draft-ietf-lwig-minimal-esp-05.txt   draft-ietf-lwig-minimal-esp-06.txt 
Light-Weight Implementation Guidance (lwig) D. Migault Light-Weight Implementation Guidance (lwig) D.M. Migault
Internet-Draft Ericsson Internet-Draft Ericsson
Intended status: Informational T. Guggemos Intended status: Informational T.G. Guggemos
Expires: October 15, 2021 LMU Munich Expires: 27 January 2022 LMU Munich
April 13, 2021 26 July 2021
Minimal ESP Minimal ESP
draft-ietf-lwig-minimal-esp-05 draft-ietf-lwig-minimal-esp-06
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. Instead, a minimal implementation is expected to be RFC 4303 ESP. Instead, a minimal implementation is expected to be
optimized for constrained environment while remaining interoperable optimized for constrained environment while remaining interoperable
with implementations of RFC 4303 ESP. Some constrains include with implementations of RFC 4303 ESP. Some constraints include
limiting the number of flash writes, handling frequent wakeup / sleep limiting the number of flash writes, handling frequent wakeup / sleep
states, limiting wakeup time, or reducing the use of random states, limiting wakeup time, or reducing the use of random
generation. generation.
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. RFC 4303 remains the authoritative description. protocol. RFC 4303 remains the authoritative description.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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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 October 15, 2021. This Internet-Draft will expire on 27 January 2022.
Copyright Notice Copyright Notice
Copyright (c) 2021 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.
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carefully, as they describe your rights and restrictions with respect and restrictions with respect to this document. Code Components
to this document. Code Components extracted from this document must extracted from this document must include Simplified BSD License text
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described in the Simplified BSD License.
Table of Contents Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Security Parameter Index (SPI) (32 bit) . . . . . . . . . . . 3
3. Security Parameter Index (SPI) (32 bit) . . . . . . . . . . . 4 2.1. Considerations over SPI generation . . . . . . . . . . . 4
3.1. Considerations over SPI generation . . . . . . . . . . . 5 3. Sequence Number(SN) (32 bit) . . . . . . . . . . . . . . . . 5
4. Sequence Number(SN) (32 bit) . . . . . . . . . . . . . . . . 6 4. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Next Header (8 bit) . . . . . . . . . . . . . . . . . . . . . 9
6. Next Header (8 bit) . . . . . . . . . . . . . . . . . . . . . 9 6. ICV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. ICV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. Cryptographic Suites . . . . . . . . . . . . . . . . . . . . 10
8. Cryptographic Suites . . . . . . . . . . . . . . . . . . . . 10 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. Security Considerations . . . . . . . . . . . . . . . . . . . 12 10. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 12 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 11.1. Normative References . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . 13 11.2. Informative References . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Requirements Notation 1. Introduction
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 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 (TFC)
padding.
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 multipurpose Operating Systems (OS). The ESP and the kernel of major multipurpose Operating Systems (OS). The ESP and
IPsec suite is usually implemented in a complete way to fit multiple IPsec suite is usually implemented in a complete way to fit multiple
purpose usage of these OS. However, completeness of the IPsec suite purpose usage of these OS. However, completeness of the IPsec suite
as well as multipurpose scope of these OS is often performed at the as well as multipurpose scope of these OS is often performed at the
expense of resources, or performance. As a result, constrained expense of resources, or performance. As a result, constrained
devices are likely to have their own implementation of ESP optimized devices are likely to have their own implementation of ESP optimized
and adapted to their specificities such as limiting the number of and adapted to their specificities such as limiting the number of
flash writes (for each packet or across wake time), handling frequent flash writes (for each packet or across wake time), handling frequent
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+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov- + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-
| | Padding (0-255 bytes) | |ered* | | Padding (0-255 bytes) | |ered*
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | Pad Length | Next Header | v v | | Pad Length | Next Header | v v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
| Integrity Check Value-ICV (variable) | | Integrity Check Value-ICV (variable) |
~ ~ ~ ~
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: ESP Packet Description Figure 1: ESP Packet Description
3. Security Parameter Index (SPI) (32 bit) 2. Security Parameter Index (SPI) (32 bit)
According to the [RFC4303], the SPI is a mandatory 32 bits field and According to the [RFC4303], the SPI is a mandatory 32 bits field and
is not allowed to be removed. is not allowed to be removed.
