Signed HTTP Exchanges Implementation Checkpoints
draft-yasskin-httpbis-origin-signed-exchanges-impl-01
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| Document | Type |
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| Authors | Jeffrey Yasskin , Kouhei Ueno | ||
| Last updated | 2018-07-23 (Latest revision 2018-04-06) | ||
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draft-yasskin-httpbis-origin-signed-exchanges-impl-01
Network Working Group J. Yasskin
Internet-Draft K. Ueno
Intended status: Standards Track Google
Expires: January 21, 2019 July 20, 2018
Signed HTTP Exchanges Implementation Checkpoints
draft-yasskin-httpbis-origin-signed-exchanges-impl-01
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/ [1].
The source code and issues list for this draft can be found in
https://github.com/WICG/webpackage [2].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 21, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Signing an exchange . . . . . . . . . . . . . . . . . . . . . 4
3.1. The Signature Header . . . . . . . . . . . . . . . . . . 4
3.1.1. Examples . . . . . . . . . . . . . . . . . . . . . . 5
3.2. CBOR representation of exchange headers . . . . . . . . . 6
3.2.1. Example . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Loading a certificate chain . . . . . . . . . . . . . . . 7
3.4. Canonical CBOR serialization . . . . . . . . . . . . . . 9
3.5. Signature validity . . . . . . . . . . . . . . . . . . . 9
3.6. Updating signature validity . . . . . . . . . . . . . . . 12
3.6.1. Examples . . . . . . . . . . . . . . . . . . . . . . 13
3.7. The Accept-Signature header . . . . . . . . . . . . . . . 14
3.7.1. Integrity identifiers . . . . . . . . . . . . . . . . 15
3.7.2. Key type identifiers . . . . . . . . . . . . . . . . 16
3.7.3. Key value identifiers . . . . . . . . . . . . . . . . 16
3.7.4. Examples . . . . . . . . . . . . . . . . . . . . . . 16
4. Cross-origin trust . . . . . . . . . . . . . . . . . . . . . 17
4.1. Stateful header fields . . . . . . . . . . . . . . . . . 18
4.2. Certificate Requirements . . . . . . . . . . . . . . . . 19
5. Transferring a signed exchange . . . . . . . . . . . . . . . 20
5.1. Same-origin response . . . . . . . . . . . . . . . . . . 20
5.1.1. Significant headers for a same-origin response . . . 20
5.1.2. The Signed-Headers Header . . . . . . . . . . . . . . 21
5.2. HTTP/2 extension for cross-origin Server Push . . . . . . 22
5.3. application/signed-exchange format . . . . . . . . . . . 22
5.3.1. Cross-origin trust in application/signed-exchange . . 23
5.3.2. Example . . . . . . . . . . . . . . . . . . . . . . . 23
6. Security considerations . . . . . . . . . . . . . . . . . . . 23
7. Privacy considerations . . . . . . . . . . . . . . . . . . . 23
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 24
8.1. Internet Media Type application/signed-exchange . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.1. Normative References . . . . . . . . . . . . . . . . . . 25
9.2. Informative References . . . . . . . . . . . . . . . . . 27
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 28
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 30
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
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 [3] ([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 [4] 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 [5].
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.
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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.
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"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
(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="mi-draft2";
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 "MI-Draft2" response header 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
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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 an exchange "exchange"'s headers is the
CBOR ([RFC7049]) array with the following content:
1. The map mapping:
* The byte string ':method' to the byte string containing
"exchange"'s request's method.
* The byte string ':url' to the byte string containing
"exchange"'s request's effective request URI, which MUST be an
absolute URL (Section 2) with a scheme of "https".
* For each request header field in "exchange" except for the
"Host" header field, the header field's lowercase name as a
byte string to the header field's value as a byte string.
Note: "Host" is excluded because it is already part of the
effective request URI.
2. The map mapping:
* the byte string ':status' to the byte string containing
"exchange"'s response's 3-digit status code, and
* for each response header field in "exchange", the header
field's lowercase name as a byte string to the header field's
value as a byte string.
