Network Working Group                                    M. JethanandaniInternet-Draft                                            Kloud ServicesUpdates: 5880 (if approved)                                   S. AgarwalIntended status: Standards Track                      Cisco Systems, IncExpires: June 19, 2021                                         A. Mishra                                                            O3b Networks                                                               A. Saxena                                                       Ciena Corporation                                                                A. Dekok                                                     Network RADIUS SARL                                                       December 16, 2020                      Secure BFD Sequence Numbers               draft-ietf-bfd-secure-sequence-numbers-07Abstract   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.   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 19, 2021.Copyright Notice   Copyright (c) 2020 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 respectJethanandani, et al.      Expires June 19, 2021                 [Page 1]Internet-Draft        Securing next sequence number        December 2020   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.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 . . . . . . . . . . . . . . . . . . . . .   4   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5     8.2.  Informative References  . . . . . . . . . . . . . . . . .   5   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   61.  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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   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 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 result is   computed, using a shared key, on the sequence number.  ThatJethanandani, et al.      Expires June 19, 2021                 [Page 2]Internet-Draft        Securing next sequence number        December 2020   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   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 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   number   [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 symmetric key algorithm that   would create a 32 bit ciphertext.  The 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 sending the sequence   number, the sender encrypts the sequence number using it as input to   the symmetric algorithm to produce 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 the expected 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.Jethanandani, et al.      Expires June 19, 2021                 [Page 3]Internet-Draft        Securing next sequence number        December 2020   K: symmetric key   S: sequence number   S': encrypted sequence number OR ciphertext result   O: original RFC 5880 packet with monotonically increasing sequence   number   f(S, K) = S', where f is a symmetric encryption algorithm   f(S', K) = S, where f is a symmetric decryption algorithm                     Sender                Receiver                     [O] [S'] [A] -------- [A] [S] [O]   The above diagram describes how the sender encrypts and receiver   decrypts the sequence number.  The sender starts by taking the   monotonically increasing (but non linear) sequence number and   encrypting it using a symmetric encryption algorithm.  The resulting   ciphertext replaces the sequence 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 as part of receiver   authentication.  On successful authentication, it decrypts the   ciphertext with the same key used to encrypt it, in order to obtain   the original sequence number.  If it is greater than the previously   received 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 in   ciphertext.  Therefore, there is some impact on the system and its   performance while 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   RFC.Jethanandani, et al.      Expires June 19, 2021                 [Page 4]Internet-Draft        Securing next sequence number        December 20206.  Security Considerations   In a symmetric key algorithm, the key is shared between the 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 will require these keys to be refreshed often, which poses a   further challenge.  Therefore, it is difficult to change the keys   during the operation of a BFD session without affecting the stability   of the BFD session.  Therefore, it is recommended to administratively   disable the BFD session before changing the keys.  If the keys are   not changed frequently, an attacker can try to guess the key to   launch a replay attack.   This method allows the BFD end-points to detect a malicious packet   (the decrypted sequence number will not be in sequence).  The   behavior of the session, when such a packet is detected, is based on   the implementation.  A flood of such malicious packets may cause a   BFD session to be operationally down.   The 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 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.  References8.1.  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,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,              <https://www.rfc-editor.org/info/rfc5880>.8.2.  Informative ReferencesJethanandani, et al.      Expires June 19, 2021                 [Page 5]Internet-Draft        Securing next sequence number        December 2020   [I-D.ietf-bfd-optimizing-authentication]              Jethanandani, M., Mishra, A., Saxena, A., and M. Bhatia,              "Optimizing BFD Authentication", draft-ietf-bfd-              optimizing-authentication-11 (work in progress), July              2020.Authors' Addresses   Mahesh Jethanandani   Kloud Services   Email: mjethanandani@gmail.com   Sonal Agarwal   Cisco Systems, Inc   170 W. Tasman Drive   San Jose, CA  95070   USA   Email: agarwaso@cisco.com   URI:   www.cisco.com   Ashesh Mishra   O3b Networks   Email: mishra.ashesh@gmail.com   Ankur Saxena   Ciena Corporation   3939 North First Street   San Jose, CA  95134   USA   Email: ankurpsaxena@gmail.com   Alan DeKok   Network RADIUS SARL   100 Centrepointe Drive #200   Ottowa, ON  K2G 6B1   Canada   Email: aland@freeradius.orgJethanandani, et al.      Expires June 19, 2021                 [Page 6]