TLS 1.3 Extension for Certificate-based Authentication with an External Pre-Shared Key
draft-housley-tls-tls13-cert-with-extern-psk-02

Network Working Group                                         R. Housley
Internet-Draft                                            Vigil Security
Intended status: Standards Track                      September 26, 2018
Expires: March 30, 2019

TLS 1.3 Extension for Certificate-based Authentication with an External
                             Pre-Shared Key
            draft-housley-tls-tls13-cert-with-extern-psk-02

Abstract

   This document specifies a TLS 1.3 extension that allows a server to
   authenticate with a certificate while also providing a pre-shared key
   (PSK) as an input to the key schedule.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 30, 2019.

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1.  Introduction

   The TLS 1.3 [RFC8446] handshake protocol provides two mutually
   exclusive forms of server authentication.  First, the server can be
   authenticated by providing a signature certificate and creating a
   valid digital signature to demonstrate that it possesses the
   corresponding private key.  Second, the server can be authenticated
   by demonstrating that it possesses a pre-shared key (PSK) that was
   established by a previous handshake.  A PSK that is established in
   this fashion is called a resumption PSK.  A PSK that is established
   by any other means is called an external PSK.  This document
   specifies a TLS 1.3 extension permitting certificate-based server
   authentication to be combined with either of these two types of PSK
   as an input to the TLS 1.3 key schedule.

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.

3.  Motivation and Design Rationale

   The motivation for using a certificate with an external PSK is
   different than the motivation for using a certificate with a
   resumption PSK.

3.1.  Certificate With External PSK

   The invention of a large-scale quantum computer would pose a serious
   challenge for the cryptographic algorithms that are widely deployed
   today, including the digital signature algorithms that are used to
   authenticate the server in the TLS 1.3 handshake protocol and key
   agreement algorithm used to establish a pairwise shared secret
   between the client and server.  It is an open question whether or not
   it is feasible to build a large-scale quantum computer, and if so,
   when that might happen.  However, if such a quantum computer is
   invented, many of the cryptographic algorithms and the security
   protocols that use them would become vulnerable.

   The TLS 1.3 handshake protocol employs key agreement algorithms that
   could be broken by the invention of a large-scale quantum computer
   [I-D.hoffman-c2pq].  These algorithms include Diffie-Hellman (DH)
   [DH] and Elliptic Curve Diffie-Hellman (ECDH) [IEEE1363].  As a
   result, an adversary that stores a TLS 1.3 handshake protocol

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   exchange today could decrypt the associated encrypted communications
   in the future when a large-scale quantum computer becomes available.

   When a certificate is used for authentication and a strong external
   PSK is used in conjunction with a key agreement algorithm, today's
   communications can be protected from the future invention of a large-
   scale quantum computer.  The strong external PSK and the shared
   secret from the key agreement algorithms are both provided as inputs
   to the TLS 1.3 key schedule, which preserves the authentication
   provided by the existing certificate and digital signature
   mechanisms, and requires the attacker to learn the external PSK as
   well as the shared secret to break confidentiality.

3.2.  Certificate With Resumption PSK

   There are two motivations for using a certificate with a resumption
   PSK.

   In the first situation, the client seeks corroboration that the
   server has access to the private key associated with the certificate.
   That is, the server uses the same certificate in this handshake as
   was used to establish the resumption PSK.  Successful completion of
   the handshake requires the server to produce a valid signature in the
   CertificateVerify handshake message.

   In the second situation, the server wishes to use a different
   certificate for the resumption handshake, which allows the resumed
   session to be associated with a different server identity.
   Successful completion of the handshake requires the server to produce
   a valid signature in the CertificateVerify message that can be
   validated with the public key in the certificate that is provided in
   the Certificate handshake message.

3.3.  Design Considerations With Early Data

   When a client provides early data and makes use of a certificate with
   a resumption PSK, the server MUST use the same certificate, public
   key, and private key as in the original handshake.  Doing otherwise
   would create an ambiguity about the server identity that received the
   early data.  For this reason, the handshake fails if the client sends
   early data and the server uses a different certificate with a
   resumption PSK.

4.  Extension Overview

   This section provides a brief overview of the "tls_cert_with_psk"
   extension.

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   The client includes the "tls_cert_with_psk" extension in the
   ClientHello message.  The "tls_cert_with_psk" extension MUST
   accompanied by the "key_share", "psk_key_exchange_modes", and
   "pre_shared_key" extensions.  The "pre_shared_key" extension MUST be
   the last extension in the ClientHello message, and it provides a list
   of PSK identifiers that the client is willing to use with this
   server.  If the "tls_cert_with_psk" extension is used with a
   resumption PSK and the "early_data" extension, then the client MUST
   check that the server provided the same certificate as was used in
   the initial handshake.  These extensions are all described in
   Section 4.2 of [RFC8446].

   If the server is willing to use one of the PSKs listed in the
   "pre_shared_key" extension and perform certificate-based
   authentication, then the server includes the "tls_cert_with_psk"
   extension in the ServerHello message.  The "tls_cert_with_psk"
   extension MUST be accompanied by the "key_share" and "pre_shared_key"
   extensions.  If none of the PSKs in the list provided by the client
   is acceptable to the server, then the "tls_cert_with_psk" extension
   is omitted from the ServerHello message.

   The successful negotiation of the "tls_cert_with_psk" extension
   requires the TLS 1.3 key schedule processing to include both the
   selected PSK and the (EC)DHE shared secret value.  As a result, the
   Early Secret, Handshake Secret, and Master Secret values all depend
   upon the value of the selected PSK.

   The authentication of the server and optional authentication of the
   client depend upon the ability to generate a signature that can be
   validated with the public key in their certificates.  The
   authentication processing is not changed in any way by the selected
   PSK.

   As required by Section 4.2.11 of [RFC8446], each external PSK is
   associated with a single Hash algorithm.  The hash algorithm MUST be
   set when the external PSK is established, with a default of SHA-256
   if no hash algorithm is specified during establishment.

   Resumption PSKs are established via the ticket mechanism described in
   Section 4.6.1 of [RFC8446].  The hash algorithm associated with the
   resumption PSK MUST be the same KDF hash algorithm as that used to
   establish the initial session.  This is the KDF hash algorithm of the
   session where the ticket was established.

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5.  Certificate with PSK Extension

   This section specifies the "tls_cert_with_psk" extension, which MAY
   appear in the ClientHello message and ServerHello message.  It MUST
   NOT appear in any other messages.  The "tls_cert_with_psk" extension
   MUST NOT appear in the ServerHello message unless "tls_cert_with_psk"
   extension appeared in the preceding ClientHello message.  If an
   implementation recognizes the "tls_cert_with_psk" extension and
   receives it in any other message, then the implementation MUST abort
   the handshake with an "illegal_parameter" alert.

   The TLS 1.3 general extension mechanisms enable clients and servers
   to negotiate the use of specific extensions.  Clients request
   extended functionality from servers with the extensions field in the
   ClientHello message.  If the server responds with a HelloRetryRequest
   message, then the client sends another ClientHello message as
   described in Section 4.1.2 of [RFC8446], and it MUST include the same
   "tls_cert_with_psk" extension as the original ClientHello message or
   abort the handshake.

   Many server extensions are carried in the EncryptedExtensions
   message; however, the "tls_cert_with_psk" extension is carried in the
   ServerHello message.  It is only present in the ServerHello message
   if the server recognizes the "tls_cert_with_psk" extension and the
   server possesses one of the PSKs offered by the client in the
   "pre_shared_key" extension in the ClientHello message.

   The Extension structure is defined in [RFC8446]; it is repeated here
   for convenience.

     struct {
         ExtensionType extension_type;
         opaque extension_data<0..2^16-1>;
     } Extension;

   The "extension_type" identifies the particular extension type, and
   the "extension_data" contains information specific to the particular
   extension type.

   This document specifies the "tls_cert_with_psk" extension, adding one
   new type to ExtensionType:

     enum {
         tls_cert_with_psk(TBD), (65535)
     } ExtensionType;

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   In an initial handshake, the "tls_cert_with_psk" extension is
   relevant when the client and server possess an external PSK in common
   that can be used as an input to the TLS 1.3 key schedule.  In a
   subsequent handshake, the "tls_cert_with_psk" extension is relevant
   when the client and server possess a resumptions PSK in common and
   server authentication with a certificate is desired.  The
   "tls_cert_with_psk" extension has the following syntax:

     struct {
         select (Handshake.msg_type) {
             case client_hello: Empty;
             case server_hello: Empty;
         };
     } CertWithPSK;

   To use a PSK with certificates, clients MUST provide the
   "tls_cert_with_psk" extension, and it MUST be accompanied by the
   "key_share", "psk_key_exchange_modes", and "pre_shared_key"
   extensions in the ClientHello.  If clients offer a
   "tls_cert_with_psk" extension without all of these other extensions,
   servers MUST abort the handshake.  The client MAY also find it useful
   to include the the "supported_groups" extension.  If clients offer a
   "early_data" extension during a resumption handshake, then clients
   MUST confirm that the server uses the same certificate, public key,
   and private key as in the handshake that established the resumption
   PSK.  Note that Section 4.2 of [RFC8446] allows extensions to appear
   in any order, with the exception of the "pre_shared_key" extension,
   which MUST be the last extension in the ClientHello.  Also, there
   MUST NOT be more than one instance of any extension in the
   ClientHello message.

