Network Working Group C. Do
Internet-Draft W. Kolodziejak
Obsoletes: 7298 (if approved) J. Chroboczek
Updates: 6126bis (if approved) IRIF, University of Paris-Diderot
Intended status: Standards Track March 9, 2019
Expires: September 10, 2019

HMAC authentication for the Babel routing protocol


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.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on September 10, 2019.

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Table of Contents

1. Introduction

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:

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 Datagram Transport Layer Security (DTLS) protocol [I-D.ietf-babel-dtls], is 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.

1.1. Applicability

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, and 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 persistent storage except for what might be required for storing cryptographic keys.

1.2. Assumptions and security properties

The correctness of the protocol relies on the following assumptions: [RFC4086] and sufficiently large indices and nonces, by using a reliable hardware clock, or by rekeying whenever a collision becomes likely.

The first assumption is a property of the HMAC being used. The second assumption can be met either by using a robust random number generator

If the assumptions above are met, the protocol described in this document has the following properties:

While this protocol makes serious efforts to mitigate the effects of a denial of service attack, it does not fully protect against such attacks.

1.3. Specification of Requirements

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Conceptual overview of the protocol

When a node B sends out a Babel packet through an interface that is configured for HMAC 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 HMAC 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 peer 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 invulnerable to replay attacks (see Section 1.2 above).

3. Data Structures

Every Babel node maintains a set of conceptual data structures described in [RFC6126bis] Section 3.2. This protocol extends these data structures as follows.

3.1. The Interface Table

Every Babel node maintains an interface table, as described in [RFC6126bis] Section 3.2.3. Implementations of this protocol MUST allow each interface to be provisioned with a set of one or more HMAC keys and the associated HMAC algorithms (see Section 4.1). In order to allow incremental deployment of this protocol (see Appendix A), implementations SHOULD allow an interface to be configured in a mode in which it participates in the HMAC authentication protocol but accepts packets that are not authentified.

This protocol extends each entry in this table that is associated with an interface on which HMAC authentication has been configured with two new pieces of data:

The Index and PC are initialised to arbitrary values chosen so as to ensure that a given (Index, PC) pair is never reused. Typically, the initial Index will be chosen as a random string of sufficient length, and the initial PC will be set to 0.

3.2. The Neighbour table

Every Babel node maintains a neighbour table, as described in [RFC6126bis] Section 3.2.4. This protocol extends each entry in this table with two new pieces of data: Section 4.3. The Nonce and expiry timer are initially undefined, and used as described in Section

The Index and PC are initially undefined, and are managed as described in

4. Protocol Operation

4.1. HMAC computation

A Babel node computes an HMAC as follows.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|                                                               |
+                                                               +
|                                                               |
+                          Src address                          +
|                                                               |
+                                                               +
|                                                               |
|           Src port            |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
+                                                               +
|                         Dest address                          |
+                                                               +
|                                                               |
+                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               |           Dest port           |
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|                          Src address                          |
|           Src port            |        Dest address           |
|                               |           Dest port           |

First, the node builds a pseudo-header that will participate in HMAC computation but will not be sent. If the packet was carried over IPv6, the pseudo-header has the following format:

Fields :

Src address
The source IP address of the packet.
Src port
The source UDP port number of the packet.
Dest address
The destination IP address of the packet.
Src port
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 with one of the implemented hash algorithms. Every implementation MUST implement HMAC-SHA256 as defined in [RFC6234] and Section 2 of [RFC2104], SHOULD implement keyed BLAKE2s [RFC7693], and MAY implement other HMAC algorithms.

4.2. Packet Transmission

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 [RFC6126bis]). 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:

4.3. Packet Reception

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:

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.

4.3.1. Challenge Requests and Replies

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. Sending challenges

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; the limit SHOULD default to one challenge request every 300ms, and MAY be configurable. Replying to challenges

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 from which the Challenge Request was sent.

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. Receiving challenge replies

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.

4.4. Expiring per-neighbour state

The per-neighbour (Index, PC) pair is maintained in the neighbour table, and is normally discarded when the neighbour table entry expires. Implementations MUST ensure that an (Index, PC) pair is discarded within a finite time since the last time a packet has been accepted. In particular, unsuccessful challenges MUST NOT prevent an (Index, PC) pair from being discarded for unbounded periods of time.

A possible implementation strategy for implementations that use a Hello history (Appendix A of [RFC6126bis]) is to discard the (Index, PC) pair whenever the Hello history becomes empty. Another implementation strategy is to use a timer that is reset whenever a packet is accepted, and to discard the (Index, PC) pair whenever the timer expires. If the latter strategy is being used, the timer SHOULD default to a value of 5 min, and MAY be configurable.

5. Packet Format


 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|   Type = 16   |    Length     |     HMAC...

Fields :

Set to 16 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 Section 4.2 of [RFC6126bis]), and MUST be ignored if it is found in the packet body.

5.2. PC TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|   Type = 17   |    Length     |             PC
                                |            Index...

Fields :

Set to 17 to indicate a PC TLV.
The length of the body, exclusive of the Type and Length fields.
The Packet Counter (PC), a 32-bit (4 octet) unsigned integer which is increased with every packet sent over this interface. A fresh index MUST be generated whenever the PC overflows.
The sender's Index, an opaque string of 0 to 32 octets.

Indices are limited to a size of 32 octets: a node MUST NOT send a TLV with an index of size strictly larger than 32 octets, and a node MAY silently ignore a PC TLV with an index of size strictly larger than 32 octets.

