HTTP                                                           M. Bishop
Intended status: Standards Track                             N. Sullivan
Expires: May 3, 2018                                          Cloudflare
                                                              M. Thomson
                                                        October 30, 2017

             Secondary Certificate Authentication in HTTP/2


   TLS provides fundamental mutual authentication services for HTTP,
   supporting up to one server certificate and up to one client
   certificate associated to the session to prove client and server
   identities as necessary.  This draft provides mechanisms for
   providing additional such certificates at the HTTP layer when these
   constraints are not sufficient.

   Many HTTP servers host content from several origins.  HTTP/2
   [RFC7540] permits clients to reuse an existing HTTP connection to a
   server provided that the secondary origin is also in the certificate
   provided during the TLS [I-D.ietf-tls-tls13] handshake.

   In many cases, servers will wish to maintain separate certificates
   for different origins but still desire the benefits of a shared HTTP
   connection.  Similarly, servers may require clients to present
   authentication, but have different requirements based on the content
   the client is attempting to access.

   This document describes how TLS exported authenticators
   [I-D.ietf-tls-exported-authenticator] can be used to provide proof of
   ownership of additional certificates to the HTTP layer to support
   both scenarios.

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

Bishop, et al.             Expires May 3, 2018                  [Page 1]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   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 3, 2018.

Copyright Notice

   Copyright (c) 2017 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
   ( 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Server Certificate Authentication . . . . . . . . . . . .   3
     1.2.  Client Certificate Authentication . . . . . . . . . . . .   4
       1.2.1.  HTTP/1.1 using TLS 1.2 and previous . . . . . . . . .   5
       1.2.2.  HTTP/1.1 using TLS 1.3  . . . . . . . . . . . . . . .   6
       1.2.3.  HTTP/2  . . . . . . . . . . . . . . . . . . . . . . .   6
     1.3.  HTTP-Layer Certificate Authentication . . . . . . . . . .   7
     1.4.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   8
   2.  Discovering Additional Certificates at the HTTP/2 Layer . . .   8
     2.1.  Indicating support for HTTP-layer certificate
           authentication  . . . . . . . . . . . . . . . . . . . . .   8
     2.2.  Making certificates or requests available . . . . . . . .   8
     2.3.  Requiring certificate authentication  . . . . . . . . . .   9
   3.  Certificates Frames for HTTP/2  . . . . . . . . . . . . . . .  11
     3.1.  The CERTIFICATE_NEEDED frame  . . . . . . . . . . . . . .  11
     3.2.  The USE_CERTIFICATE Frame . . . . . . . . . . . . . . . .  12
     3.3.  The CERTIFICATE_REQUEST Frame . . . . . . . . . . . . . .  13
     3.4.  The CERTIFICATE Frame . . . . . . . . . . . . . . . . . .  14
       3.4.1.  Exported Authenticator Characteristics  . . . . . . .  15
   4.  Indicating failures during HTTP-Layer Certificate
       Authentication  . . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
     5.1.  Impersonation . . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  Fingerprinting  . . . . . . . . . . . . . . . . . . . . .  17

Bishop, et al.             Expires May 3, 2018                  [Page 2]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

     5.3.  Denial of Service . . . . . . . . . . . . . . . . . . . .  17
     5.4.  Confusion About State . . . . . . . . . . . . . . . . . .  17
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  HTTP/2 SETTINGS_HTTP_CERT_AUTH Setting  . . . . . . . . .  18
     6.2.  New HTTP/2 Frames . . . . . . . . . . . . . . . . . . . .  18
     6.3.  New HTTP/2 Error Codes  . . . . . . . . . . . . . . . . .  19
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   HTTP clients need to know that the content they receive on a
   connection comes from the origin that they intended to retrieve in
   from.  The traditional form of server authentication in HTTP has been
   in the form of X.509 certificates provided during the TLS RFC5246
   [I-D.ietf-tls-tls13] handshake.

   Many existing HTTP [RFC7230] servers also have authentication
   requirements for the resources they serve.  Of the bountiful
   authentication options available for authenticating HTTP requests,
   client certificates present a unique challenge for resource-specific
   authentication requirements because of the interaction with the
   underlying TLS layer.

   TLS 1.2 [RFC5246] supports one server and one client certificate on a
   connection.  These certificates may contain multiple identities, but
   only one certificate may be provided.

1.1.  Server Certificate Authentication

   Section 9.1.1 of [RFC7540] describes how connections may be used to
   make requests from multiple origins as long as the server is
   authoritative for both.  A server is considered authoritative for an
   origin if DNS resolves the origin to the IP address of the server and
   (for TLS) if the certificate presented by the server contains the
   origin in the Subject Alternative Names field.

