TLS H. Tschofenig Internet-Draft M. Brossard Intended status: Standards Track Arm Limited Expires: May 7, 2020 November 04, 2019 Using CBOR Web Tokens (CWTs) in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) draft-tschofenig-tls-cwt-01 Abstract The TLS protocol supports different credentials, including pre-shared keys, raw public keys, and X.509 certificates. For use with public key cryptography developers have to decide between raw public keys, which require out-of-band agreement and full-fletched X.509 certificates. For devices where the reduction of code size is important it is desirable to minimize the use of X.509-related libraries. With the CBOR Web Token (CWT) a structure has been defined that allows CBOR-encoded claims to be protected with CBOR Object Signing and Encryption (COSE). This document registers a new value to the "TLS Certificate Types" sub-registry to allow TLS and DTLS to use CWTs. Conceptually, CWTs can be seen as a certificate format (when with public key cryptography) or a Kerberos ticket (when used with symmetric key cryptography). 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 May 7, 2020. Tschofenig & Brossard Expires May 7, 2020 [Page 1] Internet-Draft CWTs in TLS/DTLS November 2019 Copyright Notice Copyright (c) 2019 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. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3 3. The CWT Certificate Type . . . . . . . . . . . . . . . . . . 3 4. Representation and Verification the Identity of Application Services . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Security and Privacy Considerations . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 7.2. Informative References . . . . . . . . . . . . . . . . . 6 Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 8 Appendix B. Working Group Information . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction The CBOR Web Token (CWT) [RFC8392] was defined as the CBOR-based version of the JSON Web Token (JWT) [RFC7519]. JWT is used extensively on Web application and for use with Internet of Things Tschofenig & Brossard Expires May 7, 2020 [Page 2] Internet-Draft CWTs in TLS/DTLS November 2019 environments the believe is that a more lightweight encoding, namely CBOR, is needed. CWTs, like JWTs, contain claims and those claims are protected against modifications using COSE [RFC8152]. CWTs are flexible with regard to the use of cryptography and hence CWTs may be protected using a keyed message digest, or a digital signature. One of the claims allows keys to be included, as described in [I-D.ietf-ace-cwt-proof-of-possession]. This specification makes use of these proof-of-possession claims in CWTs. This document mandates a minimum number of claims to be present in a CWT. There may, however, be a number of additional claims present in a CWT. An example of a token with a larger number of claims is the Entity Attestation Token (EAT), which may also be encoded as a CWT. Fundamentally, there are two types of keys that can be used with CWTs: - Asymmetric keys: In this case a CWT contains a COSE_Key [RFC8152] representing an asymmetric public key. To protect the CWT against modifications the CWT also needs to be digitally signed. - Symmetric keys: In this case a CWT contains the Encrypted_COSE_Key [RFC8152] structure representing a symmetric key encrypted to a key known to the recipient using COSE_Encrypt or COSE_Encrypt0. Again, to protect the CWT against modifications a keyed message digest is used. The CWT also allows mixing symmetric and asymmetric crypto although this is less likely to be used in practice. Exchanging CWTs in the TLS / DTLS handshake offers an alternative to the use of raw public keys and X.509 certificates. Compared to raw public keys, CWTs allow more information to be included via the use of claims. Compared to X.509 certificates CBOR offers an alternative encoding format, which may also be used by the application layer thereby potentially reducing the overall code size requirements. 2. 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 RFC 2119 [RFC2119]. 3. The CWT Certificate Type This document defines a new value to the "TLS Certificate Types" sub- registry and the value is defined as follows. Tschofenig & Brossard Expires May 7, 2020 [Page 3] Internet-Draft CWTs in TLS/DTLS November 2019 /* Managed by IANA */ enum { X509(0), RawPublicKey(2), CWT(TBD), (255) } CertificateType; struct { select (certificate_type) { /* CWT "certificate type" defined in this document.