Manufacturer Usage Description (MUD) (D)TLS Profiles for IoT Devices
draft-ietf-opsawg-mud-tls-01

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Document Type Active Internet-Draft (opsawg WG)
Authors Tirumaleswar Reddy.K  , Dan Wing  , Blake Anderson 
Last updated 2020-10-05 (latest revision 2020-09-22)
Replaces draft-reddy-opsawg-mud-tls
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OPSAWG WG                                                       T. Reddy
Internet-Draft                                                    McAfee
Intended status: Standards Track                                 D. Wing
Expires: April 8, 2021                                            Citrix
                                                             B. Anderson
                                                                   Cisco
                                                         October 5, 2020

  Manufacturer Usage Description (MUD) (D)TLS Profiles for IoT Devices
                      draft-ietf-opsawg-mud-tls-01

Abstract

   This memo extends the Manufacturer Usage Description (MUD)
   specification to incorporate (D)TLS profile parameters.  This allows
   a network security service to identify unexpected (D)TLS usage, which
   can indicate the presence of unauthorized software or malware on an
   endpoint.

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 April 8, 2021.

Copyright Notice

   Copyright (c) 2020 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

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   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 . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of MUD (D)TLS profiles for IoT devices . . . . . . .   5
   4.  (D)TLS 1.3 Handshake  . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Full (D)TLS 1.3 Handshake Inspection  . . . . . . . . . .   6
     4.2.  Encrypted DNS . . . . . . . . . . . . . . . . . . . . . .   6
   5.  (D)TLS Profile YANG Module  . . . . . . . . . . . . . . . . .   7
     5.1.  Tree Structure  . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Processing of the MUD (D)TLS Profile  . . . . . . . . . . . .  16
   7.  MUD File Example  . . . . . . . . . . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  19
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     12.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   Encryption is necessary to enhance the privacy of end users using IoT
   devices.  TLS [RFC8446] and DTLS [I-D.ietf-tls-dtls13] are the
   dominant protocols providing encryption for IoT device traffic.
   Unfortunately, in conjunction with IoT applications' rise of
   encryption, malware is also using encryption which thwarts network-
   based analysis such as deep packet inspection (DPI).  Other
   mechanisms are needed to detect malware running on an IoT device.

   Malware frequently uses its own libraries for its activities, and
   those libraries are re-used much like any other software engineering
   project. [malware] indicates that there are observable differences in
   how malware uses encryption compared with how non-malware uses
   encryption.  There are several interesting findings specific to
   (D)TLS which were found common to malware:

   o  Older and weaker cryptographic parameters (e.g.,
      TLS_RSA_WITH_RC4_128_SHA).

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   o  TLS server name indication (SNI) extension and server certificates
      are composed of subjects with characteristics of a domain
      generation algorithm (DGA) (e.g., www.33mhwt2j.net).

   o  Higher use of self-signed certificates compared with typical
      legitimate software.

   o  Discrepancies in the SNI TLS extension and the DNS names in the
      SubjectAltName (SAN) X.509 extension in the server certificate
      message.

   o  Discrepancies in the key exchange algorithm and the client public
      key length in comparison with legitimate flows.  As a reminder,
      the Client Key Exchange message has been removed from TLS 1.3.

   o  Lower diversity in TLS client advertised extensions compared to
      legitimate clients.

   o  Using privacy enhancing technologies like Tor, Psiphon, Ultrasurf
      (see [malware-tls]), and evasion techniques such as ClientHello
      randomization.

   o  Using DNS-over-HTTPS (DoH) [RFC8484] to avoid detection by malware
      DNS filtering services [malware-doh].  Specifically, malware may
      not use the DoH server provided by the local network.

   If observable (D)TLS profile parameters are used, the following
   functions are possible which have a positive impact on the local
   network security:

   o  Permit intended DTLS or TLS use and block malicious DTLS or TLS
      use.  This is superior to the layers 3 and 4 ACLs of Manufacturer
      Usage Description Specification (MUD) [RFC8520] which are not
      suitable for broad communication patterns.

   o  Ensure TLS certificates are valid.  Several TLS deployments have
      been vulnerable to active Man-In-The-Middle (MITM) attacks because
      of the lack of certificate validation or vulnerability in the
      certificate validation function (see [cryto-vulnerability]).  By
      observing (D)TLS profile parameters, a network element can detect
      when the TLS SNI mismatches the SubjectAltName and when the
      server's certificate is invalid.  In TLS 1.2, the ClientHello,
      ServerHello and Certificate messages are all sent in clear-text.
      This check is not possible with TLS 1.3, which encrypts the
      Certificate message thereby hiding the server identity from any
      intermediary.  In TLS 1.3, the server certificate validation
      functions should be executed within an on-path TLS proxy, if such
      a proxy exists.