   [RFC7838] enables a step of abstraction from the DNS resolution.  If
   both hosts have provided an Alternative Service at hostnames which
   resolve to the IP address of the server, they are considered
   authoritative just as if DNS resolved the origin itself to that
   address.  However, the server's one TLS certificate is still required
   to contain the name of each origin in question.

Bishop, et al.             Expires May 3, 2018                  [Page 3]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   [I-D.ietf-httpbis-origin-frame] relaxes the requirement to perform
   the DNS lookup if already connected to a server with an appropriate
   certificate which claims support for a particular origin.

   Servers which host many origins often would prefer to have separate
   certificates for some sets of origins.  This may be for ease of
   certificate management (the ability to separately revoke or renew
   them), due to different sources of certificates (a CDN acting on
   behalf of multiple origins), or other factors which might drive this
   administrative decision.  Clients connecting to such origins cannot
   currently reuse connections, even if both client and server would
   prefer to do so.

   Because the TLS SNI extension is exchanged in the clear, clients
   might also prefer to retrieve certificates inside the encrypted
   context.  When this information is sensitive, it might be
   advantageous to request a general-purpose certificate or anonymous
   ciphersuite at the TLS layer, while acquiring the "real" certificate
   in HTTP after the connection is established.

1.2.  Client Certificate Authentication

   For servers that wish to use client certificates to authenticate
   users, they might request client authentication during or immediately
   after the TLS handshake.  However, if not all users or resources need
   certificate-based authentication, a request for a certificate has the
   unfortunate consequence of triggering the client to seek a
   certificate, possibly requiring user interaction, network traffic, or
   other time-consuming activities.  During this time, the connection is
   stalled in many implementations.  Such a request can result in a poor
   experience, particularly when sent to a client that does not expect
   the request.

   The TLS 1.3 CertificateRequest can be used by servers to give clients
   hints about which certificate to offer.  Servers that rely on
   certificate-based authentication might request different certificates
   for different resources.  Such a server cannot use contextual
   information about the resource to construct an appropriate TLS
   CertificateRequest message during the initial handshake.

   Consequently, client certificates are requested at connection
   establishment time only in cases where all clients are expected or
   required to have a single certificate that is used for all resources.
   Many other uses for client certificates are reactive, that is,
   certificates are requested in response to the client making a

Bishop, et al.             Expires May 3, 2018                  [Page 4]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

1.2.1.  HTTP/1.1 using TLS 1.2 and previous

   In HTTP/1.1, a server that relies on client authentication for a
   subset of users or resources does not request a certificate when the
   connection is established.  Instead, it only requests a client
   certificate when a request is made to a resource that requires a
   certificate.  TLS 1.2 [RFC5246] accomodates this by permitting the
   server to request a new TLS handshake, in which the server will
   request the client's certificate.

   Figure 1 shows the server initiating a TLS-layer renegotiation in
   response to receiving an HTTP/1.1 request to a protected resource.

   Client                                      Server
      -- (HTTP) GET /protected -------------------> *1
      <---------------------- (TLS) HelloRequest -- *2
      -- (TLS) ClientHello ----------------------->
      <------------------ (TLS) ServerHello, ... --
      <---------------- (TLS) CertificateRequest -- *3
      -- (TLS) ..., Certificate ------------------> *4
      -- (TLS) Finished -------------------------->
      <-------------------------- (TLS) Finished --
      <--------------------------- (HTTP) 200 OK -- *5

    Figure 1: HTTP/1.1 Reactive Certificate Authentication with TLS 1.2

   In this example, the server receives a request for a protected
   resource (at *1 on Figure 1).  Upon performing an authorization
   check, the server determines that the request requires authentication
   using a client certificate and that no such certificate has been

   The server initiates TLS renegotiation by sending a TLS HelloRequest
   (at *2).  The client then initiates a TLS handshake.  Note that some
   TLS messages are elided from the figure for the sake of brevity.

   The critical messages for this example are the server requesting a
   certificate with a TLS CertificateRequest (*3); this request might
   use information about the request or resource.  The client then
   provides a certificate and proof of possession of the private key in
   Certificate and CertificateVerify messages (*4).

   When the handshake completes, the server performs any authorization
   checks a second time.  With the client certificate available, it then
   authorizes the request and provides a response (*5).

Bishop, et al.             Expires May 3, 2018                  [Page 5]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

1.2.2.  HTTP/1.1 using TLS 1.3

   TLS 1.3 [I-D.ietf-tls-tls13] introduces a new client authentication
   mechanism that allows for clients to authenticate after the handshake
   has been completed.  For the purposes of authenticating an HTTP
   request, this is functionally equivalent to renegotiation.  Figure 2
   shows the simpler exchange this enables.