*/ case CWT: opaque cwt_data<1..2^24-1>; /* RawPublicKey defined in RFC 7250*/ case RawPublicKey: opaque ASN.1_subjectPublicKeyInfo<1..2^24-1>; /* X.509 certificate defined in RFC 5246*/ case X.509: opaque cert_data<1..2^24-1>; }; Extension extensions<0..2^16-1>; } CertificateEntry; 4. Representation and Verification the Identity of Application Services RFC 6125 [RFC6125] provides guidance for matching identifiers used in X.509 certificates against a reference identifier, i.e. an identifier constructed from a source domain and optionally an application service type. Different types of identifiers have been defined over time, such as CN-IDs, DNS-IDs, SRV-IDs, and URI-IDs, and they may be carried in different fields inside the X.509 certificate, such as in the Common Name or in the subjectAltName extension. For CWTs issued to servers the following rule applies: To claim conformance with this specification an implementation MUST populate the Subject claim with the value of the Server Name Indication (SNI) extension. The Subject claim is of type StringOrURI. If it is string an equality match is used between the Subject claim value and the SNI. If the value contains a URI then the URI schema must be matched against the service being requested and the remaining part of the URI is matched against the SNI in an equality match (since the SNI only defines Hostname types). Tschofenig & Brossard Expires May 7, 2020 [Page 4] Internet-Draft CWTs in TLS/DTLS November 2019 For CWTs issued to clients the application service interacting with the TLS/DTLS stack on the server side is responsible for authenticating the client. No specific rules apply but the Subject and the Audience claims are likely to be good candidates for authorization policy checks. Note: Verification of the Not Before and the Expiration Time claims MUST be performed to determine the validity of the received CWT. 5. Security and Privacy Considerations The security and privacy characteristics of this extension are best described in relationship to certificates (when asymmetric keys are used) and to Kerberos tickets (when symmetric keys are used) since the main difference is in the encoding. When creating proof-of-possession keys the recommendations for state- of-the-art key sizes and algorithms have to be followed. For TLS/ DTLS those algorithm recommendations can be found in [RFC7925] and [RFC7525]. CWTs without proof-of-possession keys MUST NOT be used. When CWTs are used with TLS 1.3 [RFC8446] and DTLS 1.3 [I-D.ietf-tls-dtls13] additional privacy properties are provided since most handshake messages are encrypted. 6. IANA Considerations IANA is requested to add a new value to the "TLS Certificate Types" sub-registry for CWTs. 7. References 7.1. Normative References [I-D.ietf-ace-cwt-proof-of-possession] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. Tschofenig, "Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of- possession-11 (work in progress), October 2019. [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-dtls13-33 (work in progress), October 2019. Tschofenig & Brossard Expires May 7, 2020 [Page 5] Internet-Draft CWTs in TLS/DTLS November 2019 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., Weiler, S., and T. Kivinen, "Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, June 2014, . [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", RFC 8152, DOI 10.17487/RFC8152, July 2017, . [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, May 2018, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . 7.2. Informative References [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011, . [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, . [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, . [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, . Tschofenig & Brossard Expires May 7, 2020 [Page 6] Internet-Draft CWTs in TLS/DTLS November 2019 7.3. URIs [1] mailto:tls@ietf.org [2] https://www1.ietf.org/mailman/listinfo/tls [3] https://www.ietf.org/mail-archive/web/tls/current/index.html Tschofenig & Brossard Expires May 7, 2020 [Page 7] Internet-Draft CWTs in TLS/DTLS November 2019 Appendix A. History RFC EDITOR: PLEASE REMOVE THE THIS SECTION - -01: Minor editorial bugfixes and reference updates; refreshing draft - -00: Initial version Appendix B. Working Group Information The discussion list for the IETF TLS working group is located at the e-mail address tls@ietf.org [1]. Information on the group and information on how to subscribe to the list is at https://www1.ietf.org/mailman/listinfo/tls [2] Archives of the list can be found at: https://www.ietf.org/mail- archive/web/tls/current/index.html [3] Authors' Addresses Hannes Tschofenig Arm Limited EMail: hannes.tschofenig@arm.com Mathias Brossard Arm Limited EMail: Mathias.Brossard@arm.com Tschofenig & Brossard Expires May 7, 2020 [Page 8]