The SPI has a local significance to index the Security Association The SPI has a local significance to index the Security Association
(SA). From [RFC4301] section 4.1, nodes supporting only unicast (SA). From [RFC4301] section 4.1, nodes supporting only unicast
communications can index their SA only using the SPI. On the other communications can index their SA only using the SPI. On the other
hand, nodes supporting multicast communications must also use the IP hand, nodes supporting multicast communications must also use the IP
addresses and thus SA lookup needs to be performed using the longest addresses and thus SA lookup needs to be performed using the longest
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 internally
and it MUST NOT be sent on the wire. and it must not be sent on the wire.
[RFC4303] does not require the SPI to be randomly generated over 32 [RFC4303] does not require the SPI to be randomly generated over 32
bits. However, this is the RECOMMENDED way to generate SPIs as it bits. However, this is the recommended way to generate SPIs as it
provides some privacy benefits and avoids, for example, correlation provides some privacy benefits and avoids, for example, correlation
between ESP communications. To randomly generate a 32 bit SPI, the between ESP communications. To randomly generate a 32 bit SPI, the
node generates a random 32 bit value, checks does not fall in the node generates a random 32 bit valueand checks it does not fall in
0-255 range. If the SPI has an acceptable value, it is used to index the 0-255 range. If the SPI has an acceptable value, it is used to
the inbound session, otherwise the SPI is re-generated until an index the inbound session, otherwise the SPI is re-generated until an
acceptable value is found. acceptable value is found.
However, some constrained nodes may be less concerned by the privacy However, some constrained nodes may be less concerned by the privacy
properties associated to SPIs randomly generated. Examples of such properties associated to SPIs randomly generated. Examples of such
nodes might include sensors looking to reduce their code complexity, nodes might include sensors looking to reduce their code complexity,
in which case the use of a predictive function to generate the SPI in which case the use of a predictive function to generate the SPI
might be preferred over the generation and handling of random values. might be preferred over the generation and handling of random values.
An example of such predictable function may consider the combination An example of such predictable function may consider the combination
of a fixed value and the memory address of the SAD structure. For of a fixed value and the memory address of the SAD structure. For
every incoming packet, the node will be able to point the SAD every incoming packet, the node will be able to point the SAD
structure directly from the SPI value. This avoids having a separate structure directly from the SPI value. This avoids having a separate
and additional binding between SPI and SAD entries that is involved and additional binding between SPI and SAD entries that is involved
for every incoming packet. for every incoming packet.
3.1. Considerations over SPI generation 2.1. Considerations over SPI generation
SPI that are not randomly generated over 32 bits MAY lead to privacy SPI that are not randomly generated over 32 bits may lead to privacy
and security concerns. As a result, the use of alternative designs and security concerns. As a result, the use of alternative designs
requires careful security and privacy reviews. This section provides requires careful security and privacy reviews. This section provides
some considerations upon the adoption of alternative designs. some considerations upon the adoption of alternative designs.
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 it sends traffic. As SPI is only used used by the remote peer when it sends traffic. As SPI is 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. Similarly, the privacy set of SPIs used for its inbound traffic. Similarly, the privacy
concerns associated with the generation of nonrandom SPI is also concerns associated with the generation of nonrandom SPI is also
limited to the incoming traffic. limited to the incoming traffic.
When alternate designs are considered, it is likely that the number When alternate designs are considered, it is likely that the number
of possible SPIs will be limited. This limit should both consider of possible SPIs will be limited. This limit should both consider
the number of inbound SAs - possibly per IP addresses - as well as 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 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 implement a key update with the SPI indicating the key is being used.
which key is being used. This can typically be implemented by a SPI For example, a SPI might be encoded with the Security Association
being encoded with the Security Association Database (SAD) entry on a Database (SAD) entry on a subset of bytes (for example 3 bytes),
subset of bytes (for example 3 bytes), while the remaining byte is while the remaining byte indicates the rekey index.
left to indicate the rekey index.
The use of a smaller number of SPIs across communications comes with The use of a smaller number of SPIs across communications comes with
privacy and security concerns. Typically some specific values or privacy and security concerns. Typically some specific values or
subset of SPI values may reveal the models or manufacturer of the subset of SPI values may reveal the models or manufacturer of the
node implementing ESP. This may raise some privacy issues as an node implementing ESP. This may raise some privacy issues as an
observer is likely to be able to determine the constrained devices of observer is likely to be able to determine the constrained devices of
the network. In some cases, these nodes may host a very limited the network. In some cases, these nodes may host a very limited
number of applications - typically a single application - in which number of applications - typically a single application - in which
case the SPI would provide some information related to the case the SPI would provide some information related to the
application of the user. In addition, the device or application may application of the user. In addition, the device or application may
be associated with some vulnerabilities, in which case specific SPI be associated with some vulnerabilities, in which case specific SPI
values may be used by an attacker to discover vulnerabilities. values may be used by an attacker to discover vulnerabilities.
While the use of randomly generated SPI may reduce the leakage or While the use of randomly generated SPIs may reduce the leakage or
privacy or security related information by ESP itself, these privacy of security related information by ESP itself, these
information may also be leaked otherwise and a privacy analysis information may also be leaked otherwise and a privacy analysis
should consider at least the type of information as well the traffic should consider at least the type of information as well the traffic
pattern. Typically, temperature sensors, wind sensors, used outdoors pattern. Typically, temperature sensors, wind sensors, used outdoors
do not leak privacy sensitive information and mosty of its traffic is do not leak privacy sensitive information and mosty of its traffic is
expected to be outbound traffic. When used indoors, a sensor that expected to be outbound traffic. When used indoors, a sensor that
reports every minute an encrypted status of the door (closed or reports every minute an encrypted status of the door (closed or
opened) leaks truly little privacy sensitive information outside the opened) leaks truly little privacy sensitive information outside the
local network. local network.
4. Sequence Number(SN) (32 bit) 3. 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 than the SN of packet A. packet B is strictly greater than the SN of packet A.
Some constrained devices may establish communication with specific Some constrained 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 an 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 constrained device may avoid maintaining this context and sent. A constrained device may avoid maintaining this context and
use another source that is known to always increase. Typically, use another source that is known to always increase. Typically,
constrained nodes using 802.15.4 Time Slotted Channel Hopping (TSCH), constrained 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. A lot of IoT devices are in a of their clock to generate the SN. A lot of IoT devices are in a
sleep state most of the time wake up and are only awake to perform a 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 specific operation before going back to sleep. They do have separate
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greater than what it was last time they woke up. Using time for SN greater than what it was last time they woke up. Using time for SN
would guarantee a strictly increasing function and avoid storing any would guarantee a strictly increasing function and avoid storing any
additional values or context related to the SN. When the use of a additional values or context related to the SN. When the use of a
clock is considered, one should take care that packets associated clock is considered, one should take care that packets associated
with a given SA are not sent with the same time value. Note however with a given SA are not sent with the same time value. Note however
that standard receivers are generally configured with incrementing that standard receivers are generally configured with incrementing
counters and, if not appropriately configured, the use of a counters and, if not appropriately configured, the use of a
significantly larger SN may result in the packet out of the significantly larger SN may result in the packet out of the
receiver's windows and that packet being discarded. receiver's windows and that packet being discarded.