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   o  Support new communication patterns.  An IoT device can learn a new
      capability, and the new capability can change the way the IoT
      device communicates with other devices located in the local
      network and Internet.  There would be an inaccurate policy if an
      IoT device rapidly changes the IP addresses and domain names it
      communicates with while the MUD ACLs were slower to update.  In
      such a case, observable (D)TLS profile parameters can be used to
      permit intended use and to block malicious behavior from the IoT
      device.

   This document extends MUD [RFC8520] to model observable (D)TLS
   profile parameters.  Using these (D)TLS profile parameters, an active
   MUD-enforcing network security service (e.g., firewall) can identify
   MUD non-compliant (D)TLS behavior indicating outdated cryptography or
   malware.  This detection can prevent malware downloads, block access
   to malicious domains, enforce use of strong ciphers, stop data
   exfiltration, etc.  In addition, organizations may have policies
   around acceptable ciphers and certificates for the websites the IoT
   devices connect to.  Examples include no use of old and less secure
   versions of TLS, no use of self-signed certificates, deny-list or
   accept-list of Certificate Authorities, valid certificate expiration
   time, etc.  These policies can be enforced by observing the (D)TLS
   profile parameters.  Network security services can use the IoT
   device's (D)TLS profile parameters to identify legitimate flows by
   observing (D)TLS sessions, and can make inferences to permit
   legitimate flows and to block malicious or insecure flows.  The
   proposed technique is also suitable in deployments where decryption
   techniques are not ideal due to privacy concerns, non-cooperating
   end-points, and expense.

2.  Terminology

   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.

   "(D)TLS" is used for statements that apply to both Transport Layer
   Security [RFC8446] and Datagram Transport Layer Security [RFC6347].
   Specific terms are used for any statement that applies to either
   protocol alone.

   'DoH/DoT' refers to DNS-over-HTTPS and/or DNS-over-TLS.

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3.  Overview of MUD (D)TLS profiles for IoT devices

   In Enterprise networks, protection and detection are typically done
   both on end hosts and in the network.  Host security agents have deep
   visibility on the devices where they are installed, whereas the
   network has broader visibility.  Installing host security agents may
   not be a viable option on IoT devices, and network-based security is
   an efficient means to protect such IoT devices.  If the IoT device
   supports a MUD (D)TLS profile, the (D)TLS profile parameters of the
   IoT device can be used by a middlebox to detect and block malware
   communication, while at the same time preserving the privacy of
   legitimate uses of encryption.  The middlebox need not proxy (D)TLS
   but can passively observe the parameters of (D)TLS handshakes from
   IoT devices and gain visibility into TLS 1.2 parameters and partial
   visibility into TLS 1.3 parameters.  Malicious agents can try to use
   the (D)TLS profile parameters of legitimate agents to evade
   detection, but it becomes a challenge to mimic the behavior of
   various IoT device types and IoT device models from several
   manufacturers.  In other words, malware developers will have to
   develop malicious agents per IoT device type, manufacturer and model,
   infect the device with the tailored malware agent and will have keep
   up with updates to the device's (D)TLS profile parameters over time.
   Furthermore, the malware's command and control server certificates
   need to be signed by the same certifying authorities trusted by the
   IoT devices.  Typically, IoT devices have an infrastructure that
   supports a rapid deployment of updates, and malware agents will have
   a near-impossible task of similarly deploying updates and continuing
   to mimic the TLS behavior of the IoT device it has infected.
   However, if the IoT device has reached end-of-life and the IoT
   manufcaturer will not issue a firmware or software update to the
   Thing or will not update the MUD file, the "is-supported" attribute
   defined in Section 3.6 of [RFC8520] can be used by the MUD manager to
   identify the IoT manufcaturer no longer supports the device.  The
   end-of-life of a device does not necessarily mean that it is
   defective; rather, it denotes a need to replace and upgrade the
   network to next-generation devices for additional functionality.  The
   network security service will have to rely on other techniques
   discussed in Section 8 to identify malicious connections until the
   device is replaced.

   Compromised IoT devices are typically used for launching DDoS attacks
   (Section 3 of [RFC8576]).  For example, DDoS attacks like Slowloris
   and Transport Layer Security (TLS) re-negotiation can be blocked if
   the victim's server certificate is not be signed by the same
   certifying authorities trusted by the IoT device.

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4.  (D)TLS 1.3 Handshake

   In (D)TLS 1.3, full (D)TLS handshake inspection is not possible since
   all (D)TLS handshake messages excluding the ClientHello message are
   encrypted.  (D)TLS 1.3 has introduced new extensions in the handshake
   record layers called Encrypted Extensions.  Using these extensions
   handshake messages will be encrypted and network security services
   (such as a firewall) are incapable to decipher the handshake, and
   thus cannot view the server certificate.  However, the ClientHello
   and ServerHello still have some fields visible, such as the list of
   supported versions, named groups, cipher suites, signature algorithms
   and extensions in ClientHello, and chosen cipher in the ServerHello.
   For instance, if the malware uses evasion techniques like ClientHello
   randomization, the observable list of cipher suites and extensions
   offered by the malware agent in the ClientHello message will not
   match the list of cipher suites and extensions offered by the
   legitimate client in the ClientHello message, and the middlebox can
   block malicious flows without acting as a (D)TLS 1.3 proxy.

