draft-ietf-lwig-minimal-esp-02.txt   draft-ietf-lwig-minimal-esp-03.txt 
Light-Weight Implementation Guidance (lwig) D. Migault Light-Weight Implementation Guidance (lwig) D. Migault
Internet-Draft Ericsson Internet-Draft Ericsson
Intended status: Informational T. Guggemos Intended status: Informational T. Guggemos
Expires: May 6, 2021 LMU Munich Expires: September 25, 2021 LMU Munich
November 2, 2020 March 24, 2021
Minimal ESP Minimal ESP
draft-ietf-lwig-minimal-esp-02 draft-ietf-lwig-minimal-esp-03
Abstract Abstract
This document describes a minimal implementation of the IP This document describes a minimal implementation of the IP
Encapsulation Security Payload (ESP) defined in RFC 4303. Its Encapsulation Security Payload (ESP) defined in RFC 4303. Its
purpose is to enable implementation of ESP with a minimal set of purpose is to enable implementation of ESP with a minimal set of
options to remain compatible with ESP as described in RFC 4303. A 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 minimal version of ESP is not intended to become a replacement of the
RFC 4303 ESP, but instead to enable a limited implementation to RFC 4303 ESP. Instead, a minimal implementation is expected to be
interoperate with implementations of RFC 4303 ESP. 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 This document describes what is required from RFC 4303 ESP as well as
various ways to optimize compliance with RFC 4303 ESP. various ways to optimize compliance with RFC 4303 ESP.
This document does not update or modify RFC 4303, but provides a This document does not update or modify RFC 4303, but provides a
compact description of how to implement the minimal version of the compact description of how to implement the minimal version of the
protocol. If this document and RFC 4303 conflicts then RFC 4303 is protocol. If this document and RFC 4303 conflicts, then RFC 4303 is
the authoritative description. the authoritative description.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 6, 2021. This Internet-Draft will expire on September 25, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2 1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Security Parameter Index (SPI) (32 bit) . . . . . . . . . . . 4 3. Security Parameter Index (SPI) (32 bit) . . . . . . . . . . . 4
4. Sequence Number(SN) (32 bit) . . . . . . . . . . . . . . . . 6 4. Sequence Number(SN) (32 bit) . . . . . . . . . . . . . . . . 6
5. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Next Header (8 bit) . . . . . . . . . . . . . . . . . . . . . 8 6. Next Header (8 bit) . . . . . . . . . . . . . . . . . . . . . 9
7. ICV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 7. ICV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Cryptographic Suites . . . . . . . . . . . . . . . . . . . . 9 8. Cryptographic Suites . . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Security Considerations . . . . . . . . . . . . . . . . . . . 11 10. Security Considerations . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 11 11. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . 11 12.1. Normative References . . . . . . . . . . . . . . . . . . 12
12.2. Informative References . . . . . . . . . . . . . . . . . 12 12.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Document Change Log . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Requirements Notation 1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Introduction 2. Introduction
ESP [RFC4303] is part of the IPsec protocol suite [RFC4301]. IPsec ESP [RFC4303] is part of the IPsec protocol suite [RFC4301]. IPsec
is used to provide confidentiality, data origin authentication, is used to provide confidentiality, data origin authentication,
connectionless integrity, an anti-replay service (a form of partial connectionless integrity, an anti-replay service (a form of partial
sequence integrity) and limited traffic flow confidentiality. sequence integrity) and limited traffic flow confidentiality.
