| Light-Weight Implementation Guidance (lwig) | D. Migault |
| Internet-Draft | Ericsson |
| Intended status: Informational | T. Guggemos |
| Expires: April 24, 2019 | LMU Munich |
| October 21, 2018 |
Minimal ESP
draft-mglt-lwig-minimal-esp-07
This document describes a minimal implementation of the IP Encapsulation Security Payload (ESP) defined in RFC 4303. Its purpose is to enable implementation of ESP with a minimal set of options to remain compatible with ESP as described in RFC 4303. A minimal version of ESP is not intended to become a replacement of the RFC 4303 ESP, but instead to enable a limited implementation to interoperate with implementations of RFC 4303 ESP.
This document describes what is required from RFC 4303 ESP as well as various ways to optimize compliance with RFC 4303 ESP.
This document does not update or modify RFC 4303, but provides a compact description of how to implement the minimal version of the protocol. If this document and RFC 4303 conflicts then RFC 4303 is the authoritative description.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 24, 2019.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
ESP [RFC4303] is part of the IPsec suite protocol [RFC4301]. IPsec is used to provide confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity) and limited traffic flow confidentiality.
Figure 1 describes an ESP Packet. Currently ESP is implemented in the kernel of major multi purpose Operating Systems (OS). The ESP and IPsec suite is usually implemented in a complete way to fit multiple purpose usage of these OS. However, completeness of the IPsec suite as well as multi purpose scope of these OS is often performed at the expense of resources, or a lack of performance. As a result, constraint devices are likely to have their own implementation of ESP optimized and adapted to their specificities. With the adoption of IPsec by IoT devices with minimal IKEv2 [RFC7815] and ESP Header Compression (EHC) with [I-D.mglt-ipsecme-diet-esp] or [I-D.mglt-ipsecme-ikev2-diet-esp-extension], it becomes crucial that ESP implementation designed for constraint devices remain inter-operable with the standard ESP implementation to avoid a fragmented usage of ESP. This document describes the the minimal properties and ESP implementation needs to meet.
For each field of the ESP packet represented in Figure 1 this document provides recommendations and guidance for minimal implementations. The primary purpose of Minimal ESP is to remain interoperable with other nodes implementing RFC 4303 ESP, while limiting the standard complexity of the implementation.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---- | Security Parameters Index (SPI) | ^Int. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov- | Sequence Number | |ered +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ---- | Payload Data* (variable) | | ^ ~ ~ | | | | |Conf. + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov- | | Padding (0-255 bytes) | |ered* +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Pad Length | Next Header | v v +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------ | Integrity Check Value-ICV (variable) | ~ ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: ESP Packet Description
According to the [RFC4303], the SPI is a mandatory 32 bits field and is not allowed to be removed.
The SPI has a local significance to index the Security Association (SA). From [RFC4301] section 4.1, nodes supporting only unicast communications can index their SA only using the SPI. On the other hand, nodes supporting multicast communications must also use the IP addresses and thus SA lookup needs to be performed using the longest match.
For nodes supporting only unicast communications, it is RECOMMENDED to index SA with the SPI only. Some other local constraints on the node may require a combination of the SPI as well as other parameters to index the SA.
It is RECOMMENDED to randomly generate the SPI indexing each inbound session. A random generation provides a stateless way to generate the SPIs, while keeping the probability of collision between SPIs relatively low. In case of collision, the SPI is simply re-generated.
However, for some constraint nodes, generating a random SPI may consume to much resource, in which case SPI can be generated using predictable functions or even a fix value. In fact, the SPI does not need to be random. Generating non random SPI MAY lead to privacy and security concerns. As a result, this alternative should be considered for devices that would be strongly impacted by the generation of a random SPI and after understanding the privacy and security impact of generating non random SPI.
When a constraint node uses fix value for SPIs, it imposes some limitations on the number of inbound SA. This limitation can be alleviate by how the SA look up is performed. When fix SPI are used, it is RECOMMENDED the constraint node has as many SPI values as ESP session per host IP address, and that SA lookup includes the IP addresses.
Note that SPI value is used only for inbound traffic, as such the SPI negotiated with IKEv2 [RFC7296] or [RFC7815] by a peer, is the value used by the remote peer when its sends traffic. As SPI are only used for inbound traffic by the peer, this allows each peer to manage the set of SPIs used for its inbound traffic.
The use of fix SPI MUST NOT be considered as a way to avoid strong random generators. Such generator will be required in order to provide strong cryptographic protection and follow the randomness requirements for security described in [RFC4086]. Instead, the use of a fix SPI should only considered as a way to overcome the resource limitations of the node, when this is feasible.
The use of a limited number of fix SPI or non random SPIs come with security or privacy drawbacks. Typically, a passive attacker may derive information such as the number of constraint devices connecting the remote peer, and in conjunction with data rate, the attacker may eventually determine the application the constraint device is associated to. If the SPI is fixed by a manufacturer or by some software application, the SPI may leak in an obvious way the type of sensor, the application involved or the model of the constraint device. When identification of the application or the hardware is associated to privacy, the SPI MUST be randomly generated. However, one needs to realize that in this case this is likely to be sufficient and a thorough privacy analysis is required. More specifically, traffic pattern MAY leak sufficient information in itself. In other words, privacy leakage is a complex and the use of random SPI is unlikely to be sufficient.
As the general recommendation is to randomly generate the SPI, constraint device that will use a limited number of fix SPI are expected to be very constraint devices with very limited capabilities, where the use of randomly generated SPI may prevent them to implement IPsec. In this case the ability to provision non random SPI enables these devices to secure their communications. These devices, due to there limitations, are expected to provide limited information and how the use of non random SPI impacts privacy requires further analysis. Typically temperature sensors, wind sensors, used outdoor do not leak privacy sensitive information. When used indoor, the privacy information is stored in the encrypted data and as such does not leak privacy.
As far as security is concerned, revealing the type of application or model of the constraint device could be used to identify the vulnerabilities the constraint device is subject to. This is especially sensitive for constraint devices where patches or software updates will be challenging to operate. As a result, these devices may remain vulnerable for relatively long period. In addition, predictable SPI enable an attacker to forge packets with a valid SPI. Such packet will not be rejected due to an SPI mismatch, but instead after the signature check which requires more resource and thus make DoS more efficient, especially for devices powered by batteries.
Values 0-255 SHOULD NOT be used. Values 1-255 are reserved and 0 is only allowed to be used internal and it MUST NOT be send on the wire.
[RFC4303] mentions :
According to [RFC4303], the Sequence Number (SN) is a mandatory 32 bits field in the packet.
The SN is set by the sender so the receiver can implement anti-replay protection. The SN is derived from any strictly increasing function that guarantees: if packet B is sent after packet A, then SN of packet B is strictly greater then the SN of packet A.
Some constraint devices may establish communication with specific devices, like a specific gateway, or nodes similar to them. As a result, the sender may know whereas the receiver implements anti-replay protection or not. Even though the sender may know the receiver does not implement anti replay protection, the sender MUST implement a always increasing function to generate the SN.
Usually, SN is generated by incrementing a counter for each packet sent. A constraint device may avoid maintaining this context and use another source that is known to always increase. Typically, constraint nodes using 802.15.4 Time Slotted Channel Hopping (TSCH), whose communication is heavily dependent on time, can take advantage of their clock to generate the SN. This 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, one should take care that packets associated to a given SA are not sent with the same time value.
For inbound traffic, it is RECOMMENDED to provide a anti-replay protection,and the size of the window depends on the ability of the network to deliver packet out of order. As a result, in environment where out of order packets is not possible the window size can be set to one. However, while RECOMMENDED, there is no requirements to implement an anti replay protection mechanism implemented by IPsec. A node MAY drop anti-replay protection provided by IPsec, and instead implement its own internal mechanism.
[RFC4303] mentions :
The purpose of padding is to respect the 32 bit alignment of ESP. ESP MUST have at least one padding byte Pad Length that indicates the padding length. ESP padding bytes are generated by a succession of unsigned bytes starting with 1, 2, 3 with the last byte set to Pad Length, where Pad Length designates the length of the padding bytes.
Checking the padding structure is not mandatory, so the constraint device may not proceed to such checks, however, in order to interoperate with existing ESP implementations, it MUST build the padding bytes as recommended by ESP.
In some situation the padding bytes may take a fix value. This would typically be the case when the Data Payload is of fix size.
[RFC4303] mentions :
ESP [RFC4303] also provides Traffic Flow Confidentiality (TFC) as a way to perform padding to hide traffic characteristics, which differs from respecting a 32 bit alignment. TFC is not mandatory and MUST be negotiated with the SA management protocol. TFC has not yet being widely adopted for standard ESP traffic. One possible reason is that it requires to shape the traffic according to one traffic pattern that needs to be maintained. This is likely to require extra processing as well as providing a "well recognized" traffic shape which could end up being counterproductive. As such TFC is not expected to be supported by a minimal ESP implementation.
As a result, TFC cannot not be enabled with minimal, and communication protection that were relying on TFC will be more sensitive to traffic shaping. This could expose the application as well as the devices used to a passive monitoring attacker. Such information could be used by the attacker in case a vulnerability is disclosed on the specific device. In addition, some application use - such as health applications - may also reveal important privacy oriented informations.
Some constraint nodes that have limited battery life time may also prefer avoiding sending extra padding bytes. However the same nodes may also be very specific to an application and device. As a result, they are also likely to be the main target for traffic shaping. In most cases, the payload carried by these nodes is quite small, and the standard padding mechanism may also be used as an alternative to TFC, with a sufficient trade off between the require energy to send additional payload and the exposure to traffic shaping attacks. In addition, the information leaked by the traffic shaping may also be addressed by the application level. For example, it is preferred to have a sensor sending some information at regular time interval, rather when an specific event is happening. Typically a sensor monitoring the temperature, or a door is expected to send regularly the information - i.e. the temperature of the room or whether the door is closed or open) instead of only sending the information when the temperature has raised or when the door is being opened.
According to [RFC4303], the Next Header is a mandatory 8 bits field in the packet. Next header is intended to specify the data contained in the payload as well as dummy packet. In addition, the Next Header may also carry an indication on how to process the packet [I-D.nikander-esp-beet-mode].
The ability to generate and receive dummy packet is required by [RFC4303]. For interoperability, it is RECOMMENDED a minimal ESP implementation discards dummy packets. Note that such recommendation only applies for nodes receiving packets, and that nodes designed to only send data may not implement this capability.
As the generation of dummy packets is subject to local management and based on a per-SA basis, a minimal ESP implementation may not generate such dummy packet. More especially, in constraint environment sending dummy packets may have too much impact on the device life time, and so may be avoided. On the other hand, constraint nodes may be dedicated to specific applications, in which case, traffic pattern may expose the application or the type of node. For these nodes, not sending dummy packet may have some privacy implication that needs to be measured. However, for the same reasons exposed in Section 5 traffic shaping at the IPsec layer may also introduce some traffic pattern, and on constraint devices the application is probably the most appropriated layer to limit the risk of leaking information by traffic shaping.
In some cases, devices are dedicated to a single application or a single transport protocol, in which case, the Next Header has a fix value.
Specific processing indications have not been standardized yet [I-D.nikander-esp-beet-mode] and is expected to result from an agreement between the peers. As a result, it is not expected to be part of a minimal implementation of ESP.
[RFC4303] mentions :
The ICV depends on the crypto-suite used. Currently recommended [RFC8221] only recommend crypto-suites with an ICV which makes the ICV a mandatory field.
As detailed in Section 8 we recommend to use authentication, the ICV field is expected to be present that is to say with a size different from zero. This makes it a mandatory field which size is defined by the security recommendations only.
[RFC4303] mentions :
The cryptographic suites implemented are an important component of ESP. The recommended suites to use are expect to evolve over time and implementer SHOULD follow the recommendations provided by [RFC8221] and updates. Recommendations are provided for standard nodes as well as constraint nodes.
This section lists some of the criteria that may be considered. The list is not expected to be exhaustive and may also evolve overtime. As a result, the list is provided as indicative:
There are no IANA consideration for this document.
Security considerations are those of [RFC4303]. In addition, this document provided security recommendations an guidances over the implementation choices for each fields.
The authors would like to thank Daniel Palomares, Scott Fluhrer, Tero Kivinen, Valery Smyslov, Yoav Nir, Michael Richardson for their valuable comments.
| [I-D.mglt-ipsecme-diet-esp] | Migault, D., Guggemos, T., Bormann, C. and D. Schinazi, "ESP Header Compression and Diet-ESP", Internet-Draft draft-mglt-ipsecme-diet-esp-06, May 2018. |
| [I-D.mglt-ipsecme-ikev2-diet-esp-extension] | Migault, D., Guggemos, T. and D. Schinazi, "Internet Key Exchange version 2 (IKEv2) extension for the ESP Header Compression (EHC) Strategy", Internet-Draft draft-mglt-ipsecme-ikev2-diet-esp-extension-01, June 2018. |
| [I-D.nikander-esp-beet-mode] | Nikander, P. and J. Melen, "A Bound End-to-End Tunnel (BEET) mode for ESP", Internet-Draft draft-nikander-esp-beet-mode-09, August 2008. |
[RFC Editor: This section is to be removed before publication]
-00: First version published.
-01: Clarified description
-02: Clarified description