Light-Weight Implementation Guidance (lwig)                   D. Migault
Internet-Draft                                                  Ericsson
Intended status: Informational                               T. Guggemos
Expires: May 6, September 25, 2021                                   LMU Munich
                                                        November 2, 2020
                                                          March 24, 2021

                              Minimal ESP
                     draft-ietf-lwig-minimal-esp-02
                     draft-ietf-lwig-minimal-esp-03

Abstract

   This document describes a minimal implementation of the IP
   Encapsulation Security Payload (ESP) defined in RFC 4303.  Its
   purpose is to enable implementation of ESP with a minimal set of
   options to remain compatible with ESP as described in RFC 4303.  A
   minimal version of ESP is not intended to become a replacement of the
   RFC 4303 ESP, but instead to enable ESP.  Instead, a limited minimal implementation is expected to
   interoperate be
   optimized for constrained environment while remaining interoperable
   with implementations of RFC 4303 ESP.  Constrains include among other
   limiting the number of flash writes, handling frequent wakeup / sleep
   states, limiting wakeup time, or reducing the use of random
   generation.

   This document describes what is required from RFC 4303 ESP as well as
   various ways to optimize compliance with RFC 4303 ESP.

   This document does not update or modify RFC 4303, but provides a
   compact description of how to implement the minimal version of the
   protocol.  If this document and RFC 4303 conflicts conflicts, then RFC 4303 is
   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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 6, September 25, 2021.

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Table of Contents

   1.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Security Parameter Index (SPI) (32 bit) . . . . . . . . . . .   4
   4.  Sequence Number(SN) (32 bit)  . . . . . . . . . . . . . . . .   6
   5.  Padding . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Next Header (8 bit) . . . . . . . . . . . . . . . . . . . . .   8   9
   7.  ICV . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Cryptographic Suites  . . . . . . . . . . . . . . . . . . . .   9  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   11. Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . .  11  12
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11  12
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11  12
     12.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Document Change Log  . . . . . . . . . . . . . . . .  14  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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 (a form of partial
   sequence integrity) and limited traffic flow confidentiality.

   Figure 1 describes an ESP Packet.  Currently ESP is implemented in
   the kernel of major multi purpose 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 multi purpose multipurpose scope of these OS is often performed at the
   expense of resources, or a lack of performance.  As a result,
   constraint devices are likely to have their own implementation of ESP
   optimized and adapted to their specificities. specificities such as limiting the
   number of 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 constraint devices remain inter-
   operable with the standard ESP implementation to avoid a fragmented
   usage of ESP.  This document describes the the minimal properties and ESP
   implementation needs to meet.

   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

3.  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.

   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, it is RECOMMENDED
   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.  Some other local constraints
   on the node may require a combination of the SPI as well as other
   parameters 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 internal
   and it MUST NOT be send sent on the wire.

   It is RECOMMENDED to index each inbound session with a SPI randomly
   generate over 32 bits.  Upon the generation of a SPI the peer checks
   the SPI is not already used and 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.  A random generation provides a stateless way to generate
   the SPIs, while keeping the probability of collision between SPIs
   relatively low.

   However, for some constrained nodes, generating and handling 32 bit
   random SPI may consume too much resource, in which case SPI can be
   generated using predictable functions or end up in a using a subset
   of the possible values for SPI.  In fact, the SPI does not
   necessarily need to be randomly generated.  A node provisioned with
   keys by a third party - e.g. that does not generate them - and that
   uses a transform that does not needs random data may not have such
   random generators.  However, non random nonrandom SPI and restricting their
   possible values MAY lead to privacy and security concerns.  As a
   result, this alternative should be considered for devices that would
   be strongly impacted by the generation of a random SPI and after
   understanding the privacy and security impact of generating non
   random nonrandom
   SPI.

   When a constrained node limits the number of possible SPIs 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 proceed to clean key update and the SPI value
   may be used to indicate which key is being used.  This can typically
   be implemented by a SPI being encoded with the SAD Security Association
   Database (SAD) entry on a subset of bytes (for example 3 bytes),
   while the remaining byte is left to indicate the rekey index.

   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 its sends traffic.  As SPI are only used
   for inbound traffic by the peer, this allows each peer to manage the
   set of SPIs used for its inbound traffic.

   The use of a limited number of SPIs or non random nonrandom SPIs come with
   security or privacy drawbacks.  Typically, a passive attacker may
   derive information such as the number of constraint devices
   connecting the remote peer, and in conjunction with data rate, the
   attacker may eventually determine the application the constraint
   device is associated to.  If the SPIs are set by a manufacturer or by
   some software application, the SPI may leak in an obvious way the
   type of sensor, the application involved or the model of the
   constraint device.  When identification of the application or the
   hardware is associated to privacy, the SPI MUST be randomly
   generated.  However, one needs to realize that in this case this is
   likely not to be sufficient and a thorough privacy analysis is
   required.  More specifically, traffic pattern may also leak sufficient information in
   itself.
   significant privacy sensitive information.  In other words, privacy
   leakage is a complex and the use of random SPI is unlikely to be
   sufficient.

   As the general recommendation is to randomly generate the SPI,
   constraint devices that will use a (very) limited number of SPIs are
   expected to be very constraint devices with very limited
   capabilities, where the use of randomly generated SPI may prevent
   them to implement IPsec.  In this case the ability to provision non
   random
   nonrandom SPI enables these devices to secure their communications.
   For example, the SPI could be -at least partially - generated based
   on the SAD structure in memory which would simplify the
   implementation.  These devices, due to there their limitations, are
   expected to provide limited information and how the use of non random nonrandom
   SPI impacts privacy requires further analysis.  Typically  Typically,
   temperature sensors, wind sensors, used outdoor do not leak privacy
   sensitive information.  When used indoor, a sensor that reports every
   minute an encrypted status of the door (closed or opened) leaks truly
   little privacy leakage sensitive information outside the local network may be
   limited. network.

   As far as security is concerned, revealing the type of application or
   model of the constraint device could be used to identify the
   vulnerabilities the constraint device is subject to.  This is
   especially sensitive for constraint devices where patches or software
   updates will be challenging to operate.  As a result, these devices
   may remain vulnerable for relatively long period.  In addition,
   predictable SPIs enable an attacker to forge packets with a valid
   SPI.  Such packet will not be rejected due to an SPI mismatch, but
   instead after the signature check which requires more resource and
   thus make DoS more efficient, especially for devices powered by
   batteries.

4.  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 then than the SN of packet A.

   Some constraint 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 anti-replay protection, the sender MUST
   implement a an always increasing function to generate the SN.

   Usually, SN is generated by incrementing a counter for each packet
   sent.  A constraint device may avoid maintaining this context and use
   another source that is known to always increase.  Typically,
   constraint 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.  This  The problem with regular sequence
   numbers is that you need to store them to stable storage every time
   you go sleep.  Storing the SA information (keys, SPIs etc) to the
   flash once after the creation of the SA can be done, as that is just
   one flash write per SA creation.  Synchronizing sequence number to
   flash after every packet would quickly wear out the flash, and likely
   slow down the system greatly, as writing to flash is not as fast as
   reading.  Note, that lots of these 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 certain timeout, and most likely
   also timers that start running when the device was booted up, so they
   might have concept of time with certain granularity.  They might not
   have real time clocks or any information how their internal clock
   relates to real world clock, and 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, function and avoid storing any additional values or
   context related to the SN.  When the use of a clock is considered,
   one should take care that packets associated to a given SA are not
   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 may
   result in the packet out of the receiver's windows and that packet
   being discarded.

   For inbound traffic, it is RECOMMENDED that any receiver provide a
   anti-replay protection, and the size of the window depends on the
   ability of the network to deliver packet out of order.  As a result,
   in environment where out of order packets is not possible the window
   size can be set to one.  However, while RECOMMENDED, there is no
   requirements to implement an anti replay anti-replay protection mechanism
   implemented by IPsec.  If an IoT device such as a temperature sensor
   is sending a temperature every 60 seconds, the implementation of an
   anti-replay mechanism requires the sensor to receive an
   acknowledgment for the receiver as well as storing an additional
   value across sleep time.  Such design is likely to be non optimal in
   term of limiting the number of read/write on a flash card as well as
   limiting the time the sensor needs to be awake.  On the other hand,
   the impact provided by an anti-replay mechanisms implemented on the
   sensor is very limited and unlikely to change the way of working of
   the sensor.  A node MAY drop anti-replay protection provided by
   IPsec, and instead implement its own internal mechanism.

   SN can be encoded over 32 bits or 64 bits - known as Extended
   Sequence Number (ESN).  As per [RFC4303], the support 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 life
   time
   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 primary determined by the security associated
   to 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.  In a constrainted constrained environment, it is likely that the
   implementation of a rekey mechanism is preferred over the use of ESN.

5.  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.

   Checking the padding structure is not mandatory, so the constraint
   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 fix 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 TFC is not
   expected to be supported by a minimal ESP implementation.

   As a result, TFC cannot be enabled with minimal ESP, and
   communication protection that were rely 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
   informations.
   information.

   Some constrained nodes that have limited battery life time lifetime may also
   prefer avoiding sending extra padding bytes.  However  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 trade off 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 when an a specific event is happening.  Typically  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.

6.  Next Header (8 bit)

   According to [RFC4303], the Next Header is a mandatory 8 bits field
   in the packet.  Next header is intended to specify the data contained
   in the payload as well as dummy packet. 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 packet is required by
   [RFC4303].  For interoperability, it is RECOMMENDED a minimal ESP implementation discards MUST
   discard dummy packets.  Note that such recommendation only applies
   for nodes receiving packets, and that nodes designed to only send
   data may 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 constraint
   environment sending dummy packets may have too much impact on the
   device life time, 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 5 traffic shaping at the IPsec layer may also
   introduce some traffic pattern, and on constraint 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 fix
   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 is not expected to be
   part of a minimal implementation of ESP.

7.  ICV

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

   As detailed in Section 8 we recommend to use using authentication, the ICV
   field is expected to be present that is to say with a size different
   from zero.  This makes it a mandatory field which size is defined by
   the security recommendations only.

8.  Cryptographic Suites

   The cryptographic suites implemented are an important component of
   ESP.  The recommended suites algorithms to use are expected to evolve over
   time and implementers SHOULD follow the recommendations provided by
   [RFC8221] and updates.  Recommendations are provided for standard
   nodes as well as constrained nodes.

   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 indicative:

   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 guidances guidance and recommendations.  The chosen encryption
       algorithm transforms 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 later 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, it is RECOMMENDED to consider transforms that are
       nonce misuse resistant such as AES-GCM-SIV for example[RFC8452]

   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
       weaken
       weakened for interoperability.  [RFC8221] and successors consider
       the life cycle of encryption algorithm transforms sufficiently
       long to provide interoperability.  Constraint 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 to 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 may
       consider re-using specific libraries or to take advantage of
       hardware acceleration provided by the device.  For example 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 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 an guidances and guidance over the
   implementation choices for each fields. field.

   The security of a communication provided by ESP is closely related to
   the security associated to the management of that key.  This usually
   include mechanisms to prevent a nonce to repeat 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 of key management
   protocols such as for example IKEv2 [RFC7296] or minimal IKEv2
   [RFC7815].  Such mechanisms are responsible to negotiate fresh
   session keys as well as prevent a session key being use beyond its
   life time.
   lifetime.  When such mechanisms cannot be implemented and the session
   key is, for example, provisioned, the nodes MUST ensure that keys are
   not used beyond their life time lifetime and that the appropriated 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 guidances guidance to build random
   generators based on deterministic random functions.

11.  Acknowledgment

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

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

   [I-D.mglt-ipsecme-diet-esp]
              Migault, D., Guggemos, T., Bormann, C., and D. Schinazi,
              "ESP Header Compression and Diet-ESP", draft-mglt-ipsecme-
              diet-esp-07 (work in progress), March 2019.

   [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", draft-mglt-ipsecme-ikev2-
              diet-esp-extension-01 (work in progress), June 2018.

   [I-D.nikander-esp-beet-mode]
              Nikander, P. and J. Melen, "A Bound End-to-End Tunnel
              (BEET) mode for ESP", draft-nikander-esp-beet-mode-09
              (work in progress), August 2008.

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

Appendix A.  Document Change Log

   [RFC Editor: This section is to be removed before publication]

   -00: First version published.

   -01: Clarified description

   -02: Clarified description

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, Bavaria
   Germany

   EMail: guggemos@mnm-team.org