   Client                                      Server
      -- (HTTP) GET /protected ------------------->
      <---------------- (TLS) CertificateRequest --
      -- (TLS) Certificate, CertificateVerify,
                  Finished ----------------------->
      <--------------------------- (HTTP) 200 OK --

    Figure 2: HTTP/1.1 Reactive Certificate Authentication with TLS 1.3

   TLS 1.3 does not support renegotiation, instead supporting direct
   client authentication.  In contrast to the TLS 1.2 example, in TLS
   1.3, a server can simply request a certificate.

1.2.3.  HTTP/2

   An important part of the HTTP/1.1 exchange is that the client is able
   to easily identify the request that caused the TLS renegotiation.
   The client is able to assume that the next unanswered request on the
   connection is responsible.  The HTTP stack in the client is then able
   to direct the certificate request to the application or component
   that initiated that request.  This ensures that the application has
   the right contextual information for processing the request.

   In HTTP/2, a client can have multiple outstanding requests.  Without
   some sort of correlation information, a client is unable to identify
   which request caused the server to request a certificate.

   Thus, the minimum necessary mechanism to support reactive certificate
   authentication in HTTP/2 is an identifier that can be use to
   correlate an HTTP request with a request for a certificate.  Since
   streams are used for individual requests, correlation with a stream
   is sufficient.

   [RFC7540] prohibits renegotiation after any application data has been
   sent.  This completely blocks reactive certificate authentication in
   HTTP/2 using TLS 1.2.  If this restriction were relaxed by an
   extension or update to HTTP/2, such an identifier could be added to
   TLS 1.2 by means of an extension to TLS.  Unfortunately, many TLS 1.2
   implementations do not permit application data to continue during a

Bishop, et al.             Expires May 3, 2018                  [Page 6]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   renegotiation.  This is problematic for a multiplexed protocol like

1.3.  HTTP-Layer Certificate Authentication

   This draft defines HTTP/2 frames to carry the relevant certificate
   messages, enabling certificate-based authentication of both clients
   and servers independent of TLS version.  This mechanism can be
   implemented at the HTTP layer without breaking the existing interface
   between HTTP and applications above it.

   This could be done in a naive manner by replicating the TLS messages
   as HTTP/2 frames on each stream.  However, this would create needless
   redundancy between streams and require frequent expensive signing
   operations.  Instead, TLS Exported Authenticators
   [I-D.ietf-tls-exported-authenticator] are exchanged on stream zero
   and the on-stream frames incorporate them by reference as needed.

   TLS Exported Authenticators are structured messages that can be
   exported by either party of a TLS connection and validated by the
   other party.  An authenticator message can be constructed by either
   the client or the server given an established TLS connection, a
   certificate, and a corresponding private key.  Exported
   Authenticators use the message structures from section 4.4 of
   [I-D.ietf-tls-tls13], but different parameters.

   Each Authenticator is computed using a Handshake Context and Finished
   MAC Key derived from the TLS session.  The Handshake Context is
   identical for both parties of the TLS connection, while the Finished
   MAC Key is dependent on whether the Authenticator is created by the
   client or the server.

   Successfully verified Authenticators result in certificate chains,
   with verified possession of the corresponding private key, which can
   be supplied into a collection of available certificates.  Likewise,
   descriptions of desired certificates can be supplied into these
   collections.  These pre-supplied elements are then available for
   automatic use (in some situations) or for reference by individual

   Section 2 describes how the feature is employed, defining means to
   detect support in peers (Section 2.1), make certificates and requests
   available (Section 2.2), and indicate when streams are blocked
   waiting on an appropriate certificate (Section 2.3).  Section 3
   defines the required frame types, which parallel the TLS 1.3 message
   exchange.  Finally, Section 4 defines new error types which can be
   used to notify peers when the exchange has not been successful.

Bishop, et al.             Expires May 3, 2018                  [Page 7]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

1.4.  Terminology

   RFC 2119 [RFC2119] defines the terms "MUST", "MUST NOT", "SHOULD" and

2.  Discovering Additional Certificates at the HTTP/2 Layer

   A certificate chain with proof of possession of the private key
   corresponding to the end-entity certificate is sent as a single
   "CERTIFICATE" frame (see Section 3.4) on stream zero.  Once the
   holder of a certificate has sent the chain and proof, this
   certificate chain is cached by the recipient and available for future
   use.  If the certificate is marked as "AUTOMATIC_USE", the
   certificate may be used by the recipient to authorize any current or
   future request.  Otherwise, the recipient requests the required
   certificate on each stream, but the previously-supplied certificates
   are available for reference without having to resend them.

   Likewise, the details of a request are sent on stream zero and stored
   by the recipient.  These details will be referenced by subsequent

   Data sent by each peer is correlated by the ID given in each frame.
   This ID is unrelated to values used by the other peer, even if each
   uses the same ID in certain cases.

