Independent Submission N. Cam-Winget

Independent Submission N. Cam-Winget
Request for Comments: 9150 Cisco Systems
Category: Informational J. Visoky
ISSN: 2070-1721 ODVA
April 2022

TLS 1.3 Authentication and Integrity-Only Cipher Suites

Abstract

This document defines the use of cipher suites for TLS 1.3 based on
Hashed Message Authentication Code (HMAC). Using these cipher suites
provides server and, optionally, mutual authentication and data
authenticity, but not data confidentiality. Cipher suites with these
properties are not of general applicability, but there are use cases,
specifically in Internet of Things (IoT) and constrained
environments, that do not require confidentiality of exchanged
messages while still requiring integrity protection, server
authentication, and optional client authentication. This document
gives examples of such use cases, with the caveat that prior to using
these integrity-only cipher suites, a threat model for the situation
at hand is needed, and a threat analysis must be performed within
that model to determine whether the use of integrity-only cipher
suites is appropriate. The approach described in this document is
not endorsed by the IETF and does not have IETF consensus, but it is
presented here to enable interoperable implementation of a reduced-
security mechanism that provides authentication and message integrity
without supporting confidentiality.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9150.

Copyright Notice

Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents

1. Introduction
2. Terminology
3. Applicability Statement
4. Cryptographic Negotiation Using Integrity-Only Cipher Suites
5. Record Payload Protection for Integrity-Only Cipher Suites
6. Key Schedule when Using Integrity-Only Cipher Suites
7. Error Alerts
8. IANA Considerations
9. Security and Privacy Considerations
10. References
10.1. Normative References
10.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

There are several use cases in which communications privacy is not
strictly needed, although authenticity of the communications
transport is still very important. For example, within the
industrial automation space, there could be TCP or UDP communications
that command a robotic arm to move a certain distance at a certain
speed. Without authenticity guarantees, an attacker could modify the
packets to change the movement of the robotic arm, potentially
causing physical damage. However, the motion control commands are
not always considered to be sensitive information, and thus there is
no requirement to provide confidentiality. Another Internet of
Things (IoT) example with no strong requirement for confidentiality
is the reporting of weather information; however, message
authenticity is required to ensure integrity of the message.

There is no requirement to encrypt messages in environments where the
information is not confidential, such as when there is no
intellectual property associated with the processes, or where the
threat model does not indicate any outsider attacks (such as in a
closed system). Note, however, that this situation will not apply
equally to all use cases (for example, in one case a robotic arm
might be used for a process that does not involve any intellectual
property but in another case might be used in a different process
that does contain intellectual property). Therefore, it is important
that a user or system developer carefully examine both the
sensitivity of the data and the system threat model to determine the
need for encryption before deploying equipment and security
protections.

Besides having a strong need for authenticity and no need for
confidentiality, many of these systems also have a strong requirement
for low latency. Furthermore, several classes of IoT devices
(industrial or otherwise) have limited processing capability.
However, these IoT systems still gain great benefit from leveraging
TLS 1.3 for secure communications. Given the reduced need for
confidentiality, TLS 1.3 cipher suites [RFC8446] that maintain data
integrity without confidentiality are described in this document.
These cipher suites are not meant for general use, as they do not
meet the confidentiality and privacy goals of TLS. They should only
be used in cases where confidentiality and privacy are not a concern
and there are constraints on using cipher suites that provide the
full set of security properties. The approach described in this
document is not endorsed by the IETF and does not have IETF
consensus, but it is presented here to enable interoperable
implementation of a reduced-security mechanism that provides
authentication and message integrity with supporting confidentiality.

2. Terminology

This document adopts the conventions for normative language to
provide clarity of instructions to the implementer. 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.

3. Applicability Statement

The two cipher suites defined in this document, which are based on
Hashed Message Authentication Code (HMAC) SHAs [RFC6234], are
intended for a small limited set of applications where
confidentiality requirements are relaxed and the need to minimize the
number of cryptographic algorithms is prioritized. This section
describes some of those applicable use cases.

Use cases in the industrial automation industry, while requiring data
integrity, often do not require confidential communications. Mainly,
data communicated to unmanned machines to execute repetitive tasks
does not convey private information. For example, there could be a
system with a robotic arm that paints the body of a car. This
equipment is used within a car manufacturing plant and is just one
piece of equipment in a multi-step manufacturing process. The
movements of this robotic arm are likely not a trade secret or
sensitive intellectual property, although some portions of the
manufacturing of the car might very well contain sensitive
intellectual property. Even the mixture for the paint itself might
be confidential, but the mixing is done by a completely different
piece of equipment and therefore communication to/from that equipment
can be encrypted without requiring the communication to/from the
robotic arm to be encrypted. Modern manufacturing often has
segmented equipment with different levels of risk related to
intellectual property, although nearly every communication
interaction has strong data authenticity requirements.