4.1.  Full (D)TLS 1.3 Handshake Inspection

   To obtain more visibility into negotiated TLS 1.3 parameters, a
   middlebox can act as a (D)TLS 1.3 proxy.  A middlebox can act as a
   (D)TLS proxy for the IoT devices owned and managed by the IT team in
   the Enterprise network and the (D)TLS proxy must meet the security
   and privacy requirements of the organization.  In other words, the
   scope of middlebox acting as a (D)TLS proxy is restricted to
   Enterprise network owning and managing the IoT devices.  The
   middlebox would have to follow the behaviour detailed in Section 9.3
   of [RFC8446] to act as a compliant (D)TLS 1.3 proxy.

   To further increase privacy, encrypted client hello
   [I-D.ietf-tls-esni] prevents passive observation of the TLS Server
   Name Indication extension.  To effectively provide that privacy
   protection, SNI encryption needs to be used in conjunction with DNS
   encryption (e.g., DoH).  A middlebox (e.g., firewall) passively
   inspecting an encrypted SNI (D)TLS handshake cannot observe the
   encrypted SNI nor observe the encrypted DNS traffic.

4.2.  Encrypted DNS

   A common usage pattern for certain type of IoT devices (e.g., light
   bulb) is for it to "call home" to a service that resides on the
   public Internet, where that service is referenced through a domain
   name (A or AAAA record).  As discussed in Manufacturer Usage
   Description Specification [RFC8520], because these devices tend to
   require access to very few sites, all other access should be
   considered suspect.  If an IoT device is pre-configured to use public

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   DoH/DoT server, the MUD policy enforcement point is moved to that
   public server, which cannot enforce the MUD policy based on domain
   names (Section 8 of [RFC8520]).  If the DNS query is not accessible
   for inspection, it becomes quite difficult for the infrastructure to
   suspect anything.  Thus the use of a public DoH/DoT server is
   incompatible with MUD in general.  A local DoH/DoT server is
   necessary to allow MUD policy enforcement on the local network
   [I-D.reddy-add-enterprise].

5.  (D)TLS Profile YANG Module

   This document specifies a YANG module for representing (D)TLS
   profile.  The (D)TLS profile YANG module provides a method for
   network security services to observe the (D)TLS profile parameters in
   the (D)TLS handshake to permit intended use and to block malicious
   behavior.  This module uses the common YANG types defined in
   [RFC6991], the rules defined in [RFC8519], and the cryptographic
   types defined in [I-D.ietf-netconf-crypto-types].  See [RFC7925] for
   (D)TLS 1.2 and [I-D.ietf-uta-tls13-iot-profile] for DTLS 1.3
   recommendations related to IoT devices, and [RFC7525] for additional
   (D)TLS 1.2 recommendations.

   The (D)TLS parameters in each (D)TLS profile include the following:

   o  Profile name

   o  (D)TLS versions supported by the IoT device.

   o  List of supported cipher suites.  For (D)TLS1.2, [RFC7925]
      recommends AEAD ciphers for IoT devices.

   o  List of supported extension types

   o  List of trust anchor certificates used by the IoT device.  If the
      server certificate is signed by one of the trust anchors, the
      middlebox continues with the connection as normal.  Otherwise, the
      middlebox will react as if the server certificate validation has
      failed and takes appropriate action (e.g, block the (D)TLS
      session).  An IoT device can use a private trust anchor to
      validate a server's certificate (e.g., the private trust anchor
      can be preloaded at manufacturing time on the IoT device and the
      IoT device fetches the firmware image from the Firmware server
      whose certificate is signed by the private CA).  This empowers the
      middlebox to reject TLS sessions to servers that the IoT device
      does not trust.

   o  List of SPKI pin set pre-configured on the client to validate
      self-signed server certificates or raw public keys.  A SPKI pin

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      set is a cryptographic digest to "pin" public key information in a
      manner similar to HTTP Public Key Pinning (HPKP) [RFC7469].  If
      SPKI pin set is present in the (D)TLS profile of a IoT device and
      the server certificate does not pass the PKIX certification path
      validation, the middlebox computes the SPKI Fingerprint for the
      public key found in the server's certificate (or in the raw public
      key, if the server provides that instead).  If a computed
      fingerprint exactly matches one of the SPKI pin sets in the (D)TLS
      profile, the middlebox continues with the connection as normal.
      Otherwise, the middlebox will act on the SPKI validation failure
      and takes appropriate action.

   o  Cryptographic hash algorithm used to generate the SPKI pinsets

   o  List of pre-shared key exchange modes

   o  List of named groups (DHE or ECDHE) supported by the client

   o  List signature algorithms the client can validate in X.509 server
      certificates

   o  List signature algorithms the client is willing to accept for
      CertificateVerify message (Section 4.2.3 of [RFC8446]).  For
      example, a TLS client implementation can support different sets of
      algorithms for certificates and in TLS to signal the capabilities
      in "signature_algorithms_cert" and "signature_algorithms"
      extensions.

   o  List of supported application protocols (e.g., h3, h2, http/1.1
      etc.)

   o  List of certificate compression algorithms (defined in
      [I-D.ietf-tls-certificate-compression])

   o  List of the distinguished names [X501] of acceptable certificate
      authorities, represented in DER-encoded format [X690] (defined in
      Section 4.2.4 of [RFC8446])

   GREASE [RFC8701] sends random values on TLS parameters to ensure
   future extensibility of TLS extensions.  Similar random values might
   be extended to other TLS parameters.  Thus, the (D)TLS profile
   parameters defined in the YANG module by this document MUST NOT
   include the GREASE values for extension types, named groups,
   signature algorithms, (D)TLS versions, pre-shared key exchange modes,
   cipher suites and for any other TLS parameters defined in future
   RFCs.

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   The (D)TLS profile does not include parameters like compression
   methods for data compression, [RFC7525] recommends disabling TLS-
   level compression to prevent compression-related attacks.  In TLS
   1.3, only the "null" compression method is allowed (Section 4.1.2 of
   [RFC8446]).

   Note: The TLS and DTLS IANA registries are available from
   <https://www.iana.org/assignments/tls-parameters/tls-parameters.txt>
   and <https://www.iana.org/assignments/tls-extensiontype-values/tls-
   extensiontype-values.txt>.  The values for all the parameters in the
   YANG module excluding the supported_versions parameter are defined in
   the TLS and DTLS IANA registries.  The TLS and DTLS IANA registry
   does not maintain (D)TLS version numbers.

5.1.  Tree Structure

   This document augments the "ietf-mud" MUD YANG module defined in
   [RFC8520] for signaling the IoT device (D)TLS profile.  This document
   defines the YANG module "iana-opsawg-mud-tls-profile", which has the
   following tree structure:

module: iana-opsawg-mud-tls-profile
  augment /acl:acls/acl:acl/acl:aces/acl:ace/acl:matches:
    +--rw client-profile
       +--rw tls-dtls-profiles* [profile-name]
          +--rw profile-name                   string
          +--rw supported_tls_versions*        tls-version
          +--rw supported_dtls_versions*       dtls-version
          +--rw cipher-suites* [cipher aead]
          |  +--rw cipher    cipher-algorithm
          |  +--rw aead      aead-algorithm
          +--rw extension-types*               extension-type
          +--rw acceptlist-ta-certs*           ct:trust-anchor-cert-cms
          +--rw SPKI
          |  +--rw SPKI-pin-sets*         SPKI-pin-set
          |  +--rw SPKI-hash-algorithm?   iha:hash-algorithm-type
          +--rw psk-key-exchange-modes*        psk-key-exchange-mode {tls-1_3 or dtls-1_3}?
          +--rw supported-groups*              supported-group
          +--rw signature-algorithms-cert*     signature-algorithm {tls-1_3 or dtls-1_3}?
          +--rw signature-algorithms*          signature-algorithm
          +--rw application-protocols*         application-protocol
          +--rw cert-compression-algorithms*   cert-compression-algorithm {tls-1_3 or dtls-1_3}?
          +--rw certificate_authorities*       certificate_authority {tls-1_3 or dtls-1_3}?

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5.2.  YANG Module

<CODE BEGINS> file "iana-opsawg-mud-tls-profile@2020-09-25.yang"
module iana-opsawg-mud-tls-profile {
   yang-version 1.1;
   namespace "urn:ietf:params:xml:ns:yang:iana-opsawg-mud-tls-profile";
   prefix mud-tls-profile;

   import ietf-crypto-types {
     prefix ct;
     reference "draft-ietf-netconf-crypto-types-01:
                Common YANG Data Types for Cryptography";
   }

   import iana-hash-algs {
     prefix iha;
     reference
          "RFC XXXX: Common YANG Data Types for Hash algorithms";
   }

   import ietf-access-control-list {
     prefix acl;
     reference
       "RFC 8519: YANG Data Model for Network Access
                  Control Lists (ACLs)";
   }

   organization
     "IETF Operations and Management Area Working Group Working Group";
   contact
      "Editor:  Konda, Tirumaleswar Reddy
               <mailto:TirumaleswarReddy_Konda@McAfee.com>";

   description
     "This module contains YANG definition for the IoT device
      (D)TLS profile.

      Copyright (c) 2019 IETF Trust and the persons identified as
      authors of the code.  All rights reserved.

      Redistribution and use in source and binary forms, with or
      without modification, is permitted pursuant to, and subject
      to the license terms contained in, the Simplified BSD License
      set forth in Section 4.c of the IETF Trust's Legal Provisions
      Relating to IETF Documents
      (http://trustee.ietf.org/license-info).

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      This version of this YANG module is part of RFC XXXX; see
      the RFC itself for full legal notices.";

   revision 2019-06-12 {
     description
       "Initial revision";
   }

   typedef extension-type {
     type uint16;
     description "Extension type";
   }

    typedef supported-group {
     type uint16;
     description "Named group (DHE or ECDHE)";
   }

   typedef SPKI-pin-set {
     type binary;
     description "Subject Public Key Info pin set";
   }

   typedef signature-algorithm {
     type uint16;
     description "Signature algorithm";
   }

   typedef psk-key-exchange-mode {
     type uint8;
     description "pre-shared key exchange mode";
   }

   typedef client-public-key-length {
     type uint8;
     description "client public key length";
   }

   typedef application-protocol {
     type string;
     description "application protocol";
   }

   typedef cert-compression-algorithm {
     type uint16;
     description "certificate compression algorithm";
   }

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   typedef certificate_authority {
     type binary;
     description "Distinguished Name of Certificate authority";
   }

   typedef cipher-algorithm {
     type uint8;
     description "Cipher Algorithm";
   }

   typedef aead-algorithm {
     type uint8;
     description "AEAD Algorithm";
   }

   typedef tls-version {
       type enumeration {
         enum tls-1.2 {
           value 1;
           description
               "TLS Protocol Version 1.2.";
           reference
              "RFC 5246: The Transport Layer Security (TLS) Protocol
                                   Version 1.2";
         }
         enum tls-1.3 {
           value 2;
           description
               "TLS Protocol Version 1.3.";
           reference
              "RFC 8446: The Transport Layer Security (TLS) Protocol
                                   Version 1.3";
         }
      }
      description
           "Indicates the TLS version.";
   }

   typedef dtls-version {
     type enumeration {
         enum dtls-1.2 {
           value 1;
           description
               "DTLS Protocol Version 1.2.";
           reference
              "RFC 6346: Datagram Transport Layer Security 1.2";
         }
         enum dtls-1.3 {

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           value 2;
           description
               "DTLS Protocol Version 1.3.";
           reference
               "draft-ietf-tls-dtls13: Datagram Transport Layer Security 1.3";
         }
     }
     description
       "Indicates the DTLS version.";
   }

   feature tls-1_2 {
     description
       "TLS Protocol Version 1.2 is supported.";
       reference
         "RFC 5246: The Transport Layer Security (TLS) Protocol
                    Version 1.2";
   }

   feature tls-1_3 {
     description
       "TLS Protocol Version 1.3 is supported.";
       reference
         "RFC 8446: The Transport Layer Security (TLS) Protocol
                    Version 1.3";
   }

   feature dtls-1_2 {
       description
          "DTLS Protocol Version 1.2 is supported.";
       reference
          "RFC 6346: Datagram Transport Layer Security Version 1.2";
   }

   feature dtls-1_3 {
       description
         "DTLS Protocol Version 1.3 is supported.";
       reference
         "draft-ietf-tls-dtls13: Datagram Transport Layer Security 1.3";
   }

   grouping client-profile {
     description
       "A grouping for (D)TLS profiles.";
     container client-profile {
       list tls-dtls-profiles {
         key "profile-name";
         description

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          "A list of (D)TLS version profiles supported by the client.";
        leaf profile-name {
          type string {
            length "1..64";
          }
          description
            "The name of (D)TLS profile; space and special
            characters are not allowed.";
         }
         leaf-list supported_versions {
           type uint16;
           description
             "(D)TLS versions supported by the client";
         }
         list cipher-suites {
            key "cipher aead";
            leaf cipher {
               type cipher-algorithm;
               description "Cipher";
            }
            leaf aead {
               type aead-algorithm;
               description "AEAD";
           }
          description "Cipher Suites";
         }
         leaf-list extension-types {
           type extension-type;
           description "Extension Types";
         }
         leaf-list acceptlist-ta-certs {
           type ct:trust-anchor-cert-cms;
           description
             "A list of trust anchor certificates used by the client.";
         }
         container SPKI {
            leaf-list SPKI-pin-sets {
                type SPKI-pin-set;
                description
                  "A list of SPKI pin sets pre-configured on the client
                  to validate self-signed server certificate or
                  raw public key.";
            }
            leaf SPKI-hash-algorithm {
               type iha:hash-algorithm-type;
              description
                "cryptographic hash algorithm used to generate the
                SPKI pinset.";

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            }
         }
         leaf-list psk-key-exchange-modes {
           if-feature "tls-1_3 or dtls-1_3";
           type psk-key-exchange-mode;
           description
             "pre-shared key exchange modes";
         }
         leaf-list supported-groups {
            type supported-group;
            description
             "A list of named groups supported by the client.";
         }
         leaf-list signature-algorithms-cert {
            if-feature "tls-1_3 or dtls-1_3";
            type signature-algorithm;
            description
             "A list signature algorithms the client can validate
              in X.509 certificates.";
         }
         leaf-list signature-algorithms {
            type signature-algorithm;
            description
             "A list signature algorithms the client can validate
              in the CertificateVerify message.";
         }
         leaf-list application-protocols {
            type application-protocol;
            description
             "A list application protocols supported by the client";
         }
         leaf-list cert-compression-algorithms {
            if-feature "tls-1_3 or dtls-1_3";
            type cert-compression-algorithm;
            description
             "A list certificate compression algorithms
              supported by the client";
         }
         leaf-list certificate_authorities {
            if-feature "tls-1_3 or dtls-1_3";
            type certificate_authority;
            description
             "A list of the distinguished names of certificate authorities
              acceptable to the client";
         }
   }
  }
 }

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 augment "/acl:acls/acl:acl/acl:aces/acl:ace/acl:matches" {
   description
     "MUD (D)TLS specific matches.";
   uses client-profile;
 }
}

6.  Processing of the MUD (D)TLS Profile

   The following text outlines the rules for a network security service
   (e.g., firewall) to follow to process the MUD (D)TLS Profile:

   o  If the (D)TLS parameter observed in a (D)TLS session is not
      specified in the MUD (D)TLS profile and the parameter is
      recognized by the firewall, it can identify unexpected (D)TLS
      usage, which can indicate the presence of unauthorized software or
      malware on an endpoint.  The firewall can take several actions
      like block the (D)TLS session or raise an alert to quarantine and
      remediate the compromised device.  For example, if the cipher
      suite TLS_RSA_WITH_AES_128_CBC_SHA in the ClientHello message is
      not specified in the MUD (D)TLS profile and the cipher suite is
      recognized by the firewall, it can identify unexpected TLS usage.

   o  If the (D)TLS parameter observed in a (D)TLS session is not
      specified in the MUD (D)TLS profile and the (D)TLS parameter is
      not recognized by the firewall, it can ignore the unrecognized
      parameter and the correct behavior is not to block the (D)TLS
      session.  The behaviour is functionally equivalent to the
      description in Section 9.3 of [RFC8446] to ignore all unrecognized
      cipher suites, extensions, and other parameters.  For example, if
      the cipher suite TLS_CHACHA20_POLY1305_SHA256 in the ClientHello
      message is not specified in the MUD (D)TLS profile and the cipher
      suite is not recognized by the firewall, it can ignore the
      unrecognized cipher suite.

   o  Deployments update at different rates, so an updated MUD (D)TLS
      profile may support newer parameters.  If the firewall does not
      recognize the newer parameters, an alert should be triggered to
      the firewall vendor and the IoT device owner or administrator.  A
      firewall must be readily updatable, so that when ossification
      problems are discovered, they can be addressed quickly.  Most
      importantly, if the firewall is not readily updatable, its
      efficacy to identify emerging malware will decrease with time.
      For example, if the cipher suite TLS_AES_128_CCM_8_SHA256 in the
      ClientHello message is specified in the MUD (D)TLS profile and the
      cipher suite is not recognized by the firewall, an alert will be
      triggered.

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7.  MUD File Example

   The example below contains (D)TLS profile parameters for a IoT device
   used to reach servers listening on port 443 using TCP transport.
   JSON encoding of YANG modelled data [RFC7951] is used to illustrate
   the example.

   {
      "ietf-mud:mud": {
        "mud-version": 1,
         "mud-url": "https://example.com/IoTDevice",
         "last-update": "2019-18-06T03:56:40.105+10:00",
         "cache-validity": 100,
         "is-supported": true,
         "systeminfo": "IoT device name",
         "from-device-policy": {
            "access-lists": {
              "access-list": [
                {
                  "name": "mud-7500-profile"
                }
              ]
            }
         },
        "ietf-access-control-list:acls": {
          "acl": [
            {
              "name": "mud-7500-profile",
              "type": "ipv6-acl-type",
              "aces": {
                "ace": [
                  {
                    "name": "cl0-frdev",
                    "matches": {
                      "ipv6": {
                        "protocol": 6
                      },
                      "tcp": {
                        "ietf-mud:direction-initiated": "from-device",
                        "destination-port": {
                          "operator": "eq",
                          "port": 443
                        }
                      },
                      "iana-opsawg-mud-tls-profile:client-profile" : {
                        "tls-dtls-profiles" : [
                           {
                              "supported-tls-versions" : [2],

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                              "cipher-suites" : [
                                {
                                  "cipher": 19,
                                  "aead":  1
                                 },
                                 {
                                  "cipher": 19,
                                  "aead":  2
                                 }
                               ],
                              "extension-types" : [10,11,13,16,24],
                              "supported-groups" : [29]
                           }
                         ]
                      },
                      "actions": {
                         "forwarding": "accept"
                      }
                  }
               }
             ]
            }
           }
          ]
        }
      }
   }

   The following illustrates the example scenarios for processing the
   above profile:

   o  If the extension type "encrypt_then_mac" (code point 22) [RFC7366]
      in the ClientHello message is recognized by the firewall, it can
      identify unexpected TLS usage.

   o  If the extension type "token_binding" (code point 24) [RFC8472] in
      the ClientHello message is not recognized by the firewall, it can
      ignore the unrecognized extension.  Because the extension type
      token_binding is specified in the profile, an alert will be
      triggered to the firewall vendor and the IoT device owner or
      administrator to notify the firewall is not up to date.

8.  Security Considerations

   Security considerations in [RFC8520] need to be taken into
   consideration.  Although it is challenging for a malware to mimic the
   TLS behavior of various IoT device types and IoT device models from
   several manufacturers, malicious agents have a very low probability

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   of using the same (D)TLS profile parameters as legitimate agents on
   the IoT device to evade detection.  Network security services should
   also rely on contextual network data to detect false negatives.  In
   order to detect such malicious flows, anomaly detection (deep
   learning techniques on network data) can be used to detect malicious
   agents using the same (D)TLS profile parameters as legitimate agent
   on the IoT device.  In anomaly detection, the main idea is to
   maintain rigorous learning of "normal" behavior and where an
   "anomaly" (or an attack) is identified and categorized based on the
   knowledge about the normal behavior and a deviation from this normal
   behavior.

9.  Privacy Considerations

   Privacy considerations discussed in Section 16 of [RFC8520] to not
   reveal the MUD URL to an atacker need to be taken into consideration.
   The MUD URL can be stored in Trusted Execution Environment (TEE) for
   secure operation, enhanced data security, and prevent exposure to
   unauthorized software.

   The middlebox acting as a (D)TLS proxy must immediately delete the
   decrypted data upon completing any necessary inspection functions.
   TLS proxy potentially has access to a user's PII (Personally
   identifiable information) and PHI (Protected Health Information).
   The TLS proxy must not store, process or modify PII data.  For
   example, IT administrator can configure the middlebox to bypass
   payload inspection for a connection destined to a specific service
   due to privacy compliance requirements.  In addition, mechanisms
   based on object security can be used by IoT devices to enable end-to-
   end security and the middlebox will not have any access to the packet
   data.  For example, Object Security for Constrained RESTful
   Environments (OSCORE) [RFC8613] is a proposal that protects CoAP
   messages by wrapping them in the COSE format [RFC8152].

10.  IANA Considerations

   Each normative YANG module MUST be registered in both the "IETF XML
   Registry" [RFC3688] and the "YANG Module Names" registry [RFC6020].

   This document requests IANA to register the following URIs in the
   "ns" subregistry within the "IETF XML Registry" [RFC3688]:

         URI: urn:ietf:params:xml:ns:yang:iana-opsawg-mud-tls-profile
         Registrant Contact: The IESG.
         XML: N/A; the requested URI is an XML namespace.

   This document requests IANA to register the following YANG modules in
   the "YANG Module Names" subregistry [RFC6020] within the "YANG

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   Parameters" registry.  IANA is requested to create an IANA-maintained
   YANG Module called "iana-opsawg-mud-tls-profile", based on the
   contents of Section 5, which will allow for new (D)TLS parameters and
   (D)TLS versions.  The registration procedure will be Expert Review or
   Specification Required, as defined by [RFC8126].

      name: iana-opsawg-mud-tls-profile
      namespace: urn:ietf:params:xml:ns:yang:iana-opsawg-mud-tls-profile
      maintained by IANA: Y
      prefix: mud-tls-profile
      reference: RFC XXXX

11.  Acknowledgments

   Thanks to Flemming Andreasen, Shashank Jain, Michael Richardson,
   Piyush Joshi, Eliot Lear, Harsha Joshi, Qin Wu, Mohamed Boucadair,
   Ben Schwartz, Eric Rescorla, Panwei William, Nick Lamb and Nick
   Harper for the discussion and comments.

12.  References

12.1.  Normative References

   [I-D.ietf-netconf-crypto-types]
              Watsen, K., "YANG Data Types and Groupings for
              Cryptography", draft-ietf-netconf-crypto-types-18 (work in
              progress), August 2020.

   [I-D.ietf-tls-certificate-compression]
              Ghedini, A. and V. Vasiliev, "TLS Certificate
              Compression", draft-ietf-tls-certificate-compression-10
              (work in progress), January 2020.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-38 (work in progress), May
              2020.

   [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>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

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   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,
              <https://www.rfc-editor.org/info/rfc6991>.

   [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>.

   [RFC8519]  Jethanandani, M., Agarwal, S., Huang, L., and D. Blair,
              "YANG Data Model for Network Access Control Lists (ACLs)",
              RFC 8519, DOI 10.17487/RFC8519, March 2019,
              <https://www.rfc-editor.org/info/rfc8519>.

   [RFC8701]  Benjamin, D., "Applying Generate Random Extensions And
              Sustain Extensibility (GREASE) to TLS Extensibility",
              RFC 8701, DOI 10.17487/RFC8701, January 2020,
              <https://www.rfc-editor.org/info/rfc8701>.

   [X690]     ITU-T, "Information technology - ASN.1 encoding Rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ISO/IEC 8825-1:2002, 2002.

12.2.  Informative References

   [cryto-vulnerability]
              Perez, B., "Exploiting the Windows CryptoAPI
              Vulnerability", January 2020,
              <https://media.defense.gov/2020/Jan/14/2002234275/-1/-1/0/
              CSA-WINDOWS-10-CRYPT-LIB-20190114.PDF>.

   [I-D.ietf-tls-esni]
              Rescorla, E., Oku, K., Sullivan, N., and C. Wood, "TLS
              Encrypted Client Hello", draft-ietf-tls-esni-07 (work in
              progress), June 2020.

   [I-D.ietf-uta-tls13-iot-profile]
              Tschofenig, H. and T. Fossati, "TLS/DTLS 1.3 Profiles for
              the Internet of Things", draft-ietf-uta-tls13-iot-
              profile-00 (work in progress), June 2020.

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   [I-D.reddy-add-enterprise]
              Reddy.K, T. and D. Wing, "DNS-over-HTTPS and DNS-over-TLS
              Server Deployment Considerations for Enterprise Networks",
              draft-reddy-add-enterprise-00 (work in progress), June
              2020.

   [malware]  Anderson, B., Paul, S., and D. McGrew, "Deciphering
              Malware's use of TLS (without Decryption)", July 2016,
              <https://arxiv.org/abs/1607.01639>.

   [malware-doh]
              Cimpanu, C., "First-ever malware strain spotted abusing
              new DoH (DNS over HTTPS) protocol", July 2019,
              <https://www.zdnet.com/article/first-ever-malware-strain-
              spotted-abusing-new-doh-dns-over-https-protocol/>.

   [malware-tls]
              Anderson, B. and D. McGrew, "TLS Beyond the Browser:
              Combining End Host and Network Data to Understand
              Application Behavior", October 2019,
              <https://dl.acm.org/citation.cfm?id=3355601>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
              <https://www.rfc-editor.org/info/rfc7366>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <https://www.rfc-editor.org/info/rfc7469>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016,
              <https://www.rfc-editor.org/info/rfc7925>.

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   [RFC7951]  Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              RFC 7951, DOI 10.17487/RFC7951, August 2016,
              <https://www.rfc-editor.org/info/rfc7951>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

   [RFC8472]  Popov, A., Ed., Nystroem, M., and D. Balfanz, "Transport
              Layer Security (TLS) Extension for Token Binding Protocol
              Negotiation", RFC 8472, DOI 10.17487/RFC8472, October
              2018, <https://www.rfc-editor.org/info/rfc8472>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [RFC8520]  Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
              Description Specification", RFC 8520,
              DOI 10.17487/RFC8520, March 2019,
              <https://www.rfc-editor.org/info/rfc8520>.

   [RFC8576]  Garcia-Morchon, O., Kumar, S., and M. Sethi, "Internet of
              Things (IoT) Security: State of the Art and Challenges",
              RFC 8576, DOI 10.17487/RFC8576, April 2019,
              <https://www.rfc-editor.org/info/rfc8576>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [X501]     "Information Technology - Open Systems Interconnection -
              The Directory: Models", ITU-T X.501, 1993.

Authors' Addresses

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   Tirumaleswar Reddy
   McAfee, Inc.
   Embassy Golf Link Business Park
   Bangalore, Karnataka  560071
   India

   Email: kondtir@gmail.com

   Dan Wing
   Citrix Systems, Inc.
   4988 Great America Pkwy
   Santa Clara, CA  95054
   USA

   Email: danwing@gmail.com

   Blake Anderson
   Cisco Systems, Inc.
   170 West Tasman Dr
   San Jose, CA  95134
   USA

   Email: blake.anderson@cisco.com

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