interface on which HMAC protection is configured, the TLV aggregation logic MUST take into account the overhead due to PC TLVs (one in each packet) and HMAC TLVs (one per configured key). Before sending a packet, the following actions are performed: a PC TLV containing the Packet Counter and Index associated with the outgoing interface is appended to the packet body; the packet counter is incremented by a strictly positive amount (typically just 1); if the packet counter overflows, a new index is generated; for each key configured on the interface, an HMAC is computed as specified in above, and an HMAC TLV is appended to the packet trailer.

When a packet is received on an interface that is configured for HMAC protection, the following steps are performed before the packet is passed to normal processing: First, the receiver checks whether the trailer of the received packet carries at least one HMAC TLV; if not, the packet is immediately dropped and processing stops. Then, for each key configured on the receiving interface, the implementation computes the HMAC of the packet. It then compares every generated HMAC against every HMAC included in the packet; if there is at least one match, the packet passes the HMAC test; if there is none, the packet is silently dropped and processing stops at this point. In order to avoid memory exhaustion attacks, an entry in the Neighbour Table MUST NOT be created before the HMAC test has passed successfully. The HMAC of the packet MUST NOT be computed for each HMAC TLV contained in the packet, but only once for each configured key. The packet body is then parsed a first time. During this "preparse" phase, the packet body is traversed and all TLVs are ignored except PC TLVs, Challenge Requests and Challenge Replies. When a PC TLV is encountered, the enclosed PC and Index are saved for later processing; if multiple PCs are found, only the first one is processed, the remaining ones are silently ignored. If a Challenge Request is encountered, a Challenge Reply is scheduled, as described in , and if a Challenge Reply is encountered, it is tested for validity as described in and a note is made of the result of the test. The preparse phase above has yielded two pieces of data: the PC and Index from the first PC TLV, and a bit indicating whether the packet contains a successful Challenge Reply. If the packet does not contain a PC TLV, the packet is dropped and processing stops at this point. If the packet contains a successful Challenge Reply, then the PC and Index contained in the PC TLV are stored in the Neighbour Table entry corresponding to the sender (which may need to be created at this stage). If there is no entry in the Neighbour Table corresponding to the sender, or if such an entry exists but contains no Index, or if the Index it contains is different from the Index contained in the PC TLV, then a challenge is sent as described in , processing stops at this stage, and the packet is dropped. At this stage, the Index contained in the PC TLV is equal to the Index in the Neighbour Table entry corresponding to the sender. The receiver compares the received PC with the PC contained in the Neighbour Table; if the received PC smaller or equal than the PC contained in the Neighbour Table, the packet is silently dropped and processing stops (no challenge is sent in this case, since the mismatch might be caused by harmless packet reordering on the link). Otherwise, the PC contained in the Neighbour Table entry is set to the received PC, and the packet is accepted. After the packet has been accepted, it is processed as normal, except that any PC, Challenge Request and Challenge Reply TLVs that it contains are silently ignored.

During the preparse stage, the receiver might encounter a mismatched Index, to which it will react by scheduling a Challenge Request. It might encounter a Challenge Request TLV, to which it will reply with a Challenge Reply TLV. Finally, it might encounter a Challenge Reply TLV, which it will attempt to match with a previously sent Challenge Request TLV in order to update the Neighbour Table entry corresponding to the sender of the packet.

When it encounters a mismatched Index during the preparse phase, a node picks a nonce that it has never used before, for example by drawing a sufficiently large random string of bytes or by consulting a strictly monotonic hardware clock. It stores the nonce in the entry of the Neighbour Table of the neighbour (the entry might need to be created at this stage), initialises the neighbour's challenge expiry timer to 30 seconds, and sends a Challenge Request TLV to the unicast address corresponding to the neighbour. A node MAY aggregate a Challenge Request with other TLVs; in other words, if it has already buffered TLVs to be sent to the unicast address of the sender of the neighbour, it MAY send the buffered TLVs in the same packet as the Challenge Request. However, it MUST arrange for the Challenge Request to be sent in a timely manner, as any packets received from that neighbour will be silently ignored until the challenge completes. Since a challenge may be prompted by a replayed packet, a node MUST impose a rate limitation to the challenges it sends; a limit of one challenge every 300ms for each neighbour is suggested.

When it encounters a Challenge Request during the preparse phase, a node constructs a Challenge Reply TLV by copying the Nonce from the Challenge Request into the Challenge Reply. It sends the Challenge Reply to the unicast address of the sender of the Challenge Reply. A node MAY aggregate a Challenge Reply with other TLVs; in other words, if it has already buffered TLVs to be sent to the unicast address of the sender of the Challenge Request, it MAY send the buffered TLVs in the same packet as the Challenge Reply. However, it MUST arrange for the Challenge Reply to be sent in a timely manner (within a few seconds), and SHOULD NOT send any other packets over the same interface before sending the Challenge Reply, as those would be dropped by the challenger. A challenge sent to a multicast address MUST be silently ignored.