2.1.  Indicating support for HTTP-layer certificate authentication

   Clients and servers that will accept requests for HTTP-layer
   certificate authentication indicate this using the HTTP/2

   The initial value for the "SETTINGS_HTTP_CERT_AUTH" setting is 0,
   indicating that the peer does not support HTTP-layer certificate
   authentication.  If a peer does support HTTP-layer certificate
   authentication, the value is 1.

2.2.  Making certificates or requests available

   When a peer has advertised support for HTTP-layer certificates as in
   Section 2.1, either party can supply additional certificates into the
   connection at any time.  These certificates then become available for
   the peer to consider when deciding whether a connection is suitable
   to transport a particular request.

   Available certificates which have the "AUTOMATIC_USE" flag set MAY be
   used by the recipient without further notice.  This means that
   clients or servers which predict a certificate will be required could

Bishop, et al.             Expires May 3, 2018                  [Page 8]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   pre-supply the certificate without being asked.  Regardless of
   whether "AUTOMATIC_USE" is set, these certificates are available for
   reference by future "USE_CERTIFICATE" frames.

   Client                                      Server
      <-------- (stream 0) CERTIFICATE (AU flag) --
      -- (stream N) GET /from-new-origin --------->
      <----------------------- (stream N) 200 OK --

                  Figure 3: Proactive Server Certificate

   Client                                      Server
      -- (stream 0) CERTIFICATE (AU flag) -------->
      -- (streams 1,3) GET /protected ------------>
      <-------------------- (streams 1,3) 200 OK --

                  Figure 4: Proactive Client Certificate

   Likewise, either party can supply a "CERTIFICATE_REQUEST" that
   outlines parameters of a certificate they might request in the
   future.  It is important to note that this does not currently request
   such a certificate, but makes the contents of the request available
   for reference by a future "CERTIFICATE_NEEDED" frame.

2.3.  Requiring certificate authentication

   As defined in [RFC7540], when a client finds that a https:// origin
   (or Alternative Service [RFC7838]) to which it needs to make a
   request has the same IP address as a server to which it is already
   connected, it MAY check whether the TLS certificate provided contains
   the new origin as well, and if so, reuse the connection.

   If the TLS certificate does not contain the new origin, but the
   server has claimed support for that origin (with an ORIGIN frame, see
   [I-D.ietf-httpbis-origin-frame]) and advertised support for HTTP-
   layer certificates (see Section 2.1), it MAY send a
   "CERTIFICATE_NEEDED" frame on the stream it will use to make the
   request.  (If the request parameters have not already been made
   available using a "CERTIFICATE_REQUEST" frame, the client will need
   to send the "CERTIFICATE_REQUEST" in order to generate the
   "CERTIFICATE_NEEDED" frame.)  The stream represents a pending request
   to that origin which is blocked until a valid certificate is

Bishop, et al.             Expires May 3, 2018                  [Page 9]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   The request is blocked until the server has responded with a
   "USE_CERTIFICATE" frame pointing to a certificate for that origin.
   If the certificate is already available, the server SHOULD
   immediately respond with the appropriate "USE_CERTIFICATE" frame.
   (If the certificate has not already been transmitted, the server will
   need to make the certificate available as described in Section 2.2
   before completing the exchange.)

   If the server does not have the desired certificate, it MUST respond
   with an empty "USE_CERTIFICATE" frame.  In this case, or if the
   server has not advertised support for HTTP-layer certificates, the
   client MUST NOT send any requests for resources in that origin on the
   current connection.

   Client                                      Server
      <----------------------- (stream 0) ORIGIN --
      -- (stream 0) CERTIFICATE_REQUEST ---------->
      -- (stream N) CERTIFICATE_NEEDED ----------->
      <------------------ (stream 0) CERTIFICATE --
      <-------------- (stream N) USE_CERTIFICATE --
      -- (stream N) GET /from-new-origin --------->
      <----------------------- (stream N) 200 OK --

                  Figure 5: Client-Requested Certificate

   Likewise, on each stream where certificate authentication is
   required, the server sends a "CERTIFICATE_NEEDED" frame, which the
   client answers with a "USE_CERTIFICATE" frame indicating the
   certificate to use.  If the request parameters or the responding
   certificate are not already available, they will need to be sent as
   described in Section 2.2 as part of this exchange.

   Client                                      Server
      <---------- (stream 0) CERTIFICATE_REQUEST --
      -- (stream N) GET /protected --------------->
      <----------- (stream N) CERTIFICATE_NEEDED --
      -- (stream 0) CERTIFICATE ------------------>
      -- (stream N) USE_CERTIFICATE -------------->
      <----------------------- (stream N) 200 OK --

               Figure 6: Reactive Certificate Authentication

   A server SHOULD provide certificates for an origin before pushing
   resources from it or supplying content referencing the origin.  If a

Bishop, et al.             Expires May 3, 2018                 [Page 10]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   client receives a "PUSH_PROMISE" referencing an origin for which it
   has not yet received the server's certificate, the client MUST verify
   the server's possession of an appropriate certificate by sending a
   "CERTIFICATE_NEEDED" frame on the pushed stream to inform the server
   that progress is blocked until the request is satisfied.  The client
   MUST NOT use the pushed resource until an appropriate certificate has
   been received and validated.

