[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04

Network Working Group                                         J. Yasskin
Internet-Draft                                                    Google
Intended status: Standards Track                           June 14, 2018
Expires: December 16, 2018


                         Signed HTTP Exchanges
             draft-yasskin-http-origin-signed-responses-04

Abstract

   This document specifies how a server can send an HTTP request/
   response pair, known as an exchange, with signatures that vouch for
   that exchange's authenticity.  These signatures can be verified
   against an origin's certificate to establish that the exchange is
   authoritative for an origin even if it was transferred over a
   connection that isn't.  The signatures can also be used in other ways
   described in the appendices.

   These signatures contain countermeasures against downgrade and
   protocol-confusion attacks.

Note to Readers

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].

   The source code and issues list for this draft can be found in
   https://github.com/WICG/webpackage [2].

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 December 16, 2018.




Yasskin                 Expires December 16, 2018               [Page 1]


Internet-Draft            Signed HTTP Exchanges                June 2018


Copyright Notice

   Copyright (c) 2018 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Signing an exchange . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  The Signature Header  . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Examples  . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.2.  Open Questions  . . . . . . . . . . . . . . . . . . .   8
     3.2.  CBOR representation of exchange headers . . . . . . . . .   8
       3.2.1.  Example . . . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Loading a certificate chain . . . . . . . . . . . . . . .  10
     3.4.  Canonical CBOR serialization  . . . . . . . . . . . . . .  11
     3.5.  Signature validity  . . . . . . . . . . . . . . . . . . .  12
       3.5.1.  Open Questions  . . . . . . . . . . . . . . . . . . .  15
     3.6.  Updating signature validity . . . . . . . . . . . . . . .  15
       3.6.1.  Examples  . . . . . . . . . . . . . . . . . . . . . .  16
     3.7.  The Accept-Signature header . . . . . . . . . . . . . . .  18
       3.7.1.  Integrity identifiers . . . . . . . . . . . . . . . .  19
       3.7.2.  Key type identifiers  . . . . . . . . . . . . . . . .  19
       3.7.3.  Key value identifiers . . . . . . . . . . . . . . . .  19
       3.7.4.  Examples  . . . . . . . . . . . . . . . . . . . . . .  20
       3.7.5.  Open Questions  . . . . . . . . . . . . . . . . . . .  20
   4.  Cross-origin trust  . . . . . . . . . . . . . . . . . . . . .  21
     4.1.  Stateful header fields  . . . . . . . . . . . . . . . . .  22
     4.2.  Certificate Requirements  . . . . . . . . . . . . . . . .  23
   5.  Transferring a signed exchange  . . . . . . . . . . . . . . .  24
     5.1.  Same-origin response  . . . . . . . . . . . . . . . . . .  24
       5.1.1.  Significant headers for a same-origin response  . . .  24
       5.1.2.  The Signed-Headers Header . . . . . . . . . . . . . .  25
     5.2.  HTTP/2 extension for cross-origin Server Push . . . . . .  26
       5.2.1.  Indicating support for cross-origin Server Push . . .  26
       5.2.2.  NO_TRUSTED_EXCHANGE_SIGNATURE error code  . . . . . .  26
       5.2.3.  Validating a cross-origin Push  . . . . . . . . . . .  26



Yasskin                 Expires December 16, 2018               [Page 2]


Internet-Draft            Signed HTTP Exchanges                June 2018


     5.3.  application/signed-exchange format  . . . . . . . . . . .  27
       5.3.1.  Cross-origin trust in application/signed-exchange . .  28
       5.3.2.  Example . . . . . . . . . . . . . . . . . . . . . . .  28
       5.3.3.  Open Questions  . . . . . . . . . . . . . . . . . . .  29
   6.  Security considerations . . . . . . . . . . . . . . . . . . .  29
     6.1.  Over-signing  . . . . . . . . . . . . . . . . . . . . . .  29
       6.1.1.  Session fixation  . . . . . . . . . . . . . . . . . .  30
       6.1.2.  Misleading content  . . . . . . . . . . . . . . . . .  30
     6.2.  Off-path attackers  . . . . . . . . . . . . . . . . . . .  30
     6.3.  Downgrades  . . . . . . . . . . . . . . . . . . . . . . .  30
     6.4.  Signing oracles are permanent . . . . . . . . . . . . . .  31
     6.5.  Unsigned headers  . . . . . . . . . . . . . . . . . . . .  31
     6.6.  application/signed-exchange . . . . . . . . . . . . . . .  31
   7.  Privacy considerations  . . . . . . . . . . . . . . . . . . .  32
   8.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  32
     8.1.  Signature Header Field Registration . . . . . . . . . . .  32
     8.2.  Accept-Signature Header Field Registration  . . . . . . .  33
     8.3.  Signed-Headers Header Field Registration  . . . . . . . .  33
     8.4.  HTTP/2 Settings . . . . . . . . . . . . . . . . . . . . .  33
     8.5.  HTTP/2 Error code . . . . . . . . . . . . . . . . . . . .  33
     8.6.  Internet Media Type application/signed-exchange . . . . .  34
     8.7.  Internet Media Type application/cert-chain+cbor . . . . .  35
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  36
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  38
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  40
   Appendix A.  Use cases  . . . . . . . . . . . . . . . . . . . . .  41
     A.1.  PUSHed subresources . . . . . . . . . . . . . . . . . . .  41
     A.2.  Explicit use of a content distributor for subresources  .  41
     A.3.  Subresource Integrity . . . . . . . . . . . . . . . . . .  42
     A.4.  Binary Transparency . . . . . . . . . . . . . . . . . . .  42
     A.5.  Static Analysis . . . . . . . . . . . . . . . . . . . . .  43
     A.6.  Offline websites  . . . . . . . . . . . . . . . . . . . .  43
   Appendix B.  Requirements . . . . . . . . . . . . . . . . . . . .  43
     B.1.  Proof of origin . . . . . . . . . . . . . . . . . . . . .  43
       B.1.1.  Certificate constraints . . . . . . . . . . . . . . .  43
       B.1.2.  Signature constraints . . . . . . . . . . . . . . . .  44
       B.1.3.  Retrieving the certificate  . . . . . . . . . . . . .  44
     B.2.  How much to sign  . . . . . . . . . . . . . . . . . . . .  45
       B.2.1.  Conveying the signed headers  . . . . . . . . . . . .  45
     B.3.  Response lifespan . . . . . . . . . . . . . . . . . . . .  46
       B.3.1.  Certificate revocation  . . . . . . . . . . . . . . .  46
       B.3.2.  Response downgrade attacks  . . . . . . . . . . . . .  46
     B.4.  Low implementation complexity . . . . . . . . . . . . . .  47
       B.4.1.  Limited choices . . . . . . . . . . . . . . . . . . .  47
       B.4.2.  Bounded-buffering integrity checking  . . . . . . . .  47
   Appendix C.  Determining validity using cache control . . . . . .  48
     C.1.  Example of updating cache control . . . . . . . . . . . .  48



Yasskin                 Expires December 16, 2018               [Page 3]


Internet-Draft            Signed HTTP Exchanges                June 2018


     C.2.  Downsides of updating cache control . . . . . . . . . . .  49
   Appendix D.  Change Log . . . . . . . . . . . . . . . . . . . . .  49
   Appendix E.  Acknowledgements . . . . . . . . . . . . . . . . . .  51
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  51

1.  Introduction

   Signed HTTP exchanges provide a way to prove the authenticity of a
   resource in cases where the transport layer isn't sufficient.  This
   can be used in several ways:

   o  When signed by a certificate ([RFC5280]) that's trusted for an
      origin, an exchange can be treated as authoritative for that
      origin, even if it was transferred over a connection that isn't
      authoritative (Section 9.1 of [RFC7230]) for that origin.  See
      Appendix A.1 and Appendix A.2.

   o  A top-level resource can use a public key to identify an expected
      publisher for particular subresources, a system known as
      Subresource Integrity ([SRI]).  An exchange's signature provides
      the matching proof of authorship.  See Appendix A.3.

   o  A signature can vouch for the exchange in some way, for example
      that it appears in a transparency log or that static analysis
      indicates that it omits certain attacks.  See Appendix A.4 and
      Appendix A.5.

   Subsequent work toward the use cases in
   [I-D.yasskin-webpackage-use-cases] will provide a way to group signed
   exchanges into bundles that can be transmitted and stored together,
   but single signed exchanges are useful enough to standardize on their
   own.

2.  Terminology

   Author  The entity that wrote the content in a particular resource.
      This specification deals with publishers rather than authors.

   Publisher  The entity that controls the server for a particular
      origin [RFC6454].  The publisher can get a CA to issue
      certificates for their private keys and can run a TLS server for
      their origin.

   Exchange (noun)  An HTTP request/response pair.  This can either be a
      request from a client and the matching response from a server or
      the request in a PUSH_PROMISE and its matching response stream.
      Defined by Section 8 of [RFC7540].




Yasskin                 Expires December 16, 2018               [Page 4]


Internet-Draft            Signed HTTP Exchanges                June 2018


   Intermediate  An entity that fetches signed HTTP exchanges from a
      publisher or another intermediate and forwards them to another
      intermediate or a client.

   Client  An entity that uses a signed HTTP exchange and needs to be
      able to prove that the publisher vouched for it as coming from its
      claimed origin.

   Unix time  Defined by [POSIX] section 4.16 [3].

   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.  Signing an exchange

   In the response of an HTTP exchange the server MAY include a
   "Signature" header field (Section 3.1) holding a list of one or more
   parameterised signatures that vouch for the content of the exchange.
   Exactly which content the signature vouches for can depend on how the
   exchange is transferred (Section 5).

   The client categorizes each signature as "valid" or "invalid" by
   validating that signature with its certificate or public key and
   other metadata against the exchange's headers and content
   (Section 3.5).  This validity then informs higher-level protocols.

   Each signature is parameterised with information to let a client
   fetch assurance that a signed exchange is still valid, in the face of
   revoked certificates and newly-discovered vulnerabilities.  This
   assurance can be bundled back into the signed exchange and forwarded
   to another client, which won't have to re-fetch this validity
   information for some period of time.

3.1.  The Signature Header

   The "Signature" header field conveys a list of signatures for an
   exchange, each one accompanied by information about how to determine
   the authority of and refresh that signature.  Each signature directly
   signs the exchange's headers and identifies one of those headers that
   enforces the integrity of the exchange's payload.

   The "Signature" header is a Structured Header as defined by
   [I-D.ietf-httpbis-header-structure].  Its value MUST be a
   parameterised list (Section 3.3 of
   [I-D.ietf-httpbis-header-structure]).  Its ABNF is:



Yasskin                 Expires December 16, 2018               [Page 5]


Internet-Draft            Signed HTTP Exchanges                June 2018


   Signature = sh-param-list

   Each parameterised identifier in the list MUST have parameters named
   "sig", "integrity", "validity-url", "date", and "expires".  Each
   parameterised identifier MUST also have either "cert-url" and "cert-
   sha256" parameters or an "ed25519key" parameter.  This specification
   gives no meaning to the identifier itself, which can be used as a
   human-readable identifier for the signature (see
   Section 3.1.2, Paragraph 1).  The present parameters MUST have the
   following values:

   "sig"  Binary content (Section 3.9 of
      [I-D.ietf-httpbis-header-structure]) holding the signature of most
      of these parameters and the exchange's headers.

