Meticulous Keyed ISAAC for BFD Optimized Authentication
draft-ietf-bfd-secure-sequence-numbers-26

Network Working Group                                           A. Dekok
Internet-Draft                                        InkBridge Networks
Intended status: Experimental                            M. Jethanandani
Expires: 11 April 2026                                    Kloud Services
                                                              S. Agarwal
                                                      Cisco Systems, Inc
                                                               A. Mishra
                                                    Aalyria Technologies
                                                                 J. Haas
                                                                     HPE
                                                          8 October 2025

        Meticulous Keyed ISAAC for BFD Optimized Authentication
               draft-ietf-bfd-secure-sequence-numbers-26

Abstract

   This document describes a BFD Optimized Authentication Mode,
   Meticulous Keyed ISAAC Authentication.  This mode can be used to
   authenticate some BFD packets with less CPU time cost than using MD5
   or SHA1, with the tradeoff of decreased security.  This mechanism
   cannot be used to signal state changes, but it can be used to
   maintain a session in the Up state.

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 April 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Meticulous Keying . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.3.  Note to RFC Editor  . . . . . . . . . . . . . . . . . . .   4
   2.  Experimental updates to RFC 5880  . . . . . . . . . . . . . .   4
   3.  Architecture of the Auth Type Method  . . . . . . . . . . . .   6
     3.1.  Rationale for ISAAC and Operational Overview  . . . . . .   7
   4.  Meticulous Keyed ISAAC Authentication Types . . . . . . . . .   9
     4.1.  Meticulous Keyed ISAAC Authentication, ISAAC Format . . .   9
     4.2.  Meticulous Keyed ISAAC Authentication, MD5 Format . . . .  10
     4.3.  Meticulous Keyed ISAAC Authentication, SHA1 Format  . . .  12
   5.  New State variables for Meticulous Keyed ISAAC
           Authentications . . . . . . . . . . . . . . . . . . . . .  13
   6.  Procedures for BFD Authentication using Meticulous Keyed ISAAC,
           MD5 or SHA1 Formats . . . . . . . . . . . . . . . . . . .  14
   7.  Procedures for BFD Authentication using Meticulous Keyed ISAAC,
           ISAAC Format  . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Secret Key  . . . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  Transition to using ISAAC . . . . . . . . . . . . . . . . . .  17
   10. Seeding ISAAC . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Sender Variable Initialization . . . . . . . . . . . . .  21
     10.2.  Receiver Variable Initialization . . . . . . . . . . . .  22
   11. Operation . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     11.1.  Page Flipping  . . . . . . . . . . . . . . . . . . . . .  24
     11.2.  Multiple Keys  . . . . . . . . . . . . . . . . . . . . .  25
   12. Transition away from using ISAAC  . . . . . . . . . . . . . .  25
   13. The YANG Model  . . . . . . . . . . . . . . . . . . . . . . .  26
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
     14.1.  BFD Auth Types . . . . . . . . . . . . . . . . . . . . .  28
     14.2.  IETF XML Registry  . . . . . . . . . . . . . . . . . . .  28
     14.3.  The YANG Module Names Registry . . . . . . . . . . . . .  28
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  29
     15.1.  Protocol Security Considerations . . . . . . . . . . . .  29
       15.1.1.  Spoofing . . . . . . . . . . . . . . . . . . . . . .  29
       15.1.2.  Re-Use of keys . . . . . . . . . . . . . . . . . . .  30
     15.2.  YANG Security Considerations . . . . . . . . . . . . . .  31
   16. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  31

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   17. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     18.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   BFD [RFC5880] (Section 6.7) defines a number of authentication
   mechanisms, including Simple Password, and various other methods
   based on MD5 and SHA1 hashes.  The benefit of using cryptographic
   hashes is that they are secure.  The downside to cryptographic hashes
   is that they are expensive and time consuming on resource-constrained
   hardware.

   When BFD packets are unauthenticated, it is possible for an attacker
   to forge, modify, and/or replay packets on a link.  These attacks
   have a number of side effects.  They can cause parties to believe
   that a link is down, or they can cause parties to believe that the
   link is up when it is, in fact, down.

   [I-D.ietf-bfd-optimizing-authentication] defines procedures that
   enable better scaling of authentication for BFD by splitting BFD
   authentication work between more computationally intensive
   authentication used for significant changes, and less computationally
   intensive authentication for packets validating that the session is
   in the Up state.  See that document for general performance and
   security considerations.

   This document provides the definition of BFD optimized authentication
   modes using the existing MD5 (Section 6.7.3 of [RFC5880]) and SHA1
   (Section 6.7.4 of [RFC5880]) authentication mechanisms for the more
   computationally intensive work.  It also defines methods for using a
   mechanism, ISAAC [ISAAC], for the less computationally intensive
   mechanism.

   ISAAC requires only a few CPU operations per generated 32-bit number,
   can take a large secret key as a seed, and it has an extremely long
   cycle length.  These properties make it ideal for use in BFD.

   ISAAC+ [ISAAC_] documents some cryptanalysis of the ISAAC mechanism.
   This analysis addressed an issue with initial seeding, and the method
   proposed here incorporates recomendations to address that attack.

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1.1.  Meticulous Keying

   RFC5880 [RFC5880] uses the term "meticulous keyed" and "meticulous
   keying" without defining those terms.  That meaning of that term is
   found by examining the definition of the Sequence Number from BFD
   [RFC5880] (Section 4.2):

   Sequence Number
      The sequence number for this packet.  For Keyed MD5
      Authentication, this value is incremented occasionally.  For
      Meticulous Keyed MD5 Authentication, this value is incremented for
      each successive packet transmitted for a session.  This provides
      protection against replay attacks.

   In this context, the term "meticulous" means that the Sequence number
   is incremented on every new packet which is sent.  The term "keyed"
   means that the packets are authenticated via the use of a secret key
   or keys which are known to both sender and receiver.  The term
   "meticulous keyed" therefore refers to BFD authentication type where
   each subsequently transmitted packet has a sequence number one
   greater than the immediately prior one, and can be authenticated.

1.2.  Requirements Language

   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.

1.3.  Note to RFC Editor

   This document uses several placeholder values throughout the
   document.  Please replace them as follows and remove this note before
   publication.

   RFC XXXX, where XXXX is the number assigned to this document at the
   time of publication.

   2025-10-08 with the actual date of the publication of this document.

2.  Experimental updates to RFC 5880

   This document describes an experimental update to BFD [RFC5880].
   This experiment is intended to provide additional insights into what
   happens when the authentication method defined in this document is
   used.

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   This document is classified as Experimental and is not part of the
   IETF Standards Track.  Implementations based on this document should
   not be considered as compliant with BFD [RFC5880] and should not
   assume interoperability with other implementations that conform to
   this document.

   Some of the state variables in BFD [RFC5880] (Section 6.8.1), are
   related to the authentication type being used for a particular
   session.  However, the definitions given in BFD [RFC5880] are
   specific to Keyed MD5 or SHA1 Authentication, which limit their
   utility for new authentication types.  For the purpose of the
   experiment, this specification updates the definition of some of the
   state variables as given below.

