TLS/DTLS 1.3 Profiles for the Internet of Things
draft-ietf-uta-tls13-iot-profile-01

UTA                                                        H. Tschofenig
Internet-Draft                                                T. Fossati
Updates: 7925 (if approved)                                  Arm Limited
Intended status: Standards Track                        22 February 2021
Expires: 26 August 2021

            TLS/DTLS 1.3 Profiles for the Internet of Things
                  draft-ietf-uta-tls13-iot-profile-01

Abstract

   This document is a companion to RFC 7925 and defines TLS/DTLS 1.3
   profiles for Internet of Things devices.  It also updates RFC 7925
   with regards to the X.509 certificate profile.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/thomas-fossati/draft-tls13-iot
   (https://github.com/thomas-fossati/draft-tls13-iot).

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
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   This Internet-Draft will expire on 26 August 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   3
   2.  Credential Types  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Session Resumption  . . . . . . . . . . . . . . . . . . . . .   4
   5.  Compression . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Perfect Forward Secrecy . . . . . . . . . . . . . . . . . . .   4
   7.  Keep-Alive  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   8.  Timeouts  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   9.  Random Number Generation  . . . . . . . . . . . . . . . . . .   5
   10. Server Name Indication (SNI)  . . . . . . . . . . . . . . . .   5
   11. Maximum Fragment Length Negotiation . . . . . . . . . . . . .   5
   12. Crypto Agility  . . . . . . . . . . . . . . . . . . . . . . .   5
   13. Key Length Recommendations  . . . . . . . . . . . . . . . . .   5
   14. 0-RTT Data  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   15. Certificate Profile . . . . . . . . . . . . . . . . . . . . .   6
     15.1.  All Certificates . . . . . . . . . . . . . . . . . . . .   6
       15.1.1.  Version  . . . . . . . . . . . . . . . . . . . . . .   6
       15.1.2.  Serial Number  . . . . . . . . . . . . . . . . . . .   6
       15.1.3.  Signature  . . . . . . . . . . . . . . . . . . . . .   6
       15.1.4.  Issuer . . . . . . . . . . . . . . . . . . . . . . .   6
       15.1.5.  Validity . . . . . . . . . . . . . . . . . . . . . .   7
       15.1.6.  subjectPublicKeyInfo . . . . . . . . . . . . . . . .   7
     15.2.  Root CA Certificate  . . . . . . . . . . . . . . . . . .   7
     15.3.  Intermediate CA Certificate  . . . . . . . . . . . . . .   7
     15.4.  End Entity Certificate . . . . . . . . . . . . . . . . .   7
       15.4.1.  Client Certificate Subject . . . . . . . . . . . . .   8
   16. Certificate Revocation Checks . . . . . . . . . . . . . . . .   8
     16.1.  Open Issues  . . . . . . . . . . . . . . . . . . . . . .   8
   17. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   18. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   19. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   20. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     20.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     20.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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1.  Introduction

   This document defines a profile of DTLS 1.3 [I-D.ietf-tls-dtls13] and
   TLS 1.3 [RFC8446] that offers communication security services for IoT
   applications and is reasonably implementable on many constrained
   devices.  Profile thereby means that available configuration options
   and protocol extensions are utilized to best support the IoT
   environment.

   For IoT profiles using TLS/DTLS 1.2 please consult [RFC7925].  This
   document re-uses the communication pattern defined in [RFC7925] and
   makes IoT-domain specific recommendations for version 1.3 (where
   necessary).

   TLS 1.3 has been re-designed and several previously defined
   extensions are not applicable to the new version of TLS/DTLS anymore.
   This clean-up also simplifies this document.  Furthermore, many
   outdated ciphersuites have been omitted from the TLS/DTLS 1.3
   specification.

1.1.  Conventions and 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.

2.  Credential Types

   In accordance with the recommendations in [RFC7925], a compliant
   implementation MUST implement TLS_AES_128_CCM_8_SHA256.  It SHOULD
   implement TLS_CHACHA20_POLY1305_SHA256.

   Pre-shared key based authentication is integrated into the main TLS/
   DTLS 1.3 specification and has been harmonized with session
   resumption.

   A compliant implementation supporting authentication based on
   certificates and raw public keys MUST support digital signatures with
   ecdsa_secp256r1_sha256.  A compliant implementation MUST support the
   key exchange with secp256r1 (NIST P-256) and SHOULD support key
   exchange with X25519.

   A plain PSK-based TLS/DTLS client or server MUST implement the
   following extensions:

   *  supported_versions

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   *  cookie
   *  server_name
   *  pre_shared_key
   *  psk_key_exchange_modes

   For TLS/DTLS clients and servers implementing raw public keys and/or
   certificates the guidance for mandatory-to-implement extensions
   described in Section 9.2 of [RFC8446] MUST be followed.

3.  Error Handling

   TLS 1.3 simplified the Alert protocol but the underlying challenge in
   an embedded context remains unchanged, namely what should an IoT
   device do when it encounters an error situation.  The classical
   approach used in a desktop environment where the user is prompted is
   often not applicable with unattended devices.  Hence, it is more
   important for a developer to find out from which error cases a device
   can recover from.

4.  Session Resumption

   TLS 1.3 has built-in support for session resumption by utilizing PSK-
   based credentials established in an earlier exchange.

5.  Compression

   TLS 1.3 does not have support for compression.

6.  Perfect Forward Secrecy

   TLS 1.3 allows the use of PFS with all ciphersuites since the support
   for it is negotiated independently.

7.  Keep-Alive

   The discussion in Section 10 of [RFC7925] is applicable.

8.  Timeouts

   The recommendation in Section 11 of [RFC7925] is applicable.  In
   particular this document RECOMMENDED to use an initial timer value of
   9 seconds with exponential back off up to no less then 60 seconds.

   Question: DTLS 1.3 now offers per-record retransmission and therefore
   introduces much less congestion risk associated with spurious
   retransmissions.  Hence, should we relax the 9s initial timeout?

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9.  Random Number Generation

   The discussion in Section 12 of [RFC7925] is applicable with one
   exception: the ClientHello and the ServerHello messages in TLS 1.3 do
   not contain gmt_unix_time component anymore.

10.  Server Name Indication (SNI)

   This specification mandates the implementation of the SNI extension.
   Where privacy requirements require it, the encrypted SNI extension
   [I-D.ietf-tls-esni] prevents an on-path attacker to determine the
   domain name the client is trying to connect to.  Note, however, that
   the extension is still at an experimental state.

11.  Maximum Fragment Length Negotiation

   The Maximum Fragment Length Negotiation (MFL) extension has been
   superseded by the Record Size Limit (RSL) extension [RFC8449].
   Implementations in compliance with this specification MUST implement
   the RSL extension and SHOULD use it to indicate their RAM
   limitations.

12.  Crypto Agility

   The recommendations in Section 19 of [RFC7925] are applicable.

13.  Key Length Recommendations

   The recommendations in Section 20 of [RFC7925] are applicable.

14.  0-RTT Data

   When clients and servers share a PSK, TLS/DTLS 1.3 allows clients to
   send data on the first flight ("early data").  This features reduces
   communication setup latency but requires application layer protocols
   to define its use with the 0-RTT data functionality.

   For HTTP this functionality is described in [RFC8470].  This document
   specifies the application profile for CoAP, which follows the design
   of [RFC8470].

   For a given request, the level of tolerance to replay risk is
   specific to the resource it operates upon (and therefore only known
   to the origin server).  In general, if processing a request does not
   have state-changing side effects, the consequences of replay are not
   significant.  The server can choose whether it will process early
   data before the TLS handshake completes.

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   It is RECOMMENDED that origin servers allow resources to explicitly
   configure whether early data is appropriate in requests.