   "integrity"  A string (Section 3.7 of
      [I-D.ietf-httpbis-header-structure]) containing the lowercase name
      of the response header field that guards the response payload's
      integrity.

   "cert-url"  A string (Section 3.7 of
      [I-D.ietf-httpbis-header-structure]) containing an absolute-URL
      string [4] ([URL]).

   "cert-sha256"  Binary content (Section 3.9 of
      [I-D.ietf-httpbis-header-structure]) holding the SHA-256 hash of
      the first certificate found at "cert-url".

   "ed25519key"  Binary content (Section 3.9 of
      [I-D.ietf-httpbis-header-structure]) holding an Ed25519 public key
      ([RFC8032]).

   "validity-url"  A string (Section 3.7 of
      [I-D.ietf-httpbis-header-structure]) containing an absolute-URL
      string [5] ([URL]).

   "date" and "expires"  An integer (Section 3.5 of
      [I-D.ietf-httpbis-header-structure]) representing a Unix time.

   The "cert-url" parameter is _not_ signed, so intermediates can update
   it with a pointer to a cached version.

3.1.1.  Examples

   The following header is included in the response for an exchange with
   effective request URI "https://example.com/resource.html".  Newlines
   are added for readability.




Yasskin                 Expires December 16, 2018               [Page 6]


Internet-Draft            Signed HTTP Exchanges                June 2018


Signature:
 sig1;
  sig=*MEUCIQDXlI2gN3RNBlgFiuRNFpZXcDIaUpX6HIEwcZEc0cZYLAIga9DsVOMM+g5YpwEBdGW3sS+bvnmAJJiSMwhuBdqp5UY=*;
  integrity="mi";
  validity-url="https://example.com/resource.validity.1511128380";
  cert-url="https://example.com/oldcerts";
  cert-sha256=*W7uB969dFW3Mb5ZefPS9Tq5ZbH5iSmOILpjv2qEArmI=*;
  date=1511128380; expires=1511733180,
 sig2;
  sig=*MEQCIGjZRqTRf9iKNkGFyzRMTFgwf/BrY2ZNIP/dykhUV0aYAiBTXg+8wujoT4n/W+cNgb7pGqQvIUGYZ8u8HZJ5YH26Qg=*;
  integrity="mi";
  validity-url="https://example.com/resource.validity.1511128380";
  cert-url="https://example.com/newcerts";
  cert-sha256=*J/lEm9kNRODdCmINbvitpvdYKNQ+YgBj99DlYp4fEXw=*;
  date=1511128380; expires=1511733180,
 srisig;
  sig=*lGZVaJJM5f2oGczFlLmBdKTDL+QADza4BgeO494ggACYJOvrof6uh5OJCcwKrk7DK+LBch0jssDYPp5CLc1SDA=*
  integrity="mi";
  validity-url="https://example.com/resource.validity.1511128380";
  ed25519key=*zsSevyFsxyZHiUluVBDd4eypdRLTqyWRVOJuuKUz+A8=*
  date=1511128380; expires=1511733180,
 thirdpartysig;
  sig=*MEYCIQCNxJzn6Rh2fNxsobktir8TkiaJYQFhWTuWI1i4PewQaQIhAMs2TVjc4rTshDtXbgQEOwgj2mRXALhfXPztXgPupii+=*;
  integrity="mi";
  validity-url="https://thirdparty.example.com/resource.validity.1511161860";
  cert-url="https://thirdparty.example.com/certs";
  cert-sha256=*UeOwUPkvxlGRTyvHcsMUN0A2oNsZbU8EUvg8A9ZAnNc=*;
  date=1511133060; expires=1511478660,

   There are 4 signatures: 2 from different secp256r1 certificates
   within "https://example.com/", one using a raw ed25519 public key
   that's also controlled by "example.com", and a fourth using a
   secp256r1 certificate owned by "thirdparty.example.com".

   All 4 signatures rely on the "MI" response header to guard the
   integrity of the response payload.

   The signatures include a "validity-url" that includes the first time
   the resource was seen.  This allows multiple versions of a resource
   at the same URL to be updated with new signatures, which allows
   clients to avoid transferring extra data while the old versions don't
   have known security bugs.

   The certificates at "https://example.com/oldcerts" and
   "https://example.com/newcerts" have "subjectAltName"s of
   "example.com", meaning that if they and their signatures validate,
   the exchange can be trusted as having an origin of
   "https://example.com/".  The publisher might be using two



Yasskin                 Expires December 16, 2018               [Page 7]


Internet-Draft            Signed HTTP Exchanges                June 2018


   certificates because their readers have disjoint sets of roots in
   their trust stores.

   The publisher signed with all three certificates at the same time, so
   they share a validity range: 7 days starting at 2017-11-19 21:53 UTC.

   The publisher then requested an additional signature from
   "thirdparty.example.com", which did some validation or processing and
   then signed the resource at 2017-11-19 23:11 UTC.
   "thirdparty.example.com" only grants 4-day signatures, so clients
   will need to re-validate more often.

3.1.2.  Open Questions

   [I-D.ietf-httpbis-header-structure] provides a way to parameterise
   identifiers but not other supported types like binary content.  If
   the "Signature" header field is notionally a list of parameterised
   signatures, maybe we should add a "parameterised binary content"
   type.

   Should the cert-url and validity-url be lists so that intermediates
   can offer a cache without losing the original URLs?  Putting lists in
   dictionary fields is more complex than
   [I-D.ietf-httpbis-header-structure] allows, so they're single items
   for now.

3.2.  CBOR representation of exchange headers

   To sign an exchange's headers, they need to be serialized into a byte
   string.  Since intermediaries and distributors (Appendix A.2) might
   rearrange, add, or just reserialize headers, we can't use the literal
   bytes of the headers as this serialization.  Instead, this section
   defines a CBOR representation that can be embedded into other CBOR,
   canonically serialized (Section 3.4), and then signed.

   The CBOR representation of an exchange "exchange"'s headers is the
   CBOR ([RFC7049]) array with the following content:

   1.  The map mapping:

       *  The byte string ':method' to the byte string containing
          "exchange"'s request's method.

       *  The byte string ':url' to the byte string containing
          "exchange"'s request's effective request URI, which MUST be an
          absolute-URL string [6] ([URL]).





Yasskin                 Expires December 16, 2018               [Page 8]


Internet-Draft            Signed HTTP Exchanges                June 2018


       *  For each request header field in "exchange" except for the
          "Host" header field, the header field's lowercase name as a
          byte string to the header field's value as a byte string.

          Note: "Host" is excluded because it is already part of the
          effective request URI.

   2.  The map mapping:

       *  the byte string ':status' to the byte string containing
          "exchange"'s response's 3-digit status code, and

       *  for each response header field in "exchange", the header
          field's lowercase name as a byte string to the header field's
          value as a byte string.

3.2.1.  Example

   Given the HTTP exchange:

GET / HTTP/1.1
Host: example.com
Accept: */*

HTTP/1.1 200
Content-Type: text/html
MI: mi-sha256=20addcf7368837f616d549f035bf6784ea6d4bf4817a3736cd2fc7a763897fe3
Signed-Headers: "content-type", "mi"

<!doctype html>
<html>
...

   The cbor representation consists of the following item, represented
   using the extended diagnostic notation from [I-D.ietf-cbor-cddl]
   appendix G:















Yasskin                 Expires December 16, 2018               [Page 9]


Internet-Draft            Signed HTTP Exchanges                June 2018


[
  {
    ':url': 'https://example.com/',
    'accept': '*/*',
    ':method': 'GET',
  },
  {
    'mi': 'mi-sha256=20addcf7368837f616d549f035bf6784ea6d4bf4817a3736cd2fc7a763897fe3',
    ':status': '200',
    'content-type': 'text/html'
  }
]

3.3.  Loading a certificate chain

   The resource at a signature's "cert-url" MUST have the "application/
   cert-chain+cbor" content type, MUST be canonically-encoded CBOR
   (Section 3.4), and MUST match the following CDDL:

   cert-chain = [
     "&#128220;&#9939;", ; U+1F4DC U+26D3
     + {
       cert: bytes,
       ? ocsp: bytes,
       ? sct: bytes,
       * tstr => any,
     }
   ]

   The first item in the CBOR array is treated as the end-entity
   certificate, and the client will attempt to build a path ([RFC5280])
   to it from a trusted root using the other certificates in the chain.

   1.  Each "cert" value MUST be a DER-encoded X.509v3 certificate
       ([RFC5280]).  Other key/value pairs in the same array item define
       properties of this certificate.

   2.  The first certificate's "ocsp" value if any MUST be a complete,
       DER-encoded OCSP response for that certificate (using the ASN.1
       type "OCSPResponse" defined in [RFC6960]).  Subsequent
       certificates MUST NOT have an "ocsp" value.

   3.  Each certificate's "sct" value MUST be a
       "SignedCertificateTimestampList" for that certificate as defined
       by Section 3.3 of [RFC6962].

   Loading a "cert-url" takes a "forceFetch" flag.  The client MUST:




Yasskin                 Expires December 16, 2018              [Page 10]


Internet-Draft            Signed HTTP Exchanges                June 2018


   1.  Let "raw-chain" be the result of fetching ([FETCH]) "cert-url".
       If "forceFetch" is _not_ set, the fetch can be fulfilled from a
       cache using normal HTTP semantics [RFC7234].  If this fetch
       fails, return "invalid".

   2.  Let "certificate-chain" be the array of certificates and
       properties produced by parsing "raw-chain" using the CDDL above.
       If any of the requirements above aren't satisfied, return
       "invalid".  Note that this validation requirement might be
       impractical to completely achieve due to certificate validation
       implementations that don't enforce DER encoding or other standard
       constraints.

   3.  Return "certificate-chain".

3.4.  Canonical CBOR serialization

   Within this specification, the canonical serialization of a CBOR item
   uses the following rules derived from Section 3.9 of [RFC7049] with
   erratum 4964 applied:

   o  Integers and the lengths of arrays, maps, and strings MUST use the
      smallest possible encoding.

   o  Items MUST NOT be encoded with indefinite length.

   o  The keys in every map MUST be sorted in the bytewise lexicographic
      order of their canonical encodings.  For example, the following
      keys are correctly sorted:

      1.  10, encoded as 0A.

      2.  100, encoded as 18 64.

      3.  -1, encoded as 20.

      4.  "z", encoded as 61 7A.

      5.  "aa", encoded as 62 61 61.

      6.  [100], encoded as 81 18 64.

      7.  [-1], encoded as 81 20.

      8.  false, encoded as F4.






Yasskin                 Expires December 16, 2018              [Page 11]


Internet-Draft            Signed HTTP Exchanges                June 2018


   Note: this specification does not use floating point, tags, or other
   more complex data types, so it doesn't need rules to canonicalize
   those.

3.5.  Signature validity

   The client MUST parse the "Signature" header field as the
   parameterised list (Section 4.2.3 of
   [I-D.ietf-httpbis-header-structure]) described in Section 3.1.  If an
   error is thrown during this parsing or any of the requirements
   described there aren't satisfied, the exchange has no valid
   signatures.  Otherwise, each member of this list represents a
   signature with parameters.