For inbound traffic, it is RECOMMENDED that any receiver provides For inbound traffic, it is recommended that any receiver provides
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 packets out of order. As a result, ability of the network to deliver packets out of order. As a result,
in an environment where out of order packets is not possible the in an environment where out of order packets is not possible the
window size can be set to one. However, while RECOMMENDED, there are window size can be set to one. However, while recommended, there are
no requirements to implement an anti-replay protection mechanism no requirements to implement an anti-replay protection mechanism
implemented by IPsec. Similarly to the SN the implementation of anti implemented by IPsec. Similarly to the SN the implementation of anti
replay protection may require the device to write the received SN for replay protection may require the device to write the received SN for
every packet, which may in some cases come with the same drawbacks as every packet, which may in some cases come with the same drawbacks as
those exposed for SN. As a result, some implementations MAY drop an those exposed for SN. As a result, some implementations may drop a
non required anti replay protection especially when the necessary non required anti replay protection especially when the necessary
resource involved overcomes the benefit of the mechanism. A typical resource involved overcomes the benefit of the mechanism. These
example might consider an IoT device such as a temperature sensor resources need also to balance that absence of anti-replay mechanism,
that is sending a temperature every 60 seconds, and that receives an may lead to unnecessary integrity check operations that might be
acknowledgment from the receiver. In such cases, the ability to significantly more expensive as well. A typical example might
spoof and replay an acknowledgement is of limited interest and may consider an IoT device such as a temperature sensor that is sending a
not justify the implementation of an anti replay mechanism. temperature every 60 seconds, and that receives an acknowledgment
Receiving peers may also implement their own anti-replay mechanism. from the receiver. In such cases, the ability to spoof and replay an
Typically, when the sending peer is using SN based on time, anti- acknowledgement is of limited interest and may not justify the
replay may be implemented by discarding any packets that present a SN implementation of an anti replay mechanism. Receiving peers may also
whose value is too much in the past. Note that such mechanisms may implement their own anti-replay mechanism. Typically, when the
consider clock drifting in various ways in addition to acceptable sending peer is using SN based on time, anti-replay may be
delay induced by the network to avoid the anti replay windows implemented by discarding any packets that present a SN whose value
rejecting legitimate packets. When a packet is received at a regular is too much in the past. Note that such mechanisms may consider
time interval, some variant of time based mechanisms may not even use clock drifting in various ways in addition to acceptable delay
the value of the SN, but instead only consider the receiving time of induced by the network to avoid the anti replay windows rejecting
the packet. legitimate packets. When a packet is received at a regular time
interval, some variant of time based mechanisms may not even use the
value of the SN, but instead only consider the receiving time of the
packet.
SN can be encoded over 32 bits or 64 bits - known as Extended 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 of 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 incremented for each packet, the number of packets sent over the
lifetime 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 primarily determined by the security messages being sent is primarily determined by the security
associated with the key rather than the SN. The security of all data associated with the key rather than the SN. The security of all data
protected under a given key decreases slightly with each message and protected under a given key decreases slightly with each message and
a node MUST ensure the limit is not reached - even though the SN 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 would permit it. Estimation of the maximum number of packets to be
sent by a node is always challenging and as such should be considered sent by a node is always challenging and as such should be considered
cautiously as nodes could be online for much more time than expected. cautiously as nodes could be online for much more time than expected.
Even for constrained devices, it is RECOMMENDED to implement some Even for constrained devices, it is recommended to implement some
rekey mechanisms (see Section 10). rekey mechanisms (see Section 9).
5. Padding 4. 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
padding bytes. padding bytes.
Checking the padding structure is not mandatory, so the constrained Checking the padding structure is not mandatory, so the constrained
device may not proceed to such checks, however, in order to device may not proceed to such checks, however, in order to
interoperate with existing ESP implementations, it MUST build the interoperate with existing ESP implementations, it must build the
padding bytes as recommended by ESP. padding bytes as recommended by ESP.
In some situation the padding bytes may take a fix value. This would In some situation the padding bytes may take a fixed value. This
typically be the case when the Data Payload is of fix size. would typically be the case when the Data Payload is of fix size.
