Network Working Group                                           A. DekokInternet-Draft                                       Network RADIUS SARLUpdates: 5880 (if approved)                              M. JethanandaniIntended status: Standards Track                          Kloud ServicesExpires: 5 January 2025                                       S. Agarwal                                                      Cisco Systems, Inc                                                               A. Mishra                                                    Aalyria Technologies                                                               A. Saxena                                                       Ciena Corporation                                                             4 July 2024             Meticulous Keyed ISAAC for BFD Authentication               draft-ietf-bfd-secure-sequence-numbers-16Abstract   This document describes a new BFD Authentication mechanism,   Meticulous Keyed ISAAC.  This mechanism can be used to authenticate   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 as an authenticated signal   to maintain a session in the the "Up" state.   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 5 January 2025.Copyright Notice   Copyright (c) 2024 IETF Trust and the persons identified as the   document authors.  All rights reserved.Dekok, et al.            Expires 5 January 2025                 [Page 1]Internet-Draft            ISAAC Authentication                 July 2024   This document is subject to BCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents (https://trustee.ietf.org/   license-info) in effect on the date of publication of this document.   Please review these documents carefully, as they describe your rights   and restrictions with respect to this document.  Code Components   extracted from this document must include Revised BSD License text as   described in Section 4.e of the Trust Legal Provisions and are   provided without warranty as described in the Revised BSD License.Table of Contents   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3   3.  Updating RFC 5880 . . . . . . . . . . . . . . . . . . . . . .   3   4.  Architecture of the Auth Type Method  . . . . . . . . . . . .   4   5.  Meticulous Keyed ISAAC Authentication Format  . . . . . . . .   5   6.  Meticulous Keyed ISAAC Authentication . . . . . . . . . . . .   6   7.  New State variables for Meticulous Keyed ISAAC  . . . . . . .   8   8.  Secret Key  . . . . . . . . . . . . . . . . . . . . . . . . .   9   9.  Transition to using ISAAC . . . . . . . . . . . . . . . . . .  10   10. Seeding ISAAC . . . . . . . . . . . . . . . . . . . . . . . .  10     10.1.  Sender Variable Initialization . . . . . . . . . . . . .  12     10.2.  Receiver Variable Initialization . . . . . . . . . . . .  13   11. Operation . . . . . . . . . . . . . . . . . . . . . . . . . .  14     11.1.  Page Flipping  . . . . . . . . . . . . . . . . . . . . .  15   12. Transition away from using ISAAC  . . . . . . . . . . . . . .  16   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17   14. Security Considerations . . . . . . . . . . . . . . . . . . .  17     14.1.  Spoofing . . . . . . . . . . . . . . . . . . . . . . . .  18     14.2.  Re-Use of keys . . . . . . . . . . . . . . . . . . . . .  18   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19     16.1.  Normative References . . . . . . . . . . . . . . . . . .  19     16.2.  Informative References . . . . . . . . . . . . . . . . .  19   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  201.  Introduction   BFD [RFC5880] (Section 6.7.2) 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 believeDekok, et al.            Expires 5 January 2025                 [Page 2]Internet-Draft            ISAAC Authentication                 July 2024   that a link is down, or they can cause parties to believe that the   link is up when it is, in fact, down.  The goal of this specification   is to use a simple method to prevent spoofing of the BFD session   being "Up".  We therefore define a fast Auth Type method which allows   parties to securely signal that they are still in the Up state.   This document proposes the use of an Authentication method which   provides meticulous keying, but which has less impact on resource   constrained systems.  The algorithm chosen is a seeded pseudo-random   number generator named ISAAC [ISAAC].  ISAAC has been subject to   significant cryptanalysis in the past thirty years, most notably   ISAAC+ [ISAAC_].  The only issue found was with initial seeding, and   the method proposed here is safe from known attacks.  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 period.   These properties make it ideal for use in BFD.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.  Updating RFC 5880   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.  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.   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.   bfd.AuthType:Dekok, et al.            Expires 5 January 2025                 [Page 3]Internet-Draft            ISAAC Authentication                 July 2024      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.  Note that the session MAY change      AuthType during a session.  For example, where the session      transitions from one with strong authentication to a less strong      one, or vice versa.      Packets which indicate a state transition MUST use a stronger      authentication method.  When the bfd.SessionState value is Up,      packets MAY use a less strong 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.  The initial      value is unimportant.   bfd.XmitAuthSeq:      A 32-bit unsigned integer containing the next sequence number for      for the Authentication Section which will be transmitted.  This      variable MUST be initialized to a random 32-bit value.   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.4.  Architecture of the Auth Type Method   When BFD uses authentication, methods using MD5 or SHA1 are CPU   intensive, and can negatively impact systems with limited   computational power.   However, once the session transitions into the Up state, there is no   need to authenticate every packet.  An optimized authentication   mechanism as described in Optimizing BFD Authentication   [I-D.ietf-bfd-optimizing-authentication], permits BFD to use a   relaxed authentication, that satisfies the ability to provde a less   expensive authentication, but strong enough that periodic   reauthentication is not strictly required to prevent a person-in-the-   middle attack.Dekok, et al.            Expires 5 January 2025                 [Page 4]Internet-Draft            ISAAC Authentication                 July 2024   We use ISAAC here as a way to generate an infinite stream of pseudo-   random numbers, referred to here as "Auth-Key"s.  With Meticulous   Keyed ISAAC, 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, then only the sending party could have generated it.  The   sender is therefore authentic, even if the packet contents have   potentially been modified in transit.   Note that with this Auth Type method, the full packet contents are   not signed or authenticated.  Therefore, the Meticulous Keyed ISAAC   method MUST NOT be used to signal BFD state changes.  For BFD state   changes, and a more optimized way to authenticate packets, please   refer to BFD Authentication [I-D.ietf-bfd-optimizing-authentication].   Instead, the packets containing Meticulous Keyed ISAAC are only a   signal that the sending party is still alive, and that the sending   party is authentic.  That is, this Auth Type method MUST only be used   when bfd.SessionState=Up, and the State (Sta) field equals 3 (Up).5.  Meticulous Keyed ISAAC Authentication Format   If the Authentication Present (A) bit is set in the header, and the   State (Sta) field equals 3 (Up), and the Authentication Type field   contains TBD1 (Meticulous Keyed ISAAC), 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  |   Reserved    |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |                        Sequence Number                        |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |                             Seed                              |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |                            Auth-Key                           |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Auth Type:      The Authentication Type, which in this case is TBD1 (Meticulous      Keyed ISAAC).  If the State (Sta) field value is not 3 (Up), then      Meticulous Keyed ISAAC MUST NOT be used.   Auth Len:      The length of the Authentication Section, in bytes.  For      Meticulous Keyed ISAAC authentication, the length is 16.Dekok, et al.            Expires 5 January 2025                 [Page 5]Internet-Draft            ISAAC Authentication                 July 2024   Auth Key ID:      The authentication key ID in use for this packet.  This allows      multiple secret keys to be active simultaneously.   Reserved:      This field MUST be set to zero on transmit, and ignored on      receipt.   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 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", and is   the only one who could have sent the message.6.  Meticulous Keyed ISAAC Authentication   In this method of 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, the sequence number is incremented on every packet.Dekok, et al.            Expires 5 January 2025                 [Page 6]Internet-Draft            ISAAC Authentication                 July 2024   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)   Transmission Using Meticulous Keyed ISAAC Authentication      The Auth Type field MUST be set to one of two TBDs (Optimized MD5      Meticulous Keyed ISAAC Authentication or Optimized SHA-1      Meticulous Keyed ISAAC Authentication).  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.      For Meticulous Keyed ISAAC, bfd.XmitAuthSeq MUST be incremented 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 for a packet.   Receipt using Meticulous Keyed ISAAC Authentication      If the received BFD Control packet does not contain an      Authentication Section, or the Auth Type is not correct (one of      two possible TBDs, Optimized MD5 Meticulous Keyed ISAAC      Authentication or Optimized SHA-1 Meticulous Keyed ISAAC      Authentication), 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.      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.Dekok, et al.            Expires 5 January 2025                 [Page 7]Internet-Draft            ISAAC Authentication                 July 2024      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 matches 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.7.  New State variables for Meticulous Keyed ISAAC   This document defines a few new state variables for use with   Meticulous Keyed ISAAC.   bfd.MetKeyIsaacRcvKeyKnown:      A boolean value which indicates whether or not the system knows      the receive key for the Meticulous Keyed ISAAC Auth Type method.      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 Auth Type method, 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.Dekok, et al.            Expires 5 January 2025                 [Page 8]Internet-Draft            ISAAC Authentication                 July 2024   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.   bfd.MetKeyIsaacXmitKeyKnown:      A boolean value which indicates whether or not the system knows      the xmit key for the Meticulous Keyed ISAAC Auth Type method.  The      initial value is "false".  This value is changed to "true" when a      party starts to transmit using the Meticulous Keyed ISAAC Auth      Type method.   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.8.  Secret Key   The security of this 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.  The Meticulous Keyed ISAAC authentication   method permits systems to have multiple Secret Keys configured, but   we do not discuss how those keys are managed or used.  We do,   however, require that a session MUST NOT change the Auth Key ID for   Meticulous Keyed ISAAC, 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 KeyDekok, et al.            Expires 5 January 2025                 [Page 9]Internet-Draft            ISAAC Authentication                 July 2024   If this Auth Type method was defined as being initialized without a   per-session Seed, then an attacker could pre-compute the ISAAC states   for many keys, and perform an off-line dictionary attack.  The use of   the Seed makes these attacks unfeasable.   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 adminsitrators   different Secret Keys for each Auth Type.9.  Transition to using ISAAC   Once a session transitions to the Up state, the packets MAY contain   AuthType supporting Meticulous Keyed ISAAC.  A system receiving such   a packet will initialize the ISAAC PRNG state using the Seed from the   packet.  A system originating such a packet 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.   There is no negotiation when using this Auth Type method.  A sending   system simply starts sending packets which contain Auth Type   supporting Meticulous Keyed ISAAC.   Similarly, a receiving system sees that it has received a packet   contains AuthType of Meticulous Keyed ISAAC 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.   When there is a transition to using ISAAC the first time, the initial   state has to be seeded.  The next section describes this seeding   process.10.  Seeding ISAAC   The Seed field is used to 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, and the Secret Key, to initialize ISAAC.Dekok, et al.            Expires 5 January 2025                [Page 10]Internet-Draft            ISAAC Authentication                 July 2024   The value of the Seed field MUST be derived from a secure 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 |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   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.  The length of   the Secret Key MUST be 1016 octets or less.  The Counter field is use   to ensure the the initial seeding of ISAAC avoids the seeding issues   discussed in ISAAC+ [ISAAC_].   The ISAAC "page" is initialized by filling it with repeated copies of   the above structure.  For each complete copy of the structure, the   Counter field is incremented, starting from zero (0).  This process   may finish with a partial copy of the above structure, in which case   only a prefix of the structure is copied.   Once the ISAAC "page" is initialized, the data is processed throught   the "randinit()" function of 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 needs to be done in the BFD "fast   path".Dekok, et al.            Expires 5 January 2025                [Page 11]Internet-Draft            ISAAC Authentication                 July 2024   The following figure gives Seed and Your-Discriminator as 32-bit hex   values, and the Secret Key as an eleven-character string.  The   subsequent figure shows the first eight Sequence numbers and   corresponding Auth Key values which were generated using the above   initial values.   Seed    0x0bfd5eed   Y-Disc  0x4002d15c   Key     RFC5880June   Sequence Auth Key   0        4e13bbfc   1        8f80176a   2        56ea9eeb   3        6950dc88   4        46e6ada6   5        5a823dc9   6        221d032d   7        d1beedf3   Note that this construct requires that the "Your Discriminator" field   not change during a session.  However, it does allow the "My   Discriminator" field to change as permitted by RFC5880 Section 6.3   [RFC5880]   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 RFC5880 Section 10 [RFC5880].   There is no way to signal or negotiate Seed changes.  The receiving   party MUST remember the current Seed value, and then detect if the   Seed changes.  Note that the Seed value MUST NOT change unless   sending party has signalled a BFD state change with a packet that is   authenticated using a stronger authentication method.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.Dekok, et al.            Expires 5 January 2025                [Page 12]Internet-Draft            ISAAC Authentication                 July 2024   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.10.2.  Receiver Variable Initialization   When a system receives packets with the Meticulous Keyed ISAAC   authentication type 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 sender the bfd.RcvAuthSeq number from the current packet to be      sent 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.Dekok, et al.            Expires 5 January 2025                [Page 13]Internet-Draft            ISAAC Authentication                 July 2024   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 has to search through the first "N" Auth Keys   derived from its calculated ISAAC state in order to find one which   matches.  If no key matches the Auth Key in the packets, the packet   is deemed to be inauthentic, and is discarded.   If a calculated key at index "I" 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.   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.   This document does not make provisions for dealing with the case of   losing more than 256 packets.  Implementors should limit the value of   "Detect Multi" to a small number in order to keep the number of lost   packets within an acceptable limit.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.Dekok, et al.            Expires 5 January 2025                [Page 14]Internet-Draft            ISAAC Authentication                 July 2024   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.   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 processes 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.c.  Both pages should be maintained at all   times.   This process has a number of benefits.  First, the "next" can be   calculated asynchronously, and does not have to be done in the BFD   "fast path".  At 60 packets per second, the system has approximately   four (4) seconds to calculate the "next" page.Dekok, et al.            Expires 5 January 2025                [Page 15]Internet-Draft            ISAAC Authentication                 July 2024   Second, having the "next" page always available 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 "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 notify an asynchronous system   to calculate the "next" page.  Such asynchronous calculations are   preferable to calculating the next page in the BFD fast path.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 strong Auth   Type.  The second situation is where one party wishes to temporarily   signal that it is still Up, using a strong Auth Type.   Since the Meticulous Keyed ISAAC authentication method does not   provide for full packet integrity checks, it may be desirable for a   party to periodically use a strong Auth Type.  The switch to a   different Auth Type can be done at any time during a session.  The   different Auth Type can signal that the session is still in the Up   state.   It is RECOMMENDED that implementations periodically use a strong Auth   Type for packets which maintain the session in an Up state.  See BFD   Authentication [I-D.ietf-bfd-optimizing-authentication] for   appropriate procedures.   The nature of the Meticulous Keyed ISAAC method means that there is   no issue with this switch, so long as it is for a small number of   packets.  From the point of view of the Meticulous Keyed ISAAC state   machine, this switch can be handled similarly to a lost packet.  The   state machine simply notices that instead of Sequence Number value   being one more than the last value used for ISAAC, it is larger by   two.  The ISAAC state machine then calculates the index into the   current "page", and uses the found number to validate (or send) the   Auth Key.   If the non-ISAAC Auth Type instead runs for extended periods of time,   then the ISAAC process must continue "in the background" in order to   maintain synchronization.  This process is needed because this method   does not provide for a way to reinitialize the ISAAC method with new   Seed value.Dekok, et al.            Expires 5 January 2025                [Page 16]Internet-Draft            ISAAC Authentication                 July 202413.  IANA Considerations   For IANA Consideration, please refer to the IANA Considerations   section of Optimizing BFD Authentication   [I-D.ietf-bfd-optimizing-authentication].   Note to RFC Editor: this section may be removed on publication as an   RFC.14.  Security Considerations   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 random numbers, with one generated by each   party to the BFD session.  An attacker cannot simply perform an off-   line brute-force dictionary attack to discover the key.  Instead, any   analysis has to include the particular 64 bits of entropy used for a   particular session.  As a result, dictionary attacks are more   difficult than they would be if the PRNG generator depended on   nothing more than the Secret Key.   The security of this proposal depends strongly on ISAAC.  This   generator has been analyzed for almost three decades, and has not   been broken.  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 is infeasibe for resource   constrained systems.   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.  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.   That is, while the Auth Key ID field provides for the use of multiple   keys simultaneously, there is no way for each party to signal which   Key IDs are supported.Dekok, et al.            Expires 5 January 2025                [Page 17]Internet-Draft            ISAAC Authentication                 July 2024   The Auth Type method defined here allows the BFD end-points to detect   a malicious packet, as the calculated hash value will not match the   value found in the packet.  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.14.1.  Spoofing   When Meticulous Keyed ISAAC is used, it is possible for an attacker   who can see the packets to observe a particular Auth Key value, and   then copy it to a different packet as a on-path attacker attack.   However, the usefulness of such an attack is limited by the   requirements that these packets must not signal state changes in the   BFD session, and that the Auth Key changes on every packet.   Performing such an attack would require an attacker to have the   following information and capabilities:      This is man-in-the-middle active attack.      The attacker has the contents of a stable packet      The attacker has managed to deduce the ISAAC key and knows which      per-packet key is being used.   The attack is therefore limited to keeping the BFD session up when it   would otherwise drop.   However, the usual actual attack which we are protecting BFD from is   availability.  That is, the attacker is trying to shut down then   connection when the attacked parties are trying to keep it up.  As a   result, the attacks here seem to be irrelevant in practice.14.2.  Re-Use of keys   The strength of the Auth-Type methods is significantly different   between the strong one like 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?Dekok, et al.            Expires 5 January 2025                [Page 18]Internet-Draft            ISAAC Authentication                 July 2024   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 immediatly drop.  Requiring instead that the keys   be identical means that no such misconfiguration is possible.   We believe that the use of the same key is acceptable, as the Auth   Type defined for ISAAC depends on 64 bits of random data.  The use of   this randomness increases the difficulty of breaking the key, and   makes off-line dictionary attacks infeasible.15.  Acknowledgements   The authors would like to thank Jeff Haas and Reshad Rahman for their   reviews of and suggestions for the document.16.  References16.1.  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-17, 1 July 2024,              <https://datatracker.ietf.org/doc/html/draft-ietf-bfd-              optimizing-authentication-17>.   [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>.16.2.  Informative References   [ISAAC]    Jenkins, R. J., "ISAAC",              http://www.burtleburtle.net/bob/rand/isaac.html, 1996.   [ISAAC_]   Aumasson, J-P., "On the pseudo-random generator ISAAC",              https://eprint.iacr.org/2006/438.pdf, 2006.Dekok, et al.            Expires 5 January 2025                [Page 19]Internet-Draft            ISAAC Authentication                 July 2024Authors' Addresses   Alan DeKok   Network RADIUS SARL   100 Centrepointe Drive #200   Ottawa ON K2G 6B1   Canada   Email: aland@freeradius.org   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   Ankur Saxena   Ciena Corporation   3939 North First Street   San Jose, CA 95134   United States of America   Email: ankurpsaxena@gmail.comDekok, et al.            Expires 5 January 2025                [Page 20]