   The client MUST use the following algorithm to determine whether each
   signature with parameters is invalid or potentially-valid for an
   "exchange".  Potentially-valid results include:

   o  The signed headers of the exchange so that higher-level protocols
      can avoid relying on unsigned headers, and

   o  Either a certificate chain or a public key so that a higher-level
      protocol can determine whether it's actually valid.

   This algorithm accepts a "forceFetch" flag that avoids the cache when
   fetching URLs.  A client that determines that a potentially-valid
   certificate chain is actually invalid due to an expired OCSP response
   MAY retry with "forceFetch" set to retrieve an updated OCSP from the
   original server.

   1.  Let "payload" be the payload body (Section 3.3 of [RFC7230]) of
       "exchange".  Note that the payload body is the message body with
       any transfer encodings removed.

   2.  Let:

       *  "signature" be the signature (binary content in the
          parameterised identifier's "sig" parameter).

       *  "integrity" be the signature's "integrity" parameter.

       *  "validity-url" be the signature's "validity-url" parameter.

       *  "cert-url" be the signature's "cert-url" parameter, if any.

       *  "cert-sha256" be the signature's "cert-sha256" parameter, if
          any.




Yasskin                 Expires December 16, 2018              [Page 12]


Internet-Draft            Signed HTTP Exchanges                June 2018


       *  "ed25519key" be the signature's "ed25519key" parameter, if
          any.

       *  "date" be the signature's "date" parameter, interpreted as a
          Unix time.

       *  "expires" be the signature's "expires" parameter, interpreted
          as a Unix time.

   3.  If "integrity" names a header field that is not present in
       "exchange"'s response headers or which the client cannot use to
       check the integrity of "payload" (for example, the header field
       is new and hasn't been implemented yet), then return "invalid".
       If the selected header field provides integrity guarantees weaker
       than SHA-256, return "invalid".  If validating integrity using
       the selected header field requires the client to process records
       larger than TBD bytes, return "invalid".  Clients MUST implement
       at least the "MI" ([I-D.thomson-http-mice]) header field with its
       "mi-sha256" content encoding.

   4.  Set "publicKey" and "signing-alg" depending on which key fields
       are present:

       1.  If "cert-url" is present:

           1.  Let "certificate-chain" be the result of loading the
               certificate chain at "cert-url" passing the "forceFetch"
               flag (Section 3.3).  If this returns "invalid", return
               "invalid".

           2.  Let "main-certificate" be the first certificate in
               "certificate-chain".

           3.  Set "publicKey" to "main-certificate"'s public key.

           4.  If "publicKey" is an RSA key, return "invalid".

           5.  If "publicKey" is a key using the secp256r1 elliptic
               curve, set "signing-alg" to ecdsa_secp256r1_sha256 as
               defined in Section 4.2.3 of [I-D.ietf-tls-tls13].

           6.  Otherwise, either return "invalid" or set "signing-alg"
               to a non-legacy signing algorithm defined by TLS 1.3 or
               later ([I-D.ietf-tls-tls13]).  This choice MUST depend
               only on "publicKey"'s type and not on any other context.

       2.  If "ed25519key" is present, set "publicKey" to "ed25519key"
           and "signing-alg" to ed25519, as defined by [RFC8032]



Yasskin                 Expires December 16, 2018              [Page 13]


Internet-Draft            Signed HTTP Exchanges                June 2018


   5.  If "expires" is more than 7 days (604800 seconds) after "date",
       return "invalid".

   6.  If the current time is before "date" or after "expires", return
       "invalid".

   7.  Let "message" be the concatenation of the following byte strings.
       This matches the [I-D.ietf-tls-tls13] format to avoid cross-
       protocol attacks if anyone uses the same key in a TLS certificate
       and an exchange-signing certificate.

       1.  A string that consists of octet 32 (0x20) repeated 64 times.

       2.  A context string: the ASCII encoding of "HTTP Exchange 1".

           Note: RFC EDITOR PLEASE DELETE THIS NOTE; The implementation
           of the final RFC MUST use this context string, but
           implementations of drafts MUST NOT use it and MUST use
           another draft-specific string beginning with "HTTP Exchange 1
           " instead.  This ensures that signers can predict how their
           signatures will be used.

       3.  A single 0 byte which serves as a separator.

       4.  The bytes of the canonical CBOR serialization (Section 3.4)
           of a CBOR map mapping:

           1.  If "cert-sha256" is set:

               1.  The text string "cert-sha256" to the byte string
                   value of "cert-sha256".

           2.  The text string "validity-url" to the byte string value
               of "validity-url".

           3.  The text string "date" to the integer value of "date".

           4.  The text string "expires" to the integer value of
               "expires".

           5.  The text string "headers" to the CBOR representation
               (Section 3.2) of "exchange"'s headers.

   8.  If "cert-url" is present and the SHA-256 hash of "main-
       certificate"'s "cert_data" is not equal to "cert-sha256" (whose
       presence was checked when the "Signature" header field was
       parsed), return "invalid".




Yasskin                 Expires December 16, 2018              [Page 14]


Internet-Draft            Signed HTTP Exchanges                June 2018


       Note that this intentionally differs from TLS 1.3, which signs
       the entire certificate chain in its Certificate Verify
       (Section 4.4.3 of [I-D.ietf-tls-tls13]), in order to allow
       updating the stapled OCSP response without updating signatures at
       the same time.

   9.  If "signature" is a valid signature of "message" by "publicKey"
       using "signing-alg", return "potentially-valid" with whichever is
       present of "certificate-chain" or "ed25519key".  Otherwise,
       return "invalid".

   Note that the above algorithm can determine that an exchange's
   headers are potentially-valid before the exchange's payload is
   received.  Similarly, if "integrity" identifies a header field like
   "MI" ([I-D.thomson-http-mice]) that can incrementally validate the
   payload, early parts of the payload can be determined to be
   potentially-valid before later parts of the payload.  Higher-level
   protocols MAY process parts of the exchange that have been determined
   to be potentially-valid as soon as that determination is made but
   MUST NOT process parts of the exchange that are not yet potentially-
   valid.  Similarly, as the higher-level protocol determines that parts
   of the exchange are actually valid, the client MAY process those
   parts of the exchange and MUST wait to process other parts of the
   exchange until they too are determined to be valid.

3.5.1.  Open Questions

   Should the signed message use the TLS format (with an initial 64
   spaces) even though these certificates can't be used in TLS servers?

3.6.  Updating signature validity

   Both OCSP responses and signatures are designed to expire a short
   time after they're signed, so that revoked certificates and signed
   exchanges with known vulnerabilities are distrusted promptly.

   This specification provides no way to update OCSP responses by
   themselves.  Instead, clients need to re-fetch the "cert-url"
   (Section 3.5, Paragraph 4) to get a chain including a newer OCSP
   response.

   The "validity-url" parameter (Paragraph 6) of the signatures provides
   a way to fetch new signatures or learn where to fetch a complete
   updated exchange.

   Each version of a signed exchange SHOULD have its own validity URLs,
   since each version needs different signatures and becomes obsolete at
   different times.



Yasskin                 Expires December 16, 2018              [Page 15]


Internet-Draft            Signed HTTP Exchanges                June 2018


   The resource at a "validity-url" is "validity data", a CBOR map
   matching the following CDDL ([I-D.ietf-cbor-cddl]):

   validity = {
     ? signatures: [ + bytes ]
     ? update: {
       ? size: uint,
     }
   ]

   The elements of the "signatures" array are parameterised identifiers
   (Section 4.2.4 of [I-D.ietf-httpbis-header-structure]) meant to
   replace the signatures within the "Signature" header field pointing
   to this validity data.  If the signed exchange contains a bug severe
   enough that clients need to stop using the content, the "signatures"
   array MUST NOT be present.

   If the the "update" map is present, that indicates that a new version
   of the signed exchange is available at its effective request URI
   (Section 5.5 of [RFC7230]) and can give an estimate of the size of
   the updated exchange ("update.size").  If the signed exchange is
   currently the most recent version, the "update" SHOULD NOT be
   present.

   If both the "signatures" and "update" fields are present, clients can
   use the estimated size to decide whether to update the whole resource
   or just its signatures.

3.6.1.  Examples

   For example, say a signed exchange whose URL is "https://example.com/
   resource" has the following "Signature" header field (with line
   breaks included and irrelevant fields omitted for ease of reading).


















Yasskin                 Expires December 16, 2018              [Page 16]


Internet-Draft            Signed HTTP Exchanges                June 2018


Signature:
 sig1;
  sig=*MEUCIQ...*;
  ...
  validity-url="https://example.com/resource.validity.1511157180";
  cert-url="https://example.com/oldcerts";
  date=1511128380; expires=1511733180,
 sig2;
  sig=*MEQCIG...*;
  ...
  validity-url="https://example.com/resource.validity.1511157180";
  cert-url="https://example.com/newcerts";
  date=1511128380; expires=1511733180,
 thirdpartysig;
  sig=*MEYCIQ...*;
  ...
  validity-url="https://thirdparty.example.com/resource.validity.1511161860";
  cert-url="https://thirdparty.example.com/certs";
  date=1511478660; expires=1511824260

   At 2017-11-27 11:02 UTC, "sig1" and "sig2" have expired, but
   "thirdpartysig" doesn't exipire until 23:11 that night, so the client
   needs to fetch "https://example.com/resource.validity.1511157180"
   (the "validity-url" of "sig1" and "sig2") to update those signatures.
   This URL might contain:

{
  "signatures": [
    'sig1; '
    'sig=*MEQCIC/I9Q+7BZFP6cSDsWx43pBAL0ujTbON/+7RwKVk+ba5AiB3FSFLZqpzmDJ0NumNwN04pqgJZE99fcK86UjkPbj4jw==*; '
    'validity-url="https://example.com/resource.validity.1511157180"; '
    'integrity="mi"; '
    'cert-url="https://example.com/newcerts"; '
    'cert-sha256=*J/lEm9kNRODdCmINbvitpvdYKNQ+YgBj99DlYp4fEXw=*; '
    'date=1511733180; expires=1512337980'
  ],
  "update": {
    "size": 5557452
  }
}

   This indicates that the client could fetch a newer version at
   "https://example.com/resource" (the original URL of the exchange), or
   that the validity period of the old version can be extended by
   replacing the first two of the original signatures (the ones with a
   validity-url of "https://example.com/resource.validity.1511157180")
   with the single new signature provided.  (This might happen at the




Yasskin                 Expires December 16, 2018              [Page 17]


Internet-Draft            Signed HTTP Exchanges                June 2018


   end of a migration to a new root certificate.)  The signatures of the
   updated signed exchange would be:

Signature:
 sig1;
  sig=*MEQCIC...*;
  ...
  validity-url="https://example.com/resource.validity.1511157180";
  cert-url="https://example.com/newcerts";
  date=1511733180; expires=1512337980,
 thirdpartysig;
  sig=*MEYCIQ...*;
  ...
  validity-url="https://thirdparty.example.com/resource.validity.1511161860";
  cert-url="https://thirdparty.example.com/certs";
  date=1511478660; expires=1511824260

   "https://example.com/resource.validity.1511157180" could also expand
   the set of signatures if its "signatures" array contained more than 2
   elements.