3.  Certificates Frames for HTTP/2

   correlated by their "Request-ID" field.  Subsequent
   "CERTIFICATE_NEEDED" frames with the same "Request-ID" value MAY be
   sent on other streams where the sender is expecting a certificate
   with the same parameters.

   The "CERTIFICATE", and "USE_CERTIFICATE" frames are correlated by
   their "Cert-ID" field.  Subsequent "USE_CERTIFICATE" frames with the
   same "Cert-ID" MAY be sent in response to other "CERTIFICATE_NEEDED"
   frames and refer to the same certificate.

   "Request-ID" and "Cert-ID" are sender-local, and the use of the same
   value by the other peer does not imply any correlation between their
   frames.  These values MUST be unique per sender over the lifetime of
   the connection.


   The "CERTIFICATE_NEEDED" frame (0xFRAME-TBD1) is sent to indicate
   that the HTTP request on the current stream is blocked pending
   certificate authentication.  The frame includes a request identifier
   which can be used to correlate the stream with a previous
   "CERTIFICATE_REQUEST" frame sent on stream zero.  The
   "CERTIFICATE_REQUEST" describes the certificate the sender requires
   to make progress on the stream in question.

   The "CERTIFICATE_NEEDED" frame contains 2 octets, which is the
   authentication request identifier, "Request-ID".  A peer that
   receives a "CERTIFICATE_NEEDED" of any other length MUST treat this
   as a stream error of type "PROTOCOL_ERROR".  Frames with identical
   request identifiers refer to the same "CERTIFICATE_REQUEST".

   A server MAY send multiple "CERTIFICATE_NEEDED" frames on the same
   stream.  If a server requires that a client provide multiple
   certificates before authorizing a single request, each required
   certificate MUST be indicated with a separate "CERTIFICATE_NEEDED"
   frame, each of which MUST have a different request identifier
   (referencing different "CERTIFICATE_REQUEST" frames describing each

Bishop, et al.             Expires May 3, 2018                 [Page 11]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   required certificate).  To reduce the risk of client confusion,
   servers SHOULD NOT have multiple outstanding "CERTIFICATE_NEEDED"
   frames on the same stream at any given time.

   Clients MUST NOT send multiple "CERTIFICATE_NEEDED" frames on the
   same stream.

   The "CERTIFICATE_NEEDED" frame MUST NOT be sent to a peer which has
   not advertised support for HTTP-layer certificate authentication.

   The "CERTIFICATE_NEEDED" frame MUST NOT be sent on stream zero, and
   MUST NOT be sent on a stream in the "half-closed (local)" state
   [RFC7540].  A client that receives a "CERTIFICATE_NEEDED" frame on a
   stream which is not in a valid state SHOULD treat this as a stream
   error of type "PROTOCOL_ERROR".


   The "USE_CERTIFICATE" frame (0xFRAME-TBD4) is sent in response to a
   "CERTIFICATE_NEEDED" frame to indicate which certificate is being
   used to satisfy the requirement.

   A "USE_CERTIFICATE" frame with no payload refers to the certificate
   provided at the TLS layer, if any.  If no certificate was provided at
   the TLS layer, the stream should be processed with no authentication,
   likely returning an authentication-related error at the HTTP level
   (e.g. 403) for servers or routing the request to a new connection for

   Otherwise, the "USE_CERTIFICATE" frame contains the two-octet "Cert-
   ID" of the certificate the sender wishes to use.  This MUST be the ID
   of a certificate for which proof of possession has been presented in
   a "CERTIFICATE" frame.  Recipients of a "USE_CERTIFICATE" frame of
   any other length MUST treat this as a stream error of type
   "PROTOCOL_ERROR".  Frames with identical certificate identifiers
   refer to the same certificate chain.

   The "USE_CERTIFICATE" frame MUST NOT be sent on stream zero or a
   stream on which a "CERTIFICATE_NEEDED" frame has not been received.
   Receipt of a "USE_CERTIFICATE" frame in these circumstances SHOULD be
   treated as a stream error of type "PROTOCOL_ERROR".  Each
   "USE_CERTIFICATE" frame should reference a preceding "CERTIFICATE"
   frame.  Receipt of a "USE_CERTIFICATE" frame before the necessary
   frames have been received on stream zero MUST also result in a stream
   error of type "PROTOCOL_ERROR".