Another use case that is closely related is that of fine-grained time
updates. Motion systems often rely on time synchronization to ensure
proper execution. Time updates are essentially public; there is no
threat from an attacker knowing the time update information. This
should make intuitive sense to those not familiar with these
applications; rarely if ever does time information present a serious
attack surface dealing with privacy. However, the authenticity is
still quite important. The consequences of maliciously modified time
data can vary from mere denial of service to actual physical damage,
depending on the particular situation and attacker capability. As
these time synchronization updates are very fine-grained, it is again
important for latency to be very low.

A third use case deals with data related to alarms. Industrial
control sensing equipment can be configured to send alarm information
when it meets certain conditions -- for example, temperature goes
above or below a given threshold. Oftentimes, this data is used to
detect certain out-of-tolerance conditions, allowing an operator or
automated system to take corrective action. Once again, in many
systems the reading of this data doesn't grant the attacker
information that can be exploited; it is generally just information
regarding the physical state of the system. At the same time, being
able to modify this data would allow an attacker to either trigger
alarms falsely or cover up evidence of an attack that might allow for
detection of their malicious activity. Furthermore, sensors are
often low-powered devices that might struggle to process encrypted
and authenticated data. These sensors might be very cost sensitive
such that there is not enough processing power for data encryption.
Sending data that is just authenticated but not encrypted eases the
burden placed on these devices yet still allows the data to be
protected against any tampering threats. A user can always choose to
pay more for a sensor with encryption capability, but for some, data
authenticity will be sufficient.

A fourth use case considers the protection of commands in the railway
industry. In railway control systems, no confidentiality
requirements are applied for the command exchange between an
interlocking controller and a railway equipment controller (for
instance, a railway point controller of a tram track where the
position of the controlled point is publicly available). However,
protecting the integrity and authenticity of those commands is vital;
otherwise, an adversary could change the target position of the point
by modifying the commands, which consequently could lead to the
derailment of a passing train. Furthermore, requirements for
providing flight-data recording of the safety-related network traffic
can only be fulfilled through using authenticity-only ciphers as,
typically, the recording is used by a third party responsible for the
analysis after an accident. The analysis requires such third party
to extract the safety-related commands from the recording.

The fifth use case deals with data related to civil aviation
airplanes and ground communication. Pilots can send and receive
messages to/from ground control, such as airplane route-of-flight
updates, weather information, controller and pilot communication, and
airline back-office communication. Similarly, the Air Traffic
Control (ATC) service uses air-to-ground communication to receive the
surveillance data that relies on (is dependent on) downlink reports
from an airplane's avionics. This communication occurs automatically
in accordance with contracts established between the ATC ground
system and the airplane's avionics. Reports can be sent whenever
specific events occur or specific time intervals are reached. In
many systems, the reading of this data doesn't grant the attacker
information that can be exploited; it is generally just information
regarding the states of the airplane, controller pilot communication,
meteorological information, etc. At the same time, being able to
modify this data would allow an attacker to either put aircraft in
the wrong flight trajectory or provide false information to ground
control that might delay flights, damage property, or harm life.
Sending data that is not encrypted but is authenticated allows the
data to be protected against any tampering threats. Data
authenticity is sufficient for the air traffic, weather, and
surveillance information exchanges between airplanes and the ground
systems.

The above use cases describe the requirements where confidentiality
is not needed and/or interferes with other requirements. Some of
these use cases are based on devices that come with a small runtime
memory footprint and reduced processing power; therefore, the need to
minimize the number of cryptographic algorithms used is a priority.
Despite this, it is noted that memory, performance, and processing
power implications of any given algorithm or set of algorithms are
highly dependent on hardware and software architecture. Therefore,
although these cipher suites may provide performance benefits, they
will not necessarily provide these benefits in all cases on all
platforms. Furthermore, in some use cases, third-party inspection of
data is specifically needed, which is also supported through the lack
of confidentiality mechanisms.