3.7.  The Accept-Signature header

   "Signature" header fields cost on the order of 300 bytes for ECDSA
   signatures, so servers might prefer to avoid sending them to clients
   that don't intend to use them.  A client can send the "Accept-
   Signature" header field to indicate that it does intend to take
   advantage of any available signatures and to indicate what kinds of
   signatures it supports.

   When a server receives an "Accept-Signature" header field in a client
   request, it SHOULD reply with any available "Signature" header fields
   for its response that the "Accept-Signature" header field indicates
   the client supports.  However, if the "Accept-Signature" value
   violates a requirement in this section, the server MUST behave as if
   it hadn't received any "Accept-Signature" header at all.

   The "Accept-Signature" header field is a Structured Header as defined
   by [I-D.ietf-httpbis-header-structure].  Its value MUST be a
   parameterised list (Section 3.3 of
   [I-D.ietf-httpbis-header-structure]).  Its ABNF is:

   Accept-Signature = sh-param-list

   The order of identifiers in the "Accept-Signature" list is not
   significant.  Identifiers, ignoring any initial "-" character, MUST
   NOT be duplicated.




Yasskin                 Expires December 16, 2018              [Page 18]


Internet-Draft            Signed HTTP Exchanges                June 2018


   Each identifier in the "Accept-Signature" header field's value
   indicates that a feature of the "Signature" header field
   (Section 3.1) is supported.  If the identifier begins with a "-"
   character, it instead indicates that the feature named by the rest of
   the identifier is not supported.  Unknown identifiers and parameters
   MUST be ignored because new identifiers and new parameters on
   existing identifiers may be defined by future specifications.

3.7.1.  Integrity identifiers

   Identifiers starting with "mi/" indicate that the client supports the
   "MI" header field ([I-D.thomson-http-mice]) with the parameter from
   the HTTP MI Parameter Registry registry named in lower-case by the
   rest of the identifier.  For example, "mi/mi-blake2" indicates
   support for Merkle integrity with the as-yet-unspecified mi-blake2
   parameter, and "-digest/mi-sha256" indicates non-support for Merkle
   integrity with the mi-sha256 content encoding.

   If the "Accept-Signature" header field is present, servers SHOULD
   assume support for "mi/mi-sha256" unless the header field states
   otherwise.

3.7.2.  Key type identifiers

   Identifiers starting with "ecdsa/" indicate that the client supports
   certificates holding ECDSA public keys on the curve named in lower-
   case by the rest of the identifier.

   If the "Accept-Signature" header field is present, servers SHOULD
   assume support for "ecdsa/secp256r1" unless the header field states
   otherwise.

3.7.3.  Key value identifiers

   The "ed25519key" identifier has parameters indicating the public keys
   that will be used to validate the returned signature.  Each
   parameter's name is re-interpreted as binary content (Section 3.9 of
   [I-D.ietf-httpbis-header-structure]) encoding a prefix of the public
   key.  For example, if the client will validate signatures using the
   public key whose base64 encoding is
   "11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo=", valid "Accept-
   Signature" header fields include:

Accept-Signature: ..., ed25519key; *11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo=*
Accept-Signature: ..., ed25519key; *11qYAYKxCrfVS/7TyWQHOg==*
Accept-Signature: ..., ed25519key; *11qYAQ==*
Accept-Signature: ..., ed25519key; **




Yasskin                 Expires December 16, 2018              [Page 19]


Internet-Draft            Signed HTTP Exchanges                June 2018


   but not

   Accept-Signature: ..., ed25519key; *11qYA===*

   because 5 bytes isn't a valid length for encoded base64, and not

   Accept-Signature: ..., ed25519key; 11qYAQ

   because it doesn't start or end with the "*"s that indicate binary
   content.

   Note that "ed25519key; **" is an empty prefix, which matches all
   public keys, so it's useful in subresource integrity (Appendix A.3)
   cases like "<link rel=preload as=script href="...">" where the public
   key isn't known until the matching "<script src="..."
   integrity="...">" tag.

3.7.4.  Examples

   Accept-Signature: mi/mi-sha256

   states that the client will accept signatures with payload integrity
   assured by the "MI" header and "mi-sha256" content encoding and
   implies that the client will accept integrity assured by the "Digest:
   SHA-256" header and signatures from ECDSA keys on the secp256r1
   curve.

   Accept-Signature: -ecdsa/secp256r1, ecdsa/secp384r1

   states that the client will accept ECDSA keys on the secp384r1 curve
   but not the secp256r1 curve and payload integrity assured with the
   "MI: mi-sha256" header field.

3.7.5.  Open Questions

   Is an "Accept-Signature" header useful enough to pay for itself?  If
   clients wind up sending it on most requests, that may cost more than
   the cost of sending "Signature"s unconditionally.  On the other hand,
   it gives servers an indication of which kinds of signatures are
   supported, which can help us upgrade the ecosystem in the future.

   Is "Accept-Signature" the right spelling, or do we want to imitate
   "Want-Digest" (Section 4.3.1 of [RFC3230]) instead?

   Do I have the right structure for the identifiers indicating feature
   support?





Yasskin                 Expires December 16, 2018              [Page 20]


Internet-Draft            Signed HTTP Exchanges                June 2018


4.  Cross-origin trust

   To determine whether to trust a cross-origin exchange, the client
   takes a "Signature" header field (Section 3.1) and the "exchange".
   The client MUST parse the "Signature" header into a list of
   signatures according to the instructions in Section 3.5, and run the
   following algorithm for each signature, stopping at the first one
   that returns "valid".  If any signature returns "valid", return
   "valid".  Otherwise, return "invalid".

   1.  If the signature's "validity-url" parameter (Paragraph 6) is not
       same-origin [7] with "exchange"'s effective request URI
       (Section 5.5 of [RFC7230]), return "invalid".

   2.  Use Section 3.5 to determine the signature's validity for
       "exchange", getting "certificate-chain" back.  If this returned
       "invalid" or didn't return a certificate chain, return "invalid".

   3.  If "exchange"'s request method is not safe (Section 4.2.1 of
       [RFC7231]) or not cacheable (Section 4.2.3 of [RFC7231]), return
       "invalid".

   4.  If "exchange"'s headers contain a stateful header field, as
       defined in Section 4.1, return "invalid".

   5.  Let "authority" be the host component of "exchange"'s effective
       request URI.

   6.  Validate the "certificate-chain" using the following substeps.
       If any of them fail, re-run Section 3.5 once over the signature
       with the "forceFetch" flag set, and restart from step 2.  If a
       substep fails again, return "invalid".

       1.  Use "certificate-chain" to validate that its first entry,
           "main-certificate" is trusted as "authority"'s server
           certificate ([RFC5280] and other undocumented conventions).
           Let "path" be the path that was used from the "main-
           certificate" to a trusted root, including the "main-
           certificate" but excluding the root.

       2.  Validate that "main-certificate" has the CanSignHttpExchanges
           extension (Section 4.2).

       3.  Validate that "main-certificate" has an "ocsp" property
           (Section 3.3) with a valid OCSP response whose lifetime
           ("nextUpdate - thisUpdate") is less than 7 days ([RFC6960]).
           Note that this does not check for revocation of intermediate




Yasskin                 Expires December 16, 2018              [Page 21]


Internet-Draft            Signed HTTP Exchanges                June 2018


           certificates, and clients SHOULD implement another mechanism
           for that.

       4.  Validate that valid SCTs from trusted logs are available from
           any of:

           +  The "SignedCertificateTimestampList" in "main-
              certificate"'s "sct" property (Section 3.3),

           +  An OCSP extension in the OCSP response in "main-
              certificate"'s "ocsp" property, or

           +  An X.509 extension in the certificate in "main-
              certificate"'s "cert" property,

           as described by Section 3.3 of [RFC6962].

   7.  Return "valid".

4.1.  Stateful header fields

   As described in Section 6.1, a publisher can cause problems if they
   sign an exchange that includes private information.  There's no way
   for a client to be sure an exchange does or does not include private
   information, but header fields that store or convey stored state in
   the client are a good sign.

   A stateful request header field informs the server of per-client
   state.  These include but are not limited to:

   o  "Authorization", [RFC7235]

   o  "Cookie", [RFC6265]

   o  "Cookie2", [RFC2965]

   o  "Proxy-Authorization", [RFC7235]

   o  "Sec-WebSocket-Key", [RFC6455]

   A stateful response header field modifies state, including
   authentication status, in the client.  The HTTP cache is not
   considered part of this state.  These include but are not limited to:

   o  "Authentication-Control", [RFC8053]

   o  "Authentication-Info", [RFC7615]




Yasskin                 Expires December 16, 2018              [Page 22]


Internet-Draft            Signed HTTP Exchanges                June 2018


   o  "Optional-WWW-Authenticate", [RFC8053]

   o  "Proxy-Authenticate", [RFC7235]

   o  "Proxy-Authentication-Info", [RFC7615]

   o  "Sec-WebSocket-Accept", [RFC6455]

   o  "Set-Cookie", [RFC6265]

   o  "Set-Cookie2", [RFC2965]

   o  "SetProfile", [W3C.NOTE-OPS-OverHTTP]

   o  "WWW-Authenticate", [RFC7235]

4.2.  Certificate Requirements

   We define a new X.509 extension, CanSignHttpExchanges to be used in
   the certificate when the certificate permits the usage of signed
   exchanges.  When this extension is not present the client MUST NOT
   accept a signature from the certificate as proof that a signed
   exchange is authoritative for a domain covered by the certificate.
   When it is present, the client MUST follow the validation procedure
   in Section 4.

      id-ce-canSignHttpExchanges OBJECT IDENTIFIER ::= { TBD }

      CanSignHttpExchanges ::= NULL

   Note that this extension contains an ASN.1 NULL (bytes "05 00")
   because some implementations have bugs with empty extensions.

   Leaf certificates without this extension need to be revoked if the
   private key is exposed to an unauthorized entity, but they generally
   don't need to be revoked if a signing oracle is exposed and then
   removed.

   CA certificates, by contrast, need to be revoked if an unauthorized
   entity is able to make even one unauthorized signature.

   Certificates with this extension MUST be revoked if an unauthorized
   entity is able to make even one unauthorized signature.

   Conforming CAs MUST mark this extension as critical, and clients MUST
   NOT accept certificates with this extension in TLS connections
   (Section 4.4.2.2 of [I-D.ietf-tls-tls13]).  This simplifies security
   analysis of this protocol and avoids encouraging server operators to



Yasskin                 Expires December 16, 2018              [Page 23]


Internet-Draft            Signed HTTP Exchanges                June 2018


   put exchange-signing keys on servers exposed directly to the
   internet.

   RFC EDITOR PLEASE DELETE THE REST OF THE PARAGRAPHS IN THIS SECTION

   id-ce-google OBJECT IDENTIFIER ::= { 1 3 6 1 4 1 11129 }
   id-ce-testCanSignHttpExchanges OBJECT IDENTIFIER ::= { id-ce-google 2 1 22 }

   Implementations of drafts of this specification MAY recognize the
   "id-ce-testCanSignHttpExchanges" OID as identifying the
   CanSignHttpExchanges extension.  This OID might or might not be used
   as the final OID for the extension, so certificates including it
   might need to be reissued once the final RFC is published.

5.  Transferring a signed exchange

   A signed exchange can be transferred in several ways, of which three
   are described here.

