| Network Working Group | J. Yasskin |
| Internet-Draft | K. Ueno |
| Intended status: Standards Track | |
| Expires: October 8, 2018 | April 06, 2018 |
Signed HTTP Exchanges Implementation Checkpoints
draft-yasskin-httpbis-origin-signed-exchanges-impl-00
This document describes checkpoints of [I-D.yasskin-http-origin-signed-responses] to synchronize implementation between clients, intermediates, and publishers.
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.
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 October 8, 2018.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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.
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.
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.
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-02]. Its value MUST be a list (Section 4.8 of [I-D.ietf-httpbis-header-structure-02]) of parameterised labels (Section 4.4 of [I-D.ietf-httpbis-header-structure-02]), and the list MUST contain exactly one element.
Each parameterised label MUST have parameters named “sig”, “integrity”, “validityUrl”, “date”, and “expires”. Each parameterised label MUST also have “certUrl” and “certSha256” parameters. This specification gives no meaning to the label 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:
The “certUrl” parameter is not signed, so intermediates can update it with a pointer to a cached version.
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=*t7LoYw6vwL2FSZRNJPYdNdYjfZSQkaCQeqpBD1whcy/6AAamVJ2OryXoXv6ACVBQgPV13o5de9oOVcOGGMX9fsf2ve1UDw/ITpeimB7n3zcuDEePzIcPbUnicicN2yodZAfr5il7BBJTs8L+V2ZERI16nJfrOZOvUfhvuUaMDGQXx5StIj7XLiX7/caxPz5ctwglgVAwCmoVPhmYFLq391O+hEssHSk2xkY6r/D9V2cKMikBBOTZ+JFyrnS/f2B4li7YASIY0YX64ifCmCw97cQTngXax6Upoie44IAe+6JngOie9JlDgcMF3YZ1uxNGWl9VwlalSwWgi1YA9Ff7mQ; integrity="mi"; validityUrl="https://example.com/resource.validity.1511128380"; certUrl="https://example.com/certs"; certSha256=*W7uB969dFW3Mb5ZefPS9Tq5ZbH5iSmOILpjv2qEArmI; date=1511128380; expires=1511733180
The signatures uses a 2048-bit RSA certificate within https://example.com/.
It relies on the MI response header to guard the integrity of the response payload.
The signature includes a “validityUrl” 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/.
[I-D.ietf-httpbis-header-structure-02] provides a way to parameterise labels 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 certUrl and validityUrl 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-02] allows, so they’re single items for now.
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:
Given the HTTP exchange:
GET https://example.com/ HTTP/1.1 Accept: */* HTTP/1.1 200 Content-Type: text/html Content-Encoding: mi-sha256 MI: mi-sha256=20addcf7368837f616d549f035bf6784ea6d4bf4817a3736cd2fc7a763897fe3 <0x0000000000004000><!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/'
':method': 'GET',
},
{
'mi': 'mi-sha256=20addcf7368837f616d549f035bf6784ea6d4bf4817a3736cd2fc7a763897fe3',
':status': '200',
'content-type': 'text/html'
'content-encoding': 'mi-sha256',
}
]
The resource at a signature’s certUrl MUST contain a TLS 1.3 Certificate message (Section 4.4.2 of [I-D.ietf-tls-tls13]) containing X.509v3 certificates.
Parsing notes:
The client MUST ignore unknown or unexpected extensions.
Loading a certUrl takes a forceFetch flag. The client MUST:
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.
The client MUST parse the Signature header field as the list of parameterised values (Section 4.8.1 of [I-D.ietf-httpbis-header-structure-02]) 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.
Set
signing-alg to the result of applying this function to the type of main-certificate’s public key. If the function is undefined on this input, 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.
Should the signed message use the TLS format (with an initial 64 spaces) even though these certificates can’t be used in TLS servers?
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.
The “validityUrl” 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 “validityUrl” 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 labels (Section 4.4 of [I-D.ietf-httpbis-header-structure-02]) 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.
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...; ... validityUrl="https://example.com/resource.validity.1511157180"; certUrl="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 validityUrl of sig1) to update that signatures. This URL might contain:
{
"signatures": [
'sig1; '
'sig=*MEQCIC/I9Q+7BZFP6cSDsWx43pBAL0ujTbON/+7RwKVk+ba5AiB3FSFLZqpzmDJ0NumNwN04pqgJZE99fcK86UjkPbj4jw; '
'validityUrl="https://example.com/resource.validity.1511157180"; '
'integrity="mi"; '
'certUrl="https://example.com/newcerts"; '
'certSha256=*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...; ... validityUrl="https://example.com/resource.validity.1511157180"; certUrl="https://example.com/newcerts"; date=1511733180; expires=1512337980
This section isn’t implemented.
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”.
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:
For this draft, no new X.509 extension is required.
A signed exchange can be transferred in several ways, of which three are described here.
Receiving a Signature header as part of a normal HTTP exchange is not implemented.
Cross origin push is not implemented.
To parse a resource with content type application/signed-exchange;v=b0, the client MUST run the following algorithm:
Read 3 bytes and interpret them as a big-endian integer headerLength.
If headerLength is larger than 524288 (512kB), parsing MUST fail.
Read headerLength bytes, and parse them as a CBOR item. If this item isn’t canonically encoded (Section 3.4) or doesn’t match the following CDDL, parsing MUST fail:
signed-exchange-header = [
{ ':method': bytes,
':url': bytes,
* bytes => bytes,
},
{ ':status': bytes,
'signature': bytes,
* bytes => bytes,
},
]
The first element of the array is interpreted as the exchange’s request headers with lowercase names, with the request method in the ‘:method’ key’s value, and the effective request URI, which MUST be an absolute-URL string ([URL]), in the ‘:url’ key’s value.
The second element of the array is interpreted as the exchange’s response headers with lowercase names, with the 3-digit response status code in the ‘:status’ key’s value.
If any header field name includes uppercase characters, parsing MUST fail.
Pass the Signature response header and the exchange with that header removed to the algorithm in Section 4. Fail if this returns “invalid”.
The remainder of the resource is the exchange’s payload, encoded with the mi-sha256 content encoding ([I-D.thomson-http-mice]). If the mi-sha256 record length (the first 8 bytes of the payload) is greater than 16kB, or if any of the integrity proofs fail validation, parsing MUST fail.
All of the security considerations from Section 6 of [I-D.yasskin-http-origin-signed-responses] apply.
In addition, because this draft does not check for certificate revocation and allows signatures from certificates that can be used in normal TLS servers with no defense against future-dated signatures, clients MUST NOT trust signed exchanges as authoritative for their claimed origin without some explicit opt-in by their user.
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.
This depends on the following IANA registration in [I-D.yasskin-http-origin-signed-responses]:
This document also registers:
Type name: application
Subtype name: signed-exchange
Required parameters:
Optional parameters: N/A
Encoding considerations: binary
Security considerations: see Section 6.6 of [I-D.yasskin-http-origin-signed-responses]
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): 82 A?
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
| [FETCH] | WHATWG, "Fetch", April 2018. |
| [HTML] | WHATWG, "HTML", April 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-02] | Nottingham, M. and P. Kamp, "Structured Headers for HTTP", Internet-Draft draft-ietf-httpbis-header-structure-02, November 2017. |
| [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. |
| [I-D.yasskin-http-origin-signed-responses] | Yasskin, J., "Signed HTTP Exchanges", Internet-Draft draft-yasskin-http-origin-signed-responses-03, March 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. |
| [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. |
| [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. |
| [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", April 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. |
draft-00
Vs. [I-D.yasskin-http-origin-signed-responses]: