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Minimal IP Encapsulating Security Payload (ESP)
RFC 9333

Document Type RFC - Informational (January 2023)
Authors Daniel Migault , Tobias Guggemos
Last updated 2023-01-13
Replaces draft-mglt-lwig-minimal-esp
RFC stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Associated WG milestone
Nov 2018
Submit the minimal ESP guidance document to the IESG for publication as an Informational RFC
Document shepherd Mohit Sethi
Shepherd write-up Show Last changed 2021-09-28
IESG IESG state RFC 9333 (Informational)
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(None)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Erik Kline
Send notices to mohit.m.sethi@ericsson.com
IANA IANA review state Version Changed - Review Needed
IANA action state No IANA Actions
RFC 9333


Internet Engineering Task Force (IETF)                        D. Migault
Request for Comments: 9333                                      Ericsson
Category: Informational                                      T. Guggemos
ISSN: 2070-1721                                               LMU Munich
                                                            January 2023

            Minimal IP Encapsulating Security Payload (ESP)

Abstract

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

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

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9333.

Copyright Notice

   Copyright (c) 2023 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 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.  Requirements Notation
   3.  Security Parameters Index (SPI)
     3.1.  Considerations for SPI Generation
   4.  Sequence Number (SN)
   5.  Padding
   6.  Next Header and "Dummy" Packets
   7.  ICV
   8.  Cryptographic Suites
   9.  IANA Considerations
   10. Security Considerations
   11. Privacy Considerations
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Acknowledgments
   Authors' Addresses

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, and limited Traffic
   Flow Confidentiality (TFC) padding.

   Figure 1 describes an ESP packet.  Currently, ESP is implemented in
   the kernel of most major multipurpose Operating Systems (OSes).  ESP
   is usually implemented with all of its features to fit the
   multipurpose usage of these OSes, at the expense of resources and
   with no considerations for code size.  Constrained devices are likely
   to have their own implementation of ESP optimized and adapted to
   their specific use, such as limiting the number of flash writes (for
   each packet or across wake time), handling frequent wakeup and sleep
   states, limiting wakeup time, and reducing the use of random
   generation.  With the adoption of IPsec by Internet of Things (IoT)
   devices with minimal IKEv2 [RFC7815] and ESP Header Compression (EHC)
   [EHC-DIET-ESP] [EHC-IKEv2], these ESP implementations MUST remain
   interoperable with standard ESP implementations.  This document
   describes the minimal properties an ESP implementation needs to meet
   to remain interoperable with ESP [RFC4303].  In addition, this
   document provides advice to implementers for implementing ESP within
   constrained environments.  This document does not update or modify
   [RFC4303].

   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 ESP [RFC4303], 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.  Requirements Notation

   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.

3.  Security Parameters Index (SPI)

   [RFC4303] defines the SPI as a mandatory 32-bit field.

   The SPI has local significance to index the Security Association
   (SA).  As described in Section 4.1 of [RFC4301], nodes supporting
   only unicast communications can index their SA using only the SPI.
   Nodes supporting multicast communications also require the use of IP
   addresses; thus, SA lookup needs to be performed using the longest
   match.

   For nodes supporting only unicast communications, indexing the SA
   using only the SPI is RECOMMENDED.  The index may be based on the
   full 32 bits of the 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 MUST NOT be sent over the wire.

   [RFC4303] does not require the 32-bit SPI to be randomly generated,
   although that is the RECOMMENDED way to generate SPIs as it provides
   some privacy and security benefits and avoids correlation between ESP
   communications.  To obtain a usable random 32-bit SPI, the node
   generates a random 32-bit value and checks it does not fall within
   the 0-255 range.  If the SPI has an acceptable value, it is used to
   index the inbound session.  Otherwise, the generated value is
   discarded, and the process repeats until a valid value is found.

   Some constrained devices are less concerned with the privacy
   properties associated with randomly generated SPIs.  Examples of such
   devices might include sensors looking to reduce their code
   complexity.  The use of a predictive function to generate the SPI
   might be preferred over the generation and handling of random values.
   An implementation of such predictable function could use the
   combination of a fixed value and the memory address of the Security
   Association Database (SAD) structure.  For every incoming packet, the
   node will be able to point to the SAD structure directly from the SPI
   value.  This avoids having a separate and additional binding and
   lookup function for the SPI to its SAD entry for every incoming
   packet.

3.1.  Considerations for SPI Generation

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

   The SPI value is only looked up for inbound traffic.  The SPI
   negotiated with IKEv2 [RFC7296] or minimal IKEv2 [RFC7815] by a peer
   is the value used by the remote peer when it sends traffic.  The main
   advantage of using a rekeying mechanism is to enable a rekey, which
   is performed by replacing an old SA with a new SA, both indexed with
   distinct SPIs.  The SPI is only used for inbound traffic by the peer,
   which allows each peer to manage the set of SPIs used for its inbound
   traffic.  The necessary number of SPIs reflects the number of inbound
   SAs as well as the ability to rekey those SAs.  Typically, rekeying
   an SA is performed by creating a new SA (with a dedicated SPI) before
   the old SA is deleted.  This results in an additional SA and the need
   to support an additional SPI.  Similarly, the privacy concerns
   associated with the generation of non-random SPIs is also limited to
   the incoming traffic.

   Alternatively, some constrained devices will not implement IKEv2 or
   minimal IKEv2 and, as such, will not be able to manage a rollover
   between two distinct SAs.  In addition, some of these constrained
   devices are likely to have a limited number of SAs; for example, they
   are likely to be indexed over 3 bytes only.  One possible way to
   enable a rekeying mechanism with these devices is to use the SPI
   where, for example, the first 3 bytes designates the SA while the
   remaining byte indicates a rekey index.  SPI numbers can be used to
   implement tracking the inbound SAs when rekeying is taking place.
   When rekeying an SPI, the new SPI could use the SPI bytes to indicate
   the rekeying index.

   The use of a small, limited set of SPI numbers across communications
   comes with privacy and security concerns.  Some specific values or
   subsets of SPI values could reveal the model or manufacturer of the
   node implementing ESP or reveal a state such as "not yet rekeyed" or
   "rekeyed 10 times".  If a constrained host uses a very limited number
   of applications, eventually a single one, the SPI itself could
   indicate what kind of traffic is transmitted (e.g., the kind of
   application typically running).  This could also be correlated with
   encrypted data size to further leak information to an observer on the
   network.  In addition, use of specific hardcoded SPI numbers could
   reveal a manufacturer or device version.  If updated devices use
   different SPI numbers, an attacker could locate vulnerable devices by
   their use of specific SPI numbers.

   A privacy analysis should consider at least the type of information
   as well as the traffic pattern before deciding whether non-random
   SPIs are safe to use.  Typically, temperature and wind sensors that
   are used outdoors do not leak privacy-sensitive information, and most
   of their traffic is expected to be outbound traffic.  When used
   indoors, a sensor that reports an encrypted status of a door (closed
   or opened) every minute might not leak sensitive information outside
   the local network.  In these examples, the privacy aspect of the
   information itself might be limited.  Being able to determine the
   version of the sensor to potentially take control of it may also have
   some limited security consequences.  Of course, this depends on the
   context in which these sensors are being used.  If the risks
   associated to privacy and security are acceptable, a non-randomized
   SPI can be used.

4.  Sequence Number (SN)

   The Sequence Number (SN) in [RFC4303] is a mandatory 32-bit 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 the following: if packet B is sent after packet A,
   then the SN of packet B is higher than the SN of packet A.

   Some constrained devices may establish communication with specific
   devices where it is known whether or not the peer implements anti-
   replay protection.  As per [RFC4303], the sender MUST still implement
   a strictly increasing function to generate the SN.