   These updated definitions are entirely compatible with the
   definitions given in BFD [RFC5880] (Section 6.8.1), and require no
   changes to existing configurations or implementations.  Instead, the
   updated definitions clarify that the state variables apply to the
   current authentication type, no matter what it is.

   The text first updates the [RFC5880] definitions, and then defines a
   new authentication type which uses these updated definitions.

   These updated definitions also mean that Authentication Sections
   SHOULD include a Sequence Number field.  Where a Sequence Number is
   not used (as with Simple Password) the variables bfd.RcvAuthSeq and
   bfd.XmitAuthSeq MUST be set to zero.  Where an Authentication
   Section uses a meticulous keyed authentication type, it MUST include
   a Sequence Number field.

   bfd.AuthType:
      The current authentication type in use for this session, as
      defined in BFD [RFC5880] (Section 4.1), or zero if no
      authentication is in use.

      When using an authentication method implementing optimized
      authentication ([I-D.ietf-bfd-optimizing-authentication]), packets
      which indicate a BFD significant change MUST use an authentication
      method which provides for full packet integrity checks with the
      more computationally intensive authentication method.  When the
      bfd.SessionState value is Up, packets MAY use a less
      computationally intensive authentication method such as Meticulous
      Keyed ISAAC.

   bfd.RcvAuthSeq:
      A 32-bit unsigned integer containing the last sequence number for
      the current Authentication Section that was received and accepted.
      The initial value is unimportant.

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   bfd.XmitAuthSeq:
      A 32-bit unsigned integer containing the next sequence number for
      the Authentication Section which will be transmitted.  This
      variable MUST be initialized to a random 32-bit value.  This value
      SHOULD be taken from a cryptographically strong pseudo-random
      number generator (CSPRNG).

   bfd.AuthSeqKnown:
      Set to 1 if the next expected Authentication Section has a
      sequence number which is known, or 0 if it is not known.  This
      variable MUST be initialized to zero.

      This variable MUST be set to zero after no packets have been
      received on this session for at least twice the Detection Time.
      This ensures that the sequence number can be resynchronized if the
      remote system restarts.

3.  Architecture of the Auth Type Method

   This document specifies two Optimized BFD
   [I-D.ietf-bfd-optimizing-authentication] authentication modes:

   *  For the more computationally intensive authentication mechanisms,
      the existing MD5 (Section 6.7.3 of [RFC5880]) and SHA1
      (Section 6.7.4 of [RFC5880]) authentication mechanisms are
      leveraged with small PDU changes necessary to carry the
      Optimization Mode encoding.  These changes are documented in
      Section 4.2 and Section 4.3 respectively.

   *  For the less computationally intensive authentication mode, this
      document defines the Meticulous Keyed ISAAC Authentication
      mechanism.  The PDU format for this mode is defined in
      Section 4.1.  The procedures for using this format are covered
      later in this document.

   ISAAC is used as a way to generate an infinite stream of pseudo-
   random numbers, referred to here as "Auth Keys".  With Meticulous
   Keyed ISAAC Authentication, these Auth Keys are used as a signal that
   the sending party is authentic.  That is, only the sending party can
   generate the correct Auth Keys.  Therefore if the receiving party
   sees a correct Auth Key in a BFD Control Packet in the Up state, then
   only the sending party could have generated it.

   Note that BFD Control Packets with the less computationally intensive
   ISAAC authentication format type are NOT signed or authenticated.
   Therefore, this format MUST NOT be used to signal BFD state changes.

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3.1.  Rationale for ISAAC and Operational Overview

   There are many CSPRNGs available.  This section explains why ISAAC
   was chosen.

   The goal for this less computationally intensive authentication was
   to provide a signal that the session was in the Up state, in the form
   of a 32-bit number which is difficult for an attacker to guess.  The
   number should be generated from a CSPRNG which produces results based
   on a seed composed of both public and private data.  Since BFD can
   have packet loss, the generator should also be "seekable", in that
   the BFD state machine should be able to query the generator (within a
   small window) for new numbers.

   This last property rules out most CSPRNGs, as they are not seekable
   by design.  That is, most CSRNGs maintain minimal state, and are
   designed to produce a long sequence of pseudo-random numbers from a
   few simple calculations.  In general, every call to the CSPRNG
   function modifies the internal state in an irreversible fashion, and
   then produces a new random number as the result.

   It could be possible to use such a generator, and then to manually
   save many results in a buffer.  This buffer could then enable
   "seeking" within a short window.  In contrast, ISAAC produces large
   sets of numbers by design, making it an integrated solution.

   Further, most CSPRNGs are designed to have small seeds.  This
   limitation means that any secret key defined by an administrator is
   not directly usable as a seed for the generator.  Instead, any secret
   key (including any per-session data) would have to be hashed before
   being used to see the generator.  For these reasons, ISAAC was
   chosen.  It can accept keys up to 8192 octets in length, which is
   more than sufficient for BFD.

   ISAAC has been subject to cryptanalysis, most notably ISAAC+
   [ISAAC_].  There are no known vulnerabilities.

   An instance of ISAAC is created for transmission and one for
   reception.  An instance is required for each direction since the
   inputs for seeding ISAAC require the locally randomly generated Seed
   value, and the current BFD Your Discriminator value for an Up
   session, and these values are distinct on each side of the BFD
   session.

   The process for using ISAAC with BFD for each direction is then as
   follows:

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   *  The administrator provides a secret key which is used to
      authenticate each party in the BFD sessions.

   *  When the session transitions into the Up state, the secret key is
      combined with per-session data to seed ISAAC.

   *  The ISAAC process produces a "page" of 256 32-bit random numbers.

   *  The BFD state machine also records a Sequence Number which is
      associated with the first entry of that page.  The combination of
      256 entries and the Sequence number allows the BFD state machine
      to "seek" within a 256-packet window with zero cost, through
      simple addition or subtraction of Sequence Numbers.

   *  If there is a lost packet, the BFD state machine simply seeks to
      the entry which is associated with the received packet, and checks
      if the received packet contains the expected number.

   *  BFD supports packet rates of hundreds of packets per second.  Even
      at those rates, 256 entries per ISAAC page provides for about a
      second of BFD operation before the next page has to be calculated.

   *  As the next page calculation is complex, and there is a long
      period of time available before the next page is needed, this
      calculation can be done in the background.

   *  If the next page calculation is started immediately after the
      current page is fully used, there should be sufficient time to
      calculate the next page as a background task, no matter what the
      packet rate.

   In summary, the ISAAC seed depends on both a secret key and per-
   session data, so it is difficult for an attacker to guess or attack
   via an off-line dictionary attack.  The generated numbers are saved
   in an array, where the BFD fast path can consume them at essentially
   zero cost.

   The only downside to this method is that it does not provide for per-
   packet integrity checks.  This limitation is addressed by mandating
   that Meticulous Keyed ISAAC Authentication is only used to signal
   that the session remains in the Up state.  The ISAAC numbers then
   signal that the originator of the packet is authentic, and the BFD
   state machine verifies that the rest of the packet is well formed,
   and matches the expected state.

   The result is an authentication method which satisfies both the needs
   of the BFD state machine, and is secure.

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4.  Meticulous Keyed ISAAC Authentication Types

4.1.  Meticulous Keyed ISAAC Authentication, ISAAC Format

   If the Authentication Present (A) bit is set in the header, and the
   Authentication Type field contains either Optimized MD5 Meticulous
   Keyed ISAAC Authentication (TBD1), or Optimized SHA-1 Meticulous
   Keyed ISAAC Authentication (TBD2), and the Optimized Authentication
   Mode field contains 2 (Section 7 of
   [I-D.ietf-bfd-optimizing-authentication]) the Authentication
   Section 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Auth Type   |   Auth Len    |  Auth Key ID  |   Opt. Mode   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sequence Number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Seed                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            Auth Key                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 1: Meticulous Keyed ISAAC Authentication Format

   Auth Type:
      The current Auth Type.  It MUST provide for meticulous keying.
      That is, an authentication type where each packet is
      authenticated, and also where the Sequence Number field is
      incremented by one (1) for every packet which is sent.

   Auth Len:
      The length of the Authentication Section, in bytes.  For
      Meticulous Keyed ISAAC Authentication, the length is 16.

   Auth Key ID:
      The authentication key ID in use for this packet.  This allows
      multiple secret keys to be active simultaneously.

   Opt Mode:
      The Optimized Authentication Mode is defined in Section 7 of
      [I-D.ietf-bfd-optimizing-authentication].  When the Auth Type is
      either Optimized MD5 Meticulous Keyed ISAAC Authentication (TBD1),
      or Optimized SHA-1 Meticulous Keyed ISAAC Authentication (TBD2),
      and the format is Meticulous Keyed ISAAC Authentication Format,
      the Optimized Authentication Mode field will be set to 2.

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   Sequence Number:
      The sequence number for this packet.  For Meticulous Keyed ISAAC
      Authentication, this value is incremented once for each successive
      packet transmitted for a session.  This provides protection
      against replay attacks.

   Seed:
      A 32-bit (4 octet) seed which is used in conjunction with the
      shared key in order to configure and initialize the ISAAC pseudo-
      random-number-generator (PRNG).  It is used to identify and secure
      different "streams" of random numbers which are generated by
      ISAAC.

   Auth Key:
      This field carries the 32-bit (4 octet) ISAAC output which is
      associated with the Sequence Number.  The ISAAC PRNG MUST be
      configured and initialized as given in Section 10, below.

      Note that the Auth Key here does not include any summary or hash
      of the BFD Control Packet.  The packet itself is completely
      unauthenticated.

   When the receiving party receives a BFD packet with an expected
   sequence number and the correct corresponding ISAAC output in the
   Auth Key field, it knows that only the authentic sending party could
   have sent that message.  The sending party is therefore Up, as it is
   the only one who could have sent the message.

4.2.  Meticulous Keyed ISAAC Authentication, MD5 Format

   If the Authentication Present (A) bit is set in the header, and the
   Authentication Type field contains Optimized MD5 Meticulous Keyed
   ISAAC Authentication (TBD1), and the Optimized Authentication Mode
   field contains 1 (Section 7 of
   [I-D.ietf-bfd-optimizing-authentication]) the Authentication
   Section 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Auth Type   |   Auth Len    |  Auth Key ID  |   Opt. Mode   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sequence Number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Auth Key/Digest...                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 2: Meticulous Keyed ISAAC Authentication, MD5 Format

   Auth Type:
      The current Auth Type.  It MUST provide for meticulous keying.
      That is, an authentication type where each packet is
      authenticated, and also where the Sequence Number field is
      incremented by one (1) for every packet which is sent.

   Auth Len:
      The length of the Authentication Section, in bytes.  For
      Meticulous Keyed ISAAC MD5 Authentication Format, the length is
      24.

   Auth Key ID:
      The authentication key ID in use for this packet.  This allows
      multiple secret keys to be active simultaneously.

   Opt Mode:
      The Optimized Authentication Mode is defined in Section 7 of
      [I-D.ietf-bfd-optimizing-authentication].  When the Auth Type is
      either Optimized MD5 Meticulous Keyed ISAAC Authentication (TBD1),
      and the format is MD5 Authentication Format, the Optimized
      Authentication Mode field will be set to 1.

   Sequence Number:
      The sequence number for this packet.  For Meticulous Keyed ISAAC
      Authentication, this value is incremented once for each successive
      packet transmitted for a session.  This provides protection
      against replay attacks.

   Auth Key/Digest:
      This field carries the 16-byte MD5 digest for the packet.  The
      procedure for calculating this field is documented in
      Section 6.7.3 of [RFC5880].

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4.3.  Meticulous Keyed ISAAC Authentication, SHA1 Format

   If the Authentication Present (A) bit is set in the header, and the
   Authentication Type field contains Optimized SHA1 Meticulous Keyed
   ISAAC Authentication (TBD2), and the Optimized Authentication Mode
   field contains 1 (Section 7 of
   [I-D.ietf-bfd-optimizing-authentication]) the Authentication
   Section 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Auth Type   |   Auth Len    |  Auth Key ID  |   Opt. Mode   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sequence Number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Auth Key/Hash...                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 3: Meticulous Keyed ISAAC Authentication, SHA1 Format

   Auth Type:
      The current Auth Type.  It MUST provide for meticulous keying.
      That is, an authentication type where each packet is
      authenticated, and also where the Sequence Number field is
      incremented by one (1) for every packet which is sent.

   Auth Len:
      The length of the Authentication Section, in bytes.  For
      Meticulous Keyed ISAAC SHA1 Authentication Format, the length is
      28.

   Auth Key ID:
      The authentication key ID in use for this packet.  This allows
      multiple secret keys to be active simultaneously.

   Opt Mode:
      The Optimized Authentication Mode is defined in Section 7 of
      [I-D.ietf-bfd-optimizing-authentication].  When the Auth Type is
      either Optimized SHA1 Meticulous Keyed ISAAC Authentication
      (TBD1), and the format is SHA1 Authentication Format, the
      Optimized Authentication Mode field will be set to 1.

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   Sequence Number:
      The sequence number for this packet.  For Meticulous Keyed ISAAC
      Authentication, this value is incremented once for each successive
      packet transmitted for a session.  This provides protection
      against replay attacks.

   Auth Key/Digest:
      This field carries the 16-byte SHA1 hash for the packet.  The
      procedure for calculating this field is documented in
      Section 6.7.4 of [RFC5880].

