Network Working Group J. Yasskin
Internet-Draft K. Ueno
Intended status: Standards Track Google
Expires: June 18, 2019 December 15, 2018

Signed HTTP Exchanges Implementation Checkpoints
draft-yasskin-httpbis-origin-signed-exchanges-impl-latest

Abstract

This document describes checkpoints of draft-yasskin-http-origin-signed-responses to synchronize implementation between clients, intermediates, and publishers.

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 June 18, 2019.

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Table of Contents

1. Introduction

Each version of this document describes a checkpoint of [I-D.yasskin-http-origin-signed-responses] that can be implemented in sync by clients, intermediates, and publishers. It defines a technique to detect which version each party has implemented so that mismatches can be detected up-front.

2. Terminology

Absolute URL
A string for which the URL parser ([URL]), when run without a base URL, returns a URL rather than a failure, and for which that URL has a null fragment. This is similar to the absolute-URL string concept defined by ([URL]) but might not include exactly the same strings.
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 single signature for an exchange, 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]), and the list MUST contain exactly one element. Its ABNF is:

Signature = sh-param-list

The parameterised identifier in the list MUST have parameters named “sig”, “integrity”, “validity-url”, “date”, “expires”, “cert-url”, and “cert-sha256”. This specification gives no meaning to the identifier itself, which can be used as a human-readable identifier for the signature. 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 a “/”-separated sequence of names starting with the lowercase name of the response header field that guards the response payload’s integrity. The meaning of subsequent names depends on the response header field, but for the “digest” header field, the single following name is the name of the digest algorithm that guards the payload’s integrity.
“cert-url”
A string (Section 3.7 of [I-D.ietf-httpbis-header-structure]) containing an absolute URL (Section 2) with a scheme of “https” or “data”.
“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”.
“validity-url”
A string (Section 3.7 of [I-D.ietf-httpbis-header-structure]) containing an absolute URL (Section 2) with a scheme of “https”.
“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="digest/mi-sha256-03";
  validity-url="https://example.com/resource.validity.1511128380";
  cert-url="https://example.com/oldcerts";
  cert-sha256=*W7uB969dFW3Mb5ZefPS9Tq5ZbH5iSmOILpjv2qEArmI=*;
  date=1511128380; expires=1511733180

The signature uses a secp256r1 certificate within https://example.com/.

It relies on the Digest response header with the mi-sha256-03 digest algorithm to guard the integrity of the response payload.

The signature includes 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 certificate at https://example.com/certs has a subjectAltName of example.com, meaning that if it and its signature validate, the exchange can be trusted as having an origin of https://example.com/.

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 a set of request and response metadata and 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
Digest: mi-sha256-03=dcRDgR2GM35DluAV13PzgnG6+pvQwPywfFvAu1UeFrs=
Signed-Headers: "content-type", "digest"

<!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:

[
  {
    'accept': '*/*',
    ':method': 'GET',
  },
  {
    'digest': 'mi-sha256-03=dcRDgR2GM35DluAV13PzgnG6+pvQwPywfFvAu1UeFrs=',
    ':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 map (second 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 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 if any 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’s

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. Set publicKey and signing-alg depending on which key fields are present:
    1. Assert: 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, return “invalid”.
  4. If expires is more than 7 days (604800 seconds) after date, return “invalid”.
  5. If the current time is before date or after expires, return “invalid”.
  6. 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 b2”.

      Note: As this is a snapshot of a draft of [I-D.yasskin-http-origin-signed-responses], it uses a distinct context string.
    3. A single 0 byte which serves as a separator.
    4. If cert-sha256 is set, a byte holding the value 32 followed by the 32 bytes of the value of cert-sha256. Otherwise a 0 byte.
    5. The 8-byte big-endian encoding of the length in bytes of validity-url, followed by the bytes of validity-url.
    6. The 8-byte big-endian encoding of date.
    7. The 8-byte big-endian encoding of expires.
    8. The 8-byte big-endian encoding of the length in bytes of requestUrl, followed by the bytes of requestUrl.
    9. The 8-byte big-endian encoding of the length in bytes of headers, followed by the bytes of headers.
  7. 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.
  8. If signature is not a valid signature of message by publicKey using signing-alg, return “invalid”.
  9. If integrity does not match “digest/mi-sha256-03”, return “invalid”.
  10. If payload doesn’t match the integrity information in the header described by integrity, 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 and parameter like Digest: mi-sha256-03 ([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.

3.6. 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

At 2017-11-27 11:02 UTC, sig1 has expired, so the client needs to fetch https://example.com/resource.validity.1511157180 (the validity-url of sig1) if it wishes to update that signature. This URL might contain:

{
  "signatures": [
    'sig1; '
    'sig=*MEQCIC/I9Q+7BZFP6cSDsWx43pBAL0ujTbON/+7RwKVk+ba5AiB3FSFLZqpzmDJ0NumNwN04pqgJZE99fcK86UjkPbj4jw==*; '
    'validity-url="https://example.com/resource.validity.1511157180"; '
    'integrity="digest/mi-sha256-03"'
    '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 original signature with the new signature provided. The signature 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

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 “digest/” indicate that the client supports the Digest header field ({{!RFC3230) with the parameter from the HTTP Digest Algorithm Values Registry registry named in lower-case by the rest of the identifier. For example, “digest/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 “digest/mi-sha256-03” 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 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: digest/mi-sha256-03

states that the client will accept signatures with payload integrity assured by the Digest header and mi-sha256-03 digest algorithm and implies that the client will accept 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 Digest: mi-sha256-03 header field.

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’s

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 requestUrl, return “invalid”.
  2. Use Section 3.5 to determine the signature’s validity for requestUrl, headers, and payload, getting certificate-chain back. If this returned “invalid” or didn’t return a certificate chain, return “invalid”.
  3. Let exchange be the exchange metadata and headers parsed out of headers.
  4. 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”.
  5. If exchange’s headers contain a stateful header field, as defined in Section 4.1, return “invalid”.
  6. Let authority be the host component of requestUrl.
  7. 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].
  8. Return “valid”.

4.1. Stateful header fields

As described in Section 6.1 of [I-D.yasskin-http-origin-signed-responses], 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.

   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 NOT mark this extension as critical.

Clients MUST NOT accept certificates with this extension in TLS connections (Section 4.4.2.2 of [I-D.ietf-tls-tls13]).

This draft of the specification identifies the CanSignHttpExchanges extension with the id-ce-canSignHttpExchangesDraft OID:

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

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 calls the validation procedure in Section 3.5) to validate the Signature header field for an exchange represented as a normal HTTP request/response pair, it MUST pass:

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. Serialized headers for a same-origin response

The serialized headers of an exchange represented as a normal HTTP request/response pair (Section 2.1 of [RFC7230] or Section 8.1 of [RFC7540]) are the canonical serialization (Section 3.4) of the CBOR representation (Section 3.2) of the following request and response metadata and 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 serialized headers.

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.

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

Cross origin push is not implemented.

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-b2” followed by a 0 byte, to serve as a file signature. This is redundant with the MIME type, and recipients that receive both MUST check that they match and stop parsing if they don’t.

    Note: As this is a snapshot of a draft of [I-D.yasskin-http-origin-signed-responses], it uses a distinct file signature.
  2. 2 bytes storing a big-endian integer fallbackUrlLength.
  3. fallbackUrlLength bytes holding a fallbackUrl, which MUST be an absolute URL with a scheme of “https”.

    Note: The byte location of the fallback URL is intended to remain invariant across versions of the application/signed-exchange format so that parsers encountering unknown versions can always find a URL to redirect to.

    Issue: Should this fallback information also include the method?
  4. 3 bytes storing a big-endian integer sigLength. If this is larger than 16384 (16*1024), parsing MUST fail.
  5. 3 bytes storing a big-endian integer headerLength. If this is larger than 524288 (512*1024), parsing MUST fail.
  6. sigLength bytes holding the Signature header field’s value (Section 3.1).
  7. headerLength bytes holding signedHeaders, 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.
  8. 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’s value, fallbackUrl as the effective request URI, signedHeaders, 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-b2\0<2-byte length of the following url string>
https://example.com/<3-byte length of the following header
value><3-byte length of the encoding of the
following array>sig1; sig=*...; integrity="digest/mi-sha256-03"; ...[
  {
    ':method': 'GET',
    'accept', '*/*'
  },
  {
    ':status': '200',
    'content-type': 'text/html'
  }
]<!doctype html>\r\n<html>...

6. Security considerations

All of the security considerations from Section 6 of [I-D.yasskin-http-origin-signed-responses] apply.

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

This depends on the following IANA registrations in [I-D.yasskin-http-origin-signed-responses]:

This document also modifies the registration for:

8.1. Internet Media Type application/signed-exchange

Type name: application

Subtype name: signed-exchange

Required parameters:

Magic number(s): 73 78 67 31 2D 62 32 00

The other fields are the same as the registration in [I-D.yasskin-http-origin-signed-responses].

9. References

9.1. Normative References

[FETCH] WHATWG, "Fetch", December 2018.
[I-D.ietf-cbor-cddl] Birkholz, H., Vigano, C. and C. Bormann, "Concise data definition language (CDDL): a notational convention to express CBOR and JSON data structures", Internet-Draft draft-ietf-cbor-cddl-06, November 2018.
[I-D.ietf-httpbis-header-structure] Nottingham, M. and P. Kamp, "Structured Headers for HTTP", Internet-Draft draft-ietf-httpbis-header-structure-09, December 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.yasskin-http-origin-signed-responses] Yasskin, J., "Signed HTTP Exchanges", Internet-Draft draft-yasskin-http-origin-signed-responses-04, June 2018.
[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.
[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.
[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", December 2018.

9.2. Informative References

[I-D.thomson-http-mice] Thomson, M. and J. Yasskin, "Merkle Integrity Content Encoding", Internet-Draft draft-thomson-http-mice-03, August 2018.
[I-D.yasskin-http-origin-signed-responses-03] Yasskin, J., "Signed HTTP Exchanges", Internet-Draft draft-yasskin-http-origin-signed-responses-03, March 2018.
[I-D.yasskin-http-origin-signed-responses-04] Yasskin, J., "Signed HTTP Exchanges", Internet-Draft draft-yasskin-http-origin-signed-responses-04, June 2018.
[RFC2965] Kristol, D. and L. Montulli, "HTTP State Management Mechanism", RFC 2965, DOI 10.17487/RFC2965, October 2000.
[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.
[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.
[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. Change Log

draft-02

Vs. draft-01:

draft-01

Vs. [I-D.yasskin-http-origin-signed-responses-04]:

draft-00

Vs. [I-D.yasskin-http-origin-signed-responses-03]:

Appendix B. Acknowledgements

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

Authors' Addresses

Jeffrey Yasskin Google EMail: jyasskin@chromium.org
Kouhei Ueno Google EMail: kouhei@chromium.org