Figure 1 describes an ESP Packet. Currently ESP is implemented in Figure 1 describes an ESP Packet. Currently ESP is implemented in
the kernel of major multi purpose Operating Systems (OS). The ESP the kernel of major multipurpose Operating Systems (OS). The ESP and
and IPsec suite is usually implemented in a complete way to fit IPsec suite is usually implemented in a complete way to fit multiple
multiple purpose usage of these OS. However, completeness of the purpose usage of these OS. However, completeness of the IPsec suite
IPsec suite as well as multi purpose scope of these OS is often as well as multipurpose scope of these OS is often performed at the
performed at the expense of resources, or a lack of performance. As expense of resources, or a lack of performance. As a result,
a result, constraint devices are likely to have their own constraint devices are likely to have their own implementation of ESP
implementation of ESP optimized and adapted to their specificities. optimized and adapted to their specificities such as limiting the
With the adoption of IPsec by IoT devices with minimal IKEv2 number of flash writes (for each packet or across wake
[RFC7815] and ESP Header Compression (EHC) with time), handling frequent wakeup and sleep state, limiting wakeup
[I-D.mglt-ipsecme-diet-esp] or 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 [I-D.mglt-ipsecme-ikev2-diet-esp-extension], it becomes crucial that
ESP implementation designed for constraint devices remain inter- ESP implementation designed for constraint devices remain inter-
operable with the standard ESP implementation to avoid a fragmented operable with the standard ESP implementation to avoid a fragmented
usage of ESP. This document describes the the minimal properties and usage of ESP. This document describes the minimal properties and ESP
ESP implementation needs to meet. implementation needs to meet.
For each field of the ESP packet represented in Figure 1 this For each field of the ESP packet represented in Figure 1 this
document provides recommendations and guidance for minimal document provides recommendations and guidance for minimal
implementations. The primary purpose of Minimal ESP is to remain implementations. The primary purpose of Minimal ESP is to remain
interoperable with other nodes implementing RFC 4303 ESP, while interoperable with other nodes implementing RFC 4303 ESP, while
limiting the standard complexity of the implementation. limiting the standard complexity of the implementation.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
skipping to change at page 4, line 25 skipping to change at page 4, line 25
match. match.
For nodes supporting only unicast communications, it is RECOMMENDED 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 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 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 on the node may require a combination of the SPI as well as other
parameters to index the SA. parameters to index the SA.
Values 0-255 MUST NOT be used. As per section 2.1 of [RFC4303], 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 values 1-255 are reserved and 0 is only allowed to be used internal
and it MUST NOT be send on the wire. and it MUST NOT be sent on the wire.
It is RECOMMENDED to index each inbound session with a SPI randomly 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 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 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 the SPI has an acceptable value, it is used to index the inbound
session, otherwise the SPI is re-generated until an acceptable value session, otherwise the SPI is re-generated until an acceptable value
is found. A random generation provides a stateless way to generate is found. A random generation provides a stateless way to generate
the SPIs, while keeping the probability of collision between SPIs the SPIs, while keeping the probability of collision between SPIs
relatively low. relatively low.
However, for some constrained nodes, generating and handling 32 bit However, for some constrained nodes, generating and handling 32 bit
random SPI may consume too much resource, in which case SPI can be 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 generated using predictable functions or end up in a using a subset
of the possible values for SPI. In fact, the SPI does not of the possible values for SPI. In fact, the SPI does not
necessarily need to be randomly generated. A node provisioned with necessarily need to be randomly generated. A node provisioned with
keys by a third party - e.g. that does not generate them - and that 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 uses a transform that does not needs random data may not have such
random generators. However, non random SPI and restricting their random generators. However, nonrandom SPI and restricting their
possible values MAY lead to privacy and security concerns. As a possible values MAY lead to privacy and security concerns. As a
result, this alternative should be considered for devices that would result, this alternative should be considered for devices that would
be strongly impacted by the generation of a random SPI and after be strongly impacted by the generation of a random SPI and after
understanding the privacy and security impact of generating non understanding the privacy and security impact of generating nonrandom
random SPI. SPI.
When a constrained node limits the number of possible SPIs this limit When a constrained node limits the number of possible SPIs this limit
should both consider the number of inbound SAs - possibly per IP should both consider the number of inbound SAs - possibly per IP
addresses - as well as the ability for the node to rekey. SPI can 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 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 may be used to indicate which key is being used. This can typically
be implemented by a SPI being encoded with the SAD entry on a subset be implemented by a SPI being encoded with the Security Association
of bytes (for example 3 bytes), while the remaining byte is left to Database (SAD) entry on a subset of bytes (for example 3 bytes),
indicate the rekey index. 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 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 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 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 for inbound traffic by the peer, this allows each peer to manage the
set of SPIs used for its inbound traffic. set of SPIs used for its inbound traffic.