5.  New State variables for Meticulous Keyed ISAAC Authentications

   This document defines new state variables for use with Meticulous
   Keyed ISAAC Authentication.

   bfd.MetKeyIsaacRcvKeyKnown:
      A boolean value which indicates whether or not the system knows
      the receive key for the Meticulous Keyed ISAAC Authentication.
      The initial value is false.  This value is changed to "true" when
      a party verifies that the other party has started to use the
      Meticulous Keyed ISAAC Authentication, with an authenticated Auth
      Key.

   bfd.MetKeyIsaacRcvAuthBase:
      A 32-bit unsigned integer containing a copy of the bfd.RcvAuthSeq
      number which is associated with the current ISAAC "page" for
      authenticating received packets.

   bfd.MetKeyIsaacRcvAuthIndex:
      An 8-bit number used to index within a particular "page" of
      pseudo-random numbers.

   bfd.MetKeyIsaacRcvAuthSeed:
      A 32-bit unsigned integer containing a copy of the Seed associated
      with received packets.

   bfd.MetKeyIsaacRcvAuthData:
      A data structure which contains the ISAAC data for the received
      Auth Type method.  The format and contents of this structure are
      implementation specific, and hold the internal state of the ISAAC
      CSPRNG.

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   bfd.MetKeyIsaacXmitKeyKnown:
      A boolean value which indicates whether or not the system knows
      the xmit key for Meticulous Keyed ISAAC Authentication.  The
      initial value is false.  This value is changed to "true" when a
      party starts to transmit using Meticulous Keyed ISAAC
      Authentication.

   bfd.MetKeyIsaacXmitAuthBase:
      A 32-bit unsigned integer containing a copy of the bfd.XmitAuthSeq
      number which is associated with the current ISAAC "page" for
      authenticating sent packets.

   bfd.MetKeyIsaacXmitAuthIndex:
      An 8-bit number used to index within a particular "page" of
      pseudo-random numbers.

   bfd.MetKeyIsaacXmitAuthSeed:
      A 32-bit unsigned integer containing a copy of the Seed associated
      with sent packets.

   bfd.MetKeyIsaacXmitAuthData:
      A data structure which contains the ISAAC data for the sending
      Auth Type method.  The format and contents of this structure are
      implementation specific, and hold the internal state of the ISAAC
      CSPRNG.

6.  Procedures for BFD Authentication using Meticulous Keyed ISAAC, MD5
    or SHA1 Formats

   The transmit and receive procedures utilize the additional procedures
   documented in Section 7.1 of
   [I-D.ietf-bfd-optimizing-authentication].

   The authentication procedure for Meticulous Keyed ISAAC, MD5 Format
   is covered by Section 6.7.3 of [RFC5880] for the Meticulous Keyed MD5
   Authentication mode.

   The authentication procedure for Meticulous Keyed ISAAC, SHA1 Format
   is covered by Section 6.7.4 of [RFC5880] for the Meticulous Keyed
   SHA1 Authentication mode.

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7.  Procedures for BFD Authentication using Meticulous Keyed ISAAC,
    ISAAC Format

   In this mode of optimized authentication, one or more secret keys
   (with corresponding key IDs) are configured in each system.  One of
   the keys is used to seed the ISAAC PRNG.  The output of ISAAC is used
   to signal that the sender is authentic.  To help avoid replay
   attacks, a sequence number is also carried in each packet.  For
   Meticulous Keyed ISAAC Authentication, the sequence number MUST be
   incremented by one on every packet.

   The receiving system accepts the packet if the key ID matches one of
   the configured Keys, and the Auth Key derived from the selected Key,
   Seed, and Sequence Number matches the Auth Key carried in the packet,
   and the sequence number is strictly greater than the last sequence
   number received (modulo wrap at 2^32).  If any of these criteria do
   not match, the packet fails validation, and is discarded.

   Transmission Using Meticulous Keyed ISAAC Authentication, ISAAC
   Format

      The Auth Type field MUST be set to one of two values; Optimized
      MD5 Meticulous Keyed ISAAC Authentication (TBD1); or Optimized
      SHA-1 Meticulous Keyed ISAAC Authentication (TBD2).

      The Auth Len field MUST be set to 16.

      The Auth Key ID field MUST be set to the ID of the current
      authentication key.  The Sequence Number field MUST be set to
      bfd.XmitAuthSeq.

      The Seed field MUST be set to the value of the current seed used
      for this session.

      The Auth Key field MUST be set to the output of ISAAC, which
      depends on the secret Key, the current Seed, and the Sequence
      Number.

      The Optimized Authentication Mode field MUST be 2, the "less
      computationally intensive authentication type".  See Section 7 of
      [I-D.ietf-bfd-optimizing-authentication].

      For Meticulous Keyed ISAAC Authentication, bfd.XmitAuthSeq MUST be
      incremented by one on each packet, in a circular fashion (when
      treated as an unsigned 32-bit value).  The bfd.XmitAuthSeq MUST
      NOT be incremented by more than one per packet.

   Receipt using Meticulous Keyed ISAAC Authentication, ISAAC Format

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      If the received BFD Control packet does not contain an
      Authentication Section, or the Auth Type is not correct (either
      Optimized MD5 Meticulous Keyed ISAAC Authentication (TBD1) or
      Optimized SHA-1 Meticulous Keyed ISAAC Authentication (TBD2)),
      then the received packet MUST be discarded.

      If the Auth Key ID field does not match the ID of a configured
      authentication key, the received packet MUST be discarded.

      The Optimized Authentication Mode field MUST be 2, the "less
      computationally intensive authentication type".  See Section 7 of
      [I-D.ietf-bfd-optimizing-authentication].

      If the Auth Len field is not equal to 16, the packet MUST be
      discarded.

      If bfd.AuthSeqKnown is 1, examine the Sequence Number field.  For
      Meticulous keyed ISAAC, if the sequence number lies outside of the
      range of bfd.RcvAuthSeq+1 to bfd.RcvAuthSeq+(3*Detect Mult)
      inclusive (when treated as an unsigned 32-bit circular number
      space) the received packet MUST be discarded.

      If bfd.MetKeyIsaacRcvKeyKnown is "true" and the Seed field does
      not match the current Seed value, bfd.MetKeyIsaacRcvAuthSeed, the
      packet MUST be discarded.

      Calculate the current expected output of ISAAC, which depends on
      the secret Key, the current Seed, and the Sequence Number.  If the
      value does not match the Auth Key field, then the packet MUST be
      discarded.

      If bfd.MetKeyIsaacRcvKeyKnown is false, the ISAAC related
      variables are initialized as per Section 10.2 using the contents
      of the packet.

      Note that in some cases, calculating the expected output of ISAAC
      will result in the creation of a new "page" of 256 numbers.  This
      process will be irreversible, and will destroy the current "page".
      As a result, if the generation of a new output will create a new
      "page", the receiving party MUST save a copy of the entire ISAAC
      state before proceeding with this calculation.  If the outputs
      match, then the saved copy can be discarded, and the new ISAAC
      state is used.  If the outputs do not match, then the saved copy
      MUST be restored, and the modified copy discarded, or cached for
      later use.

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8.  Secret Key

   The security of the Meticulous Keyed ISAAC Auth Type depends on the
   Secret Key.  The Secret Key is mixed with a per-session Seed as
   discussed below.  The result is used to initialize a stream of
   pseudo-random numbers using the ISAAC random number generator.