ESP [RFC4303] also provides Traffic Flow Confidentiality (TFC) as a ESP [RFC4303] also provides Traffic Flow Confidentiality (TFC) as a
way to perform padding to hide traffic characteristics, which differs way to perform padding to hide traffic characteristics, which differs
from respecting a 32 bit alignment. TFC is not mandatory and MUST be from respecting a 32 bit alignment. TFC is not mandatory and must be
negotiated with the SA management protocol. TFC has not yet being negotiated with the SA management protocol. TFC has not yet being
widely adopted for standard ESP traffic. One possible reason is that widely adopted for standard ESP traffic. One possible reason is that
it requires to shape the traffic according to one traffic pattern it requires to shape the traffic according to one traffic pattern
that needs to be maintained. This is likely to require extra that needs to be maintained. This is likely to require extra
processing as well as providing a "well recognized" traffic shape processing as well as providing a "well recognized" traffic shape
which could end up being counterproductive. As such, it is NOT which could end up being counterproductive. As such, it is NOT
RECOMMENDED that minimal ESP implementation supports TFC. recommended that minimal ESP implementation supports TFC.
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 relying on TFC will be more
to traffic shaping. This could expose the application as well as the sensitive to traffic shaping. This could expose the application as
devices used to a passive monitoring attacker. Such information well as the devices used to a passive monitoring attacker. Such
could be used by the attacker in case a vulnerability is disclosed on information could be used by the attacker in case a vulnerability is
the specific device. In addition, some application use - such as disclosed on the specific device. In addition, some application use
health applications - may also reveal important privacy oriented - such as health applications - may also reveal important privacy
information. oriented information.
Some constrained nodes that have limited battery lifetime 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 tradeoff 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 a specific event is happening. Typically, a sensor rather than 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) 5. 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 specifies the data contained in the in the packet. Next header specifies the data contained in the
payload as well as dummy packet, i.e. packets with the Next Header payload as well as dummy packet, i.e. packets with the Next Header
with a value 59 meaning "no next header". In addition, the Next with a value 59 meaning "no next header". In addition, the Next
Header may also carry an indication on how to process the packet 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 packets is required by
[RFC4303]. For interoperability, a minimal ESP implementation MUST [RFC4303]. For interoperability, a minimal ESP implementation must
discard dummy packets without indicating an error. Note that such discard dummy packets without indicating an error. Note that such
recommendation only applies for nodes receiving packets, and that recommendation only applies for nodes receiving packets, and that
nodes designed to only send data may not implement this capability. nodes designed to only send 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 constrained generate such dummy packet. More especially, in constrained
environment sending dummy packets may have too much impact on the environment sending dummy packets may have too much impact on the
device lifetime, 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 4 traffic shaping at the IPsec layer may also
introduce some traffic pattern, and on constrained devices the introduce some traffic pattern, and on constrained 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 fixed
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 SHOULD NOT be part of a agreement between the peers. As a result, it should not be part of a
minimal implementation of ESP. minimal implementation of ESP.
7. ICV 6. ICV
The ICV depends on the cryptographic suite used. Currently [RFC8221] The ICV depends on the cryptographic suite used. Currently [RFC8221]
only recommends cryptographic suites with an ICV which makes the ICV only recommends cryptographic suites with an ICV which makes the ICV
a mandatory field. a mandatory field.
As detailed in [RFC8221] authentication or authenticated encryption As detailed in [RFC8221] authentication or authenticated encryption
are RECOMMENDED and as such the ICV field MUST be present with a size are recommended and as such the ICV field must be present with a size
different from zero. It length is defined by the security different from zero. It length is defined by the security
recommendations only. recommendations only.
8. Cryptographic Suites 7. Cryptographic Suites
The cryptographic suites implemented are an important component of The cryptographic suites implemented are an important component of
ESP. The recommended algorithms to use are expected to evolve over ESP. The recommended algorithms to use are expected to evolve over
time and implementers SHOULD follow the recommendations provided by time and implementers should follow the recommendations provided by
[RFC8221] and updates. [RFC8221] and updates.