4. Cryptographic Negotiation Using Integrity-Only Cipher Suites

The cryptographic negotiation as specified in [RFC8446],
Section 4.1.1 remains the same, with the inclusion of the following
cipher suites:

TLS_SHA256_SHA256 {0xC0,0xB4}

TLS_SHA384_SHA384 {0xC0,0xB5}

As defined in [RFC8446], TLS 1.3 cipher suites denote the
Authenticated Encryption with Associated Data (AEAD) algorithm for
record protection and the hash algorithm to use with the HMAC-based
Key Derivation Function (HKDF). The cipher suites provided by this
document are defined in a similar way, but they use the HMAC
authentication tag to model the AEAD interface, as only an HMAC is
provided for record protection (without encryption). These cipher
suites allow the use of SHA-256 or SHA-384 as the HMAC for data
integrity protection as well as its use for the HKDF. The
authentication mechanisms remain unchanged with the intent to only
update the cipher suites to relax the need for confidentiality.

Given that these cipher suites do not support confidentiality, they
MUST NOT be used with authentication and key exchange methods that
rely on confidentiality.

5. Record Payload Protection for Integrity-Only Cipher Suites

Record payload protection as defined in [RFC8446] is retained in
modified form when integrity-only cipher suites are used. Note that
due to the purposeful use of hash algorithms, instead of AEAD
algorithms, confidentiality protection of the record payload is not
provided. This section describes the mapping of record payload
structures when integrity-only cipher suites are employed.

Given that there is no encryption to be done at the record layer, the
operations "Protect" and "Unprotect" take the place of "AEAD-Encrypt"
and "AEAD-Decrypt" [RFC8446], respectively.

As integrity protection is provided over the full record, the
encrypted_record in the TLSCiphertext along with the additional_data
input to protected_data (termed AEADEncrypted data in [RFC8446]) as
defined in Section 5.2 of [RFC8446] remain the same. The
TLSCiphertext.length for the integrity cipher suites will be:

TLS_SHA256_SHA256:
TLSCiphertext.length = TLSInnerPlaintext_length + 32

TLS_SHA384_SHA384:
TLSCiphertext.length = TLSInnerPlaintext_length + 48

Note that TLSInnerPlaintext_length is not defined as an explicit
field in [RFC8446]; this refers to the length of the encoded
TLSInnerPlaintext structure.

The resulting protected_record is the concatenation of the
TLSInnerPlaintext with the resulting HMAC. Note that this is
analogous to the "encrypted_record" as defined in [RFC8446], although
it is referred to as a "protected_record" because of the lack of
confidentiality via encryption. With this mapping, the record
validation order as defined in Section 5.2 of [RFC8446] remains the
same. That is, the encrypted_record field of TLSCiphertext is set
to:

encrypted_record = TLS13-HMAC-Protected = TLSInnerPlaintext ||
HMAC(write_key, nonce || additional_data || TLSInnerPlaintext)

Here, "nonce" refers to the per-record nonce described in Section 5.3
of [RFC8446].

For DTLS 1.3, the DTLSCiphertext is set to:

encrypted_record = DTLS13-HMAC-Protected = DTLSInnerPlaintext ||
HMAC(write_key, nonce || additional_data || DTLSInnerPlaintext)

The DTLS "nonce" refers to the per-record nonce described in
Section 4.2.2 of [DTLS13].

The Protect and Unprotect operations provide the integrity protection
using HMAC SHA-256 or HMAC SHA-384 as described in [RFC6234].

Due to the lack of encryption of the plaintext, record padding does
not provide any obfuscation as to the plaintext size, although it can
be optionally included.

6. Key Schedule when Using Integrity-Only Cipher Suites

The key derivation process for integrity-only cipher suites remains
the same as that defined in [RFC8446]. The only difference is that
the keys used to protect the tunnel apply to the negotiated HMAC
SHA-256 or HMAC SHA-384 ciphers. Note that the traffic key material
(client_write_key, client_write_iv, server_write_key, and
server_write_iv) MUST be calculated as per [RFC8446], Section 7.3.
The key lengths and Initialization Vectors (IVs) for these cipher
suites are according to the hash output lengths. In other words, the
following key lengths and IV lengths SHALL be:

+===================+============+===========+
| Cipher Suite | Key Length | IV Length |
+===================+============+===========+
| TLS_SHA256_SHA256 | 32 Bytes | 32 Bytes |
+-------------------+------------+-----------+
| TLS_SHA384_SHA384 | 48 Bytes | 48 Bytes |
+-------------------+------------+-----------+

Table 1

7. Error Alerts

The error alerts as defined by [RFC8446] remain the same; in
particular:

bad_record_mac: This alert can also occur for a record whose message
authentication code cannot be validated. Since these cipher
suites do not involve record encryption, this alert will only
occur when the HMAC fails to verify.

decrypt_error: This alert, as described in [RFC8446], Section 6.2,
occurs when the signature or message authentication code cannot be
validated. Note that this error is only sent during the
handshake, not for an error in validating record HMACs.

8. IANA Considerations

IANA has registered the following cipher suites, defined by this
document, in the "TLS Cipher Suites" registry:

+===========+===================+=========+=============+
| Value | Description | DTLS-OK | Recommended |
+===========+===================+=========+=============+
| 0xC0,0xB4 | TLS_SHA256_SHA256 | Y | N |
+-----------+-------------------+---------+-------------+
| 0xC0,0xB5 | TLS_SHA384_SHA384 | Y | N |
+-----------+-------------------+---------+-------------+

Table 2

9. Security and Privacy Considerations

In general, except for confidentiality and privacy, the security
considerations detailed in [RFC8446] and [RFC5246] apply to this
document. Furthermore, as the cipher suites described in this
document do not provide any confidentiality, it is important that
they only be used in cases where there are no confidentiality or
privacy requirements and concerns; the runtime memory requirements
can accommodate support for authenticity-only cryptographic
constructs.

With the lack of data encryption specified in this specification, no
confidentiality or privacy is provided for the data transported via
the TLS session. That is, the record layer is not encrypted when
using these cipher suites, nor is the handshake. To highlight the
loss of privacy, the information carried in the TLS handshake, which
includes both the server and client certificates, while integrity
protected, will be sent unencrypted. Similarly, other TLS extensions
that may be carried in the server's EncryptedExtensions message will
only be integrity protected without provisions for confidentiality.
Furthermore, with this lack of confidentiality, any private Pre-
Shared Key (PSK) data MUST NOT be sent in the handshake while using
these cipher suites. However, as PSKs may be loaded externally,
these cipher suites can be used with the 0-RTT handshake defined in
[RFC8446], with the same security implications discussed therein
applied.

Application protocols that build on TLS or DTLS for protection (e.g.,
HTTP) may have implicit assumptions of data confidentiality. Any
assumption of data confidentiality is invalidated by the use of these
cipher suites, as no data confidentiality is provided. This applies
to any data sent over the application-data channel (e.g., bearer
tokens in HTTP), as data requiring confidentiality MUST NOT be sent
using these cipher suites.

Limits on key usage for AEAD-based ciphers are described in
[RFC8446]. However, as the cipher suites discussed here are not
AEAD, those same limits do not apply. The general security
properties of HMACs discussed in [RFC2104] and [BCK1] apply.
Additionally, security considerations on the algorithm's strength
based on the HMAC key length and truncation length further described
in [RFC4868] also apply. Until further cryptanalysis demonstrates
limitations on key usage for HMACs, general practice for key usage
recommends that implementations place limits on the lifetime of the
HMAC keys and invoke a key update as described in [RFC8446] prior to
reaching this limit.

DTLS 1.3 defines a mechanism for encrypting the DTLS record sequence
numbers. However, as these cipher suites do not utilize encryption,
the record sequence numbers are sent unencrypted. That is, the
procedure for DTLS record sequence number protection is to apply no
protection for these cipher suites.

Given the lack of confidentiality, these cipher suites MUST NOT be
enabled by default. As these cipher suites are meant to serve the
IoT market, it is important that any IoT endpoint that uses them be
explicitly configured with a policy of non-confidential
communications.

10. References

10.1. Normative References

[BCK1] Bellare, M., Canetti, R., and H. Krawczyk, "Keying Hash
Functions for Message Authentication",
DOI 10.1007/3-540-68697-5_1, 1996,
<https://link.springer.com/
chapter/10.1007/3-540-68697-5_1>.

[DTLS13] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4868] Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-
384, and HMAC-SHA-512 with IPsec", RFC 4868,
DOI 10.17487/RFC4868, May 2007,
<https://www.rfc-editor.org/info/rfc4868>.

[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

10.2. Informative References

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

Acknowledgements

The authors would like to acknowledge the work done by Industrial
Communications Standards Groups (such as ODVA) as the motivation for
this document. We would also like to thank Steffen Fries for
providing a fourth use case and Madhu Niraula for a fifth use case.
In addition, we are grateful for the advice and feedback from Joe
Salowey, Blake Anderson, David McGrew, Clement Zeller, and Peter Wu.

Authors' Addresses

Nancy Cam-Winget
Cisco Systems
3550 Cisco Way
San Jose, CA 95134
United States of America
Email: [email protected]

Jack Visoky
ODVA
1 Allen Bradley Dr
Mayfield Heights, OH 44124
United States of America
Email: [email protected]