Network Working Group                                    M. Jethanandani
Internet-Draft                                            Kloud Services
Updates: 5880 (if approved)                                   S. Agarwal
Intended status: Standards Track                      Cisco Systems, Inc
Expires: February 6, June 19, 2021                                         A. Mishra
                                                            O3b Networks
                                                               A. Saxena
                                                       Ciena Corporation
                                                                A. Dekok
                                                     Network RADIUS SARL
                                                          August 5,
                                                       December 16, 2020

                      Secure BFD Sequence Numbers


   This document describes a security enhancement for the sequence
   number used in BFD control packets.  This document updates RFC 5880.

Status of This Memo

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

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   This Internet-Draft will expire on February 6, June 19, 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Theory of operation . . . . . . . . . . . . . . . . . . . . .   2
   4.  Impact of using a hash  . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   BFD [RFC5880] section 6.7 describes the use of monotonically
   incrementing 32-bit sequence numbers for use in authentication of BFD
   packets.  While this method protects against simple replay attacks,
   the monotonically incrementing sequence numbers are predictable and
   vulnerable to more complex attack vectors.  This document proposes
   the use of non-monotonically-incrementing sequence numbers in the BFD
   authentication section to enhance the security of BFD sessions.
   Specifically, the document presents a method to generate pseudo-
   random sequence numbers on the frame by algorithmically hashing
   monotonically increasing sequence numbers.  Since the monotonically
   increasing sequence number does not appear on the wire, it is
   difficult for a third party to launch a replay attack.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Theory of operation

   Instead of inserting a monotonically, sometimes occasionally,
   increasing sequence number in BFD control packets, the ciphertext
   result from a hash symmetric key algorithm operation (Symmetric-key
   algorithms require both the sender and the recipient of a message to
   have the same shared secret key) is inserted.  The hash result is
   computed, using a shared key, on the sequence number.  That computed hash
   ciphertext result is then inserted into the sequence number field of
   the packet.  In case of BFD Authentication
   [I-D.ietf-bfd-optimizing-authentication], the sequence number used in
   computing an authenticated packet would be this new computed hash.
   ciphertext.  Even though the BFD Authentication
   [I-D.ietf-bfd-optimizing-authentication] sequence number is
   independent of this enhancement, it would benefit by using the
   computed hash. ciphertext.

   As currently defined in BFD [RFC5880], a BFD packet with
   authentication will undergo the following steps, where:

   [O]: original RFC 5880 packet with monotonically increasing sequence

   [S]: pseudo random sequence number

   [A]: Authentication

                   Sender                    Receiver

                   [O] [S] [A] ------------- [A] [S] [O]

   This document proposes that for enhanced security in sequence number
   encoding, the sender would identify a hash symmetric key algorithm (symmetric) that
   would create a 32 bit hash. ciphertext.  The hashing symmetric key is provisioned
   securely on the sender and receiver of the BFD session.  The
   mechanism of provisioning such a key is outside the scope of this
   document.  This key SHOULD be different from the symmetric key used
   to to authenticate the packet.  Instead of using sending the sequence
   number, the sender encodes encrypts the sequence number with using it as input to
   the hashing key symmetric algorithm to produce a hash. the ciphertext, which is then
   inserted in place of the sequence number.

   Upon receiving the BFD Control packet, the receiver decrypts the
   ciphertext using the same provisioned shared key to produce the
   received sequence number.  It compares the received sequence number
   against the expected sequence number.  The mechanism used for
   comparing is an implementation detail (implementations may pre-calculate pre-
   calculate the expected hashed sequence number, or decrypt the received
   sequence number before comparing against expected value).  To
   tolerate dropped frames, the receiver MUST compare the received
   sequence number against the current expected sequence number
   (previous received sequence number + 1) and N subsequent expected
   sequence numbers (where N is greater than or equal to the detect
   multiplier).  Note: The first sequence number can be obtained using
   the same logic as used in determining Local Discriminator value for
   the session or by using a random number.

   k: hashing

   K: symmetric key


   S: sequence number

   S': encrypted sequence number OR ciphertext result

   O: original RFC 5880 packet with monotonically increasing sequence
   R: remainder of packet