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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-Draft2: mi-sha256-draft2=dcRDgR2GM35DluAV13PzgnG6-pvQwPywfFvAu1UeFrs
Signed-Headers: "content-type", "mi-draft2"
<!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-draft2': 'mi-sha256-draft2=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:
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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".
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3.4. Canonical CBOR serialization
Within this specification, the canonical serialization of a CBOR item
uses the following rules derived from Section 3.9 of [RFC7049] with
erratum 4964 applied:
o Integers and the lengths of arrays, maps, and strings MUST use the
smallest possible encoding.
o Items MUST NOT be encoded with indefinite length.
o The keys in every map MUST be sorted in the bytewise lexicographic
order of their canonical encodings. For example, the following
keys are correctly sorted:
1. 10, encoded as 0A.
2. 100, encoded as 18 64.
3. -1, encoded as 20.
4. "z", encoded as 61 7A.
5. "aa", encoded as 62 61 61.
6. [100], encoded as 81 18 64.
7. [-1], encoded as 81 20.
8. false, encoded as F4.
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:
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o The signed headers of the exchange so that higher-level protocols
can avoid relying on unsigned headers, and
o Either a certificate chain or a public key so that a higher-level
protocol can determine whether it's actually valid.
This algorithm accepts a "forceFetch" flag that avoids the cache when
fetching URLs. A client that determines that a potentially-valid
certificate chain is actually invalid due to an expired OCSP response
MAY retry with "forceFetch" set to retrieve an updated OCSP from the
original server.
1. Let "payload" be the payload body (Section 3.3 of [RFC7230]) of
"exchange". Note that the payload body is the message body with
any transfer encodings removed.
2. Let:
* "signature" be the signature (binary content in the
parameterised identifier's "sig" parameter).
* "integrity" be the signature's "integrity" parameter.
* "validity-url" be the signature's "validity-url" parameter.
* "cert-url" be the signature's "cert-url" parameter, if any.
* "cert-sha256" be the signature's "cert-sha256" parameter, if
any.
* "date" be the signature's "date" parameter, interpreted as a
Unix time.
* "expires" be the signature's "expires" parameter, interpreted
as a Unix time.
3. If "integrity" names a header field other than "MI-Draft2" or
this header field is not present in "exchange"'s response
headers, then return "invalid". If validating integrity using
the selected header field requires the client to process records
larger than 16kB (for example, if the "mi-sha256-draft2" record
length is greater than 16kB), return "invalid". Clients MUST be
able to check the integrity of "payload" using the "MI-Draft2"
header field with its "mi-sha256-draft2" content encoding, which
are defined equivalently to the "MI" header field and "mi-sha256"
content encoding from [I-D.thomson-http-mice].
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4. 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".
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 b1".
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. The bytes of the canonical CBOR serialization (Section 3.4)
of a CBOR map mapping:
1. If "cert-sha256" is set:
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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
"certificate-chain". 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-Draft2" ([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.
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This specification provides no way to update OCSP responses by
themselves. Instead, clients need to re-fetch the "cert-url"
(Section 3.5, Paragraph 4) to get a chain including a newer OCSP
response.
The "validity-url" parameter (Paragraph 5) 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).
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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="mi-draft2"; '
'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
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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-draft2/" indicate that the client
supports the "MI-Draft2" header field (equivalent to "MI" in
[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-draft2/mi-blake2" indicates support for
Merkle integrity with the as-yet-unspecified mi-blake2 parameter, and
"-mi-draft2/mi-sha256-draft2" indicates non-support for Merkle
integrity with the mi-sha256-draft2 content encoding.
If the "Accept-Signature" header field is present, servers SHOULD
assume support for "mi-draft2/mi-sha256-draft2" unless the header
field states otherwise.
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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: mi-draft2/mi-sha256
states that the client will accept signatures with payload integrity
assured by the "MI-Draft2" header and "mi-sha256-draft2" content
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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-Draft2: mi-sha256-draft2" 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".
The client MUST parse the "Signature" header into a list of
signatures according to the instructions in Section 3.5, and run the
following algorithm for each signature, stopping at the first one
that returns "valid". If any signature returns "valid", return
"valid". Otherwise, return "invalid".
1. If the signature's "validity-url" parameter (Paragraph 5) is not
same-origin [6] 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-
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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 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:
o "Authorization", [RFC7235]
o "Cookie", [RFC6265]
o "Cookie2", [RFC2965]
o "Proxy-Authorization", [RFC7235]
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o "Sec-WebSocket-Key", [RFC6455]
A stateful response header field modifies state, including
authentication status, in the client. The HTTP cache is not
considered part of this state. These include but are not limited to:
o "Authentication-Control", [RFC8053]
o "Authentication-Info", [RFC7615]
o "Optional-WWW-Authenticate", [RFC8053]
o "Proxy-Authenticate", [RFC7235]
o "Proxy-Authentication-Info", [RFC7615]
o "Sec-WebSocket-Accept", [RFC6455]
o "Set-Cookie", [RFC6265]
o "Set-Cookie2", [RFC2965]
o "SetProfile", [W3C.NOTE-OPS-OverHTTP]
o "WWW-Authenticate", [RFC7235]
4.2. Certificate Requirements
We define a new X.509 extension, CanSignHttpExchanges to be used in
the certificate when the certificate permits the usage of signed
exchanges. When this extension is not present the client MUST NOT
accept a signature from the certificate as proof that a signed
exchange is authoritative for a domain covered by the certificate.
When it is present, the client MUST follow the validation procedure
in Section 4.
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.
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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 validates the
"Signature" header field for an exchange represented as a normal HTTP
request/response pair, it MUST pass only the subset of header fields
defined by Section 5.1.1 to the validation procedure (Section 3.5).
If the client relies on signature validity for any aspect of its
behavior, it MUST ignore any header fields that it didn't pass to the
validation procedure.
5.1.1. Significant headers for a same-origin response
The significant headers of an exchange represented as a normal HTTP
request/response pair (Section 2.1 of [RFC7230] or Section 8.1 of
[RFC7540]) are:
o The method (Section 4 of [RFC7231]) and effective request URI
(Section 5.5 of [RFC7230]) of the request.
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o The response status code (Section 6 of [RFC7231]) and the response
header fields whose names are listed in that exchange's "Signed-
Headers" header field (Section 5.1.2), in the order they appear in
that header field. If a response header field name from "Signed-
Headers" does not appear in the exchange's response header fields,
the exchange has no significant headers.
If the exchange's "Signed-Headers" header field is not present,
doesn't parse as a Structured Header
([I-D.ietf-httpbis-header-structure]) or doesn't follow the
constraints on its value described in Section 5.1.2, the exchange has
no significant headers.
5.1.2. 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.
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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-b1" 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. 3 bytes storing a big-endian integer "sigLength". If this is
larger than 16kB, parsing MUST fail.
3. 3 bytes storing a big-endian integer "headerLength". If this is
larger than 512kB, 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"
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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 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/ [7] 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-b1\0<3-byte length of the following header
value><3-byte length of the encoding of the
following array>sig1; sig=*...; integrity="mi-draft2"; ...[
{
':method': 'GET',
':url': 'https://example.com/',
'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".
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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]:
o The "Signature" header field
o The "Accept-Signature" header field
o The "Signed-Headers" header field
o The application/cert-chain+cbor media type
This document also modifies the registration for:
8.1. Internet Media Type application/signed-exchange
Type name: application
Subtype name: signed-exchange
Required parameters:
o v: A string denoting the version of the file format. ([RFC5234]
ABNF: "version = DIGIT/%x61-7A") The version defined in this
specification is "b1". When used with the "Accept" header field
(Section 5.3.1 of [RFC7231]), this parameter can be a comma
(,)-separated list of version strings. ([RFC5234] ABNF: "version-
list = version *( "," version )") The server is then expected to
reply with a resource using a particular version from that list.
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Note: As this is a snapshot of a draft of
[I-D.yasskin-http-origin-signed-responses], it uses a distinct
version number.
Magic number(s): 73 78 67 31 2D 62 31 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", July 2018,
<https://fetch.spec.whatwg.org/>.
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-ietf-cbor-cddl-03
(work in progress), July 2018.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-07 (work in progress),
July 2018.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
[I-D.thomson-http-mice]
Thomson, M., "Merkle Integrity Content Encoding", draft-
thomson-http-mice-02 (work in progress), October 2016.
[I-D.yasskin-http-origin-signed-responses]
Yasskin, J., "Signed HTTP Exchanges", draft-yasskin-http-
origin-signed-responses-04 (work in progress), June 2018.
[POSIX] IEEE and The Open Group, "The Open Group Base
Specifications Issue 7", name IEEE, value 1003.1-2008,
2016 Edition, 2016,
<http://pubs.opengroup.org/onlinepubs/9699919799/
basedefs/>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
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[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[URL] WHATWG, "URL", July 2018, <https://url.spec.whatwg.org/>.
9.2. Informative References
[I-D.yasskin-http-origin-signed-responses-03]
Yasskin, J., "Signed HTTP Exchanges", draft-yasskin-http-
origin-signed-responses-03 (work in progress), March 2018,
<https://tools.ietf.org/html/
draft-yasskin-http-origin-signed-responses-03>.
[I-D.yasskin-http-origin-signed-responses-04]
Yasskin, J., "Signed HTTP Exchanges", draft-yasskin-http-
origin-signed-responses-04 (work in progress), June 2018,
<https://tools.ietf.org/html/
draft-yasskin-http-origin-signed-responses-04>.
[RFC2965] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2965, DOI 10.17487/RFC2965, October 2000,
<https://www.rfc-editor.org/info/rfc2965>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, DOI 10.17487/RFC6455, December 2011,
<https://www.rfc-editor.org/info/rfc6455>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
<https://www.rfc-editor.org/info/rfc7235>.
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[RFC7615] Reschke, J., "HTTP Authentication-Info and Proxy-
Authentication-Info Response Header Fields", RFC 7615,
DOI 10.17487/RFC7615, September 2015,
<https://www.rfc-editor.org/info/rfc7615>.
[RFC8053] Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
T., and Y. Ioku, "HTTP Authentication Extensions for
Interactive Clients", RFC 8053, DOI 10.17487/RFC8053,
January 2017, <https://www.rfc-editor.org/info/rfc8053>.
[W3C.NOTE-OPS-OverHTTP]
Hensley, P., Metral, M., Shardanand, U., Converse, D., and
M. Myers, "Implementation of OPS Over HTTP", W3C NOTE
NOTE-OPS-OverHTTP, June 1997.
9.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] https://github.com/WICG/webpackage
[3] https://url.spec.whatwg.org/#concept-url-parser
[4] https://url.spec.whatwg.org/#absolute-url-string
[5] http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
V1_chap04.html#tag_04_16
[6] https://html.spec.whatwg.org/multipage/origin.html#same-origin
[7] https://example.com/
Appendix A. Change Log
draft-01
Vs. [I-D.yasskin-http-origin-signed-responses-04]:
o The MI header and mi-sha256 content-encoding are renamed to MI-
Draft2 and mi-sha256-draft2 in case [I-D.thomson-http-mice]
changes.
o Signed exchanges cannot be transmitted using HTTP/2 Push.
o Removed non-normative sections.
o The mi-sha256 encoding must have records <= 16kB.
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o The signature must be <=16kB long.
o The HTTP request and response headers together must be <=512kB.
o Versions in file signatures and context strings are "b1".
o Only 1 signature is supported.
o Removed support for ed25519 signatures.
draft-00
Vs. [I-D.yasskin-http-origin-signed-responses-03]:
o Removed non-normative sections.
o Only 1 signature is supported.
o Only 2048-bit RSA keys are supported.
o The certificate chain resource uses the TLS 1.3 Certificate
message format rather than a CBOR-based format.
o OCSP responses and SCTs are not checked.
o Certificates without the CanSignHttpExchanges extension are
allowed.
o The signature string starts with 64 0x20 octets like the TLS 1.3
signature format.
o The application/http-exchange+cbor format is replaced with a more
specialized application/signed-exchange format.
o Signed exchanges can only be transmitted using the application/
signed-exchange format, not HTTP/2 Push or plain HTTP request/
response pairs.
o Only the MI payload-integrity header is supported.
o The mi-sha256 encoding must have records <= 16kB.
o The Accept-Signature header isn't used.
o Require absolute URLs.
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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
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