5.3. Challenge Request TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|   Type = 18   |    Length     |     Nonce...

Fields :

Set to 18 to indicate a Challenge Request TLV.
The length of the body, exclusive of the Type and Length fields.
The nonce uniquely identifying the challenge, an opaque string of 0 to 192 octets.

Nonces are limited to a size of 192 octets: a node MUST NOT send a Challenge Request TLV with a nonce of size strictly larger than 192 octets, and a node MAY ignore a nonce that is of size strictly larger than 192 octets.

5.4. Challenge Reply TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|   Type = 19   |    Length     |     Nonce...

Fields :

Set to 19 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.

6. Security Considerations

This document defines a mechanism that provides basic security properties for the Babel routing protocol. The scope of this protocol is strictly limited: it only provides authentication (we assume that routing information is not confidential), it only supports symmetric keying, and it only allows for the use of a small number of symmetric keys on every link. Deployments that need more features, e.g., confidentiality or asymmetric keying, should use a more featureful security mechanism such as the one described in [I-D.ietf-babel-dtls].

This mechanism relies on two assumptions, as described in Section 1.2. First, it assumes that the hash being used is invulnerable to pre-image attacks (Section 1.1 of [RFC6039]); at the time of writing, SHA-256, which is mandatory to implement (Section 4.1), is believed to be safe against practical attacks.

Second, it assumes that indices and nonces are generated uniquely over the lifetime of a key used for HMAC computation (more precisely, indices must be unique for a given (key, source) pair, and nonces must be unique for a given (key, source, destination) triple). This property can be satisfied either by using a cryptographically secure random number generator to generate indices and nonces that contain enough entropy (64-bit values are believed to be large enough for all practical applications), or by using a reliably monotonic hardware clock. If uniqueness cannot be guaranteed (e.g., because a hardware clock has been reset), then rekeying is necessary.

The expiry mechanism mandated in Section 4.4 is required to prevent an attacker from delaying an authentic packet by an unbounded amount of time. If an attacker is able to delay the delivery of a packet (e.g., because it is located at a layer 2 switch), then the packet will be accepted as long as the corresponding (Index, PC) pair is present at the receiver. If the attacker is able to cause the (Index, PC) pair to persist for arbitrary amounts of time (e.g., by causing failed challenges), then it is able to delay the packet by arbitrary amounts of time, even after the sender has left the network.

While it is probably not possible to be immune against denial of service (DoS) attacks in general, this protocol includes a number of mechanisms designed to mitigate such attacks. In particular, reception of a packet with no correct HMAC creates no local state whatsoever (Section 4.3). Reception of a replayed packet with correct hash, on the other hand, causes a challenge to be sent; this is mitigated somewhat by requiring that challenges be rate-limited.

At first sight, sending a challenge requires retaining enough information to validate the challenge reply. However, the nonce included in a challenge request and echoed in the challenge reply can be fairly large (up to 192 octets), which should in principle permit encoding the per-challenge state as a secure "cookie" within the nonce itself.

7. IANA Considerations

IANA has allocated the following values in the Babel TLV Numbers registry:

Type Name Reference
16 HMAC this document
17 PC this document
18 Challenge Request this document
19 Challenge Reply this document

8. Acknowledgments

The protocol described in this document is based on the original HMAC protocol defined by Denis Ovsienko [RFC7298]. 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 Donald Eastlake, Toke Hoiland-Jorgensen, Florian Horn, and Dave Taht.

9. References

9.1. Normative References

[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6126bis] Chroboczek, J. and D. Schinazi, "The Babel Routing Protocol", Internet-Draft draft-ietf-babel-rfc6126bis-06, October 2018.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, May 2011.
[RFC7693] Saarinen, M-J. and J-P. Aumasson, "The BLAKE2 Cryptographic Hash and Message Authentication Code (MAC)", RFC 7693, DOI 10.17487/RFC7693, November 2015.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

9.2. Informational References

[I-D.ietf-babel-dtls] Decimo, A., Schinazi, D. and J. Chroboczek, "Babel Routing Protocol over Datagram Transport Layer Security", Internet-Draft draft-ietf-babel-dtls-01, October 2018.
[RFC4086] Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005.
[RFC6039] Manral, V., Bhatia, M., Jaeggli, J. and R. White, "Issues with Existing Cryptographic Protection Methods for Routing Protocols", RFC 6039, DOI 10.17487/RFC6039, October 2010.
[RFC7298] Ovsienko, D., "Babel Hashed Message Authentication Code (HMAC) Cryptographic Authentication", RFC 7298, DOI 10.17487/RFC7298, July 2014.

Appendix A. Incremental deployment and key rotation

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.

Appendix B. Changes from previous versions

[RFC Editor: please remove this section before publication.]

B.1. Changes since draft-ietf-babel-hmac-00

B.2. Changes since draft-ietf-babel-hmac-01

B.3. Changes since draft-ietf-babel-hmac-02

B.4. Changes since draft-ietf-babel-hmac-03

Authors' Addresses

Clara Do IRIF, University of Paris-Diderot 75205 Paris Cedex 13, France EMail:
Weronika Kolodziejak IRIF, University of Paris-Diderot 75205 Paris Cedex 13, France EMail:
Juliusz Chroboczek IRIF, University of Paris-Diderot Case 7014 75205 Paris Cedex 13, France EMail: