Light-Weight Implementation Guidance (lwig)                 D.M. Migault
Internet-Draft                                                  Ericsson
Intended status: Informational                             T.G. Guggemos
Expires: 12 May 2022                                          LMU Munich
                                                         8 November 2021


            Minimal IP Encapsulating Security Payload (ESP)
                     draft-ietf-lwig-minimal-esp-08

Abstract

   This document describes the minimal properties IP Encapsulating
   Security Payload (ESP) implementation needs to meet to remain
   interoperable with the standard RFC4303 ESP.  Such a minimal version
   of ESP is not intended to become a replacement of the RFC 4303 ESP.
   Instead, a minimal implementation is expected to be optimized for
   constrained environment while remaining interoperable with
   implementations of RFC 4303 ESP.  In addition, this document also
   provides some considerations to implement minimal ESP in a
   constrained environment which includes limiting the number of flash
   writes, handling frequent wakeup / sleep states, limiting wakeup
   time, or reducing the use of random generation.

   This document does not update or modify RFC 4303, but provides a
   compact description of how to implement the minimal version of the
   protocol.  RFC 4303 remains the authoritative description.

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 12 May 2022.







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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Security Parameter Index (SPI) (32 bit) . . . . . . . . . . .   3
     2.1.  Considerations over SPI generation  . . . . . . . . . . .   4
   3.  Sequence Number(SN) (32 bit)  . . . . . . . . . . . . . . . .   5
   4.  Padding . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Next Header (8 bit) . . . . . . . . . . . . . . . . . . . . .   9
   6.  ICV . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Cryptographic Suites  . . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   10. Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   ESP [RFC4303] is part of the IPsec protocol suite [RFC4301].  IPsec
   is used to provide confidentiality, data origin authentication,
   connectionless integrity, an anti-replay service (a form of partial
   sequence integrity) and limited traffic flow confidentiality (TFC)
   padding.

   Figure 1 describes an ESP Packet.  Currently ESP is implemented in
   the kernel of major multipurpose Operating Systems (OS).  The ESP and
   IPsec suite is usually implemented in a complete way to fit multiple
   purpose usage of these OS.  However, completeness of the IPsec suite
   as well as multipurpose scope of these OS is often performed at the
   expense of resources, or performance.  As a result, constrained
   devices are likely to have their own implementation of ESP optimized
   and adapted to their specificities such as limiting the number of



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   flash writes (for each packet or across wake time), handling frequent
   wakeup and sleep state, limiting wakeup time, or reducing the use of
   random generation.  With the adoption of IPsec by IoT devices with
   minimal IKEv2 [RFC7815] and ESP Header Compression (EHC) with
   [I-D.mglt-ipsecme-diet-esp] or
   [I-D.mglt-ipsecme-ikev2-diet-esp-extension], it becomes crucial that
   ESP implementation designed for constrained devices remains inter-
   operable with the standard ESP implementation to avoid a fragmented
   usage of ESP.  This document describes the minimal properties an ESP
   implementation needs to meet to remain interoperable with [RFC4303]
   ESP.  In addition, this document also provides a set of options to
   implement these properties under certain constrained environments.
   This document does not update or modify RFC 4303, but provides a
   compact description of how to implement the minimal version of the
   protocol.  RFC 4303 remains the authoritative description.

   For each field of the ESP packet represented in Figure 1 this
   document provides recommendations and guidance for minimal
   implementations.  The primary purpose of Minimal ESP is to remain
   interoperable with other nodes implementing RFC 4303 ESP, while
   limiting the standard complexity of the implementation.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
|               Security Parameters Index (SPI)                 | ^Int.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-
|                      Sequence Number                          | |ered
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ----
|                    Payload Data* (variable)                   | |   ^
~                                                               ~ |   |
|                                                               | |Conf.
+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-
|               |     Padding (0-255 bytes)                     | |ered*
+-+-+-+-+-+-+-+-+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |   |
|                               |  Pad Length   | Next Header   | v   v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|         Integrity Check Value-ICV   (variable)                |
~                                                               ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 1: ESP Packet Description

2.  Security Parameter Index (SPI) (32 bit)

   According to the [RFC4303], the SPI is a mandatory 32 bits field and
   is not allowed to be removed.



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   The SPI has a local significance to index the Security Association
   (SA).  From [RFC4301] section 4.1, nodes supporting only unicast
   communications can index their SA only using the SPI.  On the other
   hand, nodes supporting multicast communications must also use the IP
   addresses and thus SA lookup needs to be performed using the longest
   match.

   For nodes supporting only unicast communications, this document
   recommends to index SA with the SPI only.  The index may be based on
   the full 32 bits of SPI or a subset of these bits.  The node may
   require a combination of the SPI as well as other parameters (like
   the IP address) to index the SA.

   Values 0-255 must not be used.  As per section 2.1 of [RFC4303],
   values 1-255 are reserved and 0 is only allowed to be used internally
   and it must not be sent on the wire.

   [RFC4303] does not require the SPI to be randomly generated over 32
   bits.  However, this is the recommended way to generate SPIs as it
   provides some privacy benefits and avoids, for example, correlation
   between ESP communications.  To randomly generate a 32 bit SPI, the
   node generates a random 32 bit valueand checks it does not fall in
   the 0-255 range.  If the SPI has an acceptable value, it is used to
   index the inbound session, otherwise the SPI is re-generated until an
   acceptable value is found.

   However, some constrained nodes may be less concerned by the privacy
   properties associated to SPIs randomly generated.  Examples of such
   nodes might include sensors looking to reduce their code complexity,
   in which case the use of a predictive function to generate the SPI
   might be preferred over the generation and handling of random values.
   An example of such predictable function may consider the combination
   of a fixed value and the memory address of the SAD structure.  For
   every incoming packet, the node will be able to point the SAD
   structure directly from the SPI value.  This avoids having a separate
   and additional binding between SPI and SAD entries that is involved
   for every incoming packet.

2.1.  Considerations over SPI generation

   SPI that are not randomly generated over 32 bits may lead to privacy
   and security concerns.  As a result, the use of alternative designs
   requires careful security and privacy reviews.  This section provides
   some considerations upon the adoption of alternative designs.

   Note that SPI value is used only for inbound traffic, as such the SPI
   negotiated with IKEv2 [RFC7296] or [RFC7815] by a peer, is the value
   used by the remote peer when it sends traffic.  As SPI is only used



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   for inbound traffic by the peer, this allows each peer to manage the
   set of SPIs used for its inbound traffic.  Similarly, the privacy
   concerns associated with the generation of nonrandom SPI is also
   limited to the incoming traffic.

   When alternate designs are considered, it is likely that the number
   of possible SPIs will be limited.  This limit should both consider
   the number of inbound SAs - possibly per IP addresses - as well as
   the ability for the node to rekey.  SPI can typically be used to
   implement a key update with the SPI indicating the key is being used.
   For example, a SPI might be encoded with the Security Association
   Database (SAD) entry on a subset of bytes (for example 3 bytes),
   while the remaining byte indicates the rekey index.

   The use of a smaller number of SPIs across communications comes with
   privacy and security concerns.  Typically some specific values or
   subset of SPI values may reveal the models or manufacturer of the
   node implementing ESP.  This may raise some privacy issues as an
   observer is likely to be able to determine the constrained devices of
   the network.  In some cases, these nodes may host a very limited
   number of applications - typically a single application - in which
   case the SPI would provide some information related to the
   application of the user.  In addition, the device or application may
   be associated with some vulnerabilities, in which case specific SPI
   values may be used by an attacker to discover vulnerabilities.

   While the use of randomly generated SPIs may reduce the leakage or
   privacy of security related information by ESP itself, these
   information may also be leaked otherwise.  As a result, a privacy
   analysis should consider at least the type of information as well the
   traffic pattern before determining non random SPI can be used.
   Typically, temperature sensors, wind sensors, used outdoors may not
   leak privacy sensitive information and most of its traffic is
   expected to be outbound traffic.  When used indoors, a sensor that
   reports every minute an encrypted status of the door (closed or
   opened) may leak truly little privacy sensitive information outside
   the local network.  In both cases, if the state of the sensor doesn't
   leak privacy info, a randomized SPI is not necessary.

3.  Sequence Number(SN) (32 bit)

   According to [RFC4303], the Sequence Number (SN) is a mandatory 32
   bits field in the packet.

   The SN is set by the sender so the receiver can implement anti-replay
   protection.  The SN is derived from any strictly increasing function
   that guarantees: if packet B is sent after packet A, then SN of
   packet B is strictly greater than the SN of packet A.



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   Some constrained devices may establish communication with specific
   devices, like a specific gateway, or nodes similar to them.  As a
   result, the sender may know whereas the receiver implements anti-
   replay protection or not.  Even though the sender may know the
   receiver does not implement anti-replay protection, the sender must
   implement an always increasing function to generate the SN.

   Usually, SN is generated by incrementing a counter for each packet
   sent.  A constrained device may avoid maintaining this context and
   use another source that is known to always increase.  Typically,
   constrained nodes using 802.15.4 Time Slotted Channel Hopping (TSCH),
   whose communication is heavily dependent on time, can take advantage
   of their clock to generate the SN.  A lot of IoT devices are in a
   sleep state most of the time wake up and are only awake to perform a
   specific operation before going back to sleep.  They do have separate
   hardware that allows them to wake up after a certain timeout, and
   most likely also timers that start running when the device was booted
   up, so they might have a concept of time with certain granularity.
   This requires to store any information in a stable storage - such as
   flash memory - that can be restored across sleeps.  Storing
   information associated with the SA such as SN requires some read and
   writing operation on a stable storage after each packet is sent as
   opposed to SPI or keys that are only written at the creation of the
   SA.  Such operations are likely to wear out the flash, and slow down
   the system greatly, as writing to flash is not as fast as reading.
   Their internal clocks/timers might not be very accurate, but they
   should be enough to know that each time they wake up their time is
   greater than what it was last time they woke up.  Using time for SN
   would guarantee a strictly increasing function and avoid storing any
   additional values or context related to the SN.  Of course, one
   should only consider use of a clock to generate SNs if the
   application will inherently ensure that no two packets with a given
   SA are sent with the same time value.  Note however that standard
   receivers are generally configured with incrementing counters and, if
   not appropriately configured, the use of a significantly larger SN
   difference may result in the packet out of the receiver's windows and
   that packet being discarded.

   For inbound traffic, this document recommends that any receiver
   provides anti-replay protection, and the size of the window depends
   on the ability of the network to deliver packets out of order.  As a
   result, in an environment where out of order packets is not possible
   the window size can be set to one.  However, while recommended, there
   are no requirements to implement an anti-replay protection mechanism
   implemented by IPsec.  Similarly to the SN the implementation of anti
   replay protection may require the device to write the received SN for
   every packet, which may in some cases come with the same drawbacks as
   those exposed for SN.  As a result, some implementations may drop a



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   non required anti replay protection especially when the necessary
   resource involved overcomes the benefit of the mechanism.  These
   resources need also to balance that absence of anti-replay mechanism,
   may lead to unnecessary integrity check operations that might be
   significantly more expensive as well.  A typical example might
   consider an IoT device such as a temperature sensor that is sending a
   temperature every 60 seconds, and that receives an acknowledgment
   from the receiver.  In such cases, the ability to spoof and replay an
   acknowledgement is of limited interest and might not justify the
   implementation of an anti replay mechanism.  Receiving peers may also
   use ESP anti-replay mechanism adapted to a specific application.
   Typically, when the sending peer is using SN based on time, anti-
   replay may be implemented by discarding any packets that present a SN
   whose value is too much in the past.  Note that such mechanisms may
   consider clock drifting in various ways in addition to acceptable
   delay induced by the network to avoid the anti replay windows
   rejecting legitimate packets.  When a packet is received at a regular
   time interval, some variant of time based mechanisms may not even use
   the value of the SN, but instead only consider the receiving time of
   the packet.

   SN can be encoded over 32 bits or 64 bits - known as Extended
   Sequence Number (ESN).  As per [RFC4303], the support of ESN is not
   mandatory.  The determination of the use of ESN is based on the
   largest possible value a SN can take over a session.  When SN is
   incremented for each packet, the number of packets sent over the
   lifetime of a session may be considered.  However, when the SN is
   incremented differently - such as when time is used - the maximum
   value SN needs to be considered instead.  Note that the limit of
   messages being sent is primarily determined by the security
   associated with the key rather than the SN.  The security of all data
   protected under a given key decreases slightly with each message and
   a node must ensure the limit is not reached - even though the SN
   would permit it.  Estimation of the maximum number of packets to be
   sent by a node is always challenging and as such should be considered
   cautiously as nodes could be online for much more time than expected.
   Even for constrained devices, this document recommends to implement
   some rekey mechanisms (see Section 9).

4.  Padding

   The purpose of padding is to respect the 32 bit alignment of ESP or
   block size expected by an encryption transform - such as AES-CBC for
   example.  ESP must have at least one padding byte Pad Length that
   indicates the padding length.  ESP padding bytes are generated by a
   succession of unsigned bytes starting with 1, 2, 3 with the last byte
   set to Pad Length, where Pad Length designates the length of the
   padding bytes.



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   Checking the padding structure is not mandatory, so the constrained
   device may not proceed to such checks, however, in order to
   interoperate with existing ESP implementations, it must build the
   padding bytes as recommended by ESP.

   In some situation the padding bytes may take a fixed value.  This
   would typically be the case when the Data Payload is of fix size.

   ESP [RFC4303] also provides Traffic Flow Confidentiality (TFC) as a
   way to perform padding to hide traffic characteristics, which differs
   from respecting a 32 bit alignment.  TFC is not mandatory and must be
   negotiated with the SA management protocol.  TFC has not yet being
   widely adopted for standard ESP traffic.  One possible reason is that
   it requires to shape the traffic according to one traffic pattern
   that needs to be maintained.  This is likely to require extra
   processing as well as providing a "well recognized" traffic shape
   which could end up being counterproductive.  As such, this document
   does not recommend that minimal ESP implementation supports TFC.

   As a result, TFC cannot be enabled with minimal ESP, and
   communication protection that were relying on TFC will be more
   sensitive to traffic shaping.  This could expose the application as
   well as the devices used to a passive monitoring attacker.  Such
   information could be used by the attacker in case a vulnerability is
   disclosed on the specific device.  In addition, some application use
   - such as health applications - may also reveal important privacy
   oriented information.

   Some constrained nodes that have limited battery lifetime may also
   prefer avoiding sending extra padding bytes.  However, the same nodes
   may also be very specific to an application and device.  As a result,
   they are also likely to be the main target for traffic shaping.  In
   most cases, the payload carried by these nodes is quite small, and
   the standard padding mechanism may also be used as an alternative to
   TFC, with a sufficient tradeoff between the require energy to send
   additional payload and the exposure to traffic shaping attacks.  In
   addition, the information leaked by the traffic shaping may also be
   addressed by the application level.  For example, it is preferred to
   have a sensor sending some information at regular time interval,
   rather than when a specific event is happening.  Typically, a sensor
   monitoring the temperature, or a door is expected to send regularly
   the information - i.e. the temperature of the room or whether the
   door is closed or open) instead of only sending the information when
   the temperature has raised or when the door is being opened.







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5.  Next Header (8 bit)

   According to [RFC4303], the Next Header is a mandatory 8 bits field
   in the packet.  Next header specifies the data contained in the
   payload as well as dummy packet, i.e. packets with the Next Header
   with a value 59 meaning "no next header".  In addition, the Next
   Header may also carry an indication on how to process the packet
   [I-D.nikander-esp-beet-mode].

   The ability to generate and receive dummy packets is required by
   [RFC4303].  For interoperability, a minimal ESP implementation must
   discard dummy packets without indicating an error.  Note that such
   recommendation only applies for nodes receiving packets, and that
   nodes designed to only send data might not implement this capability.

   As the generation of dummy packets is subject to local management and
   based on a per-SA basis, a minimal ESP implementation may not
   generate such dummy packet.  More especially, in constrained
   environment sending dummy packets may have too much impact on the
   device lifetime, and so may be avoided.  On the other hand,
   constrained nodes may be dedicated to specific applications, in which
   case, traffic pattern may expose the application or the type of node.
   For these nodes, not sending dummy packet may have some privacy
   implication that needs to be measured.  However, for the same reasons
   exposed in Section 4 traffic shaping at the IPsec layer may also
   introduce some traffic pattern, and on constrained devices the
   application is probably the most appropriated layer to limit the risk
   of leaking information by traffic shaping.

   In some cases, devices are dedicated to a single application or a
   single transport protocol, in which case, the Next Header has a fixed
   value.

   Specific processing indications have not been standardized yet
   [I-D.nikander-esp-beet-mode] and is expected to result from an
   agreement between the peers.  As a result, it should not be part of a
   minimal implementation of ESP.

6.  ICV

   The ICV depends on the cryptographic suite used.  Currently [RFC8221]
   only recommends cryptographic suites with an ICV which makes the ICV
   a mandatory field.

   As detailed in [RFC8221] authentication or authenticated encryption
   are recommended and as such the ICV field must be present with a size
   different from zero.  It length is defined by the security
   recommendations only.



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7.  Cryptographic Suites

   The cryptographic suites implemented are an important component of
   ESP.  The recommended algorithms to use are expected to evolve over
   time and implementers should follow the recommendations provided by
   [RFC8221] and updates.

   This section lists some of the criteria that may be considered.  The
   list is not expected to be exhaustive and may also evolve overtime.
   As a result, the list is provided as informational:

   1.  Security: Security is the criteria that should be considered
       first for the selection of encryption algorithm transform.  The
       security of encryption algorithm transforms is expected to evolve
       over time, and it is of primary importance to follow up-to-date
       security guidance and recommendations.  The chosen encryption
       algorithm must not be known vulnerable or weak (see [RFC8221] for
       outdated ciphers).  ESP can be used to authenticate only or to
       encrypt the communication.  In the latter case, authenticated
       encryption must always be considered [RFC8221].

   2.  Resilience to nonce re-use: Some transforms -including AES-GCM -
       are very sensitive to nonce collision with a given key.  While
       the generation of the nonce may prevent such collision during a
       session, the mechanisms are unlikely to provide such protection
       across reboot.  This causes an issue for devices that are
       configured with a key.  When the key is likely to be re-used
       across reboots, this document recommends to consider algorithms
       that are nonce misuse resistant such as, for example, AES-SIV
       [RFC5297], AES-GCM-SIV [RFC8452] or Deoxys-II [DeoxysII].  Note
       however that currently none of them has yet been defined for ESP.

   3.  Interoperability: Interoperability considers the encryption
       algorithm transforms shared with the other nodes.  Note that it
       is not because an encryption algorithm transform is widely
       deployed that it is secured.  As a result, security should not be
       weakened for interoperability.  [RFC8221] and successors consider
       the life cycle of encryption algorithm transforms sufficiently
       long to provide interoperability.  Constrained devices may have
       limited interoperability requirements which makes possible to
       reduces the number of encryption algorithm transforms to
       implement.

   4.  Power Consumption and Cipher Suite Complexity: Complexity of the
       encryption algorithm transform or the energy associated with it
       are especially considered when devices have limited resources or
       are using some batteries, in which case the battery determines
       the life of the device.  The choice of a cryptographic function



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       may consider re-using specific libraries or to take advantage of
       hardware acceleration provided by the device.  For example, if
       the device benefits from AES hardware modules and uses AES-CTR,
       it may prefer AUTH_AES-XCBC for its authentication.  In addition,
       some devices may also embed radio modules with hardware
       acceleration for AES-CCM, in which case, this mode may be
       preferred.

   5.  Power Consumption and Bandwidth Consumption: Similarly to the
       encryption algorithm transform complexity, reducing the payload
       sent, may significantly reduce the energy consumption of the
       device.  As a result, encryption algorithm transforms with low
       overhead may be considered.  To reduce the overall payload size
       one may, for example:

       1.  Use of counter-based ciphers without fixed block length (e.g.
           AES-CTR, or ChaCha20-Poly1305).

       2.  Use of ciphers with capability of using implicit IVs
           [RFC8750].

       3.  Use of ciphers recommended for IoT [RFC8221].

       4.  Avoid Padding by sending payload data which are aligned to
           the cipher block length - 2 for the ESP trailer.

8.  IANA Considerations

   There are no IANA consideration for this document.

9.  Security Considerations

   Security considerations are those of [RFC4303].  In addition, this
   document provided security recommendations and guidance over the
   implementation choices for each field.

   The security of a communication provided by ESP is closely related to
   the security associated with the management of that key.  This
   usually includes mechanisms to prevent a nonce from repeating, for
   example.  When a node is provisioned with a session key that is used
   across reboot, the implementer must ensure that the mechanisms put in
   place remain valid across reboot as well.

   This document recommends to use ESP in conjunction with key
   management protocols such as for example IKEv2 [RFC7296] or minimal
   IKEv2 [RFC7815].  Such mechanisms are responsible for negotiating
   fresh session keys as well as prevent a session key being use beyond
   its lifetime.  When such mechanisms cannot be implemented and the



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   session key is, for example, provisioned, the nodes must ensure that
   keys are not used beyond their lifetime and that the appropriate use
   of the key remains across reboots - e.g. conditions on counters and
   nonces remains valid.

   When a node generates its key or when random value such as nonces are
   generated, the random generation must follow [RFC4086].  In addition
   [SP-800-90A-Rev-1] provides appropriated guidance to build random
   generators based on deterministic random functions.

10.  Acknowledgment

   The authors would like to thank Daniel Palomares, Scott Fluhrer, Tero
   Kivinen, Valery Smyslov, Yoav Nir, Michael Richardson, Thomas Peyrin,
   Eric Thormarker, Nancy Cam-Winget and Bob Briscoe for their valuable
   comments.  In particular Scott Fluhrer suggested to include the rekey
   index in the SPI.  Tero Kivinen provided also multiple clarifications
   and examples of deployment ESP within constrained devices with their
   associated optimizations.  Thomas Peyrin Eric Thormarker and Scott
   Fluhrer suggested and clarified the use of transform resilient to
   nonce misuse.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.



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   [RFC7815]  Kivinen, T., "Minimal Internet Key Exchange Version 2
              (IKEv2) Initiator Implementation", RFC 7815,
              DOI 10.17487/RFC7815, March 2016,
              <https://www.rfc-editor.org/info/rfc7815>.

   [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>.

   [RFC8221]  Wouters, P., Migault, D., Mattsson, J., Nir, Y., and T.
              Kivinen, "Cryptographic Algorithm Implementation
              Requirements and Usage Guidance for Encapsulating Security
              Payload (ESP) and Authentication Header (AH)", RFC 8221,
              DOI 10.17487/RFC8221, October 2017,
              <https://www.rfc-editor.org/info/rfc8221>.

   [RFC8750]  Migault, D., Guggemos, T., and Y. Nir, "Implicit
              Initialization Vector (IV) for Counter-Based Ciphers in
              Encapsulating Security Payload (ESP)", RFC 8750,
              DOI 10.17487/RFC8750, March 2020,
              <https://www.rfc-editor.org/info/rfc8750>.

11.2.  Informative References

   [DeoxysII] Jeremy, J. J., Ivica, I. N., Thomas, T. P., and Y. S.
              Yannick, "Deoxys v1.41", October 2016,
              <https://competitions.cr.yp.to/round3/deoxysv141.pdf>.

   [I-D.mglt-ipsecme-diet-esp]
              Migault, D., Guggemos, T., Bormann, C., and D. Schinazi,
              "ESP Header Compression and Diet-ESP", Work in Progress,
              Internet-Draft, draft-mglt-ipsecme-diet-esp-07, 11 March
              2019, <https://www.ietf.org/archive/id/draft-mglt-ipsecme-
              diet-esp-07.txt>.

   [I-D.mglt-ipsecme-ikev2-diet-esp-extension]
              Migault, D., Guggemos, T., and D. Schinazi, "Internet Key
              Exchange version 2 (IKEv2) extension for the ESP Header
              Compression (EHC) Strategy", Work in Progress, Internet-
              Draft, draft-mglt-ipsecme-ikev2-diet-esp-extension-01, 26
              June 2018, <https://www.ietf.org/archive/id/draft-mglt-
              ipsecme-ikev2-diet-esp-extension-01.txt>.









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   [I-D.nikander-esp-beet-mode]
              Nikander, P. and J. Melen, "A Bound End-to-End Tunnel
              (BEET) mode for ESP", Work in Progress, Internet-Draft,
              draft-nikander-esp-beet-mode-09, 5 August 2008,
              <https://www.ietf.org/archive/id/draft-nikander-esp-beet-
              mode-09.txt>.

   [RFC5297]  Harkins, D., "Synthetic Initialization Vector (SIV)
              Authenticated Encryption Using the Advanced Encryption
              Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
              2008, <https://www.rfc-editor.org/info/rfc5297>.

   [RFC8452]  Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV:
              Nonce Misuse-Resistant Authenticated Encryption",
              RFC 8452, DOI 10.17487/RFC8452, April 2019,
              <https://www.rfc-editor.org/info/rfc8452>.

   [SP-800-90A-Rev-1]
              Elain, E. B. and J. K. Kelsey, "Recommendation for Random
              Number Generation Using Deterministic Random Bit
              Generators", <https://csrc.nist.gov/publications/detail/
              sp/800-90a/rev-1/final>.

Authors' Addresses

   Daniel Migault
   Ericsson
   8400 boulevard Decarie
   Montreal, QC H4P 2N2
   Canada

   Email: daniel.migault@ericsson.com


   Tobias Guggemos
   LMU Munich
   MNM-Team
   Oettingenstr. 67
   80538 Munich
   Germany

   Email: guggemos@mnm-team.org









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