   A particular Secret Key is identified via the Auth Key ID field.
   This Auth Key ID is either placed in the packet by the sender, or
   verified by the receiver.  Meticulous Keyed ISAAC Authentication
   permits systems to have multiple Secret Keys configured, but we do
   not discuss how those keys are managed or used.  A session MUST NOT,
   however, change the Auth Key ID for Meticulous Keyed ISAAC
   Authentication, during a session.  There is no defined way to re-sync
   or re-initialize an ongoing session with a different Auth Key ID and
   correspondingly different Secret Key.

   For interoperability, the management interface by which the key is
   configured MUST accept ASCII strings, and SHOULD also allow for the
   configuration of any arbitrary binary string in hexadecimal form.
   Other configuration methods MAY be supported.

   The Secret Key MUST be at least eight (8) octets in length, and
   SHOULD NOT be more than 128 octets in length.

   There are no known issues with using the same secret Key for multiple
   Auth Type methods.  However, it is RECOMMENDED that administrators
   use different Secret Keys for each Auth Type.

9.  Transition to using ISAAC

   A BFD session which uses Optimized MD5 Meticulous Keyed ISAAC
   Authenticationo or Optimized SHA-1 Meticulous Keyed ISAAC
   Authentication MUST begin a session with Auth Type set to the
   relevant authentication type, and the Optimized Authentication Mode
   field set to 1.

   When a BFD session using more computationally intensive
   authentication transitions to the Up state, the first Up packet MUST
   contain an Optimized Authentication Mode field with value 1.  Since
   state transitions require full packet integrity checks, an Optimized
   Authentication Mode field with value 2 is not permitted for state
   changes.  Each party MUST continue to use the more computationally
   intensive authentication mode until the other side has confirmed the
   switch to the Up state, with a packet that also uses more
   computationally intensive authentication.

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   Once the BFD session has transitioned to the Up state, the sender MAY
   send the subsequent packets for the Up state with the Optimized
   Authentication Mode field containing value 2 using ISAAC format.

   When a system first receives a packet containing Optimized
   Authentication Mode field with value 2, it initializes the ISAAC PRNG
   state using the Seed from that packet.  A system originating a packet
   using Meticulous Keyed ISAAC Authentication will generate a Seed, and
   place it into the packet which is then sent.  Further discussion of
   initialization is below in Section 10.1 and Section 10.2.

   The first packet after the transition to the Up state is the only
   time when the ISAAC random number generator for transmission is
   initialized.  In contrast, a temporary transition away from using
   Meticulous Keyed ISAAC Authentication, ISAAC format (Section 12) and
   back, does not cause ISAAC to be re-keyed.

   There is no negotiation as to when authentication switches from the
   original type, to using Meticulous Keyed ISAAC Authentication using
   the ISAAC format.  The sender simply begins sending packets with a
   relevant Auth-Type, and with the Optimized Authentication Mode field
   set to 1.  When the sender switches to using using Meticulous Keyed
   ISAAC Authentication, ISAAC format, it sets the Optimized
   Authentication Mode field to 2, and starts performing the ISAAC
   calculations as described here.

   Similarly, a receiving system switches to using this method when it
   sees that it has received a packet contains Optimized Authentication
   Mode field set to 2 when bfd.MetKeyIsaacRcvKeyKnown variable is
   false.  The receiving system then initializes its variables, and
   authenticates the received packet, by comparing the Auth Key in the
   packet with the key it generated itself.

   The operation of those state variables MUST now satisfy the
   requirements of the new Optimized Authentication Mode.  That is, when
   changing Optimized Authentication mode in a session, the current
   value of the bfd.RcvAuthSeq and bfd.XmitAuthSeq variables is used as
   the initial value(s) for the new mode.

10.  Seeding ISAAC

   The Seed field is used to identify and secure different "streams" of
   random numbers which are generated by ISAAC.  Each session uses a
   different Seed, which is used along with the Your Discriminator field
   (Section 4.1 of [RFC5880]), and the Secret Key, to initialize ISAAC.

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   The value of the Seed field MUST be derived from a CSPRNG source.
   Exactly how this can be done is outside of the scope of this
   document.

   A new Seed value MUST be created every time a BFD session transitions
   into the Up state.  In order to prevent continuous rekeying, once the
   session is in the Up state, the Seed for a session MUST NOT be
   changed until another state transition occurs.

   The ISAAC PRNG is initialized by setting all internal variables and
   data structures to zero (0).  The PRNG is then seeded by using the
   the following structure:

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                             Seed                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Your Discriminator                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Secret Key ...            |  Counter |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 4: ISAAC Initialization Structure

   Where the Your Discriminator field is taken from the BFD packet
   defined in RFC5880 Section 4.1 [RFC5880].  This field is taken from
   the respective values used by a sending system.  For receiving
   systems, the field are taken from the received packet.  As the size
   of the buffer used to seed is limited, the length of the Secret Key
   MUST be no more than 1015 octets.  The Counter field is used to
   ensure the the initial seeding of ISAAC avoids the seeding issues
   discussed in ISAAC+ [ISAAC_].

   Whatever the API or other interface used to input the Secret Key, any
   implementation-specific internal representations of the Secret Key
   MUST NOT be used when encoding the Secret Key into the above data
   structure.  That is, there is no "length field which indicates how
   long the Secret Key is, and there is no trailing zero or NUL byte
   which indicates the end of the Secret Key.  Implementers are reminded
   that internal representations of data should not affect protocol
   operation.

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   The buffer used to initialize ISAAC filled it with repeated copies of
   the above structure.  For each complete copy of the structure, the
   Counter field is incremented, starting from zero (0).  The final
   portion of the initialization buffer holds a partial copy of the
   structure, which is however much can be accommodated in the remaining
   portion of the buffer.

   Once the ISAAC "page" is initialized, the data is processed through
   the "randinit()" function of ISAAC [ISAAC].  Pseudo-random numbers
   are then produced 32 bits at a time by calling the "isaac()"
   function.

   For the sender, this calculation can be done outside of the BFD "fast
   path" as soon as the Your Discriminator value is known.  For the
   receiver, this calculation can only be done when the Seed is received
   from the sender, and therefore the initial seeding needs to be done
   in the BFD "fast path".

   The following table gives Seed and Your Discriminator as 32-bit
   hexadecimal values, and the Secret Key as an eleven-character string.
   The subsequent table shows the first eight Sequence numbers and
   corresponding Auth Key values which were generated using the above
   initial values.

           Field       Value(s)
           ----------  ------------
           Seed        0x0bfd5eed
           Y-Disc      0x4002d15c
           Secret Key  RFC5880June
           Counter     0...50

                  Figure 5: Test Inputs for seeding ISAAC

           Sequence Auth Key
           -------- --------
           0        9af65d83
           1        44355d56
           2        9334074e
           3        b643ef59
           4        74d659f1
           5        8966dc56
           6        a1f6f9bc
           7        21895a46

                         Figure 6: Expected Outputs

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   This construct provides for 64 bits of entropy, of which 32 bits is
   controlled by each party in a BFD session.  For security, each
   implemention SHOULD randomize their discrimator fields at the start
   of a session, as discussed in Section 10 [RFC5880].

