Network Working Group J. Yasskin
Internet-Draft Google
Intended status: Standards Track June 15, 2018
Expires: December 17, 2018

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

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/.

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

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 17, 2018.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

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:

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].
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.

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:

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 ([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 ([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.

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 subjectAltNames 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 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 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:
  2. The map mapping:

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:

[
  {
    ':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 = [
  "📜⛓", ; 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:

  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:

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:

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:
  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]
  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”.

    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” to get a chain including a newer OCSP response.

The “validity-url” parameter 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.

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).

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 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.

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; **

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 Signatures 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?

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 is not same-origin 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 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:

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:

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 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:

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.

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 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.
  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/ 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 …:

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 naïve signing systems blindly add it.

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.

Clients MAY also check the “validity-url” 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.

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

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]

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:

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

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

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.
[HTML] WHATWG, "HTML", June 2018.
[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", Internet-Draft draft-ietf-cbor-cddl-02, February 2018.
[I-D.ietf-httpbis-header-structure] Nottingham, M. and P. Kamp, "Structured Headers for HTTP", Internet-Draft draft-ietf-httpbis-header-structure-06, June 2018.
[I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-28, March 2018.
[I-D.thomson-http-mice] Thomson, M., "Merkle Integrity Content Encoding", Internet-Draft draft-thomson-http-mice-02, October 2016.
[POSIX] IEEE and The Open Group, "The Open Group Base Specifications Issue 7", name IEEE, value 1003.1-2008, 2016 Edition, 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3230] Mogul, J. and A. Van Hoff, "Instance Digests in HTTP", RFC 3230, DOI 10.17487/RFC3230, January 2002.
[RFC3864] Klyne, G., Nottingham, M. and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[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.
[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.
[RFC6962] Laurie, B., Langley, A. and E. Kasper, "Certificate Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014.
[RFC7234] Fielding, R., Nottingham, M. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014.
[RFC7540] Belshe, M., Peon, R. and M. Thomson, "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[URL] WHATWG, "URL", June 2018.

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., Käsper, E., Cohney, S., Engels, S., Paar, C. and Y. Shavitt, "The DROWN Attack", 2016.
[I-D.burke-content-signature] Burke, B., "HTTP Header for digital signatures", Internet-Draft draft-burke-content-signature-00, March 2011.
[I-D.cavage-http-signatures] Cavage, M. and M. Sporny, "Signing HTTP Messages", Internet-Draft draft-cavage-http-signatures-10, May 2018.
[I-D.ietf-httpbis-http2-secondary-certs] Bishop, M., Sullivan, N. and M. Thomson, "Secondary Certificate Authentication in HTTP/2", Internet-Draft draft-ietf-httpbis-http2-secondary-certs-01, May 2018.
[I-D.thomson-http-content-signature] Thomson, M., "Content-Signature Header Field for HTTP", Internet-Draft draft-thomson-http-content-signature-00, July 2015.
[I-D.yasskin-webpackage-use-cases] Yasskin, J., "Use Cases and Requirements for Web Packages", Internet-Draft draft-yasskin-webpackage-use-cases-01, March 2018.
[RFC2965] Kristol, D. and L. Montulli, "HTTP State Management Mechanism", RFC 2965, DOI 10.17487/RFC2965, October 2000.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, DOI 10.17487/RFC6265, April 2011.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455, DOI 10.17487/RFC6455, December 2011.
[RFC7235] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Authentication", RFC 7235, DOI 10.17487/RFC7235, June 2014.
[RFC7615] Reschke, J., "HTTP Authentication-Info and Proxy-Authentication-Info Response Header Fields", RFC 7615, DOI 10.17487/RFC7615, September 2015.
[RFC8017] Moriarty, K., Kaliski, B., Jonsson, J. and A. Rusch, "PKCS #1: RSA Cryptography Specifications Version 2.2", RFC 8017, DOI 10.17487/RFC8017, November 2016.
[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.
[RFC8336] Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame", RFC 8336, DOI 10.17487/RFC8336, March 2018.
[ROBOT] Böck, H., Somorovsky, J. and C. Young, "The ROBOT Attack", 2017.
[SRI] Akhawe, D., Braun, F., Marier, F. and J. Weinberger, "Subresource Integrity", World Wide Web Consortium Recommendation REC-SRI-20160623, June 2016.
[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.

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 preloader 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 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. 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.

A.5. Static Analysis

Native app stores like the Apple App Store and the Android Play Store 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 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 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 with the signed content or PUSHed 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]).

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 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.

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, 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 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:

  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 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>
...

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

draft-03

draft-02

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