Modbus Serial Link Communication Security Protocol Reference Specification and implementation guide
draft-liu-iotops-modbus-seriallink-sec-spec-07

iotops                                                       P. Liu, Ed.
Internet-Draft                                              R. Yang, Ed.
Intended status: Informational                      Pengcheng Laboratory
Expires: 14 February 2027                                   R. Chen, Ed.
                                                            China Mobile
                                                              R. Fu, Ed.
                                                           China Telecom
                                                           Y. Zhang, Ed.
                                                    Pengcheng Laboratory
                                                          13 August 2026

      Modbus Serial Link Communication Security Protocol Reference
                 Specification and implementation guide
             draft-liu-iotops-modbus-seriallink-sec-spec-07

Abstract

   The Modbus TCP protocol has adopted TLS-based security standards;
   however, Modbus serial communication over EIA/TIA-485 multi-point
   systems, commonly used in 2-wire or 4-wire configurations, lacks
   standardized security mechanisms.  These systems support cable
   lengths exceeding 1000m at baud rates up to 9600 bit/s with AWG26 or
   thicker cables, while Category 5 cables can reach up to 600m.  As an
   application layer protocol, despite its widespread application, the
   absence of encryption and authentication in Modbus protocol via
   serial links exposes plaintext data to risks such as MIM
   interception, modification under attacks such as side-channel
   analysis etc., particularly in long-distance or bridged network
   scenarios.  Enhancing Modbus serial link security requires
   introducing proper encryption and authentication methods tailored to
   varied deployment environments onsidering the characteristics of
   serial links.  A proposed security standard guide outlines
   lightweight encryption and authentication mechanisms to improve
   confidentiality and integrity while maintaining compatibility with
   existing Modbus devices, offering a practical upgrade path for secure
   industrial control systems.

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/.

Liu, et al.             Expires 14 February 2027                [Page 1]
Internet-Draft  Security Protocol for Modbus Serial Link     August 2026

   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 14 February 2027.

Copyright Notice

   Copyright (c) 2026 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   2
   2.  Security Considerations . . . . . . . . . . . . . . . . . . .   2
   3.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Normative References  . . . . . . . . . . . . . . . . . .   3
     3.2.  Informative References  . . . . . . . . . . . . . . . . .   3
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   4

1.  IANA Considerations

   This memo includes no request to IANA.

2.  Security Considerations

   This specification defines the reference native security protocol
   specification for the case with pure serial link communication, which
   (e.g., RS485-based Modbus) remains an unresolved challenge.  Serial
   Modbus transmits data in plaintext, leaving it susceptible to
   interception, modification, and hardware-based side-channel attacks.
   There are currently no formal standards addressing security for
   Modbus over RS485, which is still widely used due to its simplicity
   and long-distance capabilities.  Currently, for any EIA/TIA-485
   multi-point system, whether it is a 2-wire or 4-wire configuration,
   the maximum length for cables with a maximum baud rate of 9600bit/s
   and AWG26 (or thicker) specifications can reach over 1000m.  For
   RS485 Modbus, a sufficiently wide cable diameter should allow for a

Liu, et al.             Expires 14 February 2027                [Page 2]
Internet-Draft  Security Protocol for Modbus Serial Link     August 2026

   maximum length of over 1000m, and for RS485 Modbus using Category 5
   cables, the maximum length can reach 600m.  This lack of encryption
   and authentication mechanisms poses risks, particularly in scenarios
   involving relays or bridged networks.  While Modbus TCP has seen
   notable advancements in security through TLS-based solutions, serial
   Modbus communication requires more attention.  Introducing
   lightweight encryption and authentication mechanisms for serial links
   could provide a practical way to enhance security and protect legacy
   systems without significant infrastructure changes.  Therefore, it is
   very necessary and valuable to improve and enhance the communication
   security of Modbus protocol under serial link mode, for reference by
   relevant institutions and organizations in various industries, in
   order to ensure the practical application security of various
   industrial control systems.  With the introduction of the Modbus
   serial link security standard, the introduction of encryption and
   authentication mechanisms in the serial link communication channel of
   the Modbus protocol significantly improves its security, providing a
   relatively simple and direct upgrade path for existing devices that
   use Modbus extensively.

3.  References

3.1.  Normative References

   [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>.

   [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>.

3.2.  Informative References

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5280>.

   [RFC8439]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
              <https://www.rfc-editor.org/rfc/rfc8439>.

   [IEC62443-3-3]
              "System Security Requirements and Security Levels",
              IEC 62443, 2013.

Liu, et al.             Expires 14 February 2027                [Page 3]
Internet-Draft  Security Protocol for Modbus Serial Link     August 2026

   [IEC62443-4-2]
              "Technical Security Requirements for IACS Components",
              IEC 62443, 2019.

   [IEC62351-3]
              "Power Systems Management and Associated Information
              Exchange – Data and Communications Security – Part 3:
              Communication Network and System Security for Profiles
              Including TCP/IP", IEC 62351, 2014.

   [IEC62351-4]
              "Power Systems Management and Associated Information
              Exchange – Data and Communications Security – Part 4:
              Profiles Including MMS and Derivatives", IEC 62351, 2018.

   [NISTSP800-82]
              "Guide to Industrial Control Systems (ICS) Security",
              NIST SP800, 2018.

   [ENISA-ICS-Security-Guidelines]
              "Good Practices for Security of Industrial Control
              Systems", ENISA ICS-Security-Guidelines, 2018.

   [ETSI-SAGE-TS35.221]
              "Specification of the 3GPP Confidentiality and Integrity
              Algorithms 128-EEA3 and 128-EIA3.Document 1:128-EEA3 and
              128-EIA3 Specification", ETSI/SAGE TS35.221, 2020.

   [ETSI-SAGE-TS35.222]
              "Specification of the 3GPP Confidentiality and Integrity
              Algorithms 128-EEA3 and 128-EIA3.Document 1:128-EEA3 and
              128-EIA3 Specification", ETSI/SAGE TS35.222, 2020.

   [NIST.FIPS.203]
              "Module-Lattice-Based Key-Encapsulation Mechanism
              Standard", NIST FIPS.203, 2024.

   [NIST.FIPS.204]
              "Module-Lattice-Based Digital Signature Standard",
              NIST FIPS.204, 2024.

Authors' Addresses

Liu, et al.             Expires 14 February 2027                [Page 4]
Internet-Draft  Security Protocol for Modbus Serial Link     August 2026

   Penghui Liu (editor)
   Pengcheng Laboratory
   No.2 Xingke 1 Street
   Shenzhen
   518055
   China
   Email: liuph@pcl.ac.cn

   Rongwei Yang (editor)
   Pengcheng Laboratory
   No.2 Xingke 1 Street
   Shenzhen
   518055
   China
   Email: yangrw@pcl.ac.cn

   Meiling Chen (editor)
   China Mobile
   No.32 Xuanwumen West Street
   Beijing
   100000
   China
   Email: chenmeiling@chinamobile.com

   Yu Fu (editor)
   China Telecom
   No. 3 Penglaiyuan South Street
   Beijing
   100000
   China
   Email: fuy44@chinatelecom.cn

   Weizhe Zhang (editor)
   Pengcheng Laboratory
   No.2 Xingke 1 Street
   Shenzhen
   518055
   China
   Email: zhangwzh@pcl.ac.cn

Liu, et al.             Expires 14 February 2027                [Page 5]