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Versions: (draft-do-babel-hmac) 00 01 02 03 04 05 06 07 08 09 10

Network Working Group                                              C. Do
Internet-Draft                                            W. Kolodziejak
Obsoletes: 7298 (if approved)                              J. Chroboczek
Intended status: Standards Track       IRIF, University of Paris-Diderot
Expires: December 22, 2019                                 June 20, 2019


           HMAC authentication for the Babel routing protocol
                        draft-ietf-babel-hmac-07

Abstract

   This document describes a cryptographic authentication mechanism for
   the Babel routing protocol that has provisions for replay avoidance.
   This document 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 December 22, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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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 . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  The Interface Table . . . . . . . . . . . . . . . . . . .   6
     3.2.  The Neighbour table . . . . . . . . . . . . . . . . . . .   6
   4.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  HMAC computation  . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Packet Transmission . . . . . . . . . . . . . . . . . . .   8
     4.3.  Packet Reception  . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Expiring per-neighbour state  . . . . . . . . . . . . . .  12
   5.  Packet Format . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  HMAC TLV  . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  PC TLV  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.3.  Challenge Request TLV . . . . . . . . . . . . . . . . . .  13
     5.4.  Challenge Reply TLV . . . . . . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informational References  . . . . . . . . . . . . . . . .  17
   Appendix A.  Incremental deployment and key rotation  . . . . . .  17
   Appendix B.  Changes from previous versions . . . . . . . . . . .  18
     B.1.  Changes since draft-ietf-babel-hmac-00  . . . . . . . . .  18
     B.2.  Changes since draft-ietf-babel-hmac-01  . . . . . . . . .  18
     B.3.  Changes since draft-ietf-babel-hmac-02  . . . . . . . . .  18
     B.4.  Changes since draft-ietf-babel-hmac-03  . . . . . . . . .  18
     B.5.  Changes since draft-ietf-babel-hmac-04  . . . . . . . . .  19
     B.6.  Changes since draft-ietf-babel-hmac-05  . . . . . . . . .  19
     B.7.  Changes since draft-ietf-babel-hmac-06  . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   By default, the Babel routing protocol trusts the information
   contained in every UDP datagram that 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;




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

   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.




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   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 [RFC4086] 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:

   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 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



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   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).








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3.  Data Structures

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

3.1.  The Interface Table

   Every Babel node maintains an interface table, as described in
   Section 3.2.3 [RFC6126bis].  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:

   o  a set of one or more HMAC keys, each associated with a given HMAC
      algorithm ; the length of each key is exactly the hash size of the
      associated HMAC algorithm (i.e., the key is not subject to the
      preprocessing described in Section 2 of [RFC2104]);

   o  a pair (Index, PC), where Index is an arbitrary string of 0 to 32
      octets, and PC is a 32-bit (4-octet) integer.

   We say that an index is fresh when it has never been used before with
   any of the keys currently configured on the interface.  The Index
   field is initialised to a fresh index, for example by drawing a
   random string of sufficient length, and the PC is initialised to an
   arbitrary value (typically 0).

3.2.  The Neighbour table

   Every Babel node maintains a neighbour table, as described in
   Section 3.2.4 of [RFC6126bis].  This protocol extends each entry in
   this table with two new pieces of data:

   o  a pair (Index, PC), where Index is a string of 0 to 32 octets, and
      PC is a 32-bit (4-octet) integer;

   o  a Nonce, which is an arbitrary string of 0 to 192 octets, and an
      associated challenge expiry timer.





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   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 the HMAC of a Babel packet 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:

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



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   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 might 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 MUST be appended to the packet body; the PC MUST be
      incremented by a strictly positive amount (typically just 1); if
      the PC overflows, a fresh index MUST be generated (as defined in
      Section 3.1); a node MUST NOT include multiple PC TLVs in a single
      packet;

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

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:





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   o  First, the receiver checks whether the trailer of the received
      packet carries at least one HMAC TLV; if not, the packet MUST be
      immediately dropped and processing stops.  Then, for each key
      configured on the receiving interface, the receiver 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 MUST
      be 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 (which should not
      happen, see Section 4.2 above), only the first one is processed,
      the remaining ones MUST be silently ignored.  If a Challenge
      Request is encountered, a Challenge Reply MUST be scheduled, as
      described in Section 4.3.1.2.  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 MUST be 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
      MUST be stored in the Neighbour Table entry corresponding to the
      sender (which may need to be created at this stage), and the
      packet is accepted.

   o  Otherwise, 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 MUST be sent
      as described in Section 4.3.1.1, the packet MUST be dropped, and
      processing stops at this stage.

   o  At this stage, the packet contains no successful challenge reply
      and 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 is smaller or equal than the PC



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      contained in the Neighbour Table, the packet MUST be 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.