The use of a limited number of SPIs or non random SPIs come with The use of a limited number of SPIs or nonrandom SPIs come with
security or privacy drawbacks. Typically, a passive attacker may security or privacy drawbacks. Typically, a passive attacker may
derive information such as the number of constraint devices derive information such as the number of constraint devices
connecting the remote peer, and in conjunction with data rate, the connecting the remote peer, and in conjunction with data rate, the
attacker may eventually determine the application the constraint attacker may eventually determine the application the constraint
device is associated to. If the SPIs are set by a manufacturer or by 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 some software application, the SPI may leak in an obvious way the
type of sensor, the application involved or the model of the type of sensor, the application involved or the model of the
constraint device. When identification of the application or the constraint device. When identification of the application or the
hardware is associated to privacy, the SPI MUST be randomly hardware is associated to privacy, the SPI MUST be randomly
generated. However, one needs to realize that in this case this is generated. However, one needs to realize that in this case this is
likely to be sufficient and a thorough privacy analysis is required. likely not to be sufficient and a thorough privacy analysis is
More specifically, traffic pattern may leak sufficient information in required. More specifically, traffic pattern may also leak
itself. In other words, privacy leakage is a complex and the use of significant privacy sensitive information. In other words, privacy
random SPI is unlikely to be sufficient. 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, As the general recommendation is to randomly generate the SPI,
constraint devices that will use a (very) limited number of SPIs are constraint devices that will use a (very) limited number of SPIs are
expected to be very constraint devices with very limited expected to be very constraint devices with very limited
capabilities, where the use of randomly generated SPI may prevent capabilities, where the use of randomly generated SPI may prevent
them to implement IPsec. In this case the ability to provision non them to implement IPsec. In this case the ability to provision
random SPI enables these devices to secure their communications. nonrandom SPI enables these devices to secure their communications.
These devices, due to there limitations, are expected to provide For example, the SPI could be -at least partially - generated based
limited information and how the use of non random SPI impacts privacy on the SAD structure in memory which would simplify the
requires further analysis. Typically temperature sensors, wind implementation. These devices, due to their limitations, are
sensors, used outdoor do not leak privacy sensitive information. expected to provide limited information and how the use of nonrandom
When used indoor, privacy leakage outside the local network may be SPI impacts privacy requires further analysis. Typically,
limited. 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 sensitive information outside the local network.
As far as security is concerned, revealing the type of application or As far as security is concerned, revealing the type of application or
model of the constraint device could be used to identify the model of the constraint device could be used to identify the
vulnerabilities the constraint device is subject to. This is vulnerabilities the constraint device is subject to. This is
especially sensitive for constraint devices where patches or software especially sensitive for constraint devices where patches or software
updates will be challenging to operate. As a result, these devices updates will be challenging to operate. As a result, these devices
may remain vulnerable for relatively long period. In addition, may remain vulnerable for relatively long period. In addition,
predictable SPIs enable an attacker to forge packets with a valid predictable SPIs enable an attacker to forge packets with a valid
SPI. Such packet will not be rejected due to an SPI mismatch, but SPI. Such packet will not be rejected due to an SPI mismatch, but
instead after the signature check which requires more resource and instead after the signature check which requires more resource and
skipping to change at page 6, line 17 skipping to change at page 6, line 21
batteries. batteries.
4. Sequence Number(SN) (32 bit) 4. Sequence Number(SN) (32 bit)
According to [RFC4303], the Sequence Number (SN) is a mandatory 32 According to [RFC4303], the Sequence Number (SN) is a mandatory 32
bits field in the packet. bits field in the packet.