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 informational:
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 guidance and recommendations. The chosen encryption security guidance and recommendations. The chosen encryption
algorithm MUST NOT be known vulnerable or weak (see [RFC8221] for algorithm must not be known vulnerable or weak (see [RFC8221] for
outdated ciphers). ESP can be used to authenticate only or to outdated ciphers). ESP can be used to authenticate only or to
encrypt the communication. In the latter case, authenticated encrypt the communication. In the latter case, authenticated
encryption must always be considered [RFC8221]. 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 algorithms that are across reboots, it is recommended to consider algorithms that are
nonce misuse resistant such as, for example, AES-SIV [RFC5297], nonce misuse resistant such as, for example, AES-SIV [RFC5297],
AES-GCM-SIV [RFC8452] or Deoxys-II [DeoxysII]. Note however that AES-GCM-SIV [RFC8452] or Deoxys-II [DeoxysII]. Note however that
currently none of them has yet been defined for ESP. currently none of them has yet been defined for ESP.
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 it is secured. As a result, security SHOULD NOT be deployed that it is secured. As a result, security should not be
weakened 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. Constrained 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 with it
especially considered when devices have limited resources or are are especially considered when devices have limited resources or
using some batteries, in which case the battery determines the are using some batteries, in which case the battery determines
life of the device. The choice of a cryptographic function may the life of the device. The choice of a cryptographic function
consider re-using specific libraries or to take advantage of may consider re-using specific libraries or to take advantage of
hardware acceleration provided by the device. For example, if hardware acceleration provided by the device. For example, if
the device benefits from AES hardware modules and uses AES-CTR, the device benefits from AES hardware modules and uses AES-CTR,
it 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
skipping to change at page 12, line 5 skipping to change at page 11, line 30
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 8. IANA Considerations
There are no IANA consideration for this document. There are no IANA consideration for this document.
10. Security Considerations 9. Security Considerations
Security considerations are those of [RFC4303]. In addition, this Security considerations are those of [RFC4303]. In addition, this
document provided security recommendations and guidance over the document provided security recommendations and guidance over the
implementation choices for each field. 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 with the management of that key. This
include mechanisms to prevent a nonce to repeat for example. When a usually includes mechanisms to prevent a nonce from repeating, for
node is provisioned with a session key that is used across reboot, example. When a node is provisioned with a session key that is used
the implementer MUST ensure that the mechanisms put in place remain across reboot, the implementer must ensure that the mechanisms put in
valid across reboot as well. place remain valid across reboot as well.
It is RECOMMENDED to use ESP in conjunction of key management It is recommended to use ESP in conjunction with 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 for negotiating 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
lifetime. When such mechanisms cannot be implemented and the session lifetime. When such mechanisms cannot be implemented and the session
key is, for example, provisioned, the nodes MUST ensure that keys are key is, for example, provisioned, the nodes must ensure that keys are
not used beyond their lifetime and that the appropriate use of the not used beyond their lifetime and that the appropriate use of the
key remains across reboots - e.g. conditions on counters and nonces key remains across reboots - e.g. conditions on counters 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 guidance 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 10. 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, Thomas Peyrin
valuable comments. In particular Scott Fluhrer suggested to include and Eric Thormarker for their valuable comments. In particular Scott
the rekey index in the SPI as well as the use of transform resilient Fluhrer suggested to include the rekey index in the SPI. Tero
to nonce misuse. Tero Kivinen provided also multiple clarifications Kivinen provided also multiple clarifications and examples of
and examples of deployment ESP within constrained devices with their deployment ESP within constrained devices with their associated
associated optimizations. optimizations. Thomas Peyrin Eric Thormarker and Scott Fluhrer
suggested and clarified the use of transform resilient to nonce
misuse.
12. References 11. References
12.1. Normative References
11.1. 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>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086, "Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005, DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>. <https://www.rfc-editor.org/info/rfc4086>.
skipping to change at page 14, line 5 skipping to change at page 13, line 27
Payload (ESP) and Authentication Header (AH)", RFC 8221, Payload (ESP) and Authentication Header (AH)", RFC 8221,
DOI 10.17487/RFC8221, October 2017, DOI 10.17487/RFC8221, October 2017,
<https://www.rfc-editor.org/info/rfc8221>. <https://www.rfc-editor.org/info/rfc8221>.