5.1.  Same-origin response

   The signature for a signed exchange can be included in a normal HTTP
   response.  Because different clients send different request header
   fields, and intermediate servers add response header fields, it can
   be impossible to have a signature for the exact request and response
   that the client sees.  Therefore, when a client validates the
   "Signature" header field for an exchange represented as a normal HTTP
   request/response pair, it MUST pass only the subset of header fields
   defined by Section 5.1.1 to the validation procedure (Section 3.5).

   If the client relies on signature validity for any aspect of its
   behavior, it MUST ignore any header fields that it didn't pass to the
   validation procedure.

5.1.1.  Significant headers for a same-origin response

   The significant headers of an exchange represented as a normal HTTP
   request/response pair (Section 2.1 of [RFC7230] or Section 8.1 of
   [RFC7540]) are:

   o  The method (Section 4 of [RFC7231]) and effective request URI
      (Section 5.5 of [RFC7230]) of the request.

   o  The response status code (Section 6 of [RFC7231]) and the response
      header fields whose names are listed in that exchange's "Signed-
      Headers" header field (Section 5.1.2), in the order they appear in
      that header field.  If a response header field name from "Signed-




Yasskin                 Expires December 16, 2018              [Page 24]


Internet-Draft            Signed HTTP Exchanges                June 2018


      Headers" does not appear in the exchange's response header fields,
      the exchange has no significant headers.

   If the exchange's "Signed-Headers" header field is not present,
   doesn't parse as a Structured Header
   ([I-D.ietf-httpbis-header-structure]) or doesn't follow the
   constraints on its value described in Section 5.1.2, the exchange has
   no significant headers.

5.1.1.1.  Open Questions

   Do the significant headers of an exchange need to include the
   "Signed-Headers" header field itself?

5.1.2.  The Signed-Headers Header

   The "Signed-Headers" header field identifies an ordered list of
   response header fields to include in a signature.  The request URL
   and response status are included unconditionally.  This allows a TLS-
   terminating intermediate to reorder headers without breaking the
   signature.  This _can_ also allow the intermediate to add headers
   that will be ignored by some higher-level protocols, but Section 3.5
   provides a hook to let other higher-level protocols reject such
   insecure headers.

   This header field appears once instead of being incorporated into the
   signatures' parameters because the signed header fields need to be
   consistent across all signatures of an exchange, to avoid forcing
   higher-level protocols to merge the header field lists of valid
   signatures.

   See Appendix B.2 for a discussion of why only the URL from the
   request is included and not other request headers.

   "Signed-Headers" is a Structured Header as defined by
   [I-D.ietf-httpbis-header-structure].  Its value MUST be a list
   (Section 3.2 of [I-D.ietf-httpbis-header-structure]).  Its ABNF is:

   Signed-Headers = sh-list

   Each element of the "Signed-Headers" list must be a lowercase string
   (Section 3.7 of [I-D.ietf-httpbis-header-structure]) naming an HTTP
   response header field.  Pseudo-header field names (Section 8.1.2.1 of
   [RFC7540]) MUST NOT appear in this list.

   Higher-level protocols SHOULD place requirements on the minimum set
   of headers to include in the "Signed-Headers" header field.




Yasskin                 Expires December 16, 2018              [Page 25]


Internet-Draft            Signed HTTP Exchanges                June 2018


5.2.  HTTP/2 extension for cross-origin Server Push

   To allow servers to Server-Push (Section 8.2 of [RFC7540]) signed
   exchanges (Section 3) signed by an authority for which the server is
   not authoritative (Section 9.1 of [RFC7230]), this section defines an
   HTTP/2 extension.

5.2.1.  Indicating support for cross-origin Server Push

   Clients that might accept signed Server Pushes with an authority for
   which the server is not authoritative indicate this using the HTTP/2
   SETTINGS parameter ENABLE_CROSS_ORIGIN_PUSH (0xSETTING-TBD).

   An ENABLE_CROSS_ORIGIN_PUSH value of 0 indicates that the client does
   not support cross-origin Push.  A value of 1 indicates that the
   client does support cross-origin Push.

   A client MUST NOT send a ENABLE_CROSS_ORIGIN_PUSH setting with a
   value other than 0 or 1 or a value of 0 after previously sending a
   value of 1.  If a server receives a value that violates these rules,
   it MUST treat it as a connection error (Section 5.4.1 of [RFC7540])
   of type PROTOCOL_ERROR.

   The use of a SETTINGS parameter to opt-in to an otherwise
   incompatible protocol change is a use of "Extending HTTP/2" defined
   by Section 5.5 of [RFC7540].  If a server were to send a cross-origin
   Push without first receiving a ENABLE_CROSS_ORIGIN_PUSH setting with
   the value of 1 it would be a protocol violation.

5.2.2.  NO_TRUSTED_EXCHANGE_SIGNATURE error code

   The signatures on a Pushed cross-origin exchange may be untrusted for
   several reasons, for example that the certificate could not be
   fetched, that the certificate does not chain to a trusted root, that
   the signature itself doesn't validate, that the signature is expired,
   etc.  This draft conflates all of these possible failures into one
   error code, NO_TRUSTED_EXCHANGE_SIGNATURE (0xERROR-TBD).

5.2.2.1.  Open Questions

   How fine-grained should this specification's error codes be?

5.2.3.  Validating a cross-origin Push

   If the client has set the ENABLE_CROSS_ORIGIN_PUSH setting to 1, the
   server MAY Push a signed exchange for which it is not authoritative,
   and the client MUST NOT treat a PUSH_PROMISE for which the server is




Yasskin                 Expires December 16, 2018              [Page 26]


Internet-Draft            Signed HTTP Exchanges                June 2018


   not authoritative as a stream error (Section 5.4.2 of [RFC7540]) of
   type PROTOCOL_ERROR, as described in Section 8.2 of [RFC7540].

   Instead, the client MUST validate such a PUSH_PROMISE and its
   response by taking the "Signature" header field from the response,
   and the exchange consisting of the PUSH_PROMISE and the response
   without that "Signature" header field, and passing them to the
   algorithm in Section 4.  If this returns "invalid", the client MUST
   treat the response as a stream error (Section 5.4.2 of [RFC7540]) of
   type NO_TRUSTED_EXCHANGE_SIGNATURE.  Otherwise, the client MUST treat
   the pushed response as if the server were authoritative for the
   PUSH_PROMISE's authority.

5.2.3.1.  Open Questions

   Is it right that "validity-url" is required to be same-origin with
   the exchange?  This allows the mitigation against downgrades in
   Section 6.3, but prohibits intermediates from providing a cache of
   the validity information.  We could do both with a list of URLs.

5.3.  application/signed-exchange format

   To allow signed exchanges to be the targets of "<link rel=prefetch>"
   tags, we define the "application/signed-exchange" content type that
   represents a signed HTTP exchange, including request metadata and
   header fields, response metadata and header fields, and a response
   payload.

   This content type consists of the concatenation of the following
   items:

   1.  The ASCII characters "sxg1" followed by a 0 byte, to serve as a
       file signature.  This is redundant with the MIME type, and
       receipients that receive both MUST check that they match and stop
       parsing if they don't.

       Note: RFC EDITOR PLEASE DELETE THIS NOTE; The implementation of
       the final RFC MUST use this file signature, but implementations
       of drafts MUST NOT use it and MUST use another implementation-
       specific string beginning with "sxg1-" and ending with a 0 byte
       instead.

   2.  3 bytes storing a big-endian integer "sigLength".  If this is
       larger than TBD, parsing MUST fail.

   3.  3 bytes storing a big-endian integer "headerLength".  If this is
       larger than TBD, parsing MUST fail.




Yasskin                 Expires December 16, 2018              [Page 27]


Internet-Draft            Signed HTTP Exchanges                June 2018


   4.  "sigLength" bytes holding the "Signature" header field's value
       (Section 3.1).

   5.  "headerLength" bytes holding the signed headers, the canonical
       serialization (Section 3.4) of the CBOR representation of the
       request and response headers of the exchange represented by the
       "application/signed-exchange" resource (Section 3.2), excluding
       the "Signature" header field.

       Note that this is exactly the bytes used when checking signature
       validity in Section 3.5.

   6.  The payload body (Section 3.3 of [RFC7230]) of the exchange
       represented by the "application/signed-exchange" resource.

       Note that the use of the payload body here means that a
       "Transfer-Encoding" header field inside the "application/signed-
       exchange" header block has no effect.  A "Transfer-Encoding"
       header field on the outer HTTP response that transfers this
       resource still has its normal effect.

5.3.1.  Cross-origin trust in application/signed-exchange

   To determine whether to trust a cross-origin exchange stored in an
   "application/signed-exchange" resource, pass the "Signature" header
   field from the non-signed header section and an exchange consisting
   of the headers from the signed headers section and the payload body,
   to the algorithm in Section 4.

5.3.2.  Example

   An example "application/signed-exchange" file representing a possible
   signed exchange with https://example.com/ [8] follows, with lengths
   represented by descriptions in "<>"s, CBOR represented in the
   extended diagnostic format defined in Appendix G of
   [I-D.ietf-cbor-cddl], and most of the "Signature" header field and
   payload elided with a ...:














Yasskin                 Expires December 16, 2018              [Page 28]


Internet-Draft            Signed HTTP Exchanges                June 2018


sxg1\0<3-byte length of the following header
value>sig1; sig=*...; integrity="mi"; ...<3-byte length of the encoding of the
following array>[
  {
    ':method': 'GET',
    ':url': 'https://example.com/',
    'accept', '*/*'
  },
  {
    ':status': '200',
    'content-type': 'text/html'
  }
]<!doctype html>\r\n<html>...

5.3.3.  Open Questions

   Should this be a CBOR format, or is the current mix of binary and
   CBOR better?

   Are the mime type, extension, and magic number right?

6.  Security considerations

6.1.  Over-signing

   If a publisher blindly signs all responses as their origin, they can
   cause at least two kinds of problems, described below.  To avoid
   this, publishers SHOULD design their systems to opt particular public
   content that doesn't depend on authentication status into signatures
   instead of signing by default.

   Signing systems SHOULD also incorporate the following mitigations to
   reduce the risk that private responses are signed:

   1.  Strip the "Cookie" request header field and other identifying
       information like client authentication and TLS session IDs from
       requests whose exchange is destined to be signed, before
       forwarding the request to a backend.

   2.  Only sign exchanges where the response includes a "Cache-Control:
       public" header.  Clients are not required to fail signature-
       checking for exchanges that omit this "Cache-Control" response
       header field to reduce the risk that naive signing systems
       blindly add it.







Yasskin                 Expires December 16, 2018              [Page 29]


Internet-Draft            Signed HTTP Exchanges                June 2018


6.1.1.  Session fixation

   Blind signing can sign responses that create session cookies or
   otherwise change state on the client to identify a particular
   session.  This breaks certain kinds of CSRF defense and can allow an
   attacker to force a user into the attacker's account, where the user
   might unintentionally save private information, like credit card
   numbers or addresses.

   This specification defends against cookie-based attacks by blocking
   the "Set-Cookie" response header, but it cannot prevent Javascript or
   other response content from changing state.

6.1.2.  Misleading content

   If a site signs private information, an attacker might set up their
   own account to show particular private information, forward that
   signed information to a victim, and use that victim's confusion in a
   more sophisticated attack.

