Light-Weight Implementation Guidance (lwig)                   D. Migault
Internet-Draft                                                  Ericsson
Intended status: Informational                               T. Guggemos
Expires: 25 November 2022                                     LMU Munich
                                                             24 May 2022


            Minimal IP Encapsulating Security Payload (ESP)
                     draft-ietf-lwig-minimal-esp-11

Abstract

   This document describes the minimal properties that an 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 environments while remaining interoperable
   with implementations of RFC 4303 ESP.  In addition, this document
   also provides some considerations for implementing minimal ESP in a
   constrained environment which includes limiting the number of flash
   writes, handling frequent wakeup / 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 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 25 November 2022.







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Copyright Notice

   Copyright (c) 2022 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.  Requirements notation . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Security Parameter Index (SPI) (32 bit) . . . . . . . . . . .   4
     3.1.  Considerations over SPI generation  . . . . . . . . . . .   4
   4.  Sequence Number(SN) (32 bit)  . . . . . . . . . . . . . . . .   6
   5.  Padding . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Next Header (8 bit) and Dummy Packets . . . . . . . . . . . .   9
   7.  ICV . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Cryptographic Suites  . . . . . . . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   12. Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . .  12
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     13.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  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.

2.  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.



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   Figure 1 describes an ESP Packet.  Currently, ESP is implemented in
   the kernel of most major multipurpose Operating Systems (OS).  ESP is
   usually implemented with all of its features to fit the multiple
   purpose 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
   state, limiting wakeup time, and 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], 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
   [RFC4303] ESP.  In addition, this document also provides advise to
   implementers for implementing ESP within constrained environments.
   This document does not update or modify RFC 4303.

   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




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

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

   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 using only the SPI.  Nodes
   supporting multicast communications also require to use the IP
   addresses and thus SA lookup need to be performed using the longest
   match.

   For nodes supporting only unicast communications, it is RECOMMENDED
   indexing the SA using only the SPI.  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 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 to 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 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 over 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 upon the adoption of alternative designs.



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   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, that is
   performed by replacing an old SA by a new SA, both indexed with
   distinct SPIs.  As the SPI is only used for inbound traffic by the
   peer, this allows each peer to manage the set of SPIs used for its
   inbound traffic.  The necessary number of SPI reflects the number of
   inbound SAs as well as the ability to rekey these SAs.  Typically,
   rekeying a 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 roll-over
   between two distinct SAs.  In addition, some of these constrained
   devices are also likely to have a limited number of SAs - likely to
   be indexed over 3 bytes only for example.  One possible way to enable
   a rekey 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 a 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
   subset of SPI values could reveal the models or manufacturer of the
   node implementing ESP.  It could also reveal some state such as "not
   yet rekeyed" or "rekeyed 10 times".  If a constrained host uses a
   very limited or even just one application, the SPI itself could
   indicate what kind of traffic (eg the kind of application typically
   running) is transmitted.  This could be further correlated by
   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 the traffic pattern before deciding whether non-random SPIs
   are safe to use.  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 an encrypted status of a door (closed or opened)
   every minute, might not leak sensitive information outside the local



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   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 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) (32 bit)

   The Sequence Number (SN) in [RFC4303] 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 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.

   The RECOMMENDED way for multipurpose ESP implementation is to
   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 contrained 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 do have separate
   hardware that allows them to wake up after a certain timeout and
   typically 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
   write operation on a stable storage after each packet is sent as
   opposed to a 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, these are good enough to guarantee that each
   time the device wakes up from sleep that their 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



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   to the time value, a RAM based counter can be used to ensure that if
   the device sends multiple packets over an SA within one wake up
   period, that the serial numbers are still increasing and unique.
   Note that standard receivers are generally configured with
   incrementing counters and, if not appropriately configured, the use
   of a significantly larger SN than the previous packet can result in
   that packet falling outside of the peer's receiver window which could
   cause that packet to be discarded.

   For inbound traffic, it is RECOMMENDED that receivers implement anti-
   replay protection.  The size of the window should depend on the
   property of the network 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 an 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 might consider an IoT device such as a
   temperature sensor that is sending a temperature measurement 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.  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.  It 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 as
   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 Extended Sequence Number (ESN).  As per [RFC4303], support
   of ESN is not mandatory and its use is negotiated via IKEv2
   [RFC7296].  A 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



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   sent by a node is not always predicatable and large margins should be
   used espcially as nodes could be online for much more time than
   expected.  Even for constrained devices, it is RECOMMENDED to
   implement some rekey 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 situation the padding bytes may take a fixed value.  This
   would typically be the case when the Data Payload 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 a 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 manifacturor or model of the
   device used to a passive monitoring attacker.  Such information can
   be used, for example, by an attacker in case a vulnerability is known
   for the specific device or application.  In addition, some
   application use - such as health applications - could leak important
   privacy oriented information.

   Constrained devices that have 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 interval, rather than when a specific event is
   happening, even if the sensor state did not change.



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6.  Next Header (8 bit) and Dummy Packets

   ESP [RFC4303] defines the Next Header as a mandatory 8 bits field in
   the packet.  The Next header, only visible after decryption,
   specifies the data contained in the payload.  In addition, the Next
   Header may also carry an indication on how to process the packet
   [I-D.nikander-esp-beet-mode].  The Next Header can point to a dummy
   packet, i.e. packets with the Next Header value set to 59 meaning "no
   next header".  The data following to "no next header" is unstructured
   dummy data.

   The ability to generate and to 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 packet
   may also reveal some patterns that can be used to identify the
   application.  For example, an application may send dummy data to hide
   some traffic pattern.  Suppose such such application sends a 1 byte
   data when a change occurs.  This results in sending a packet
   notifying a change has occurred.  Dummy packet may be used to prevent
   such information to be leaked by sending a 1 byte packet every second
   when the information is not changed.  After an upgrade the data
   becomes two bytes.  At that point, the dummy packets do not hide
   anything and having 1 byte regularly versus 2 bytes make even the
   identification of the application, version easier to identify.  This
   generaly makes the use of dummy packets more appropriated on high
   speed links.

   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.





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7.  ICV

   The ICV depends on the cryptographic suite used.  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.  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 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 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 (AEAD) is RECOMMENDED [RFC8221].

   2.  Resilience to nonce re-use: 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 re-used across reboots,
       algorithms that are nonce misuse resistant such as, for example,
       AES-SIV [RFC5297], AES-GCM-SIV [RFC8452] or Deoxys-II [DeoxysII]
       are RECOMMENDED.  Note however that currently none of these are
       yet 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 manufactoring into consideration in its recommendations
       for mandatory-to-implement ("MTI") algorithms.






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   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.  The choice of
       a cryptographic function should 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 ENCR_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 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:

       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.

9.  IANA Considerations

   There are no IANA consideration for this document.

10.  Security Considerations

   Security Considerations are those of [RFC4303].  In addition, this
   document provided security recommendations and guidance over 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.





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

   When a device generates its own key or when random value 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 result 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 some 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 take the appropriate decisions given the
   specificities of a given environment and deployment.

   The main risks 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 some very
   specific context of non randomly generated SPI might in some cases
   ease the determination of the device or the application.  Similarly,
   padding provides limited capabilities to obfuscate the traffic
   compared to those provided by TFC.  Such consequence on privacy as
   detailed in Section 5.

12.  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 including the rekey
   index in the SPI.  Tero Kivinen also provided multiple clarifications
   and examples of deployment ESP within constrained devices with their
   associated optimizations.  Thomas Peyrin Eric Thormarker and Scott



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   Fluhrer suggested and clarified the use of transform resilient to
   nonce misuse.

13.  References

13.1.  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>.






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   [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>.

13.2.  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-08, 13 May
              2022, <https://www.ietf.org/archive/id/draft-mglt-ipsecme-
              diet-esp-08.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-02, 13
              May 2022, <https://www.ietf.org/archive/id/draft-mglt-
              ipsecme-ikev2-diet-esp-extension-02.txt>.

   [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>.








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   [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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