   Note that this construct only uses the Your Discriminator field once,
   to seed ISAAC.  It therefore allows the My Discriminator field to
   change as permitted by BFD [RFC5880] (Section 6.3).

   While the Your Discriminator field may change, there is no way to
   signal or negotiate Seed changes.  The Seed is set once by each party
   after the session transitions into the Up state, and then remains
   unchanged for the duration of the session.  The receiving party MUST
   remember the current Seed value.  The Seed value MUST NOT change
   unless sending party has signalled a BFD state change with a packet
   that is authenticated using a more computationally intensive
   authentication method.  When a system receives a BFD packet
   containing Meticulous Keyed ISAAC Authentication, it MUST check that
   the received Seed contains the expected value, and if not, it MUST
   discard the packet as inauthentic.

10.1.  Sender Variable Initialization

   A system which sends packets initializes ISAAC as described above.
   The ISAAC related variables are initialized as follows:

   bfd.MetKeyIsaacXmitKeyKnown:
      This variable transitions from false to true when the sender
      decides to start using ISAAC.  The sender also initializes the
      other variables at the same time.

   bfd.MetKeyIsaacXmitAuthBase:
      The sender copies the bfd.XmitAuthSeq number from the current
      packet to be sent into this variable.

   bfd.MetKeyIsaacXmitAuthIndex:
      The sender sets this variable to zero.

   bfd.MetKeyIsaacXmitAuthSeed:
      The sender copies the current Seed value into this variable.  This
      variable is then copied into the "Seed" field of each Auth Type
      packet.

   bfd.MetKeyIsaacXmitAuthData:
      The ISAAC state for sending is encapsulated in this variable.

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10.2.  Receiver Variable Initialization

   When a system receives packets with Meticulous Keyed ISAAC
   Authentication and is able to authenticate such a packet the first
   time, the ISAAC related variables are initialized as follows:

   bfd.MetKeyIsaacRcvKeyKnown:
      This variable transitions from false to true when the receiver
      sees that the sender has started using Meticulous Keyed ISAAC
      Authentication.  The receiver also initializes the other variables
      at the same time.

   bfd.MetKeyIsaacRcvAuthBase:
      The bfd.RcvAuthSeq number from the current packet is copied into
      this variable.

   bfd.MetKeyIsaacRcvAuthIndex:
      The receiver sets this value to zero

   bfd.MetKeyIsaacRcvAuthSeed:
      The receiver copies the Seed value from the received packet into
      this variable.  Note that this copy only occurs when the
      bfd.MetKeyIsaacXmitKeyKnown variable transitions from false to
      true."

   bfd.MetKeyIsaacRcvAuthData:
      The ISAAC state for receiving is encapsulated in this variable.

   As there may be packet loss, the receiver has to take special care to
   initialize the bfd.MetKeyIsaacRcvAuthBase variable.  If there has
   been no packet loss, the bfd.MetKeyIsaacRcvAuthBase is taken directly
   from the bfd.RcvAuthSeq variable, and the bfd.MetKeyIsaacRcvAuthIndex
   is set to zero.

   If, however, the packet's Sequence Number differs from the expected
   value, then the difference "N" indicates how many packets were lost.
   The receiver then can use this difference to index into the ISAAC
   page to find the corresponding Auth Key.  If the key in the ISAAC
   page does not match the corresponding Auth Key in the packets, the
   packet fails validation, and is discarded.

   If a key found by indexing into this ISAAC page does match the Auth
   Key in the packet, then the bfd.MetKeyIsaacRcvAuthIndex field is
   initialized to this value.  The bfd.MetKeyIsaacRcvAuthBase field is
   then initialized to contain the value of bfd.RcvAuthSeq, minus the
   value of bfd.MetKeyIsaacRcvAuthIndex.  This process allows the
   pseudo-random stream to be re-synchronized in the event of lost
   packets.

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   That is, the value for bfd.MetKeyIsaacRcvAuthBase is the Sequence
   Number for first Auth Key used in this session.  This value may be
   from a lost packet, but can never the less be calculated by the
   receiver from a later packet.

11.  Operation

   Once the variables have been initialized, ISAAC will be able to
   produce 256 random numbers to use as Auth Keys, at near-zero cost.
   The AuthIndex field is incremented by one for every new Auth Key
   generated.  Each new value of the Sequence Number field (sent or
   received) is then calculated by adding the relevant AuthBase and
   AuthIndex fields.

   When all 256 numbers are consumed the AuthIndex field will wrap to
   zero.  The ISAAC mixing function is then run, which then results in
   another set of 256 random numbers.  The AuthBase variable is then
   incremented by 256, to indicate that 256 Auth Keys have been
   consumed.  This process then continues until a BFD state change.

   ISAAC can be thought of here as producing an infinite stream of
   numbers, based on a secret key, where the numbers are produced in
   "pages" of 256 32-bit values.  This property of ISAAC allows for
   essentially zero-cost "seeking" within a page.  The expensive
   operation of mixing is performed only once per 256 packets, which
   means that most BFD packet exchanges can be fast and efficient.

   The receiving party can then look at the Sequence Number to determine
   which particular PRNG value is being used in the packet.  By
   subtracting the bfd.MetKeyIsaacAuthBase from the Sequence Number
   (with possible wrapping), an expected Index can be derived, and a
   corresponding Auth Key found.  This process thus permits the two
   parties to synchronize if/when a packet or packets are lost.

   Incrementing the Sequence Number for every packet also prevents the
   re-use of any individual pseudo-random number which was derived from
   ISAAC.

   The Sequence Number can increment without bounds, though it can wrap
   once it reaches the limit of the 32-bit counter field.  ISAAC has a
   cycle length of 2^8287, so there is no issue with using more than
   2^32 values from it.

   The result of the above operation is an infinite series of numbers
   which are unguessable, and which can be used to authenticate the
   sending party.

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   Each system sending BFD packets chooses its own seed, and generates
   its own sequence of pseudo-random numbers using ISAAC, and place
   those values into the Auth Key field.  Each system receiving BFD
   packets runs a separate pseudo-random number generator, and verifies
   that the received packets contain the expected Auth Key.

11.1.  Page Flipping

   Once all 256 Auth Keys from the current page have been used, the next
   page is calculated by calling the isaac() function.  This function
   modifies the current page to create the next page, and is inherently
   destructive.  In order to prevent issues, care should be taken to
   perform this process correctly.

   It is RECOMMENDED that implementations keep both a current page, and
   a next page associated with the ISAAC state.  The next can be
   calculated by making a copy of the current page, and then calling the
   isaac() function.

   The system needs to maintain the current page at all times when
   Meticulous Keyed ISAAC Authentication is used.  The next page does
   not need to be maintained at all times, and can be calculated on
   demand.  However, in order to avoid impacting the fast path, the next
   page should be calculated in the background in an asynchronous
   manner.

   This process has a number of benefits.  First, At 60 packets per
   second, the system has approximately four (4) seconds of time to
   calculate the next page.  If the calculation is done quickly, the
   next page is available to the fast path before it is needed.

   Second, having the next page available early means that an attacker
   cannot spoof BFD packets, and force the received to spend significant
   resources calculating a next page on the BFD fast path.  Instead, the
   receiver can simply check the contents of the next page at near-zero
   cost, and discard the spoofed packet.

   When the receiver determines that it needs to move to the next page,
   it can simply swap the current and next pages (updating the BFD
   variables as appropriate), and then begin an asynchronous calculation
   of the next page.  Such asynchronous calculations are preferable to
   calculating the next page in the BFD fast path.

   This document does not make provisions for dealing with the case of
   losing more than 512 packets.  Implementors MUST limit the value of
   Detect Multi to a small enough number in order to keep the number of
   lost packets within an acceptable limit.

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11.2.  Multiple Keys

   In a keyed 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 may
   require these keys to be refreshed often, which poses a further
   challenge.

   While the Auth Key ID field provides for the provisioning of multiple
   keys simultaneously, there is no way within the BFD protocol for each
   party to signal which set of Key IDs are supported.  Any such
   signalling or negotiation needs to be done "out of band" for BFD, and
   usually via manual administrator configuration.

   The seeding mechanism for ISAAC, covered in Section 10, is carried
   out only once for a BFD session.  In order to rotate keys, it is
   REQUIRED to administratively disable the BFD session as part of
   changing the keys.  This permits the new session to be seeded as part
   of bringing up the new session.

12.  Transition away from using ISAAC

   There are two ways to transition away from using ISAAC.  One way is
   via state changes: the link either goes down due to an fault, or one
   party signals a state change via a packet signed with a more
   computationally instensive authentication.  The second situation is
   where one party wishes to temporarily signal via a more
   computationally intensive method that it is still Up, by setting the
   Optimized Authentication Mode field away from value 2 to value 1.

   The more computationally intensive authentication type provides for
   full packet integrity checks, which serves as a stronger indication
   that the session is Up, and that both parties are fully synchronized.
   This switch can be done at any time during a session.

   It is RECOMMENDED that implementations periodically switch to the
   more computationally intensive authentication type for packets which
   maintain the session in an Up state.  The interval between these
   switches SHOULD be long enough that the system still gains
   significant benefit from using Meticulous Keyed ISAAC Authentication.
   See [I-D.ietf-bfd-optimizing-authentication] for appropriate
   procedure on switching Optimized Authentication Mode.

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   When switching to the more computationally intensive authentication
   mode after ISAAC has been seeded, the Authentication Section's
   Sequence Number field will continue meticulously increasing.  In
   order to permit transition back to ISAAC as the less computationally
   intensive authentication mechanism, it is necessary for ISAAC to
   continue to generate pages appropriate for validating the received
   sequence number.

   [I-D.ietf-bfd-optimizing-authentication] describes the procedures
   that require the switch to the more computationally intensive
   authentication mode - particularly BFD Poll Sequences.

13.  The YANG Model

   This YANG module adds two identities defined in this document to the
   IETF Keychain Model [RFC8177].  One of them uses the Meticulous Keyed
   MD5 as the more computationally intensive authentication and
   Meticulous Keyed ISAAC Keyed as the less computationally intensive
   authentication.  The other uses the Meticulous Keyed SHA-1 as the
   more computationally intensive authentication and Meticulous Keyed
   ISAAC Keyed as the less computationally intensive authentication.

   <CODE BEGINS> file "ietf-bfd-met-keyed-isaac@2025-10-08.yang"
   module ietf-bfd-met-keyed-isaac {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-met-keyed-isaac";
     prefix "bfd-mki";

     import ietf-key-chain {
       prefix key-chain;
       reference
         "RFC 8177: YANG Data Model for Key Chains.";
     }

     organization
       "IETF BFD Working Group";

     contact
       "WG Web:   <https://datatracker.ietf.org/wg/bfd>
        WG List:  <rtg-bfd@ietf.org>

        Authors: Mahesh Jethanandani (mjethanandani@gmail.com)
                 Ashesh Mishra (ashesh@aalyria.com)
                 Jeffrey Haas (jhaas@juniper.net)
                 Alan Dekok (alan.dekok@inkbridge.io)
                 Sonal Agarwal (sonal@arrcus.com).";

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     description
       "This YANG module provides identities derived from the
        ietf-key-chain model for the experimental BFD Meticulous Keyed
        ISAAC authentication mechanism.

        Copyright (c) 2025 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject to
        the license terms contained in, the Revised BSD License set
        forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX
        (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself
        for full legal notices.";

     revision "2025-10-08" {
       description
         "Initial Version.";
       reference
         "RFC XXXX: Meticulous Keyed ISAAC for BFD Optimized
          Authentication.";
     }

     identity optimized-md5-meticulous-keyed-isaac {
       base key-chain:crypto-algorithm;
       description
         "BFD Optimized Authentication using Meticulous Keyed MD5 as the
          strong authentication and Meticulous Keyed ISAAC Keyed as the
          less computationally intensive authentication.";
       reference
         "RFC XXXX: Meticulous Keyed ISAAC for BFD Optimized
          Authentication.";
     }

     identity optimized-sha1-meticulous-keyed-isaac {
       base key-chain:crypto-algorithm;
       description
         "BFD Optimized Authentication using Meticulous Keyed SHA-1 as
          the strong authentication and Meticulous Keyed ISAAC Keyed as
          the less computationally intensive authentication.";
       reference
         "RFC XXXX: Meticulous Keyed ISAAC for BFD Optimized
          Authentication.";
     }

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   }
   <CODE ENDS>

14.  IANA Considerations

   This documents requests the assignment of two BFD Auth Types, one URI
   and one YANG model.

14.1.  BFD Auth Types

   This document requests an update to the registry titled "BFD
   Authentication Types".  IANA is requested to assign two new BFD
   AuthType:

   *  TBD1: Optimized MD5 Meticulous Keyed ISAAC Authentication with a
      suggested value of 7.

   *  TBD2: Optimized SHA-1 Meticulous Keyed ISAAC Authentication with a
      suggested value of 8.

14.2.  IETF XML Registry

   This document registers one URIs in the "ns" subregistry of the "IETF
   XML" registry [RFC3688].  Following the format in [RFC3688], the
   following registration is requested:

   URI: urn:ietf:params:xml:ns:yang:ietf-bfd-met-keyed-isaac
   Registrant Contact: The IESG
   XML: N/A, the requested URI is an XML namespace.

14.3.  The YANG Module Names Registry

   This document registers one YANG modules in the "YANG Module Names"
   registry [RFC6020].  Following the format in [RFC6020], the following
   registrations are requested:

   name:         ietf-bfd-met-keyed-isaac
   namespace:    urn:ietf:params:xml:ns:yang:ietf-bfd-met-keyed-isaac
   prefix:       bfd-mki
   reference:    RFC XXXX

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15.  Security Considerations

15.1.  Protocol Security Considerations

   All security considerations of [RFC5880] and
   [I-D.ietf-bfd-optimizing-authentication] apply to this document.

