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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12           Informational
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


   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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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
   "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

   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

   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

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",

   [DTLS13]   IETF Internet Drafts editor,

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

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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,

   [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,

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,

Authors' Addresses

   Nancy Cam-Winget
   Cisco Systems
   3550 Cisco Way
   San Jose, CA  95134

   Email: ncamwing@cisco.com

   Jack Visoky
   1 Allen Bradley Dr
   Mayfield Heights, OH  44124

   Email: jmvisoky@ra.rockwell.com

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