The SN is set by the sender so the receiver can implement anti-replay The SN is set by the sender so the receiver can implement anti-replay
protection. The SN is derived from any strictly increasing function protection. The SN is derived from any strictly increasing function
that guarantees: if packet B is sent after packet A, then SN of that guarantees: if packet B is sent after packet A, then SN of
packet B is strictly greater then the SN of packet A. packet B is strictly greater than the SN of packet A.
Some constraint devices may establish communication with specific Some constraint devices may establish communication with specific
devices, like a specific gateway, or nodes similar to them. As a devices, like a specific gateway, or nodes similar to them. As a
result, the sender may know whereas the receiver implements anti- result, the sender may know whereas the receiver implements anti-
replay protection or not. Even though the sender may know the replay protection or not. Even though the sender may know the
receiver does not implement anti replay protection, the sender MUST receiver does not implement anti-replay protection, the sender MUST
implement a always increasing function to generate the SN. implement an always increasing function to generate the SN.
Usually, SN is generated by incrementing a counter for each packet Usually, SN is generated by incrementing a counter for each packet
sent. A constraint device may avoid maintaining this context and use sent. A constraint device may avoid maintaining this context and use
another source that is known to always increase. Typically, another source that is known to always increase. Typically,
constraint nodes using 802.15.4 Time Slotted Channel Hopping (TSCH), constraint nodes using 802.15.4 Time Slotted Channel Hopping (TSCH),
whose communication is heavily dependent on time, can take advantage whose communication is heavily dependent on time, can take advantage
of their clock to generate the SN. This would guarantee a strictly of their clock to generate the SN. The problem with regular sequence
increasing function, and avoid storing any additional values or 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 and avoid storing any additional values or
context related to the SN. When the use of a clock is considered, 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 one should take care that packets associated to a given SA are not
sent with the same time value. Note however that standard receivers sent with the same time value. Note however that standard receivers
are generally configured with incrementing counters and, if not are generally configured with incrementing counters and, if not
appropriately configured, the use of a significantly larger SN may appropriately configured, the use of a significantly larger SN may
result in the packet out of the receiver's windows and that packet result in the packet out of the receiver's windows and that packet
being discarded. being discarded.
For inbound traffic, it is RECOMMENDED that any receiver provide a For inbound traffic, it is RECOMMENDED that any receiver provide a
anti-replay protection, and the size of the window depends on the 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, 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 in environment where out of order packets is not possible the window
size can be set to one. However, while RECOMMENDED, there is no size can be set to one. However, while RECOMMENDED, there is no
requirements to implement an anti replay protection mechanism requirements to implement an anti-replay protection mechanism
implemented by IPsec. A node MAY drop anti-replay protection implemented by IPsec. If an IoT device such as a temperature sensor
provided by IPsec, and instead implement its own internal mechanism. 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 SN can be encoded over 32 bits or 64 bits - known as Extended
Sequence Number (ESN). As per [RFC4303], the support ESN is not Sequence Number (ESN). As per [RFC4303], the support ESN is not
mandatory. The determination of the use of ESN is based on the 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 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 incremented for each packet, the number of packets sent over the
time of a session may be considered. However, when the SN is lifetime of a session may be considered. However, when the SN is
incremented differently - such as when time is used - the maximum incremented differently - such as when time is used - the maximum
value SN needs to be considered instead. Note that the limit of value SN needs to be considered instead. Note that the limit of
messages being sent is primary determined by the security associated messages being sent is primary determined by the security associated
to the key rather than the SN. The security of all data protected 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 under a given key decreases slightly with each message and a node
MUST ensure the limit is not reached - even though the SN would MUST ensure the limit is not reached - even though the SN would
permit it. In a constrainted environment, it is likely that the permit it. In a constrained environment, it is likely that the
implementation of a rekey mechanism is preferred over the use of ESN. implementation of a rekey mechanism is preferred over the use of ESN.
5. Padding 5. Padding
The purpose of padding is to respect the 32 bit alignment of ESP or 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 block size expected by an encryption transform - such as AES-CBC for
example. ESP MUST have at least one padding byte Pad Length that example. ESP MUST have at least one padding byte Pad Length that
indicates the padding length. ESP padding bytes are generated by a indicates the padding length. ESP padding bytes are generated by a
succession of unsigned bytes starting with 1, 2, 3 with the last byte 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 set to Pad Length, where Pad Length designates the length of the
skipping to change at page 8, line 5 skipping to change at page 8, line 35
which could end up being counterproductive. As such TFC is not which could end up being counterproductive. As such TFC is not
expected to be supported by a minimal ESP implementation. expected to be supported by a minimal ESP implementation.
As a result, TFC cannot be enabled with minimal ESP, and As a result, TFC cannot be enabled with minimal ESP, and
communication protection that were rely on TFC will be more sensitive communication protection that were rely on TFC will be more sensitive
to traffic shaping. This could expose the application as well as the to traffic shaping. This could expose the application as well as the
devices used to a passive monitoring attacker. Such information devices used to a passive monitoring attacker. Such information
could be used by the attacker in case a vulnerability is disclosed on could be used by the attacker in case a vulnerability is disclosed on
the specific device. In addition, some application use - such as the specific device. In addition, some application use - such as
health applications - may also reveal important privacy oriented health applications - may also reveal important privacy oriented
informations. information.
Some constrained nodes that have limited battery life time may also Some constrained nodes that have limited battery lifetime may also
prefer avoiding sending extra padding bytes. However the same nodes prefer avoiding sending extra padding bytes. However, the same nodes
may also be very specific to an application and device. As a result, 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 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 most cases, the payload carried by these nodes is quite small, and
the standard padding mechanism may also be used as an alternative to the standard padding mechanism may also be used as an alternative to
TFC, with a sufficient trade off between the require energy to send TFC, with a sufficient tradeoff between the require energy to send
additional payload and the exposure to traffic shaping attacks. In additional payload and the exposure to traffic shaping attacks. In
addition, the information leaked by the traffic shaping may also be addition, the information leaked by the traffic shaping may also be
addressed by the application level. For example, it is preferred to addressed by the application level. For example, it is preferred to
have a sensor sending some information at regular time interval, have a sensor sending some information at regular time interval,
rather when an specific event is happening. Typically a sensor rather when a specific event is happening. Typically, a sensor
monitoring the temperature, or a door is expected to send regularly monitoring the temperature, or a door is expected to send regularly
the information - i.e. the temperature of the room or whether the the information - i.e. the temperature of the room or whether the
door is closed or open) instead of only sending the information when door is closed or open) instead of only sending the information when
the temperature has raised or when the door is being opened. the temperature has raised or when the door is being opened.
6. Next Header (8 bit) 6. Next Header (8 bit)
According to [RFC4303], the Next Header is a mandatory 8 bits field 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 packet. Next header is intended to specify the data contained
in the payload as well as dummy packet. In addition, the Next Header in the payload as well as dummy packet, i.e. packets with the Next
may also carry an indication on how to process the packet 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]. [I-D.nikander-esp-beet-mode].
The ability to generate and receive dummy packet is required by The ability to generate and receive dummy packet is required by
[RFC4303]. For interoperability, it is RECOMMENDED a minimal ESP [RFC4303]. For interoperability, a minimal ESP implementation MUST
implementation discards dummy packets. Note that such recommendation discard dummy packets. Note that such recommendation only applies
only applies for nodes receiving packets, and that nodes designed to for nodes receiving packets, and that nodes designed to only send
only send data may not implement this capability. data may not implement this capability.
As the generation of dummy packets is subject to local management and As the generation of dummy packets is subject to local management and
based on a per-SA basis, a minimal ESP implementation may not based on a per-SA basis, a minimal ESP implementation may not
generate such dummy packet. More especially, in constraint generate such dummy packet. More especially, in constraint
environment sending dummy packets may have too much impact on the environment sending dummy packets may have too much impact on the
device life time, and so may be avoided. On the other hand, device lifetime, and so may be avoided. On the other hand,
constrained nodes may be dedicated to specific applications, in which constrained nodes may be dedicated to specific applications, in which
case, traffic pattern may expose the application or the type of node. case, traffic pattern may expose the application or the type of node.