   Stripping authentication information from requests before sending
   them to backends is likely to prevent the backend from showing
   attacker-specific information in the signed response.  It does not
   prevent the attacker from showing their victim a signed-out page when
   the victim is actually signed in, but while this is still misleading,
   it seems less likely to be useful to the attacker.

6.2.  Off-path attackers

   Relaxing the requirement to consult DNS when determining authority
   for an origin means that an attacker who possesses a valid
   certificate no longer needs to be on-path to redirect traffic to
   them; instead of modifying DNS, they need only convince the user to
   visit another Web site in order to serve responses signed as the
   target.  This consideration and mitigations for it are shared by the
   combination of [RFC8336] and
   [I-D.ietf-httpbis-http2-secondary-certs].

6.3.  Downgrades

   Signing a bad response can affect more users than simply serving a
   bad response, since a served response will only affect users who make
   a request while the bad version is live, while an attacker can
   forward a signed response until its signature expires.  Publishers
   should consider shorter signature expiration times than they use for
   cache expiration times.





Yasskin                 Expires December 16, 2018              [Page 30]


Internet-Draft            Signed HTTP Exchanges                June 2018


   Clients MAY also check the "validity-url" (Paragraph 6) of an
   exchange more often than the signature's expiration would require.
   Doing so for an exchange with an HTTPS request URI provides a TLS
   guarantee that the exchange isn't out of date (as long as
   Section 5.2.3.1 is resolved to keep the same-origin requirement).

6.4.  Signing oracles are permanent

   An attacker with temporary access to a signing oracle can sign "still
   valid" assertions with arbitrary timestamps and expiration times.  As
   a result, when a signing oracle is removed, the keys it provided
   access to MUST be revoked so that, even if the attacker used them to
   sign future-dated exchange validity assertions, the key's OCSP
   assertion will expire, causing the exchange as a whole to become
   untrusted.

6.5.  Unsigned headers

   The use of a single "Signed-Headers" header field prevents us from
   signing aspects of the request other than its effective request URI
   (Section 5.5 of [RFC7230]).  For example, if a publisher signs both
   "Content-Encoding: br" and "Content-Encoding: gzip" variants of a
   response, what's the impact if an attacker serves the brotli one for
   a request with "Accept-Encoding: gzip"?

   The simple form of "Signed-Headers" also prevents us from signing
   less than the full request URL.  The SRI use case (Appendix A.3) may
   benefit from being able to leave the authority less constrained.

   Section 3.5 can succeed when some delivered headers aren't included
   in the signed set.  This accommodates current TLS-terminating
   intermediates and may be useful for SRI (Appendix A.3), but is risky
   for trusting cross-origin responses (Appendix A.1, Appendix A.2, and
   Appendix A.6).  Section 5.2 requires all headers to be included in
   the signature before trusting cross-origin pushed resources, at Ryan
   Sleevi's recommendation.

6.6.  application/signed-exchange

   Clients MUST NOT trust an effective request URI claimed by an
   "application/signed-exchange" resource (Section 5.3) without either
   ensuring the resource was transferred from a server that was
   authoritative (Section 9.1 of [RFC7230]) for that URI's origin, or
   calling the algorithm in Section 5.3.1 and getting "valid" back.







Yasskin                 Expires December 16, 2018              [Page 31]


Internet-Draft            Signed HTTP Exchanges                June 2018


7.  Privacy considerations

   Normally, when a client fetches "https://o1.com/resource.js",
   "o1.com" learns that the client is interested in the resource.  If
   "o1.com" signs "resource.js", "o2.com" serves it as "https://o2.com/
   o1resource.js", and the client fetches it from there, then "o2.com"
   learns that the client is interested, and if the client executes the
   Javascript, that could also report the client's interest back to
   "o1.com".

   Often, "o2.com" already knew about the client's interest, because
   it's the entity that directed the client to "o1resource.js", but
   there may be cases where this leaks extra information.

   For non-executable resource types, a signed response can improve the
   privacy situation by hiding the client's interest from the original
   publisher.

   To prevent network operators other than "o1.com" or "o2.com" from
   learning which exchanges were read, clients SHOULD only load
   exchanges fetched over a transport that's protected from
   eavesdroppers.  This can be difficult to determine when the exchange
   is being loaded from local disk, but when the client itself requested
   the exchange over a network it SHOULD require TLS
   ([I-D.ietf-tls-tls13]) or a successor transport layer, and MUST NOT
   accept exchanges transferred over plain HTTP without TLS.

8.  IANA considerations

   TODO: possibly register the validity-url format.

8.1.  Signature Header Field Registration

   This section registers the "Signature" header field in the "Permanent
   Message Header Field Names" registry ([RFC3864]).

   Header field name: "Signature"

   Applicable protocol: http

   Status: standard

   Author/Change controller: IETF

   Specification document(s): Section 3.1 of this document






Yasskin                 Expires December 16, 2018              [Page 32]


Internet-Draft            Signed HTTP Exchanges                June 2018


8.2.  Accept-Signature Header Field Registration

   This section registers the "Accept-Signature" header field in the
   "Permanent Message Header Field Names" registry ([RFC3864]).

   Header field name: "Accept-Signature"

   Applicable protocol: http

   Status: standard

   Author/Change controller: IETF

   Specification document(s): Section 3.7 of this document

8.3.  Signed-Headers Header Field Registration

   This section registers the "Signed-Headers" header field in the
   "Permanent Message Header Field Names" registry ([RFC3864]).

   Header field name: "Signed-Headers"

   Applicable protocol: http

   Status: standard

   Author/Change controller: IETF

   Specification document(s): Section 5.1.2 of this document

8.4.  HTTP/2 Settings

   This section establishes an entry for the HTTP/2 Settings Registry
   that was established by Section 11.3 of [RFC7540]

   Name: ENABLE_CROSS_ORIGIN_PUSH

   Code: 0xSETTING-TBD

   Initial Value: 0

   Specification: This document

8.5.  HTTP/2 Error code

   This section establishes an entry for the HTTP/2 Error Code Registry
   that was established by Section 11.4 of [RFC7540]




Yasskin                 Expires December 16, 2018              [Page 33]


Internet-Draft            Signed HTTP Exchanges                June 2018


   Name: NO_TRUSTED_EXCHANGE_SIGNATURE

   Code: 0xERROR-TBD

   Description: The client does not trust the signature for a cross-
   origin Pushed signed exchange.

   Specification: This document

8.6.  Internet Media Type application/signed-exchange

   Type name: application

   Subtype name: signed-exchange

   Required parameters:

   o  v: A string denoting the version of the file format.  ([RFC5234]
      ABNF: "version = DIGIT/%x61-7A") The version defined in this
      specification is "1".  When used with the "Accept" header field
      (Section 5.3.1 of [RFC7231]), this parameter can be a comma
      (,)-separated list of version strings.  ([RFC5234] ABNF: "version-
      list = version *( "," version )") The server is then expected to
      reply with a resource using a particular version from that list.

      Note: RFC EDITOR PLEASE DELETE THIS NOTE; Implementations of
      drafts of this specification MUST NOT use simple integers to
      describe their versions, and MUST instead define implementation-
      specific strings to identify which draft is implemented.

   Optional parameters: N/A

   Encoding considerations: binary

   Security considerations: see Section 6.6

   Interoperability considerations: N/A

   Published specification: This specification (see Section 5.3).

   Applications that use this media type: N/A

   Fragment identifier considerations: N/A

   Additional information:

   Deprecated alias names for this type: N/A




Yasskin                 Expires December 16, 2018              [Page 34]


Internet-Draft            Signed HTTP Exchanges                June 2018


   Magic number(s): 73 78 67 31 00

   File extension(s): .sxg

   Macintosh file type code(s): N/A

   Person and email address to contact for further information: See
   Authors' Addresses section.

   Intended usage: COMMON

   Restrictions on usage: N/A

   Author: See Authors' Addresses section.

   Change controller: IESG

8.7.  Internet Media Type application/cert-chain+cbor

   Type name: application

   Subtype name: cert-chain+cbor

   Required parameters: N/A

   Optional parameters: N/A

   Encoding considerations: binary

   Security considerations: N/A

   Interoperability considerations: N/A

   Published specification: This specification (see Section 3.3).

   Applications that use this media type: N/A

   Fragment identifier considerations: N/A

   Additional information:

   Deprecated alias names for this type: N/A

   Magic number(s): 1*9(??) 67 F0 9F 93 9C E2 9B 93

   File extension(s): N/A

   Macintosh file type code(s): N/A



Yasskin                 Expires December 16, 2018              [Page 35]


Internet-Draft            Signed HTTP Exchanges                June 2018


   Person and email address to contact for further information: See
   Authors' Addresses section.

   Intended usage: COMMON

   Restrictions on usage: N/A

   Author: See Authors' Addresses section.

   Change controller: IESG

9.  References

9.1.  Normative References

   [FETCH]    WHATWG, "Fetch", June 2018,
              <https://fetch.spec.whatwg.org/>.

   [HTML]     WHATWG, "HTML", June 2018,
              <https://html.spec.whatwg.org/multipage>.

   [I-D.ietf-cbor-cddl]
              Birkholz, H., Vigano, C., and C. Bormann, "Concise data
              definition language (CDDL): a notational convention to
              express CBOR data structures", draft-ietf-cbor-cddl-02
              (work in progress), February 2018.

   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-06 (work in progress),
              June 2018.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

   [I-D.thomson-http-mice]
              Thomson, M., "Merkle Integrity Content Encoding", draft-
              thomson-http-mice-02 (work in progress), October 2016.

   [POSIX]    IEEE and The Open Group, "The Open Group Base
              Specifications Issue 7", name IEEE, value 1003.1-2008,
              2016 Edition, 2016,
              <http://pubs.opengroup.org/onlinepubs/9699919799/
              basedefs/>.





Yasskin                 Expires December 16, 2018              [Page 36]


Internet-Draft            Signed HTTP Exchanges                June 2018


   [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>.

   [RFC3230]  Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
              RFC 3230, DOI 10.17487/RFC3230, January 2002,
              <https://www.rfc-editor.org/info/rfc3230>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <https://www.rfc-editor.org/info/rfc6960>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <https://www.rfc-editor.org/info/rfc6962>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [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,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.



Yasskin                 Expires December 16, 2018              [Page 37]


Internet-Draft            Signed HTTP Exchanges                June 2018


   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

   [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-editor.org/info/rfc7540>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [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>.

   [URL]      WHATWG, "URL", June 2018, <https://url.spec.whatwg.org/>.

9.2.  Informative References

   [DROWN]    Aviram, N., Schinzel, S., Somorovsky, J., Heninger, N.,
              Dankel, M., Steube, J., Valenta, L., Adrian, D.,
              Halderman, J., Dukhovni, V., Kaesper, E., Cohney, S.,
              Engels, S., Paar, C., and Y. Shavitt, "The DROWN Attack",
              2016, <https://drownattack.com/>.

   [I-D.burke-content-signature]
              Burke, B., "HTTP Header for digital signatures", draft-
              burke-content-signature-00 (work in progress), March 2011.

   [I-D.cavage-http-signatures]
              Cavage, M. and M. Sporny, "Signing HTTP Messages", draft-
              cavage-http-signatures-10 (work in progress), May 2018.