   The "key_share" extension is defined in Section 4.2.8 of [RFC8446].

   The "psk_key_exchange_modes" extension is defined in Section 4.2.9 of
   [RFC8446].  The "psk_key_exchange_modes" extension restricts both the
   use of PSKs offered in this ClientHello and those which the server
   might supply via a subsequent NewSessionTicket.  As a result, clients
   MUST include the psk_dhe_ke mode for an initial handshake, and
   servers MUST select the psk_dhe_ke mode for the initial handshake.
   Servers MUST select a key exchange mode that is listed by the client
   for subsequent handshakes that include the resumption PSK from the
   initial handshake.

   The "early_data" extension is defined in Section 4.2.10 of [RFC8446].

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   The "supported_groups" extension is defined in Section 4.2.7 of
   [RFC8446].

   The "pre_shared_key" extension is defined in Section 4.2.11 of
   [RFC8446]. the syntax is repeated below for convenience.  All of the
   listed PSKs MUST be external PSKs.

     struct {
         opaque identity<1..2^16-1>;
         uint32 obfuscated_ticket_age;
     } PskIdentity;

     opaque PskBinderEntry<32..255>;

     struct {
         PskIdentity identities<7..2^16-1>;
         PskBinderEntry binders<33..2^16-1>;
     } OfferedPsks;

     struct {
         select (Handshake.msg_type) {
             case client_hello: OfferedPsks;
             case server_hello: uint16 selected_identity;
         };
     } PreSharedKeyExtension;

   The OfferedPsks contains the list of PSK identities and associated
   binders for the PSKs that the client is willing to use with the
   server.

   The identities are a list of PSK identities that the client is
   willing to negotiate with the server.  Each PSK has an associated
   identity that is known to the client and the server.  (The identity
   is also referred to as an identifier or a label.)

   The obfuscated_ticket_age is not used for external PSKs; clients
   SHOULD set this value to 0, and servers MUST ignore the value.  The
   obfuscated_ticket_age is used for resumption PSKs, and
   Section 4.2.11.1 of [RFC8446] describes how to form this value for
   identities established via the NewSessionTicket message.

   The binders are a series of HMAC values, one for each PSK offered by
   the client, in the same order as the identities list.  The HMAC value
   is computed using the binder_key, which is derived from the PSK, and
   a partial transcript of the current handshake.  Generation of the
   binder_key from the PSK is described in Section 7.1 of [RFC8446].

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   The partial transcript of the current handshake includes a partial
   ClientHello up to and including the PreSharedKeyExtension.identities
   field as described in Section 4.2.11.2 of [RFC8446].

   The selected_identity contains the PSK identity that the server
   selected from the list offered by the client.  If none of the offered
   PSKs in the list provided by the client are acceptable to the server,
   then the "tls_cert_with_psk" extension MUST be omitted from the
   ServerHello message.  The server MUST validate the binder value that
   corresponds to the selected PSK as described in Section 4.2.11.2 of
   [RFC8446].  If the binder does not validate, the server MUST abort
   the handshake with an "illegal_parameter" alert.  Servers SHOULD NOT
   attempt to validate multiple binders; rather they SHOULD select one
   of the offered PSKs and validate only the binder that corresponds to
   that PSK.

   When the "tls_cert_with_psk" extension is successfully negotiated,
   authentication of the server depends upon the ability to generate a
   signature that can be validated with the public key in the server's
   certificate.  This is accomplished by the server sending the
   Certificate and CertificateVerify messages as described in Sections
   4.4.2 and 4.4.3 of [RFC8446].

   TLS 1.3 does not permit the server to send a CertificateRequest
   message when a PSK is being used.  This restriction is removed when
   the "tls_cert_with_psk" extension is negotiated, allowing the
   certificate-based authentication for both the client and the server.
   If certificate-based client authentication is desired, this is
   accomplished by the client sending the Certificate and
   CertificateVerify messages as described in Sections 4.4.2 and 4.4.3
   of [RFC8446].

   Section 7.1 of [RFC8446] specifies the TLS 1.3 Key Schedule.  The
   successful negotiation of the "tls_cert_with_psk" extension requires
   the key schedule processing in the initial handshake to include both
   the external PSK and the (EC)DHE shared secret value.  In a
   resumption handshake, the resumption PSK MUST be used in the key
   schedule, and the (EC)DHE shared secret MAY also be used.

   If the client and the server have different values associated with
   the selected PSK identifier, then the client and the server will
   compute different values for every entry in the TLS 1.3 key schedule,
   which will lead to the termination of the connection with a
   "decrypt_error" alert.

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6.  IANA Considerations

   IANA is requested to update the TLS ExtensionType Registry to include
   "tls_cert_with_psk" with a value of TBD and the list of messages "CH,
   SH" in which the "tls_cert_with_psk" extension may appear.

7.  Security Considerations

   The Security Considerations in [RFC8446] remain relevant.

   TLS 1.3 [RFC8446] does not permit the server to send a
   CertificateRequest message when a PSK is being used.  This
   restriction is removed when the "tls_cert_with_psk" extension is
   offered by the client and accepted by the server.

   Implementations need to protect the pre-shared key (PSK).  Compromise
   of the external PSK used in the initial handshake makes the encrypted
   session content vulnerable to the future invention of a large-scale
   quantum computer.  Compromise of the resumption PSK makes the
   encrypted session content associated with subsequent sessions
   vulnerable to an attacker that knows the PSK, and it allows the
   attacker to initiate new sessions which are also vunlerable.

   Implementers should not transmit the same content on a connection
   that is protected with an external PSK and a connection that is not.
   Doing so may allow an eavesdropper to correlate the connections,
   making the content vulnerable to the future invention of a large-
   scale quantum computer.

   Deployment of a pairwise external PSK between every client and server
   is not practical.  Instead, this specification envisions an external
   PSK being distributed to a group of clients and group of severs.  At
   some point in the future a large-scale quantum computer might get
   invented, and if any member of the group has access to it, then that
   group member can recover the traffic associated with the PSK.
   However, parties outside the group cannot recover the traffic because
   the large-scale quantum computer does not assist with the discovery
   of the external PSK of reasonable size.

   Implementations must choose external PSKs with a secure key
   management technique, such as pseudo-random generation of the key or
   derivation of the key from one or more other secure keys.  The use of
   inadequate pseudo-random number generators (PRNGs) to generate
   external PSKs can result in little or no security.  An attacker may
   find it much easier to reproduce the PRNG environment that produced
   the external PSKs and searching the resulting small set of
   possibilities, rather than brute force searching the whole key space.

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   The generation of quality random numbers is difficult.  [RFC4086]
   offers important guidance in this area.

   TLS 1.3 [RFC8446] takes a conservative approach to PSKs; they are
   bound to a specific hash function and KDF.  By contrast, TLS 1.2
   [RFC5246] allows PSKs to be used with any hash function and the TLS
   1.2 PRF.  Thus, the safest approach is to use a PSK with either TLS
   1.2 or TLS 1.3.  However, any PSK that might be used with both TLS
   1.2 and TLS 1.3 must be used with only one hash function, which is
   the one that is bound for use in TLS 1.3.  This restriction is less
   than optimal when users want to provision a single PSK.  While the
   constructions used in TLS 1.2 and TLS 1.3 are both based on HMAC
   [RFC2104], the constructions are different, and there is no known way
   in which reuse of the same PSK in TLS 1.2 and TLS 1.3 that would
   produce related outputs.

8.  Acknowledgments

   Many thanks to Nikos Mavrogiannopoulos, Nick Sullivan, Martin
   Thomson, and Peter Yee for their review and comments; their efforts
   have improved this document.

9.  References

9.1.  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>.

   [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>.

9.2.  Informative References

   [DH]       Diffie, W. and M. Hellman, "New Directions in
              Cryptography", IEEE Transactions on Information
              Theory V.IT-22 n.6, June 1977.

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   [I-D.hoffman-c2pq]
              Hoffman, P., "The Transition from Classical to Post-
              Quantum Cryptography", draft-hoffman-c2pq-04 (work in
              progress), August 2018.

   [IEEE1363]
              Institute of Electrical and Electronics Engineers, "IEEE
              Standard Specifications for Public-Key Cryptography", IEEE
              Std 1363-2000, 2000.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

Author's Address

   Russ Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA  20170
   USA

   Email: housley@vigilsec.com

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