For these nodes, not sending dummy packet may have some privacy For these nodes, not sending dummy packet may have some privacy
implication that needs to be measured. However, for the same reasons implication that needs to be measured. However, for the same reasons
exposed in Section 5 traffic shaping at the IPsec layer may also exposed in Section 5 traffic shaping at the IPsec layer may also
introduce some traffic pattern, and on constraint devices the introduce some traffic pattern, and on constraint devices the
application is probably the most appropriated layer to limit the risk application is probably the most appropriated layer to limit the risk
of leaking information by traffic shaping. of leaking information by traffic shaping.
In some cases, devices are dedicated to a single application or a In some cases, devices are dedicated to a single application or a
single transport protocol, in which case, the Next Header has a fix single transport protocol, in which case, the Next Header has a fix
value. value.
Specific processing indications have not been standardized yet Specific processing indications have not been standardized yet
[I-D.nikander-esp-beet-mode] and is expected to result from an [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 agreement between the peers. As a result, it is not expected to be
part of a minimal implementation of ESP. part of a minimal implementation of ESP.
7. ICV 7. ICV
The ICV depends on the crypto-suite used. Currently [RFC8221] only The ICV depends on the cryptographic suite used. Currently [RFC8221]
recommends crypto-suites with an ICV which makes the ICV a mandatory only recommends cryptographic suites with an ICV which makes the ICV
field. a mandatory field.
As detailed in Section 8 we recommend to use authentication, the ICV As detailed in Section 8 we recommend using authentication, the ICV
field is expected to be present that is to say with a size different 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 from zero. This makes it a mandatory field which size is defined by
the security recommendations only. the security recommendations only.
8. Cryptographic Suites 8. Cryptographic Suites
The cryptographic suites implemented are an important component of The cryptographic suites implemented are an important component of
ESP. The recommended suites to use are expected to evolve over time ESP. The recommended algorithms to use are expected to evolve over
and implementers SHOULD follow the recommendations provided by time and implementers SHOULD follow the recommendations provided by
[RFC8221] and updates. Recommendations are provided for standard [RFC8221] and updates. Recommendations are provided for standard
nodes as well as constrained nodes. nodes as well as constrained nodes.
This section lists some of the criteria that may be considered. The This section lists some of the criteria that may be considered. The
list is not expected to be exhaustive and may also evolve overtime. list is not expected to be exhaustive and may also evolve overtime.
As a result, the list is provided as indicative: As a result, the list is provided as indicative:
1. Security: Security is the criteria that should be considered 1. Security: Security is the criteria that should be considered
first for the selection of encryption algorithm transform. The first for the selection of encryption algorithm transform. The
security of encryption algorithm transforms is expected to evolve security of encryption algorithm transforms is expected to evolve
over time, and it is of primary importance to follow up-to-date over time, and it is of primary importance to follow up-to-date
security guidances and recommendations. The chosen encryption security guidance and recommendations. The chosen encryption
algorithm transforms MUST NOT be known vulnerable or weak (see algorithm transforms MUST NOT be known vulnerable or weak (see
[RFC8221] for outdated ciphers). ESP can be used to authenticate [RFC8221] for outdated ciphers). ESP can be used to authenticate
only or to encrypt the communication. In the later case, only or to encrypt the communication. In the latter case,
authenticated encryption must always be considered [RFC8221]. authenticated encryption must always be considered [RFC8221].
