HMAC authentication for the Babel routing protocol
draft-ietf-babel-hmac-02

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                       December 23, 2018
Expires: June 26, 2019

           HMAC authentication for the Babel routing protocol
                        draft-ietf-babel-hmac-02

Abstract

   This document describes a cryptographic authentication 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.

   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 June 26, 2019.

Copyright Notice

   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
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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Assumptions and security properties . . . . . . . . . . .   3
     1.3.  Specification of Requirements . . . . . . . . . . . . . .   4
   2.  Conceptual overview of the protocol . . . . . . . . . . . . .   4
   3.  Data Structures . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  The Interface Table . . . . . . . . . . . . . . . . . . .   5
     3.2.  The Neighbour table . . . . . . . . . . . . . . . . . . .   6
   4.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  HMAC computation  . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Packet Transmission . . . . . . . . . . . . . . . . . . .   7
     4.3.  Packet Reception  . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Expiring per-neighbour state  . . . . . . . . . . . . . .  10
   5.  Packet Format . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  HMAC TLV  . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.2.  PC TLV  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.3.  Challenge Request TLV . . . . . . . . . . . . . . . . . .  12
     5.4.  Challenge Reply TLV . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informational References  . . . . . . . . . . . . . . . .  15
   Appendix A.  Incremental deployment and key rotation  . . . . . .  15
   Appendix B.  Changes from previous versions . . . . . . . . . . .  16
     B.1.  Changes since draft-ietf-babel-hmac-00  . . . . . . . . .  16
     B.2.  Changes since draft-ietf-babel-hmac-00  . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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:

   o  spoof a Babel packet, and redirect traffic by announcing a smaller
      metric, a larger seqno, or a longer prefix;

   o  spoof a malformed packet, which could cause an insufficiently
      robust implementation to crash or interfere with the rest of the
      network;

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   o  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 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:

   o  that the Hashed Message Authentication Code (HMAC) being used is
      invulnerable to pre-image attacks, i.e., that an attacker is
      unable to generate a packet with a correct HMAC;

   o  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.  The
   second assumption can be met either by using a robust random number
   generator and sufficiently large indices and nonces, by using a

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   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:

   o  it is invulnerable to spoofing: any packet accepted as authentic
      is the exact copy of a packet originally sent by an authorised
      node;

   o  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;

   o  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.

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 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

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   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

3.1.  The Interface Table

   Every Babel node maintains an interface table, as described in
   [RFC6126bis] Section 3.2.3.  This protocol extends the entries in
   this table with a set of HMAC keys, and a pair (Index, PC), where

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   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).

3.2.  The Neighbour table

   Every Babel node maintains a neighbour table, as described in
   [RFC6126bis] 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 Section 4.3.
   The Nonce and expiry timer are initially undefined and used as
   described in Section 4.3.1.1.

4.  Protocol Operation

4.1.  HMAC computation

   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.  If the packet was carried over
   IPv6, the pseudo-header has the following 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                          Src address                          +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Src port            |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +                                                               +
   |                         Dest address                          |
   +                                                               +
   |                                                               |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |           Dest port           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If the packet was carried over IPv4, the pseudo-header has the
   following format:

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    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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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:

   o  a PC TLV containing the PC and Index associated with the outgoing
      interface is appended to the packet body; the PC is incremented by
      a strictly positive amount (typically just 1); if the PC
      overflows, a new index is generated;

   o  for each key configured on the interface, an HMAC is computed as
      specified in Section 4.1 above, and an HMAC TLV is appended to the
      packet trailer (see Section 4.2 of [RFC6126bis]).

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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:

   o  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.

   o  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 Section 4.3.1.2, and if a Challenge Reply is
      encountered, it is tested for validity as described in
      Section 4.3.1.3 and a note is made of the result of the test.

   o  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).

   o  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 Section 4.3.1.1,
      processing stops at this stage, and the packet is dropped.

   o  At this stage, the Index contained in the PC TLV is equal to the
      Index in the Neighbour Table entry corresponding to the sender.