   [I-D.ietf-httpbis-http2-secondary-certs]
              Bishop, M., Sullivan, N., and M. Thomson, "Secondary
              Certificate Authentication in HTTP/2", draft-ietf-httpbis-
              http2-secondary-certs-01 (work in progress), May 2018.

   [I-D.thomson-http-content-signature]
              Thomson, M., "Content-Signature Header Field for HTTP",
              draft-thomson-http-content-signature-00 (work in
              progress), July 2015.





Yasskin                 Expires December 16, 2018              [Page 38]


Internet-Draft            Signed HTTP Exchanges                June 2018


   [I-D.yasskin-webpackage-use-cases]
              Yasskin, J., "Use Cases and Requirements for Web
              Packages", draft-yasskin-webpackage-use-cases-01 (work in
              progress), March 2018.

   [RFC2965]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2965, DOI 10.17487/RFC2965, October 2000,
              <https://www.rfc-editor.org/info/rfc2965>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, DOI 10.17487/RFC6455, December 2011,
              <https://www.rfc-editor.org/info/rfc6455>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [RFC7615]  Reschke, J., "HTTP Authentication-Info and Proxy-
              Authentication-Info Response Header Fields", RFC 7615,
              DOI 10.17487/RFC7615, September 2015,
              <https://www.rfc-editor.org/info/rfc7615>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [RFC8053]  Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
              T., and Y. Ioku, "HTTP Authentication Extensions for
              Interactive Clients", RFC 8053, DOI 10.17487/RFC8053,
              January 2017, <https://www.rfc-editor.org/info/rfc8053>.






Yasskin                 Expires December 16, 2018              [Page 39]


Internet-Draft            Signed HTTP Exchanges                June 2018


   [RFC8336]  Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
              RFC 8336, DOI 10.17487/RFC8336, March 2018,
              <https://www.rfc-editor.org/info/rfc8336>.

   [ROBOT]    Boeck, H., Somorovsky, J., and C. Young, "The ROBOT
              Attack", 2017, <https://robotattack.org/>.

   [SRI]      Akhawe, D., Braun, F., Marier, F., and J. Weinberger,
              "Subresource Integrity", World Wide Web Consortium
              Recommendation REC-SRI-20160623, June 2016,
              <http://www.w3.org/TR/2016/REC-SRI-20160623>.

   [W3C.NOTE-OPS-OverHTTP]
              Hensley, P., Metral, M., Shardanand, U., Converse, D., and
              M. Myers, "Implementation of OPS Over HTTP", W3C NOTE
              NOTE-OPS-OverHTTP, June 1997.

9.3.  URIs

   [1] https://lists.w3.org/Archives/Public/ietf-http-wg/

   [2] https://github.com/WICG/webpackage

   [3] http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
       V1_chap04.html#tag_04_16

   [4] https://url.spec.whatwg.org/#absolute-url-string

   [5] https://url.spec.whatwg.org/#absolute-url-string

   [6] https://url.spec.whatwg.org/#absolute-url-string

   [7] https://html.spec.whatwg.org/multipage/origin.html#same-origin

   [8] https://example.com/

   [9] https://calendar.perfplanet.com/2013/big-bad-preloader/

   [10] https://github.com/mikewest/signature-based-sri

   [11] https://github.com/mikewest/signature-based-sri/issues/5

   [12] https://www.apple.com/ios/app-store/

   [13] https://play.google.com/store

   [14] https://github.com/WICG/webpackage




Yasskin                 Expires December 16, 2018              [Page 40]


Internet-Draft            Signed HTTP Exchanges                June 2018


   [15] https://tools.ietf.org/html/rfc7540#section-8.2

   [16] https://www.imperialviolet.org/2012/02/05/crlsets.html

   [17] https://tlswg.github.io/tls13-spec/draft-ietf-tls-
        tls13.html#ocsp-and-sct

Appendix A.  Use cases

A.1.  PUSHed subresources

   To reduce round trips, a server might use HTTP/2 Push (Section 8.2 of
   [RFC7540]) to inject a subresource from another server into the
   client's cache.  If anything about the subresource is expired or
   can't be verified, the client would fetch it from the original
   server.

   For example, if "https://example.com/index.html" includes

   <script src="https://jquery.com/jquery-1.2.3.min.js">

   Then to avoid the need to look up and connect to "jquery.com" in the
   critical path, "example.com" might push that resource signed by
   "jquery.com".

A.2.  Explicit use of a content distributor for subresources

   In order to speed up loading but still maintain control over its
   content, an HTML page in a particular origin "O.com" could tell
   clients to load its subresources from an intermediate content
   distributor that's not authoritative, but require that those
   resources be signed by "O.com" so that the distributor couldn't
   modify the resources.  This is more constrained than the common CDN
   case where "O.com" has a CNAME granting the CDN the right to serve
   arbitrary content as "O.com".

   <img logicalsrc="https://O.com/img.png"
        physicalsrc="https://distributor.com/O.com/img.png">

   To make it easier to configure the right distributor for a given
   request, computation of the "physicalsrc" could be encapsulated in a
   custom element:

   <dist-img src="https://O.com/img.png"></dist-img>

   where the "<dist-img>" implementation generates an appropriate
   "<img>" based on, for example, a "<meta name="dist-base">" tag
   elsewhere in the page.  However, this has the downside that the



Yasskin                 Expires December 16, 2018              [Page 41]


Internet-Draft            Signed HTTP Exchanges                June 2018


   preloader [9] can no longer see the physical source to download it.
   The resulting delay might cancel out the benefit of using a
   distributor.

   This could be used for some of the same purposes as SRI
   (Appendix A.3).

   To implement this with the current proposal, the distributor would
   respond to the physical request to "https://distributor.com/O.com/
   img.png" with first a signed PUSH_PROMISE for "https://O.com/img.png"
   and then a redirect to "https://O.com/img.png".

A.3.  Subresource Integrity

   The W3C WebAppSec group is investigating using signatures [10] in
   [SRI].  They need a way to transmit the signature with the response,
   which this proposal provides.

   Their needs are simpler than most other use cases in that the
   "integrity="ed25519-[public-key]"" attribute and CSP-based ways of
   expressing a public key don't need that key to be wrapped into a
   certificate.

   The "ed25519key" signature parameter supports this simpler way of
   attaching a key.

   The current proposal for signature-based SRI describes signing only
   the content of a resource, while this specification requires them to
   sign the request URI as well.  This issue is tracked in
   https://github.com/mikewest/signature-based-sri/issues/5 [11].  The
   details of what they need to sign will affect whether and how they
   can use this proposal.

A.4.  Binary Transparency

   So-called "Binary Transparency" may eventually allow users to verify
   that a program they've been delivered is one that's available to the
   public, and not a specially-built version intended to attack just
   them.  Binary transparency systems don't exist yet, but they're
   likely to work similarly to the successful Certificate Transparency
   logs described by [RFC6962].

   Certificate Transparency depends on Signed Certificate Timestamps
   that prove a log contained a particular certificate at a particular
   time.  To build the same thing for Binary Transparency logs
   containing HTTP resources or full websites, we'll need a way to
   provide signatures of those resources, which signed exchanges
   provides.



Yasskin                 Expires December 16, 2018              [Page 42]


Internet-Draft            Signed HTTP Exchanges                June 2018


A.5.  Static Analysis

   Native app stores like the Apple App Store [12] and the Android Play
   Store [13] grant their contents powerful abilities, which they
   attempt to make safe by analyzing the applications before offering
   them to people.  The web has no equivalent way for people to wait to
   run an update of a web application until a trusted authority has
   vouched for it.

   While full application analysis probably needs to wait until the
   authority can sign bundles of exchanges, authorities may be able to
   guarantee certain properties by just checking a top-level resource
   and its [SRI]-constrained sub-resources.

A.6.  Offline websites

   Fully-offline websites can be represented as bundles of signed
   exchanges, although an optimization to reduce the number of signature
   verifications may be needed.  Work on this is in progress in the
   https://github.com/WICG/webpackage [14] repository.

Appendix B.  Requirements

B.1.  Proof of origin

   To verify that a thing came from a particular origin, for use in the
   same context as a TLS connection, we need someone to vouch for the
   signing key with as much verification as the signing keys used in
   TLS.  The obvious way to do this is to re-use the web PKI and CA
   ecosystem.

B.1.1.  Certificate constraints

   If we re-use existing TLS server certificates, we incur the risks
   that:

   1.  TLS server certificates must be accessible from online servers,
       so they're easier to steal or use as signing oracles than an
       offline key.  An exchange's signing key doesn't need to be
       online.

   2.  A server using an origin-trusted key for one purpose (e.g.  TLS)
       might accidentally sign something that looks like an exchange, or
       vice versa.

   These risks are considered too high, so we define a new X.509
   certificate extension in Section 4.2 that requires CAs to issue new




Yasskin                 Expires December 16, 2018              [Page 43]


Internet-Draft            Signed HTTP Exchanges                June 2018


   certificates for this purpose.  We expect at least one low-cost CA to
   be willing to sign certificates with this extension.

B.1.2.  Signature constraints

   In order to prevent an attacker who can convince the server to sign
   some resource from causing those signed bytes to be interpreted as
   something else the new X.509 extension here is forbidden from being
   used in TLS servers.  If Section 4.2 changes to allow re-use in TLS
   servers, we would need to:

   1.  Avoid key types that are used for non-TLS protocols whose output
       could be confused with a signature.  That may be just the
       "rsaEncryption" OID from [RFC8017].

   2.  Use the same format as TLS's signatures, specified in
       Section 4.4.3 of [I-D.ietf-tls-tls13], with a context string
       that's specific to this use.

   The specification also needs to define which signing algorithm to
   use.  It currently specifies that as a function from the key type,
   instead of allowing attacker-controlled data to specify it.

B.1.3.  Retrieving the certificate

   The client needs to be able to find the certificate vouching for the
   signing key, a chain from that certificate to a trusted root, and
   possibly other trust information like SCTs ([RFC6962]).  One approach
   would be to include the certificate and its chain in the signature
   metadata itself, but this wastes bytes when the same certificate is
   used for multiple HTTP responses.  If we decide to put the signature
   in an HTTP header, certificates are also unusually large for that
   context.

   Another option is to pass a URL that the client can fetch to retrieve
   the certificate and chain.  To avoid extra round trips in fetching
   that URL, it could be bundled (Appendix A.6) with the signed content
   or PUSHed (Appendix A.1) with it.  The risks from the
   "client_certificate_url" extension (Section 11.3 of [RFC6066]) don't
   seem to apply here, since an attacker who can get a client to load an
   exchange and fetch the certificates it references, can also get the
   client to perform those fetches by loading other HTML.

   To avoid using an unintended certificate with the same public key as
   the intended one, the content of the leaf certificate or the chain
   should be included in the signed data, like TLS does (Section 4.4.3
   of [I-D.ietf-tls-tls13]).




Yasskin                 Expires December 16, 2018              [Page 44]


Internet-Draft            Signed HTTP Exchanges                June 2018


B.2.  How much to sign

   The previous [I-D.thomson-http-content-signature] and
   [I-D.burke-content-signature] schemes signed just the content, while
   ([I-D.cavage-http-signatures] could also sign the response headers
   and the request method and path.  However, the same path, response
   headers, and content may mean something very different when retrieved
   from a different server.  Section 5.1.1 currently includes the whole
   request URL in the signature, but it's possible we need a more
   flexible scheme to allow some higher-level protocols to accept a
   less-signed URL.