   H1: hash of s

   H2: hash of entire packet

   A: H2 + insertion in packet

   hash(s, k)

   f(S, K) = H1

   hash((H1 + R), k) S', where f is a symmetric encryption algorithm

   f(S', K) = H2

   hash'((Packet - H2), k) == H2 ? Good packet : bad packet

   hash'(H1, k) > previously received s ? Good sequence number : bad
   sequence number S, where f is a symmetric decryption algorithm

                     Sender                Receiver

                     [O] [H1] [S'] [A] -------- [A] [H1] [S] [O]

   The above diagram describes how the sender encodes encrypts and receiver
   decrypts the sequence number.  The sender starts by taking the
   monotonically increasing (but non linear) sequence number and hashing it.  It
   encrypting it using a symmetric encryption algorithm.  The resulting
   ciphertext replaces the sequence number with the hash.  It number.  As per BFD [RFC5880], it
   then calculates the hash for the entire packet and appends the hash
   value to the end of the packet, before transmitting it.

   The receiver hashes the entire packet without H2, and compares as part of receiver
   authentication.  On successful authentication, it decrypts the
   hash value
   ciphertext with the received hash (H2).  If the hash values are
   equal, it is a good packet, else it is a bad packet.  It then
   calculates the hash on same key used to encrypt it, in order to obtain
   the received original sequence number to retreive s. number.  If it is greater than the previously
   received monotically monotonically increasing sequence number, then the receiver
   knows it's a valid sequence number.

4.  Impact of using a hash

   Under this proposal, every packet's sequence number is encoded within
   a hash.  Therefore in
   ciphertext.  Therefore, there is some impact on the system and its
   performance while encoding/decoding the hash. encryption/decryption.  As security measures go,
   this enhancement greatly increases the security of the packet with or
   without authentication of the entire packet.

5.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

6.  Security Considerations


   In a symmetric key algorithm, the proposed mechanism improves overall security of BFD
   mechanism, key is shared between the security consderations are listed below:

   Because two
   systems.  Distribution of this key to all the systems at the same
   time can be quite a cumbersome task.  BFD sessions running a fast
   rate of BFD sesions and will require these keys to be refreshed often, which poses a
   further challenge.  Therefore, it is difficult to change the keys (used for hashing the sequence number)
   during the operation of a BFD session without affecting the stability
   of the BFD session.
   It is, therefore,  Therefore, it is recommended to administratively
   disable the BFD session before changing the keys.  If the keys are
   not changed, changed frequently, an attacker can use try to guess the key to
   launch a replay attack.

   Using this

   This method allows the BFD end-points to detect a malicious packet
   (the decrypted sequence number will not be in sequence) the sequence).  The
   behavior of the session session, when such a packet is detected detected, is based on
   the implementation.  A flood of such malicious packets may cause a
   session to report
   BFD session to be operationally down.

   The hashing symmetric algorithm and key size will determine the difficulty
   for an attacker to decipher the key from the transmitted BFD frames.
   The sequential nature of the payload (sequence numbers) simplifies
   the decoding of the key.  It is, therefore, recommended to use longer
   keys or more secure hashing symmetric algorithms.

7.  Acknowledgements

   The authors would like to thank Jeff Hass and Reshad Rahman for their
   reviews of and suggestions for the document.

8.  References

8.1.  Normative References

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

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

8.2.  Informative References

              Jethanandani, M., Mishra, A., Saxena, A., and M. Bhatia,
              "Optimizing BFD Authentication", draft-ietf-bfd-
              optimizing-authentication-11 (work in progress), July

Authors' Addresses

   Mahesh Jethanandani
   Kloud Services


   Sonal Agarwal
   Cisco Systems, Inc
   170 W. Tasman Drive
   San Jose, CA  95070


   Ashesh Mishra
   O3b Networks


   Ankur Saxena
   Ciena Corporation
   3939 North First Street
   San Jose, CA  95134


   Alan DeKok
   Network RADIUS SARL
   100 Centrepointe Drive #200
   Ottowa, ON  K2G 6B1