   It is RECOMMENDED that multipurpose ESP implementations increment a
   counter for each packet sent.  However, a constrained device may
   avoid maintaining this context and use another source that is known
   to always increase.  Typically, constrained devices use 802.15.4 Time
   Slotted Channel Hopping (TSCH).  This communication is heavily
   dependent on time.  A constrained device can take advantage of this
   clock mechanism to generate the SN.  A lot of IoT devices are in a
   sleep state most of the time and wake up only to perform a specific
   operation before going back to sleep.  These devices have separate
   hardware that allows them to wake up after a certain timeout and
   typically also have timers that start running when the device is
   booted up, so they might have a concept of time with certain
   granularity.  This requires devices to store any information in
   stable storage that can be restored across sleeps (e.g., flash
   memory).  Storing information associated with the SA (such as the SN)
   requires some read and write operations on stable storage after each
   packet is sent as opposed to an SPI number or cryptographic keys that
   are only written to stable storage at the creation of the SA.  Write
   operations wear out the flash storage.  Write operations also slow
   down the system significantly, as writing to flash is much slower
   than reading from flash.  While these devices have internal clocks or
   timers that might not be very accurate, they are good enough to
   guarantee that each time the device wakes up from sleep, the time is
   greater than what it was before the device went to sleep.  Using time
   for the SN would guarantee a strictly increasing function and avoid
   storing any additional values or context related to the SN on flash.
   In addition to the time value, a RAM-based counter can be used to
   ensure that the serial numbers are still increasing and unique if the
   device sends multiple packets over an SA within one wakeup period.

   For inbound traffic, it is RECOMMENDED that receivers implement anti-
   replay protection.  The size of the window should depend on the
   network characteristic to deliver packets out of order.  In an
   environment where out-of-order packets are not possible, the window
   size can be set to one.  An ESP implementation may choose to not
   implement anti-replay protection.  An implementation of anti-replay
   protection may require the device to write the received SN for every
   packet to stable storage.  This will have the same issues as
   discussed earlier with the SN.  Some constrained device
   implementations may choose to not implement the optional anti-replay
   protection.  A typical example is an IoT device such as a temperature
   sensor that sends a temperature measurement every 60 seconds and
   receives an acknowledgment from the receiver.  In a case like this,
   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 an ESP anti-replay mechanism
   adapted to a specific application.  Typically, when the sending peer
   is using an SN based on time, anti-replay may be implemented by
   discarding any packets that present an SN whose value is too much in
   the past.  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.  Receiving
   peers could accept any SN as long as it is higher than the previously
   received SN.  Another mechanism could be used where only the received
   time on the device is used to consider a packet to be valid, without
   looking at the SN at all.

   The SN can be represented as a 32-bit number or as a 64-bit number,
   known as an "Extended Sequence Number (ESN)".  As per [RFC4303],
   support of ESN is not mandatory, and its use is negotiated via IKEv2
   [RFC7296].  An ESN is used for high-speed links to ensure there can
   be more than 2^32 packets before the SA needs to be rekeyed to
   prevent the SN from rolling over.  This assumes the SN is incremented
   by 1 for each packet.  When the SN is incremented differently -- such
   as when time is used -- rekeying needs to happen based on how the SN
   is incremented to prevent the SN from rolling over.  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 not always predictable, and large margins
   should be used, especially as nodes could be online for much more
   time than expected.  Even for constrained devices, it is RECOMMENDED
   to implement some rekeying mechanisms (see Section 10).

5.  Padding

   Padding is required to keep the 32-bit alignment of ESP.  It is also
   required for some encryption transforms that need a specific block
   size of input, such as ENCR_AES_CBC.  ESP specifies padding in the
   Pad Length byte, followed by up to 255 bytes of padding.

   Checking the padding structure is not mandatory, so constrained
   devices may omit these checks on received ESP packets.  For outgoing
   ESP packets, padding must be applied as required by ESP.

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

   ESP [RFC4303] additionally provides Traffic Flow Confidentiality
   (TFC) as a way to perform padding to hide traffic characteristics.
   TFC is not mandatory and is negotiated with the SA management
   protocol, such as IKEv2.  TFC has been widely implemented, but it is
   not widely deployed for ESP traffic.  It is NOT RECOMMENDED to
   implement TFC for minimal ESP.

   As a consequence, communication protection that relies on TFC would
   be more sensitive to traffic patterns without TFC.  This can leak
   application information as well as the manufacturer or model of the
   device used to a passive monitoring attacker.  Such information can
   be used, for example, by an attacker if a vulnerability is known for
   the specific device or application.  In addition, some applications
   (such as health applications) could leak important privacy-oriented
   information.

   Constrained devices that have a limited battery lifetime may prefer
   to avoid sending extra padding bytes.  In most cases, the payload
   carried by these devices is quite small, and the standard padding
   mechanism can be used as an alternative to TFC.  Alternatively, any
   information leak based on the size -- or presence -- of the packet
   can also be addressed at the application level before the packet is
   encrypted with ESP.  If application packets vary between 1 to 30
   bytes, the application could always send 32-byte responses to ensure
   all traffic sent is of identical length.  To prevent leaking
   information that a sensor changed state, such as "temperature
   changed" or "door opened", an application could send this information
   at regular time intervals, rather than when a specific event is
   happening, even if the sensor state did not change.