   The referenced certificate chain MUST conform to the requirements
   expressed in the "CERTIFICATE_REQUEST" to the best of the sender's

Bishop, et al.             Expires May 3, 2018                 [Page 12]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   ability.  Specifically, if the "CERTIFICATE_REQUEST" contained a non-
   empty "Cert-Extensions" element, the end-entity certificate MUST
   match with regard to the extensions recognized by the sender.

   If these requirements are not satisfied, the recipient MAY at its
   discretion either return an error at the HTTP semantic layer, or
   respond with a stream error [RFC7540] on any stream where the
   certificate is used.  Section 4 defines certificate-related error
   codes which might be applicable.


   TLS 1.3 defines the "CertificateRequest" message, which prompts the
   client to provide a certificate which conforms to certain properties
   specified by the server.  This draft defines the
   "CERTIFICATE_REQUEST" frame (0xFRAME-TBD2), which uses the same set
   of extensions to specify a desired certificate, but can be sent over
   any TLS version and can be sent by either peer.

   The "CERTIFICATE_REQUEST" frame SHOULD NOT be sent to a peer which
   has not advertised support for HTTP-layer certificate authentication.

   The "CERTIFICATE_REQUEST" frame MUST be sent on stream zero.  A
   "CERTIFICATE_REQUEST" frame received on any other stream MUST be
   rejected with a stream error of type "PROTOCOL_ERROR".

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |        Request-ID (16)        |      Extension-Count (16)     |
    |                          Extensions(?)                      ...

                Figure 7: CERTIFICATE_REQUEST frame payload

   The frame contains the following fields:

   Request-ID:  "Request-ID" is a 16-bit opaque identifier used to
      correlate subsequent certificate-related frames with this request.
      The identifier MUST be unique in the session for the sender.

   Extension-Count and Extensions:  A list of certificate selection
      criteria, represented in a series of "Extension" structures (see
      [I-D.ietf-tls-tls13] section 4.2).  This criteria MUST be used in
      certificate selection as described in [I-D.ietf-tls-tls13].  The
      number of "Extension" structures is given by the 16-bit
      "Extension-Count" field, which MAY be zero.

Bishop, et al.             Expires May 3, 2018                 [Page 13]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   Some extensions used for certificate selection allow multiple values
   (e.g.  oid_filters on Extended Key Usage).  If the sender has
   included a non-empty Extensions list, the certificate MUST match all
   criteria specified by extensions the recipient recognizes.  However,
   the recipient MUST ignore and skip any unrecognized certificate
   selection extensions.

   Servers MUST be able to recognize the "server_name" extension
   ([RFC6066]) at a minimum.  Clients MUST always specify the desired
   origin using this extension, though other extensions MAY also be

3.4.  The CERTIFICATE Frame

   The "CERTIFICATE" frame (id=0xFRAME-TBD3) provides a exported
   authenticator message from the TLS layer that provides a chain of
   certificates, associated extensions and proves possession of the
   private key corresponding to the end-entity certificate.

   The "CERTIFICATE" frame defines two flags:

   AUTOMATIC_USE (0x01):  Indicates that the certificate can be used
      automatically on future requests.

   TO_BE_CONTINUED (0x02):  Indicates that the exported authenticator
      spans more than one frame.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |          Cert-ID (16)         |   Authenticator Fragment (*)...

                    Figure 8: CERTIFICATE frame payload

   The "Exported Authenticator Fragment" field contains a portion of the
   opaque data returned from the TLS connection exported authenticator
   "authenticate" API.  See Section 3.4.1 for more details on the input
   to this API.

   This opaque data is transported in zero or more "CERTIFICATE" frames
   with the "TO_BE_CONTINUED" flag set, followed by one "CERTIFICATE"
   frame with the "TO_BE_CONTINUED" flag unset.  Each of these frames
   contains the same "Cert-ID" field, permitting them to be associated
   with each other.  Receipt of any "CERTIFICATE" frame with the same
   "Cert-ID" following the receipt of a "CERTIFICATE" frame with
   "TO_BE_CONTINUED" unset MUST be treated as a connection error of type

Bishop, et al.             Expires May 3, 2018                 [Page 14]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   If the "AUTOMATIC_USE" flag is set, the recipient MAY omit sending
   "CERTIFICATE_NEEDED" frames on future streams which would require a
   similar certificate and use the referenced certificate for
   authentication without further notice to the holder.  This behavior
   is optional, and receipt of a "CERTIFICATE_NEEDED" frame does not
   imply that previously-presented certificates were unacceptable, even
   if "AUTOMATIC_USE" was set.  Servers MUST set the "AUTOMATIC_USE"
   flag when sending a "CERTIFICATE" frame.  A server MUST NOT send
   certificates for origins which it is not prepared to service on the
   current connection.

   Upon receiving a complete series of "CERTIFICATE" frames, the
   receiver may validate the Exported Authenticator value by using the
   exported authenticator API.  This returns either an error indicating
   that the message was invalid, or the certificate chain and extensions
   used to create the message.

   The "CERTIFICATE" frame MUST be sent on stream zero.  A "CERTIFICATE"
   frame received on any other stream MUST be rejected with a stream
   error of type "PROTOCOL_ERROR".

3.4.1.  Exported Authenticator Characteristics

   The Exported Authenticator API defined in
   [I-D.ietf-tls-exported-authenticator] takes as input a certificate,
   supporting information about the certificate (OCSP, SCT, etc.), and
   an optional "certificate_request_context".  When generating exported
   authenticators for use with this extension, the
   "certificate_request_context" MUST be the two-octet Cert-ID.

   Upon receipt of a completed authenticator, an endpoint MUST check

   o  the "validate" API confirms the validity of the authenticator

   o  the "certificate_request_context" matches the Cert-ID of the
      frame(s) in which it was received

   Once the authenticator is accepted, the endpoint can perform any
   other checks for the acceptability of the certificate itself.

4.  Indicating failures during HTTP-Layer Certificate Authentication

   Because this draft permits certificates to be exchanged at the HTTP
   framing layer instead of the TLS layer, several certificate-related
   errors which are defined at the TLS layer might now occur at the HTTP

Bishop, et al.             Expires May 3, 2018                 [Page 15]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   framing layer.  In this section, those errors are restated and added
   to the HTTP/2 error code registry.

   BAD_CERTIFICATE (0xERROR-TBD1):  A certificate was corrupt, contained
      signatures that did not verify correctly, etc.

   UNSUPPORTED_CERTIFICATE (0xERROR-TBD2):  A certificate was of an
      unsupported type or did not contain required extensions

   CERTIFICATE_REVOKED (0xERROR-TBD3):  A certificate was revoked by its

   CERTIFICATE_EXPIRED (0xERROR-TBD4):  A certificate has expired or is
      not currently valid

   CERTIFICATE_GENERAL (0xERROR-TBD5):  Any other certificate-related

   As described in [RFC7540], implementations MAY choose to treat a
   stream error as a connection error at any time.  Of particular note,
   a stream error cannot occur on stream 0, which means that
   implementations cannot send non-session errors in response to
   "CERTIFICATE_REQUEST", and "CERTIFICATE" frames.  Implementations
   which do not wish to terminate the connection MAY either send
   relevant errors on any stream which references the failing
   certificate in question or process the requests as unauthenticated
   and provide error information at the HTTP semantic layer.

5.  Security Considerations

   This mechanism defines an alternate way to obtain server and client
   certificates other than in the initial TLS handshake.  While the
   signature of exported authenticator values is expected to be equally
   secure, it is important to recognize that a vulnerability in this
   code path is at least equal to a vulnerability in the TLS handshake.

5.1.  Impersonation

   This mechanism could increase the impact of a key compromise.  Rather
   than needing to subvert DNS or IP routing in order to use a
   compromised certificate, a malicious server now only needs a client
   to connect to _some_ HTTPS site under its control in order to present
   the compromised certificate.  As recommended in
   [I-D.ietf-httpbis-origin-frame], clients opting not to consult DNS
   ought to employ some alternative means to increase confidence that
   the certificate is legitimate.

Bishop, et al.             Expires May 3, 2018                 [Page 16]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   As noted in the Security Considerations of
   [I-D.ietf-tls-exported-authenticator], it difficult to formally prove
   that an endpoint is jointly authoritative over multiple certificates,
   rather than individually authoritative on each certificate.  As a
   result, clients MUST NOT assume that because one origin was
   previously colocated with another, those origins will be reachable
   via the same endpoints in the future.  Clients MUST NOT consider
   previous secondary certificates to be validated after TLS session
   resumption.  However, clients MAY proactively query for previously-
   presented secondary certificates.

5.2.  Fingerprinting

   This draft defines a mechanism which could be used to probe servers
   for origins they support, but opens no new attack versus making
   repeat TLS connections with different SNI values.  Servers SHOULD
   impose similar denial-of-service mitigations (e.g. request rate
   limits) to "CERTIFICATE_REQUEST" frames as to new TLS connections.

   While the extensions in the "CERTIFICATE_REQUEST" frame permit the
   sender to enumerate the acceptable Certificate Authorities for the
   requested certificate, it might not be prudent (either for security
   or data consumption) to include the full list of trusted Certificate
   Authorities in every request.  Senders, particularly clients, SHOULD
   send only the extensions that narrowly specify which certificates
   would be acceptable.

5.3.  Denial of Service

   Failure to provide a certificate on a stream after receiving
   "CERTIFICATE_NEEDED" blocks processing, and SHOULD be subject to
   standard timeouts used to guard against unresponsive peers.

   Validating a multitude of signatures can be computationally
   expensive, while generating an invalid signature is computationally
   cheap.  Implementations will require checks for attacks from this
   direction.  Invalid exported authenticators SHOULD be treated as a
   session error, to avoid further attacks from the peer, though an
   implementation MAY instead disable HTTP-layer certificates for the
   current connection instead.

5.4.  Confusion About State

   Implementations need to be aware of the potential for confusion about
   the state of a connection.  The presence or absence of a validated
   certificate can change during the processing of a request,
   potentially multiple times, as "USE_CERTIFICATE" frames are received.
   A server that uses certificate authentication needs to be prepared to

Bishop, et al.             Expires May 3, 2018                 [Page 17]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

   reevaluate the authorization state of a request as the set of
   certificates changes.

   Client implementations need to carefully consider the impact of
   setting the "AUTOMATIC_USE" flag.  This flag is a performance
   optimization, permitting the client to avoid a round-trip on each
   request where the server checks for certificate authentication.
   However, once this flag has been sent, the client has zero knowledge
   about whether the server will use the referenced cert for any future
   request, or even for an existing request which has not yet completed.
   Clients MUST NOT set this flag on any certificate which is not
   appropriate for currently-in-flight requests, and MUST NOT make any
   future requests on the same connection which they are not willing to
   have associated with the provided certificate.

6.  IANA Considerations

   This draft adds entries in three registries.

   The HTTP/2 "SETTINGS_HTTP_CERT_AUTH" setting is registered in
   Section 6.1.  Four frame types are registered in Section 6.2.  Six
   error codes are registered in Section 6.3.


   The SETTINGS_HTTP_CERT_AUTH setting is registered in the "HTTP/2
   Settings" registry established in [RFC7540].


   Code:  0xSETTING-TBD

   Initial Value:  0

   Specification:  This document.

6.2.  New HTTP/2 Frames

   Four new frame types are registered in the "HTTP/2 Frame Types"
   registry established in [RFC7540].  The entries in the following
   table are registered by this document.

Bishop, et al.             Expires May 3, 2018                 [Page 18]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

          | Frame Type          | Code         | Specification |
          | CERTIFICATE_NEEDED  | 0xFRAME-TBD1 | Section 3.1   |
          |                     |              |               |
          | CERTIFICATE_REQUEST | 0xFRAME-TBD2 | Section 3.3   |
          |                     |              |               |
          | CERTIFICATE         | 0xFRAME-TBD3 | Section 3.4   |
          |                     |              |               |
          | USE_CERTIFICATE     | 0xFRAME-TBD4 | Section 3.2   |

6.3.  New HTTP/2 Error Codes

   Five new error codes are registered in the "HTTP/2 Error Code"
   registry established in [RFC7540].  The entries in the following
   table are registered by this document.

        | Name                    | Code         | Specification |
        | BAD_CERTIFICATE         | 0xERROR-TBD1 | Section 4     |
        |                         |              |               |
        | UNSUPPORTED_CERTIFICATE | 0xERROR-TBD2 | Section 4     |
        |                         |              |               |
        | CERTIFICATE_REVOKED     | 0xERROR-TBD3 | Section 4     |
        |                         |              |               |
        | CERTIFICATE_EXPIRED     | 0xERROR-TBD4 | Section 4     |
        |                         |              |               |
        | CERTIFICATE_GENERAL     | 0xERROR-TBD5 | Section 4     |

7.  Acknowledgements

   Eric Rescorla pointed out several failings in an earlier revision.
   Andrei Popov contributed to the TLS considerations.

8.  References

8.1.  Normative References

              Sullivan, N., "Exported Authenticators in TLS", draft-
              ietf-tls-exported-authenticator-03 (work in progress),
              July 2017.

Bishop, et al.             Expires May 3, 2018                 [Page 19]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
              July 2017.

   [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-

   [RFC2459]  Housley, R., Ford, W., Polk, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and CRL
              Profile", RFC 2459, DOI 10.17487/RFC2459, January 1999,

   [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-

   [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,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011, <https://www.rfc-

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015, <https://www.rfc-

   [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 ISO/IEC 8825-1:2002, 2002,

Bishop, et al.             Expires May 3, 2018                 [Page 20]

Internet-Draft        Secondary Cert Auth in HTTP/2         October 2017

8.2.  Informative References

              Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
              draft-ietf-httpbis-origin-frame-04 (work in progress),
              August 2017.

   [RFC7838]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <>.

Authors' Addresses

   Mike Bishop


   Nick Sullivan


   Martin Thomson


Bishop, et al.             Expires May 3, 2018                 [Page 21]