2. Resilience to nonce re-use: Some transforms -including AES-GCM - 2. Resilience to nonce re-use: Some transforms -including AES-GCM -
are very sensitive to nonce collision with a given key. While are very sensitive to nonce collision with a given key. While
the generation of the nonce may prevent such collision during a the generation of the nonce may prevent such collision during a
session, the mechanisms are unlikely to provide such protection session, the mechanisms are unlikely to provide such protection
across reboot. This causes an issue for devices that are across reboot. This causes an issue for devices that are
configured with a key. When the key is likely to be re-used configured with a key. When the key is likely to be re-used
across reboots, it is RECOMMENDED to consider transforms that are across reboots, it is RECOMMENDED to consider transforms that are
nonce misuse resistant such as AES-GCM-SIV for example[RFC8452] nonce misuse resistant such as AES-GCM-SIV for example[RFC8452]
3. Interoperability: Interoperability considers the encryption 3. Interoperability: Interoperability considers the encryption
algorithm transforms shared with the other nodes. Note that it algorithm transforms shared with the other nodes. Note that it
is not because an encryption algorithm transform is widely is not because an encryption algorithm transform is widely
deployed that is secured. As a result, security SHOULD NOT be deployed that it is secured. As a result, security SHOULD NOT be
weaken for interoperability. [RFC8221] and successors consider weakened for interoperability. [RFC8221] and successors consider
the life cycle of encryption algorithm transforms sufficiently the life cycle of encryption algorithm transforms sufficiently
long to provide interoperability. Constraint devices may have long to provide interoperability. Constraint devices may have
limited interoperability requirements which makes possible to limited interoperability requirements which makes possible to
reduces the number of encryption algorithm transforms to reduces the number of encryption algorithm transforms to
implement. implement.
4. Power Consumption and Cipher Suite Complexity: Complexity of the 4. Power Consumption and Cipher Suite Complexity: Complexity of the
encryption algorithm transform or the energy associated to it are encryption algorithm transform or the energy associated to it are
especially considered when devices have limited resources or are especially considered when devices have limited resources or are
using some batteries, in which case the battery determines the using some batteries, in which case the battery determines the
life of the device. The choice of a cryptographic function may life of the device. The choice of a cryptographic function may
consider re-using specific libraries or to take advantage of consider re-using specific libraries or to take advantage of
hardware acceleration provided by the device. For example if the hardware acceleration provided by the device. For example, if
device benefits from AES hardware modules and uses AES-CTR, it the device benefits from AES hardware modules and uses AES-CTR,
may prefer AUTH_AES-XCBC for its authentication. In addition, it may prefer AUTH_AES-XCBC for its authentication. In addition,
some devices may also embed radio modules with hardware some devices may also embed radio modules with hardware
acceleration for AES-CCM, in which case, this mode may be acceleration for AES-CCM, in which case, this mode may be
preferred. preferred.
5. Power Consumption and Bandwidth Consumption: Similarly to the 5. Power Consumption and Bandwidth Consumption: Similarly to the
encryption algorithm transform complexity, reducing the payload encryption algorithm transform complexity, reducing the payload
sent, may significantly reduce the energy consumption of the sent, may significantly reduce the energy consumption of the
device. As a result, encryption algorithm transforms with low device. As a result, encryption algorithm transforms with low
overhead may be considered. To reduce the overall payload size overhead may be considered. To reduce the overall payload size
one may for example: one may, for example:
1. Use of counter-based ciphers without fixed block length (e.g. 1. Use of counter-based ciphers without fixed block length (e.g.
AES-CTR, or ChaCha20-Poly1305). AES-CTR, or ChaCha20-Poly1305).
2. Use of ciphers with capability of using implicit IVs 2. Use of ciphers with capability of using implicit IVs
[RFC8750]. [RFC8750].
3. Use of ciphers recommended for IoT [RFC8221]. 3. Use of ciphers recommended for IoT [RFC8221].
4. Avoid Padding by sending payload data which are aligned to 4. Avoid Padding by sending payload data which are aligned to
the cipher block length - 2 for the ESP trailer. the cipher block length - 2 for the ESP trailer.
9. IANA Considerations 9. IANA Considerations
There are no IANA consideration for this document. There are no IANA consideration for this document.
10. Security Considerations 10. Security Considerations
Security considerations are those of [RFC4303]. In addition, this Security considerations are those of [RFC4303]. In addition, this
document provided security recommendations an guidances over the document provided security recommendations and guidance over the
implementation choices for each fields. implementation choices for each field.