6.  Next Header and "Dummy" Packets

   ESP [RFC4303] defines the Next Header as a mandatory 8-bit field in
   the packet.  The Next Header, only visible after decryption,
   specifies the data contained in the payload.  In addition, the Next
   Header may carry an indication on how to process the packet
   [BEET-ESP].  The Next Header can point to a "dummy" packet, i.e., a
   packet with the Next Header value set to 59, meaning "no next
   header".  The data following "no next header" is unstructured "dummy"
   data.  (Note that this document uses the term "dummy" for consistency
   with [RFC4303].)

   The ability to generate, receive, and ignore "dummy" packets is
   required by [RFC4303].  An implementation can omit ever generating
   and sending "dummy" packets.  For interoperability, a minimal ESP
   implementation MUST be able to process and discard "dummy" packets
   without indicating an error.

   In constrained environments, sending "dummy" packets may have too
   much impact on the device lifetime, in which case, "dummy" packets
   should not be generated and sent.  On the other hand, constrained
   devices running specific applications that would leak too much
   information by not generating and sending "dummy" packets may
   implement this functionality or even implement something similar at
   the application layer.  Note also that similarly to padding and TFC
   that can be used to hide some traffic characteristics (see
   Section 5), "dummy" packets may also reveal some patterns that can be
   used to identify the application.  For example, an application may
   send "dummy" data to hide a traffic pattern.  Suppose such an
   application sends a 1-byte data when a change occurs.  This results
   in sending a packet notifying a change has occurred.  "Dummy" packets
   may be used to prevent such information from being leaked by sending
   a 1-byte packet every second when the information is not changed.
   After an upgrade, the data becomes 2 bytes.  At that point, the
   "dummy" packets do not hide anything, and having 1 byte regularly
   versus 2 bytes makes even the identification of the application
   version easier to identify.  This generally makes the use of "dummy"
   packets more appropriate on high-speed links.

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

   Specific processing indications have not been standardized yet
   [BEET-ESP] and are expected to result from an agreement between the
   peers.  As a result, they SHOULD NOT be part of a minimal
   implementation of ESP.

7.  ICV

   The ICV depends on the cryptographic suite used.  As detailed in
   [RFC8221], authentication or authenticated encryption is RECOMMENDED,
   and as such, the ICV field must be present with a size different from
   zero.  Its length is defined by the security recommendations only.

8.  Cryptographic Suites

   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 to
   select an appropriate cryptographic suite.  The list is not expected
   to be exhaustive and may also evolve over time.

   1.  Security: Security is the criteria that should be considered
       first for the selection of encryption algorithm transforms.  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 vulnerable or weak (see [RFC8221] for
       outdated ciphers).  ESP can be used to authenticate only
       (ENCR_NULL) or to encrypt the communication.  In the latter case,
       Authenticated Encryption with Associated Data (AEAD) is
       RECOMMENDED [RFC8221].

   2.  Resilience to Nonce Reuse: Some transforms, including AES-GCM,
       are vulnerable 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 sleep states or reboot.  This causes an issue for devices
       that are configured using static keys (called "manual keying"),
       and manual keying should not be used with these encryption
       algorithms.  When the key is likely to be reused across reboots,
       algorithms that are resistant to nonce misuse (for example, AES-
       SIV [RFC5297], AES-GCM-SIV [RFC8452], and Deoxys-II [DeoxysII])
       are RECOMMENDED.  Note, however, that none of these are currently
       defined for use with ESP.

   3.  Interoperability: Constrained devices usually only implement one
       or very few different encryption algorithm transforms.  [RFC8221]
       takes the life cycle of encryption algorithm transforms and
       device manufacturing into consideration in its recommendations
       for mandatory-to-implement (MTI) algorithms.