   In the algorithm described above, challenge requests are processed
   and challenges are sent before the PC/Index pair is verified against
   the neighbour table.  This simplifies the implementation somewhat
   (the node may simply schedule outgoing requests as it walks the
   packet during the preparse phase), but relies on the rate-limiting
   described in Section 4.3.1.1 to avoid sending too many challenges in
   response to replayed packets.  As an optimisation, a node MAY ignore
   all challenge requests contained in a packet except the last one, and
   it MAY ignore a challenge request in the case where it it contained
   in a packet with an Index that matches the one in the Neighbour
   Table and a PC that is smaller or equal to the one contained in the
   Neighbour Table.  Since it is still possible to replay a packet with
   an obsolete Index, the rate-limiting described in Section 4.3.1.1 is
   required even if this optimisation is implemented.

   The same is true of challenge replies.  However, since validating a
   challenge reply is extremely cheap (it's just a bitwise comparison of
   two strings of octets), a similar optimisation for challenge replies
   is not worthwile.

   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 with any of the keys
   currently configured on the relevant interface, for example by
   drawing a sufficiently large random string of bytes or by consulting
   a strictly monotonic hardware clock.  It MUST then store the nonce in
   the entry of the Neighbour Table associated to the neighbour (the



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   entry might need to be created at this stage), initialise the
   neighbour's challenge expiry timer to 30 seconds, and send 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 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 packet replayed by an
   attacker, 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.

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 MUST then send 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.

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
   MUST be 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



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   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

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 = 16   |    Length     |     HMAC...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields :

   Type      Set to 16 to indicate an HMAC TLV.

   Length    The length of the body, in octets, exclusive of the Type
             and Length fields.  The length of the body depends on the
             HMAC algorithm being used.

   HMAC      The body contains the HMAC of the packet, computed as
             described in Section 4.1.

   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.








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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 :

   Type      Set to 17 to indicate a PC TLV.

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

   PC        The Packet Counter (PC), a 32-bit (4 octet) unsigned
             integer which is increased with every packet sent over this
             interface.  A fresh index (as defined in Section 3.1) MUST
             be generated whenever the PC overflows.

   Index     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 ignore a PC TLV with an index of length strictly larger than 32
   octets.  Indices of length 0 are valid: if a node has reliable stable
   storage and the packet counter never overflows, then only one index
   is necessary, and the value of length 0 is the canonical choice.

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 :

   Type      Set to 18 to indicate a Challenge Request TLV.

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

   Nonce     The nonce uniquely identifying the challenge, an opaque
             string of 0 to 192 octets.



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   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.  Nonces of length 0 are valid: if a node has
   reliable stable storage, then it may use a sequential counter for
   generating nonces which get encoded in the minumum number of octets
   required; the value 0 is then encoded as the string of length 0.

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 :

   Type      Set to 19 to indicate a Challenge Reply TLV.

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

   Nonce     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



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   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
   repeatedly 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 Types
   registry:














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               +------+-------------------+---------------+
               | 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,
   Dave Taht and Martin Vigoureux.

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.

   [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>.



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   [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.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [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.

   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




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   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

   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-01

   o  Made BLAKE2s a recommended HMAC algorithm.

   o  Added requirement to expire per-neighbour crypto state.

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

   o  Clarified that PCs are 32-bit unsigned integers.

   o  Clarified that indices and nonces are of arbitrary size.

   o  Added reference to RFC 4086.

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

   o  Use the TLV values allocated by IANA.

   o  Fixed an issue with packets that contain a successful challenge
      reply: they should be accepted before checking the PC value.

   o  Clarified that keys are the exact value of the HMAC hash size, and
      not subject to preprocessing; this makes management more
      deterministic.







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B.5.  Changes since draft-ietf-babel-hmac-04

   o  Use normative language in more places.

B.6.  Changes since draft-ietf-babel-hmac-05

   o  Do not update RFC 6126bis.

   o  Clarify that indices and nonces of length 0 are valid.

   o  Clarify that multiple PC TLVs in a single packet are not allowed.

   o  Allow discarding challenge requests when they carry an old PC.

B.7.  Changes since draft-ietf-babel-hmac-06

   o  Do not update RFC 6126bis, for real this time.

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


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

   Email: jch@irif.fr








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