When it encounters a Challenge Reply during the preparse phase, a node consults the Neighbour Table entry corresponding to the neighbour that sent the Challenge Reply. If no challenge is in progress, i.e., if there is no Nonce stored in the Neighbour Table entry or the Challenge timer has expired, the Challenge Reply is silently ignored and the challenge has failed. Otherwise, the node compares the Nonce contained in the Challenge Reply with the Nonce contained in the Neighbour Table entry. If the two are equal (they have the same length and content), then the challenge has succeeded; otherwise, the challenge has failed.

Fields : Set to TBD to indicate an HMAC TLV The length of the body, exclusive of the Type and Length fields. The length of the body depends on the hash function used. The body contains the HMAC of the whole packet plus the pseudo header. This TLV is allowed in the packet trailer (see ), and MUST BE ignored if it is found in the packet body.

Fields : Set to TBD to indicate a PC TLV The length of the body, exclusive of the Type and Length fields. The Packet Counter (PC), which is increased with every packet sent over this interface. A new index MUST be generated whenever the PC overflows. The sender's Index.

Fields : Set to TBD to indicate a Challenge Request TLV The length of the body, exclusive of the Type and Length fields. The nonce uniquely identifying the challenge.

Fields : Set to TBD to indicate a Challenge Reply TLV The length of the body, exclusive of the Type and Length fields. The length of the body is set to the same size as the challenge request TLV length received. A copy of the nonce contained in the corresponding challenge request.

IANA is instructed to allocate the following values in the Babel TLV Numbers registry: TypeNameReference TBDHMACthis document TBDPCthis document TBDChallenge Requestthis document TBDChallenge Replythis document

The protocol described in this document is based on the original HMAC protocol defined by Denis Ovsienko . The use of a pseudo-header was suggested by David Schinazi. The use of an index to avoid replay was suggested by Markus Stenberg. The authors are also indebted to Florian Horn and Toke Hoyland-Jorgensen.

The protocol described in this document uses the packet trailer for storing HMAC TLVs. RFC 6126bis leaves the format of the packet trailer undefined. If the final version of this specification uses the packet trailer, RFC 6126bis will need to be extended with information about the format of the packet trailer. This document assumes that the packet trailer has the same format as the packet body, i.e., that it consists of a sequence of TLVs. The receiver MUST silently ignore any TLV found in the packet trailer unless its definition states that the TLV is allowed in the packet trailer.

This protocol supports incremental deployment (transitioning from an insecure network to a secured network with no service interruption) and key rotation (transitioning from a set of keys to a different set of keys). In order to perform incremental deployment, the nodes in the network are first configured in a mode where packets are sent with authentication but not checked on reception. Once all the nodes in the network are configured to send authenticated packets, nodes are reconfigured to reject unauthenticated packets. In order to perform key rotation, the new key is added to all the nodes; once this is done, both the old and the new key are sent in all packets, and packets are accepted if they are properly signed by either of the keys. At that point, the old key is removed. In order to support incremental deployment and key rotation, implementations SHOULD support an interface configuration in which they send authenticated packets but accept all packets, and SHOULD allow changing the set of keys associated with an interface without a restart.

[This appendix describes the "implicit indices" variant of the protocol, which is different and incompatible to the "explicit indices" variant described in the body of this document. This section should either be integrated into the body of the document or removed before publication of this document as an RFC, depending on which protocol variant is finally chosen.] The protocol described in the body of this document explicitly sends indices as in each packet as part of the PC TLV. Observe that, except when a challenge is required, the index sent on the wire is identical to the index stored in the Neighbour Table, and therefore doesn't need to be sent explicitly except during challenges: it is enough for it to participate in HMAC computation in order to protect against replay. The "implicit indices" variant of the protocol, due to Markus Stenberg and described in this appendix, uses this observation to avoid sending indices explicitly and thus shaves off 2 to 16 octets from almost every packet. The changes to the protocol are as follows. The pseudo-header includes the Index, and therefore has the following format:

The PC TLV no longer contains an Index, and therefore has the following format:

This TLV is now self-terminating, and therefore allows sub-TLVs. Packets containing the Challenge Reply and Challenge Request TLVs must contain an explicit index. Two encodings are possible: one uses Challenge Replies and Requests with an extra field for the sender's index, which complicates the encoding somewhat but makes these two TLVs self-terminating, the other one uses a new TLV that is used for carrying an Index, which uses up a new TLV number but makes it possible to reuse these two TLV with other protocols that require a nonce-based challenge. Packet transmission is modified as follows. If a packet contains a Challenge or a Challenge Reply, then the node inserts its index into the packet body. In any case, it uses its current index to generate the pseudo-header that will be used to compute the HMAC. (This implies that a packet must be parsed in its entirety before HMAC validation, which requires a robust parser.) Packet reception is modified as follows. Before checking the HMAC of a packet, the receiver checks whether the packet contains an explicit index. If this is the case, it uses the index contained in the packet in order to generate the pseudo header; if this is not the case, it uses the index contained in its neighbours table. If there is no index available for that neighbour (either because the table doesn't contain in an entry for this neighbour, or the entry doesn't contain an index), HMAC validation fails. The index and PC contained in the neighbours table are only updated after HMAC validation has succeeded. Since it is now impossible to differentiate between a failed HMAC and an index change, a node must send a challenge whenever HMAC validation fails. This implies that spoofed packets cause a spurious challenge, but that doesn't change the security properties of the protocol much, given that in any case replayed packets can be used to cause a spurious challenge.