[RFC8750] Migault, D., Guggemos, T., and Y. Nir, "Implicit [RFC8750] Migault, D., Guggemos, T., and Y. Nir, "Implicit
Initialization Vector (IV) for Counter-Based Ciphers in Initialization Vector (IV) for Counter-Based Ciphers in
Encapsulating Security Payload (ESP)", RFC 8750, Encapsulating Security Payload (ESP)", RFC 8750,
DOI 10.17487/RFC8750, March 2020, DOI 10.17487/RFC8750, March 2020,
<https://www.rfc-editor.org/info/rfc8750>. <https://www.rfc-editor.org/info/rfc8750>.
12.2. Informative References 11.2. Informative References
[DeoxysII] [DeoxysII] Jeremy, J. J., Ivica, I. N., Thomas, T. P., and Y. S.
Jeremy, J., Ivica, I., Thomas, T., and Y. Yannick, "Deoxys Yannick, "Deoxys v1.41", October 2016,
v1.41", October 2016,
<https://competitions.cr.yp.to/round3/deoxysv141.pdf>. <https://competitions.cr.yp.to/round3/deoxysv141.pdf>.
[I-D.mglt-ipsecme-diet-esp] [I-D.mglt-ipsecme-diet-esp]
Migault, D., Guggemos, T., Bormann, C., and D. Schinazi, Migault, D., Guggemos, T., Bormann, C., and D. Schinazi,
"ESP Header Compression and Diet-ESP", draft-mglt-ipsecme- "ESP Header Compression and Diet-ESP", Work in Progress,
diet-esp-07 (work in progress), March 2019. Internet-Draft, draft-mglt-ipsecme-diet-esp-07, 11 March
2019, <https://www.ietf.org/archive/id/draft-mglt-ipsecme-
diet-esp-07.txt>.
[I-D.mglt-ipsecme-ikev2-diet-esp-extension] [I-D.mglt-ipsecme-ikev2-diet-esp-extension]
Migault, D., Guggemos, T., and D. Schinazi, "Internet Key Migault, D., Guggemos, T., and D. Schinazi, "Internet Key
Exchange version 2 (IKEv2) extension for the ESP Header Exchange version 2 (IKEv2) extension for the ESP Header
Compression (EHC) Strategy", draft-mglt-ipsecme-ikev2- Compression (EHC) Strategy", Work in Progress, Internet-
diet-esp-extension-01 (work in progress), June 2018. Draft, draft-mglt-ipsecme-ikev2-diet-esp-extension-01, 26
June 2018, <https://www.ietf.org/archive/id/draft-mglt-
ipsecme-ikev2-diet-esp-extension-01.txt>.
[I-D.nikander-esp-beet-mode] [I-D.nikander-esp-beet-mode]
Nikander, P. and J. Melen, "A Bound End-to-End Tunnel Nikander, P. and J. Melen, "A Bound End-to-End Tunnel
(BEET) mode for ESP", draft-nikander-esp-beet-mode-09 (BEET) mode for ESP", Work in Progress, Internet-Draft,
(work in progress), August 2008. 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) [RFC5297] Harkins, D., "Synthetic Initialization Vector (SIV)
Authenticated Encryption Using the Advanced Encryption Authenticated Encryption Using the Advanced Encryption
Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
2008, <https://www.rfc-editor.org/info/rfc5297>. 2008, <https://www.rfc-editor.org/info/rfc5297>.
[RFC8452] Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV: [RFC8452] Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV:
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. B. and J. K. Kelsey, "Recommendation for Random
Generation Using Deterministic Random Bit Generators", Number Generation Using Deterministic Random Bit
<https://csrc.nist.gov/publications/detail/sp/800-90a/rev- Generators", <https://csrc.nist.gov/publications/detail/
1/final>. sp/800-90a/rev-1/final>.
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
Tobias Guggemos Tobias Guggemos
LMU Munich LMU Munich
MNM-Team MNM-Team
Oettingenstr. 67 Oettingenstr. 67
80538 Munich, Bavaria 80538 Munich
Germany Germany
EMail: guggemos@mnm-team.org Email: guggemos@mnm-team.org
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