Internet DRAFT - draft-camwinget-tls-ts13-macciphersuites
draft-camwinget-tls-ts13-macciphersuites
TLS N. Cam-Winget
Internet-Draft Cisco Systems
Intended status: Informational J. Visoky
Expires: December 19, 2021 ODVA
June 17, 2021
TLS 1.3 Authentication and Integrity only Cipher Suites
draft-camwinget-tls-ts13-macciphersuites-12
Abstract
This document defines the use of HMAC-only cipher suites for TLS 1.3,
which 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 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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 19, 2021.
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Copyright Notice
Copyright (c) 2021 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
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(https://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
4. Cryptographic Negotiation Using Integrity only Cipher Suites 6
5. Record Payload Protection for Integrity only Cipher Suites . 6
6. Key Schedule when using Integrity only Cipher Suites . . . . 8
7. Error Alerts . . . . . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security and Privacy Considerations . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative Reference . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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
which 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.
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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, this situation will not apply equally
to all use cases (for example, a robotic arm might be used in one
case for a process that does not involve any intellectual property,
but in another case 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 device
(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 [RFC8446] cipher suites 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 is 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 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", "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 HMAC SHA [RFC6234] based cipher suites defined in this
document 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.
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Use cases in the industrial automation industry, while requiring data
integrity, often do not require confidential communications. Mainly,
information 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 on intellectual
property, although nearly every communication interaction has strong
data authenticity requirements.
Another use case which 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. Often times 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 to 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
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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 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 blackbox recording of the safety related network traffic
can only be fulfilled through using authenticity-only ciphers, to be
able to provide the safety related commands to a third party, which
is responsible for the analysis after an accident.
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
update, weather information, controller and pilot communication, and
airline back office communication. Similarly, the Aviation Traffic
Control (ATC) use 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 airplane states, 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 to provide false information to ground
control that might delay flights and damage properties 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 exchange 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
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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 is
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 AEAD
algorithm for record protection and the hash algorithm to use with
the HKDF. These cipher suites are defined in a similar way, but
using 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 Hashed
Message Authentication Code (HMAC) for data integrity protection as
well as its use for HMAC-based Key Derivation Function (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
The 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, the 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", respectively, as referenced in [RFC8446]
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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 = TLSPlaintext.length + 1
(type field) + length_of_padding + 32 (HMAC) =
TLSInnerPlaintext_length + 32 (HMAC)
TLS_SHA384_SHA384: TLSCiphertext.length = TLSPlaintext.length + 1
(type field) + length_of_padding + 48 (HMAC) =
TLSInnerPlaintext_length + 48 (HMAC)
Note that TLSInnerPlaintext_length is not defined as an explicit
field in [RFC8446]; this refers to the length of the encoded
TLSInnterPlaintext structure
The resulting protected_record is the concatenation of the
TLSInnerPlaintext with the resulting HMAC. Note this analogous to
the "encrypted_record" of [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,
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.
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6. Key Schedule when using Integrity only Cipher Suites
The key derivation process for Integrity only Cipher Suites remains
the same as 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 RFC 8446, section 7.3.
The key lengths and 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 |
+-------------------+------------+-----------+
7. Error Alerts
The error alerts as defined by [RFC8446] remains the same, in
particular:
bad_record_mac: This alert can also occur for a record whose message
authentication code can not 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 can not 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 granted registration the following specifically for this
document within the TLS Cipher Suites Registry:
TLS_SHA256_SHA256 {0xC0, 0xB4} cipher suite and TLS_SHA384_SHA384
{0xC0, 0xB5} cipher suite.
Note that both of these cipher suites are registered with the DTLS-OK
column set to Y and the Recommended column set to N
No further IANA action is requested by this document.
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9. Security and Privacy Considerations
In general, except for confidentiality and privacy, the security
considerations detailed in [RFC8446] and in [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; and 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 suite, and the handshake also is not encrypted.
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 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 there applied.
Application protocols which 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 trunction length further described
in [RFC4868] also apply. Until further cryptanalysis demonstrate
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.
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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. 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.
11. References
11.1. Normative References
[BCK1] Bellare, M., Canetti, R., and H. Krawczyk, "Keyed Hash
Functions and Message Authentication",
<https://cseweb.ucsd.edu/~mihir/papers/kmd5.pdf>.
[DTLS13] IETF Internet Drafts editor,
"https://tools.ietf.org/id/draft-ietf-tls-dtls13-38.txt".
[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>.
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[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>.
11.2. Informative Reference
[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>.
Authors' Addresses
Nancy Cam-Winget
Cisco Systems
3550 Cisco Way
San Jose, CA 95134
USA
Email: ncamwing@cisco.com
Jack Visoky
ODVA
1 Allen Bradley Dr
Mayfield Heights, OH 44124
USA
Email: jmvisoky@ra.rockwell.com
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