   This specification specifies the Early-Data option, which indicates
   that the request has been conveyed in early data and that a client
   understands the 4.25 (Too Early) status code.  The semantic follows
   [RFC8470].

   +-----+---+---+---+---+-------------+--------+--------+---------+---+
   | No. | C | U | N | R | Name        | Format | Length | Default | E |
   +-----+---+---+---+---+-------------+--------+--------+---------+---+
   | TBD | x |   |   |   | Early-Data  | empty  | 0      | (none)  | x |
   +-----+---+---+---+---+-------------+--------+--------+---------+---+

           C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable,
           E=Encrypt and Integrity Protect (when using OSCORE)

                        Figure 1: Early-Data Option

15.  Certificate Profile

   This section contains updates and clarifications to the certificate
   profile defined in [RFC7925].  The content of Table 1 of [RFC7925]
   has been split by certificate "type" in order to clarify exactly what
   requirements and recommendations apply to which entity in the PKI
   hierarchy.

15.1.  All Certificates

15.1.1.  Version

   Certificates MUST be of type X.509 v3.

15.1.2.  Serial Number

   CAs SHALL generate non-sequential Certificate serial numbers greater
   than zero (0) containing at least 64 bits of output from a CSPRNG
   (cryptographically secure pseudo-random number generator).

15.1.3.  Signature

   The signature MUST be ecdsa-with-SHA256 or stronger [RFC5758].

15.1.4.  Issuer

   Contains the DN of the issuing CA.

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15.1.5.  Validity

   No maximum validity period is mandated.  Validity values are
   expressed as UTCTime in notBefore and notAfter fields, as mandated in
   [RFC5280].

   In many cases it is necessary to indicate that a certificate does not
   expire.  This is likely to be the case for manufacturer-provisioned
   certificates.  RFC 5280 provides a simple solution to convey the fact
   that a certificate has no well-defined expiration date by setting the
   notAfter to the GeneralizedTime value of 99991231235959Z.

   Some devices might not have a reliable source of time and for those
   devices it is also advisable to use certificates with no expiration
   date and to let a device management solution manage the lifetime of
   all the certificates used by the device.  While this approach does
   not utilize certificates to its widest extent, it is a solution that
   extends the capabilities offered by a raw public key approach.

15.1.6.  subjectPublicKeyInfo

   The SubjectPublicKeyInfo structure indicates the algorithm and any
   associated parameters for the ECC public key.  This profile uses the
   id-ecPublicKey algorithm identifier for ECDSA signature keys, as
   defined and specified in [RFC5480].

15.2.  Root CA Certificate

   *  basicConstraints MUST be present and MUST be marked critical.  The
      cA field MUST be set true.  The pathLenConstraint field SHOULD NOT
      be present.
   *  keyUsage MUST be present and MUST be marked critical.  Bit
      position for keyCertSign MUST be set.
   *  extendedKeyUsage MUST NOT be present.

15.3.  Intermediate CA Certificate

   *  basicConstraints MUST be present and MUST be marked critical.  The
      cA field MUST be set true.  The pathLenConstraint field MAY be
      present.
   *  keyUsage MUST be present and MUST be marked critical.  Bit
      position for keyCertSign MUST be set.
   *  extendedKeyUsage MUST NOT be present.

15.4.  End Entity Certificate

   *  extendedKeyUsage MUST be present and contain at least one of id-
      kp-serverAuth or id-kp-clientAuth.

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   *  keyUsage MAY be present and contain one of digitalSignature or
      keyAgreement.
   *  Domain names MUST NOT be encoded in the subject commonName,
      instead they MUST be encoded in a subjectAltName of type DNS-ID.
      Domain names MUST NOT contain wildcard ("*") characters.
      subjectAltName MUST NOT contain multiple names.

15.4.1.  Client Certificate Subject

   The requirement in Section 4.4.2 of [RFC7925] to only use EUI-64 for
   client certificates is lifted.

   If the EUI-64 format is used to identify the subject of a client
   certificate, it MUST be encoded in a subjectAltName of type DNS-ID as
   a string of the form "HH-HH-HH-HH-HH-HH-HH-HH" where 'H' is one of
   the symbols '0'-'9' or 'A'-'F'.

16.  Certificate Revocation Checks

   The considerations in Section 4.4.3 of [RFC7925] hold.

   Since the publication of RFC 7925 the need for firmware update
   mechanisms has been reinforced and the work on standardizing a secure
   and interoperable firmware update mechanism has made substantial
   progress, see [I-D.ietf-suit-architecture].  RFC 7925 recommends to
   use a software / firmware update mechanism to provision devices with
   new trust anchors.

   The use of device management protocols for IoT devices, which often
   include an onboarding or bootstrapping mechanism, has also seen
   considerable uptake in deployed devices and these protocols, some of
   which are standardized, allow provision of certificates on a regular
   basis.  This enables a deployment model where IoT device utilize end-
   entity certificates with shorter lifetime making certificate
   revocation protocols, like OCSP and CRLs, less relevant.

   Hence, instead of performing certificate revocation checks on the IoT
   device itself this specification recommends to delegate this task to
   the IoT device operator and to take the necessary action to allow IoT
   devices to remain operational.

16.1.  Open Issues

   A list of open issues can be found at https://github.com/thomas-
   fossati/draft-tls13-iot/issues

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17.  Security Considerations

   This entire document is about security.

18.  Acknowledgements

   We would like to thank Ben Kaduk and John Mattsson.

19.  IANA Considerations

   IANA is asked to add the Option defined in Figure 2 to the CoAP
   Option Numbers registry.

   +--------+------------+-----------+
   | Number | Name       | Reference |
   +--------+------------+-----------+
   | TBD    | Early-Data | RFCThis   |
   +--------+------------+-----------+

                        Figure 2: Early-Data Option

   IANA is asked to add the Response Code defined in Figure 3 to the
   CoAP Response Code registry.

   +--------+-------------+-----------+
   | Code   | Description | Reference |
   +--------+-------------+-----------+
   | 4.25   | Too Early   | RFCThis   |
   +--------+-------------+-----------+

                     Figure 3: Too Early Response Code

20.  References

20.1.  Normative References

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-40, 20 January 2021, <http://www.ietf.org/internet-
              drafts/draft-ietf-tls-dtls13-40.txt>.

   [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>.

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   [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/info/rfc5280>.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
              <https://www.rfc-editor.org/info/rfc5480>.

   [RFC5758]  Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
              Polk, "Internet X.509 Public Key Infrastructure:
              Additional Algorithms and Identifiers for DSA and ECDSA",
              RFC 5758, DOI 10.17487/RFC5758, January 2010,
              <https://www.rfc-editor.org/info/rfc5758>.

   [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>.

   [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>.

   [RFC8449]  Thomson, M., "Record Size Limit Extension for TLS",
              RFC 8449, DOI 10.17487/RFC8449, August 2018,
              <https://www.rfc-editor.org/info/rfc8449>.

   [RFC8470]  Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
              Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
              2018, <https://www.rfc-editor.org/info/rfc8470>.

20.2.  Informative References

   [I-D.ietf-suit-architecture]
              Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
              Firmware Update Architecture for Internet of Things", Work
              in Progress, Internet-Draft, draft-ietf-suit-architecture-
              15, 17 January 2021, <http://www.ietf.org/internet-drafts/
              draft-ietf-suit-architecture-15.txt>.

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   [I-D.ietf-tls-esni]
              Rescorla, E., Oku, K., Sullivan, N., and C. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-09, 16 December 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-tls-esni-
              09.txt>.

Authors' Addresses

   Hannes Tschofenig
   Arm Limited

   Email: Hannes.Tschofenig@gmx.net

   Thomas Fossati
   Arm Limited

   Email: Thomas.Fossati@arm.com

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