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Versions: 00 draft-ietf-babel-hmac

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                            July 2, 2018
Expires: January 3, 2019


                  Babel Cryptographic Authentification
                         draft-do-babel-hmac-00

Abstract

   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.

   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 January 3, 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
   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 . . . . . . . . . . . . . . . . . . . . . . .   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  . . . . . . . . . . . . . . . . . . . .   7
   5.  Packet Format . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  HMAC TLV  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  PC TLV  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Challenge Request TLV . . . . . . . . . . . . . . . . . .  11
     5.4.  Challenge Reply TLV . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informational References  . . . . . . . . . . . . . . . .  13
   Appendix A.  Use of the packet trailer  . . . . . . . . . . . . .  13
   Appendix B.  Incremental deployment and key rotation  . . . . . .  13
   Appendix C.  Implicit indices . . . . . . . . . . . . . . . . . .  14
   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;

   o  replay a previously captured Babel packet, which could cause
      traffic to be redirected or otherwise interfere with the network.




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   Protecting a Babel network is challenging due to the fact that the
   Babel protocol uses both unicast and multicast communication.  One
   possible approach, used notably by the Babel over DTLS protocol, is
   to require a secured version of Babel to use unicast communication
   for all semantically significant communication, and then use a
   standard unicast security protocol to protect the Babel traffic.  In
   this document, we take the opposite approach: we define a
   cryptographic extension to the Babel protocol that is able to protect
   both unicast and multicast traffic, and thus requires very few
   changes to the core protocol.

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, two during key rotation).  The protocol is
   inapplicable in situations where asymmetric keying is required, where
   the trust relationship is partial, or where large numbers of trusted
   keys are provisioned on a single link at the same time.

   This protocol supports incremental deployment (where an insecure
   Babel network is made secure with no service interruption), and it
   supports graceful key rotation (where the set of keys is changed with
   no service interruption).

   This protocol does not require synchronised clocks, it does not
   require persistently monotonic clocks, and it does not require any
   form of persistent storage.

1.2.  Assumptions and security properties

   The correctness of the protocol relies on the following assumptions:

   o  that the HMAC being used is invulnerable to spoofing, 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, and is
   therefore out-of-scope for this document.  The second assumption can
   be met either by using a robust random number generator and
   sufficiently large indices and nonces, by using a reliable hardware
   clock, or by rekeying whenever a collision becomes likely.

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




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   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
   compares the value of the PC with the one contained in the previous
   packet received from B, and unless it is strictly greater, the packet
   is discarded.

   By itself, the PC mechanism is not sufficient to protect against
   replay.  Consider a peer A that has no information about a pair B



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   (e.g., because it has recently rebooted).  Suppose that A receives a
   packet ostensibly from B carrying a given PC; since A has no
   information about B, it has no way to determine whether the packet is
   freshly generated or a replay of a previously sent packet.

   In this situation, A discards the packet and challenges B to prove
   that it knows the HMAC key.  It sends a "challenge request", a TLV
   containing a unique nonce, a value that has never been used before
   and will never be used again.  B replies to the challenge request
   with a "challenge reply", a TLV containing a copy of the nonce chosen
   by A, in a packet protected by HMAC and containing the new value of
   B's PC.  Since the nonce has never been used before, B's reply proves
   B's knowledge of the HMAC key and the freshness of the PC.

   By itself, this mechanism is safe against replay if B never resets
   its PC.  In practice, however, this is difficult to ensure, as
   persistent storage is prone to failure, and hardware clocks, even
   when available, are occasionally reset.  Suppose that B resets its PC
   to an earlier value, and sends a packet with a previously used PC n.
   A challenges B, B successfully responds to the challenge, and A
   accepts the PC equal to n + 1.  At this point, an attacker C may send
   a replayed packet with PC equal to n + 2, which will be accepted by
   A.

   Another mechanism is needed to protect against this attack.  In this
   protocol, every PC is tagged with an "index", an arbitrary string of
   octets.  Whenever B resets its PC, or whenever B doesn't know whether
   its PC has been reset, it picks an index that it has never used
   before (either by drawing it randomly or by using a reliable hardware
   clock) and starts sending PCs with that index.  Whenever A detects
   that B has changed its index, it challenges B again.