The security of a communication provided by ESP is closely related to The security of a communication provided by ESP is closely related to
the security associated to the management of that key. This usually the security associated to the management of that key. This usually
include mechanisms to prevent a nonce to repeat for example. When a include mechanisms to prevent a nonce to repeat for example. When a
node is provisioned with a session key that is used across reboot, node is provisioned with a session key that is used across reboot,
the implementer MUST ensure that the mechanisms put in place remain the implementer MUST ensure that the mechanisms put in place remain
valid across reboot as well. valid across reboot as well.
It is RECOMMENDED to use ESP in conjunction of key management It is RECOMMENDED to use ESP in conjunction of key management
protocols such as for example IKEv2 [RFC7296] or minimal IKEv2 protocols such as for example IKEv2 [RFC7296] or minimal IKEv2
[RFC7815]. Such mechanisms are responsible to negotiate fresh [RFC7815]. Such mechanisms are responsible to negotiate fresh
session keys as well as prevent a session key being use beyond its session keys as well as prevent a session key being use beyond its
life time. When such mechanisms cannot be implemented and the lifetime. When such mechanisms cannot be implemented and the session
session key is, for example, provisioned, the nodes MUST ensure that key is, for example, provisioned, the nodes MUST ensure that keys are
keys are not used beyond their life time and that the appropriated not used beyond their lifetime and that the appropriated use of the
use of the key remains across reboots - e.g. conditions on counters key remains across reboots - e.g. conditions on counters and nonces
and nonces remains valid. remains valid.
When a node generates its key or when random value such as nonces are When a node generates its key or when random value such as nonces are
generated, the random generation MUST follow [RFC4086]. In addition generated, the random generation MUST follow [RFC4086]. In addition
[SP-800-90A-Rev-1] provides appropriated guidances to build random [SP-800-90A-Rev-1] provides appropriated guidance to build random
generators based on deterministic random functions. generators based on deterministic random functions.
11. Acknowledgment 11. Acknowledgment
The authors would like to thank Daniel Palomares, Scott Fluhrer, Tero The authors would like to thank Daniel Palomares, Scott Fluhrer, Tero
Kivinen, Valery Smyslov, Yoav Nir, Michael Richardson for their Kivinen, Valery Smyslov, Yoav Nir, Michael Richardson for their
valuable comments. In particular Scott Fluhrer suggested to include valuable comments. In particular Scott Fluhrer suggested to include
the rekey index in the SPI as well as the use of transform resilient the rekey index in the SPI as well as the use of transform resilient
to nonce misuse. to nonce misuse. Tero Kivinen provided also multiple clarifications
and examples of deployment ESP within constrained devices with their
associated optimizations.
12. References 12. References
12.1. Normative References 12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
skipping to change at page 14, line 5 skipping to change at page 14, line 16
Nonce Misuse-Resistant Authenticated Encryption", Nonce Misuse-Resistant Authenticated Encryption",
RFC 8452, DOI 10.17487/RFC8452, April 2019, RFC 8452, DOI 10.17487/RFC8452, April 2019,
<https://www.rfc-editor.org/info/rfc8452>. <https://www.rfc-editor.org/info/rfc8452>.
[SP-800-90A-Rev-1] [SP-800-90A-Rev-1]
Elain, E. and J. Kelsey, "Recommendation for Random Number Elain, E. and J. Kelsey, "Recommendation for Random Number
Generation Using Deterministic Random Bit Generators", Generation Using Deterministic Random Bit Generators",
<https://csrc.nist.gov/publications/detail/sp/800-90a/rev- <https://csrc.nist.gov/publications/detail/sp/800-90a/rev-
1/final>. 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 Authors' Addresses
Daniel Migault Daniel Migault
Ericsson Ericsson
8400 boulevard Decarie 8400 boulevard Decarie
Montreal, QC H4P 2N2 Montreal, QC H4P 2N2
Canada Canada
EMail: daniel.migault@ericsson.com EMail: daniel.migault@ericsson.com
 End of changes. 44 change blocks. 
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