   The security of this proposal depends strongly on the length of the
   Secret Key, and on its entropy.  It is RECOMMENDED that the key be 16
   octets in length or more.

   The dependency on the Secret Key for security is mitigated through
   the use of two 32-bit numbers: the Your Discriminator field from the
   BFD protocol, and the ISAAC Seed.  Both numbers are procedurally
   required to be random.  These numbers serve as a nonce that inhibits
   attackers from performing an off-line brute-force dictionary attack
   to discover the key.

   The security of this proposal depends strongly on ISAAC.  Research
   shows that there are few other CSRNGs which are as simple and as fast
   as ISAAC.  For example, many other generators are based on AES, which
   may perform worse than ISAAC on resource constrained systems without
   hardware acceleration.

15.1.1.  Spoofing

   Meticulously Keyed ISAAC authentication protects vs. the spoofing of
   BFD Up packets and keeping the BFD session Up when it would otherwise
   be reset.

   In the event that Meticulously Keyed ISAAC, which is operating as the
   less computationally intensive authentication mechanism for Optimized
   BFD, is subverted, the periodic more computationally reauthentication
   mechanism will limit the time that the session is kept
   inappropriately in the Up state.  (Section 5 of
   [I-D.ietf-bfd-optimizing-authentication])

   The Meticulous Keyed ISAAC Authentication method allows the BFD end-
   points to detect a malicious packet via a number of different
   methods.  Packets which are malformed are discarded.  Packets which
   do not pass the BFD state machine [RFC5880] (Section 6.2) checks are
   discarded.  Packets which do not have the correct Sequence Number,
   Seed and Auth Key are discarded.  These discarded packets have no
   effect on the BFD state machine.

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   The correlation between the Sequence Number and the Auth Key ensures
   that each Sequence Number has a corresponding Auth Key associated
   with it.  The structure and design of the ISAAC CSPRNG ensures that
   each Auth Key is unique and is unguessable.

   Performing an attack on this authentication method would require all
   of the following to be true:

      The attacker is on-path, and can perform an active attack.

      The attacker has the contents of one or more packets.

      The attacker has deduced the Secret Key used for ISAAC, and is
      able to correlate the Sequence Number to the current ISAAC state.

   These conditions are unlikely to all be true.  If the Secret Key is
   long and complex, the search space to guess the Secret Key is too
   large to discover via brute-force.  The use of the Seed and Your
   Discriminator fields when seeding ISAAC adds 64 bits of entropy to
   each session, which further makes off-line dictionary attacks
   impractical.

15.1.2.  Re-Use of keys

   The cryptographic strength of the Auth-Type methods is significantly
   different between SHA-1 and ISAAC.  While ISAAC has had
   cryptanalysis, and has not been shown to be broken, that analysis is
   limited.  The question then is whether or not it is safe to use the
   same key for both Auth Type methods (SHA1 and ISAAC), or should we
   require different keys for each method?

   If we recommend different keys, then it is possible for the two keys
   to be configured differently on each side of a BFD link.  For
   example, a correctly configured key could allow to the BFD state
   machine to advance to Up.  Then when the session switches to using to
   weaker Auth Type with a different key, that key may not match, and
   the session would immediately drop.  Suggesting instead that the keys
   be identical means that no such misconfiguration is possible.

   Implementations are therefore free to use the same key, or different
   keys.  The use of the same key for for both more and less
   computationally intensive authentication is acceptable, as ISAAC is
   keyed not only with the authentication key, but also depends on 32
   bits of random data, along with 32 bits of a Sequence Number.  The
   use of this added randomness increases the difficulty of breaking the
   key.

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15.2.  YANG Security Considerations

   This section is modeled after the template described in Section 3.7
   of [I-D.ietf-netmod-rfc8407bis].

   The "ietf-bfd-met-keyed-isaac" YANG module defines a data model that
   is designed to be accessed via YANG-based management protocols, such
   as NETCONF [RFC6241] or RESTCONF [RFC8040].  These YANG-based
   management protocols (1) have to use a secure transport layer (e.g.,
   SSH [RFC4252] TLS [RFC8446], and QUIC [RFC9000]) and (2) have to use
   mutual authentication.

   The Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular NETCONF or
   RESTCONF users to a preconfigured subset of all available NETCONF or
   RESTCONF protocol operations and content.

   The YANG module defines a set of identities.  These identities are
   intended to be reused by other YANG modules.  The module by itself
   does not expose any data nodes that are writable, data nodes that
   contain read-only state, or RPCs.  As such, there are no additional
   security issues related to the YANG module that need to be
   considered.

16.  Contributors

   The authors of this document want to acknowledge Ankur Saxena and
   Reshad Rahman as contributors to this document.

17.  Acknowledgements

   The authors want to thank Ketan Talaulikar for his reviews and
   suggestions that have improved the document.

18.  References

18.1.  Normative References

   [I-D.ietf-bfd-optimizing-authentication]
              Jethanandani, M., Mishra, A., Saxena, A., Bhatia, M., and
              J. Haas, "Optimizing BFD Authentication", Work in
              Progress, Internet-Draft, draft-ietf-bfd-optimizing-
              authentication-33, 10 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bfd-
              optimizing-authentication-33>.

   [ISAAC]    Jenkins, R. J., "ISAAC",
              http://www.burtleburtle.net/bob/rand/isaac.html, 1996.

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   [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>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [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>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8177]  Lindem, A., Ed., Qu, Y., Yeung, D., Chen, I., and J.
              Zhang, "YANG Data Model for Key Chains", RFC 8177,
              DOI 10.17487/RFC8177, June 2017,
              <https://www.rfc-editor.org/info/rfc8177>.

18.2.  Informative References

   [I-D.ietf-netmod-rfc8407bis]
              Bierman, A., Boucadair, M., and Q. Wu, "Guidelines for
              Authors and Reviewers of Documents Containing YANG Data
              Models", Work in Progress, Internet-Draft, draft-ietf-
              netmod-rfc8407bis-28, 5 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
              rfc8407bis-28>.

   [ISAAC_]   Aumasson, J-P., "On the pseudo-random generator ISAAC",
              https://eprint.iacr.org/2006/438.pdf, 2006.

   [RFC4252]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
              January 2006, <https://www.rfc-editor.org/info/rfc4252>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

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   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

Authors' Addresses

   Alan DeKok
   InkBridge Networks
   100 Centrepointe Drive #200
   Ottawa ON K2G 6B1
   Canada
   Email: alan.dekok@inkbridge.io

   Mahesh Jethanandani
   Kloud Services
   Email: mjethanandani@gmail.com

   Sonal Agarwal
   Cisco Systems, Inc
   170 W. Tasman Drive
   San Jose, CA 95070
   United States of America
   Email: agarwaso@cisco.com
   URI:   www.cisco.com

   Ashesh Mishra
   Aalyria Technologies
   Email: ashesh@aalyria.com

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   Jeffrey Haas
   HPE
   Email: jhaas@juniper.net

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