   4.  Power Consumption and Cipher Suite Complexity: Complexity of the
       encryption algorithm transform and the energy cost associated
       with it are especially important considerations for devices that
       have limited resources or are battery powered.  The battery life
       might determine the lifetime of the entire device.  When choosing
       a cryptographic function, reusing specific libraries or taking
       advantage of hardware acceleration provided by the device should
       be considered.  For example, if the device benefits from AES
       hardware modules and uses ENCR_AES_CTR, it may prefer AUTH_AES-
       XCBC for its authentication.  In addition, some devices may embed
       radio modules with hardware acceleration for AES-CCM, in which
       case, this transform may be preferred.

   5.  Power Consumption and Bandwidth Consumption: Reducing the payload
       sent may significantly reduce the energy consumption of the
       device.  Encryption algorithm transforms with low overhead are
       strongly preferred.  To reduce the overall payload size, one may,
       for example:

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

       *  Use ciphers capable of using implicit Initialization Vectors
          (IVs) [RFC8750].

       *  Use ciphers recommended for IoT [RFC8221].

       *  Avoid padding by sending payload data that are aligned to the
          cipher block length -- 2 bytes for the ESP trailer.

9.  IANA Considerations

   This document has no IANA actions.

10.  Security Considerations

   The security considerations in [RFC4303] apply to this document as
   well.  In addition, this document provides security recommendations
   and guidance for the implementation choices for each ESP 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.

   It is RECOMMENDED 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 preventing a session key being used beyond
   its lifetime.  When such mechanisms cannot be implemented, such as
   when the session key is provisioned, the device MUST ensure that keys
   are not used beyond their lifetime and that the key remains used in
   compliance with all security requirements across reboots (e.g.,
   conditions on counters and nonces remain valid).

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

11.  Privacy Considerations

   Preventing the leakage of privacy-sensitive information is a hard
   problem to solve and usually results in balancing the information
   potentially being leaked to the cost associated with the counter
   measures.  This problem is not inherent to the minimal ESP described
   in this document and also concerns the use of ESP in general.

   This document targets minimal implementations of ESP and, as such,
   describes a minimalistic way to implement ESP.  In some cases, this
   may result in potentially revealing privacy-sensitive pieces of
   information.  This document describes these privacy implications so
   the implementer can make the appropriate decisions given the
   specificities of a given environment and deployment.

   The main risk associated with privacy is the ability to identify an
   application or a device by analyzing the traffic, which is designated
   as "traffic shaping".  As discussed in Section 3, the use in a very
   specific context of non-randomly generated SPIs might ease the
   determination of the device or the application in some cases.
   Similarly, padding provides limited capabilities to obfuscate the
   traffic compared to those provided by TFC.  Such consequences on
   privacy are detailed in Section 5.

12.  References

12.1.  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>.

   [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>.

12.2.  Informative References

   [BEET-ESP] 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://datatracker.ietf.org/doc/html/draft-nikander-esp-
              beet-mode-09>.

   [DeoxysII] Jean, J., Nikolić, I., Peyrin, T., and Y. Seurin, "Deoxys
              v1.41", October 2016,
              <https://competitions.cr.yp.to/round3/deoxysv141.pdf>.

   [EHC-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-08, 13 May
              2022, <https://datatracker.ietf.org/doc/html/draft-mglt-
              ipsecme-diet-esp-08>.

   [EHC-IKEv2]
              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-02, 13
              May 2022, <https://datatracker.ietf.org/doc/html/draft-
              mglt-ipsecme-ikev2-diet-esp-extension-02>.

   [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]
              Barker, E. and J. Kelsey, "Recommendation for Random
              Number Generation Using Deterministic Random Bit
              Generators", NIST SP 800-90A Rev 1,
              DOI 10.6028/NIST.SP.800-90Ar1, June 2015,
              <https://csrc.nist.gov/publications/detail/sp/800-90a/rev-
              1/final>.

Acknowledgments

   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 including the rekey
   index in the SPI.  Tero Kivinen also provided multiple clarifications
   and examples of ESP deployment 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.  The authors would also like to thank Mohit Sethi for
   his support as the LWIG Working Group Chair.

Authors' Addresses

   Daniel Migault
   Ericsson
   8275 Rte Transcanadienne
   Saint-Laurent QC H4S 0B6
   Canada
   Email: daniel.migault@ericsson.com

   Tobias Guggemos
   LMU Munich
   MNM-Team
   Oettingenstr. 67
   80538 Munich
   Germany
   Email: guggemos@mnm-team.org