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      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.

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.

4.3.1.1.  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; a limit of one
   challenge every 300ms for each neighbour is suggested.

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4.3.1.2.  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 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.

4.3.1.3.  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.

   Implementations that use a Hello history (Appendix A of [RFC6126bis])
   may discard the (Index, PC) pair whenever the Hello history becomes
   empty.  Other impementations may use a timer that is reset whenever a
   packet is accepted, and discard the (Index, PC) pair whenever the
   timer expires (an timeout of 5 min is suggested).

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5.  Packet Format

5.1.  HMAC 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      |    Length     |     HMAC...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields :

   Type      Set to TBD to indicate an HMAC TLV.

   Length    The length of the body, exclusive of the Type and Length
             fields.  The length of the body depends on the hash
             function used.

   HMAC      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      |    Length     |             PC
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |            Index...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields :

   Type      Set to TBD to indicate a PC TLV.

   Length    The length of the body, exclusive of the Type and Length
             fields.

   PC        The Packet Counter (PC), which is increased with every
             packet sent over this interface.  A new index MUST be
             generated whenever the PC overflows.

   Index     The sender's Index.

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   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      |    Length     |     Nonce...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields :

   Type      Set to TBD to indicate a Challenge Request TLV.

   Length    The length of the body, exclusive of the Type and Length
             fields.

   Nonce     The nonce uniquely identifying the challenge.

   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      |    Length     |     Nonce...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields :

   Type      Set to TBD to indicate a Challenge Reply TLV.

   Length    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.

   Nonce     A copy of the nonce contained in the corresponding
             challenge request.

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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 unicity 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.

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   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 is instructed to allocate the following values in the Babel TLV
   Numbers registry:

               +------+-------------------+---------------+
               | Type | Name              | Reference     |
               +------+-------------------+---------------+
               | TBD  | HMAC              | this document |
               |      |                   |               |
               | TBD  | PC                | this document |
               |      |                   |               |
               | TBD  | Challenge Request | this document |
               |      |                   |               |
               | TBD  | 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 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,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997.

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   [RFC6126bis]
              Chroboczek, J. and D. Schinazi, "The Babel Routing
              Protocol", draft-ietf-babel-rfc6126bis-06 (work in
              progress), 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,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC7693]  Saarinen, M-J., Ed. and J-P. Aumasson, "The BLAKE2
              Cryptographic Hash and Message Authentication Code (MAC)",
              RFC 7693, DOI 10.17487/RFC7693, November 2015,
              <https://www.rfc-editor.org/info/rfc7693>.

   [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",
              draft-ietf-babel-dtls-01 (work in progress), October 2018.

   [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,
              <https://www.rfc-editor.org/info/rfc6039>.

   [RFC7298]  Ovsienko, D., "Babel Hashed Message Authentication Code
              (HMAC) Cryptographic Authentication", RFC 7298,
              DOI 10.17487/RFC7298, July 2014,
              <https://www.rfc-editor.org/info/rfc7298>.

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.

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   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

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

   o  Changed the title.

   o  Removed the appendix about the packet trailer, this is now in
      rfc6126bis.

   o  Removed the appendix with implicit indices.

   o  Clarified the definitions of acronyms.

   o  Limited the size of nonces and indices.

B.2.  Changes since draft-ietf-babel-hmac-00

   o  Made BLAKE2s a recommended HMAC algorithm.

   o  Added requirement to expire per-neighbour crypto state.

Authors' Addresses

   Clara Do
   IRIF, University of Paris-Diderot
   75205 Paris Cedex 13
   France

   Email: clarado_perso@yahoo.fr

   Weronika Kolodziejak
   IRIF, University of Paris-Diderot
   75205 Paris Cedex 13
   France

   Email: weronika.kolodziejak@gmail.com

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   Juliusz Chroboczek
   IRIF, University of Paris-Diderot
   Case 7014
   75205 Paris Cedex 13
   France

   Email: jch@irif.fr

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