   The question of whether to include other request headers--primarily
   the "accept*" family--is still open.  These headers need to be
   represented so that clients wanting a different language, say, can
   avoid using the wrong-language response, but it's not obvious that
   there's a security vulnerability if an attacker can spoof them.  For
   now, the proposal (Section 3) omits other request headers.

   In order to allow multiple clients to consume the same signed
   exchange, the exchange shouldn't include the exact request headers
   that any particular client sends.  For example, a Japanese resource
   wouldn't include

   accept-language: ja-JP, ja;q=0.9, en;q=0.8, zh;q=0.7, *;q=0.5

   Instead, it would probably include just

   accept-language: ja-JP, ja

   and clients would use the same matching logic as for PUSH_PROMISE
   [15] frame headers.

B.2.1.  Conveying the signed headers

   HTTP headers are traditionally munged by proxies, making it
   impossible to guarantee that the client will see the same sequence of
   bytes as the publisher published.  In the HTTPS world, we have more
   end-to-end header integrity, but it's still likely that there are
   enough TLS-terminating proxies that the publisher's signatures would
   tend to break before getting to the client.

   There's also no way in current HTTP for the response to a client-
   initiated request (Section 8.1 of [RFC7540]) to convey the request
   headers it expected to respond to.  A PUSH_PROMISE (Section 8.2 of
   [RFC7540]) does not have this problem, and it would be possible to
   introduce a response header to convey the expected request headers.




Yasskin                 Expires December 16, 2018              [Page 45]


Internet-Draft            Signed HTTP Exchanges                June 2018


   Since proxies are unlikely to modify unknown content types, we can
   wrap the original exchange into an "application/signed-exchange"
   format (Section 5.3) and include the "Cache-Control: no-transform"
   header when sending it.

   To reduce the likelihood of accidental modification by proxies, the
   "application/signed-exchange" format includes a file signature that
   doesn't collide with other known signatures.

   To help the PUSHed subresources use case (Appendix A.1), we might
   also want to extend the "PUSH_PROMISE" frame type to include a
   signature, and that could tell intermediates not to change the
   ensuing headers.

B.3.  Response lifespan

   A normal HTTPS response is authoritative only for one client, for as
   long as its cache headers say it should live.  A signed exchange can
   be re-used for many clients, and if it was generated while a server
   was compromised, it can continue compromising clients even if their
   requests happen after the server recovers.  This signing scheme needs
   to mitigate that risk.

B.3.1.  Certificate revocation

   Certificates are mis-issued and private keys are stolen, and in
   response clients need to be able to stop trusting these certificates
   as promptly as possible.  Online revocation checks don't work [16],
   so the industry has moved to pushed revocation lists and stapled OCSP
   responses [RFC6066].

   Pushed revocation lists work as-is to block trust in the certificate
   signing an exchange, but the signatures need an explicit strategy to
   staple OCSP responses.  One option is to extend the certificate
   download (Appendix B.1.3) to include the OCSP response too, perhaps
   in the TLS 1.3 CertificateEntry [17] format.

B.3.2.  Response downgrade attacks

   The signed content in a response might be vulnerable to attacks, such
   as XSS, or might simply be discovered to be incorrect after
   publication.  Once the author fixes those vulnerabilities or
   mistakes, clients should stop trusting the old signed content in a
   reasonable amount of time.  Similar to certificate revocation, I
   expect the best option to be stapled "this version is still valid"
   assertions with short expiration times.

   These assertions could be structured as:



Yasskin                 Expires December 16, 2018              [Page 46]


Internet-Draft            Signed HTTP Exchanges                June 2018


   1.  A signed minimum version number or timestamp for a set of request
       headers: This requires that signed responses need to include a
       version number or timestamp, but allows a server to provide a
       single signature covering all valid versions.

   2.  A replacement for the whole exchange's signature.  This requires
       the publisher to separately re-sign each valid version and
       requires each version to include a different update URL, but
       allows intermediates to serve less data.  This is the approach
       taken in Section 3.

   3.  A replacement for the exchange's signature and an update for the
       embedded "expires" and related cache-control HTTP headers
       [RFC7234].  This naturally extends publishers' intuitions about
       cache expiration and the existing cache revalidation behavior to
       signed exchanges.  This is sketched and its downsides explored in
       Appendix C.

   The signature also needs to include instructions to intermediates for
   how to fetch updated validity assertions.

B.4.  Low implementation complexity

   Simpler implementations are, all things equal, less likely to include
   bugs.  This section describes decisions that were made in the rest of
   the specification to reduce complexity.

B.4.1.  Limited choices

   In general, we're trying to eliminate unnecessary choices in the
   specification.  For example, instead of requiring clients to support
   two methods for verifying payload integrity, we only require one.

B.4.2.  Bounded-buffering integrity checking

   Clients can be designed with a more-trusted network layer that
   decides how to trust resources and then provides those resources to
   less-trusted rendering processes along with handles to the storage
   and other resources they're allowed to access.  If the network layer
   can enforce that it only operates on chunks of data up to a certain
   size, it can avoid the complexity of spooling large files to disk.

   To allow the network layer to verify signed exchanges using a bounded
   amount of memory, Section 5.3 requires the signature and headers to
   be less than TBD bytes long, and Section 3.5 requires that the MI
   record size be less than TBD bytes.  This allows the network layer to
   validate a bounded chunk at a time, and pass that chunk on to a




Yasskin                 Expires December 16, 2018              [Page 47]


Internet-Draft            Signed HTTP Exchanges                June 2018


   renderer, and then forget about that chunk before processing the next
   one.

   The "Digest" header field from [RFC3230] requires the network layer
   to buffer the entire response body, so it's disallowed.

Appendix C.  Determining validity using cache control

   This draft could expire signature validity using the normal HTTP
   cache control headers ([RFC7234]) instead of embedding an expiration
   date in the signature itself.  This section specifies how that would
   work, and describes why I haven't chosen that option.

   The signatures in the "Signature" header field (Section 3.1) would no
   longer contain "date" or "expires" fields.

   The validity-checking algorithm (Section 3.5) would initialize "date"
   from the resource's "Date" header field (Section 7.1.1.2 of
   [RFC7231]) and initialize "expires" from either the "Expires" header
   field (Section 5.3 of [RFC7234]) or the "Cache-Control" header
   field's "max-age" directive (Section 5.2.2.8 of [RFC7234]) (added to
   "date"), whichever is present, preferring "max-age" (or failing) if
   both are present.

   Validity updates (Section 3.6) would include a list of replacement
   response header fields.  For each header field name in this list, the
   client would remove matching header fields from the stored exchange's
   response header fields.  Then the client would append the replacement
   header fields to the stored exchange's response header fields.

C.1.  Example of updating cache control

   For example, given a stored exchange of:

   GET  / HTTP/1.1
   Host: example.com
   Accept: */*

   HTTP/1.1 200
   Date: Mon, 20 Nov 2017 10:00:00 UTC
   Content-Type: text/html
   Date: Tue, 21 Nov 2017 10:00:00 UTC
   Expires: Sun, 26 Nov 2017 10:00:00 UTC

   <!doctype html>
   <html>
   ...




Yasskin                 Expires December 16, 2018              [Page 48]


Internet-Draft            Signed HTTP Exchanges                June 2018


   And an update listing the following headers:

   Expires: Fri, 1 Dec 2017 10:00:00 UTC
   Date: Sat, 25 Nov 2017 10:00:00 UTC

   The resulting stored exchange would be:

   GET / HTTP/1.1
   Host: example.com
   Accept: */*

   HTTP/1.1 200
   Content-Type: text/html
   Expires: Fri, 1 Dec 2017 10:00:00 UTC
   Date: Sat, 25 Nov 2017 10:00:00 UTC

   <!doctype html>
   <html>
   ...

C.2.  Downsides of updating cache control

   In an exchange with multiple signatures, using cache control to
   expire signatures forces all signatures to initially live for the
   same period.  Worse, the update from one signature's "validity-url"
   might not match the update for another signature.  Clients would need
   to maintain a current set of headers for each signature, and then
   decide which set to use when actually parsing the resource itself.

   This need to store and reconcile multiple sets of headers for a
   single signed exchange argues for embedding a signature's lifetime
   into the signature.

Appendix D.  Change Log

   RFC EDITOR PLEASE DELETE THIS SECTION.

   draft-04

   o  Update to draft-ietf-httpbis-header-structure-06.

   o  Replace the application/http-exchange+cbor format with a simpler
      application/signed-exchange format that:

      *  Doesn't require a streaming CBOR parser parse it from a network
         stream.





Yasskin                 Expires December 16, 2018              [Page 49]


Internet-Draft            Signed HTTP Exchanges                June 2018


      *  Doesn't allow request payloads or response trailers, which
         don't fit into the signature model.

      *  Allows checking the signature before parsing the exchange
         headers.

   o  Require absolute URLs.

   o  Make all identifiers in headers lower-case, as required by
      Structured Headers.

   o  Switch back to the TLS 1.3 signature format.

   o  Include the version and draft number in the signature context
      string.

   o  Remove support for integrity protection using the Digest header
      field.

   o  Limit the record size in the mi-sha256 encoding.

   o  Forbid RSA keys, and only require clients to support secp256r1
      keys.

   o  Add a test OID for the CanSignHttpExchanges X.509 extension.

   draft-03

   o  Allow each method of transferring an exchange to define which
      headers are signed, have the cross-origin methods use all headers,
      and remove the "allResponseHeaders" flag.

   o  Describe footguns around signing private content, and block
      certain headers to make it less likely.

   o  Define a CBOR structure to hold the certificate chain instead of
      re-using the TLS1.3 message.  The TLS 1.3 parser fails on
      unexpected extensions while this format should ignore them, and
      apparently TLS implementations don't expose their message parsers
      enough to allow passing a message to a certificate verifier.

   o  Require an X.509 extension for the signing certificate.

   draft-02

   o  Signatures identify a header (e.g.  Digest or MI) to guard the
      payload's integrity instead of directly signing over the payload.




Yasskin                 Expires December 16, 2018              [Page 50]


Internet-Draft            Signed HTTP Exchanges                June 2018


   o  The validityUrl is signed.

   o  Use CBOR maps where appropriate, and define how they're
      canonicalized.

   o  Remove the update.url field from signature validity updates, in
      favor of just re-fetching the original request URL.

   o  Define an HTTP/2 extension to use a setting to enable cross-origin
      Server Push.

   o  Define an "Accept-Signature" header to negotiate whether to send
      Signatures and which ones.

   o  Define an "application/http-exchange+cbor" format to fetch signed
      exchanges without HTTP/2 Push.

   o  2 new use cases.

Appendix E.  Acknowledgements

   Thanks to Ilari Liusvaara, Justin Schuh, Mark Nottingham, Mike
   Bishop, Ryan Sleevi, and Yoav Weiss for comments that improved this
   draft.

Author's Address

   Jeffrey Yasskin
   Google

   Email: jyasskin@chromium.org




















Yasskin                 Expires December 16, 2018              [Page 51]


Html markup produced by rfcmarkup 1.128, available from https://tools.ietf.org/tools/rfcmarkup/