   With this additional mechanism, this protocol is provably
   invulnerable to replay attacks (see 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
   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).






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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.  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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields :

   Src address   The source IP address of the packet.

   Src port      The source UDP port number of the packet.




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   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 as defined in Section 2 of [RFC2104] with one of the implemented
   hash algorithms.  Every implementation MUST implement HMAC-SHA256
   [RFC6234], 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 Packet Counter and Index associated with
      the outgoing interface is appended to the packet body; the packet
      counter is incremented by a strictly positive amount (typically
      just 1); if the packet counter overflows, a new index is
      generated;

   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.

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



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      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.
      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.








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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.

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.



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   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.

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 Appendix A), and MUST
   BE ignored if it is found in the packet body.

5.2.  PC TLV










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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     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.

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.

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 :




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   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.

6.  Security Considerations

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 Florian Horn and Toke Hoyland-Jorgensen.

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-05 (work in
              progress), May 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>.

   [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

   [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.  Use of the packet trailer

   The protocol described in this document uses the packet trailer for
   storing HMAC TLVs.  RFC 6126bis [RFC6126bis] leaves the format of the
   packet trailer undefined.  If the final version of this specification
   uses the packet trailer, RFC 6126bis will need to be extended with
   information about the format of the packet trailer.

   This document assumes that the packet trailer has the same format as
   the packet body, i.e., that it consists of a sequence of TLVs.  The
   receiver MUST silently ignore any TLV found in the packet trailer
   unless its definition states that the TLV is allowed in the packet
   trailer.

Appendix B.  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 C.  Implicit indices

   [This appendix describes the "implicit indices" variant of the
   protocol, which is different and incompatible to the "explicit
   indices" variant described in the body of this document.  This
   section should either be integrated into the body of the document or
   removed before publication of this document as an RFC, depending on
   which protocol variant is finally chosen.]

   The protocol described in the body of this document explicitly sends
   indices as in each packet as part of the PC TLV.  Observe that,
   except when a challenge is required, the index sent on the wire is
   identical to the index stored in the Neighbour Table, and therefore
   doesn't need to be sent explicitly except during challenges: it is
   enough for it to participate in HMAC computation in order to protect
   against replay.  The "implicit indices" variant of the protocol, due
   to Markus Stenberg and described in this appendix, uses this
   observation to avoid sending indices explicitly and thus shaves off 2
   to 16 octets from almost every packet.

   The changes to the protocol are as follows.  The pseudo-header
   includes the Index, and therefore has the following format:


















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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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Index...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   The PC TLV no longer contains an Index, and therefore 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             PC
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This TLV is now self-terminating, and therefore allows sub-TLVs.

   Packets containing the Challenge Reply and Challenge Request TLVs
   must contain an explicit index.  Two encodings are possible: one uses
   Challenge Replies and Requests with an extra field for the sender's
   index, which complicates the encoding somewhat but makes these two
   TLVs self-terminating, the other one uses a new TLV that is used for
   carrying an Index, which uses up a new TLV number but makes it
   possible to reuse these two TLV with other protocols that require a
   nonce-based challenge.

   Packet transmission is modified as follows.  If a packet contains a
   Challenge or a Challenge Reply, then the node inserts its index into



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   the packet body.  In any case, it uses its current index to generate
   the pseudo-header that will be used to compute the HMAC.  (This
   implies that a packet must be parsed in its entirety before HMAC
   validation, which requires a robust parser.)

   Packet reception is modified as follows.  Before checking the HMAC of
   a packet, the receiver checks whether the packet contains an explicit
   index.  If this is the case, it uses the index contained in the
   packet in order to generate the pseudo header; if this is not the
   case, it uses the index contained in its neighbours table.  If there
   is no index available for that neighbour (either because the table
   doesn't contain in an entry for this neighbour, or the entry doesn't
   contain an index), HMAC validation fails.

   The index and PC contained in the neighbours table are only updated
   after HMAC validation has succeeded.

   Since it is now impossible to differentiate between a failed HMAC and
   an index change, a node must send a challenge whenever HMAC
   validation fails.  This implies that spoofed packets cause a spurious
   challenge, but that doesn't change the security properties of the
   protocol much, given that in any case replayed packets can be used to
   cause a spurious challenge.

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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