Internet DRAFT - draft-yasskin-http-origin-signed-responses
draft-yasskin-http-origin-signed-responses
Network Working Group J. Yasskin
Internet-Draft Google
Intended status: Standards Track 27 July 2020
Expires: 28 January 2021
Signed HTTP Exchanges
draft-yasskin-http-origin-signed-responses-09
Abstract
This document specifies how a server can send an HTTP exchange--a
request URL, content negotiation information, and a response--with
signatures that vouch for that exchange's authenticity. These
signatures can be verified against an origin's certificate to
establish that the exchange is authoritative for an origin even if it
was transferred over a connection that isn't. The signatures can
also be used in other ways described in the appendices.
These signatures contain countermeasures against downgrade and
protocol-confusion attacks.
Note to Readers
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/
(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 (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
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 28 January 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Signing an exchange . . . . . . . . . . . . . . . . . . . . . 5
3.1. The Signature Header . . . . . . . . . . . . . . . . . . 6
3.1.1. Examples . . . . . . . . . . . . . . . . . . . . . . 7
3.1.2. Open Questions . . . . . . . . . . . . . . . . . . . 9
3.2. CBOR representation of exchange response headers . . . . 9
3.2.1. Example . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Loading a certificate chain . . . . . . . . . . . . . . . 10
3.4. Canonical CBOR serialization . . . . . . . . . . . . . . 11
3.5. Signature validity . . . . . . . . . . . . . . . . . . . 12
3.5.1. Open Questions . . . . . . . . . . . . . . . . . . . 16
3.6. Updating signature validity . . . . . . . . . . . . . . . 16
3.6.1. Examples . . . . . . . . . . . . . . . . . . . . . . 17
3.7. The Accept-Signature header . . . . . . . . . . . . . . . 19
3.7.1. Integrity identifiers . . . . . . . . . . . . . . . . 20
3.7.2. Key type identifiers . . . . . . . . . . . . . . . . 20
3.7.3. Key value identifiers . . . . . . . . . . . . . . . . 20
3.7.4. Examples . . . . . . . . . . . . . . . . . . . . . . 21
3.7.5. Open Questions . . . . . . . . . . . . . . . . . . . 21
4. Cross-origin trust . . . . . . . . . . . . . . . . . . . . . 22
4.1. Uncached header fields . . . . . . . . . . . . . . . . . 23
4.1.1. Stateful header fields . . . . . . . . . . . . . . . 24
4.2. Certificate Requirements . . . . . . . . . . . . . . . . 25
4.2.1. Extensions to the CAA Record: cansignhttpexchanges
Parameter . . . . . . . . . . . . . . . . . . . . . . 26
5. Transferring a signed exchange . . . . . . . . . . . . . . . 26
5.1. Same-origin response . . . . . . . . . . . . . . . . . . 27
5.1.1. Serialized headers for a same-origin response . . . . 27
5.1.2. The Signed-Headers Header . . . . . . . . . . . . . . 28
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5.2. HTTP/2 extension for cross-origin Server Push . . . . . . 28
5.2.1. Indicating support for cross-origin Server Push . . . 28
5.2.2. NO_TRUSTED_EXCHANGE_SIGNATURE error code . . . . . . 29
5.2.3. Validating a cross-origin Push . . . . . . . . . . . 29
5.3. application/signed-exchange format . . . . . . . . . . . 30
5.3.1. Cross-origin trust in application/signed-exchange . . 31
5.3.2. Example . . . . . . . . . . . . . . . . . . . . . . . 32
5.3.3. Open Questions . . . . . . . . . . . . . . . . . . . 32
6. Security considerations . . . . . . . . . . . . . . . . . . . 32
6.1. Over-signing . . . . . . . . . . . . . . . . . . . . . . 32
6.1.1. Session fixation . . . . . . . . . . . . . . . . . . 33
6.1.2. Misleading content . . . . . . . . . . . . . . . . . 33
6.2. Off-path attackers . . . . . . . . . . . . . . . . . . . 33
6.2.1. Mis-issued certificates . . . . . . . . . . . . . . . 33
6.2.2. Stolen private keys . . . . . . . . . . . . . . . . . 34
6.3. Downgrades . . . . . . . . . . . . . . . . . . . . . . . 35
6.4. Signing oracles are permanent . . . . . . . . . . . . . . 35
6.5. Unsigned headers . . . . . . . . . . . . . . . . . . . . 35
6.6. application/signed-exchange . . . . . . . . . . . . . . . 36
6.7. Key re-use with TLS . . . . . . . . . . . . . . . . . . . 36
6.8. Content sniffing . . . . . . . . . . . . . . . . . . . . 36
7. Privacy considerations . . . . . . . . . . . . . . . . . . . 37
7.1. Visibility of resource requests . . . . . . . . . . . . . 38
7.2. User ID transfer . . . . . . . . . . . . . . . . . . . . 39
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 39
8.1. Signature Header Field Registration . . . . . . . . . . . 39
8.2. Accept-Signature Header Field Registration . . . . . . . 39
8.3. Signed-Headers Header Field Registration . . . . . . . . 40
8.4. HTTP/2 Settings . . . . . . . . . . . . . . . . . . . . . 40
8.5. HTTP/2 Error code . . . . . . . . . . . . . . . . . . . . 40
8.6. Internet Media Type application/signed-exchange . . . . . 41
8.7. Internet Media Type application/cert-chain+cbor . . . . . 42
8.8. The cansignhttpexchanges CAA Parameter . . . . . . . . . 43
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 43
9.1. Normative References . . . . . . . . . . . . . . . . . . 43
9.2. Informative References . . . . . . . . . . . . . . . . . 46
Appendix A. Use cases . . . . . . . . . . . . . . . . . . . . . 49
A.1. PUSHed subresources . . . . . . . . . . . . . . . . . . . 49
A.2. Explicit use of a content distributor for subresources . 49
A.3. Subresource Integrity . . . . . . . . . . . . . . . . . . 50
A.4. Binary Transparency . . . . . . . . . . . . . . . . . . . 50
A.5. Static Analysis . . . . . . . . . . . . . . . . . . . . . 51
A.6. Offline websites . . . . . . . . . . . . . . . . . . . . 51
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 51
B.1. Proof of origin . . . . . . . . . . . . . . . . . . . . . 51
B.1.1. Certificate constraints . . . . . . . . . . . . . . . 51
B.1.2. Signature constraints . . . . . . . . . . . . . . . . 52
B.1.3. Retrieving the certificate . . . . . . . . . . . . . 52
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B.2. How much to sign . . . . . . . . . . . . . . . . . . . . 53
B.2.1. Conveying the signed headers . . . . . . . . . . . . 53
B.3. Response lifespan . . . . . . . . . . . . . . . . . . . . 54
B.3.1. Certificate revocation . . . . . . . . . . . . . . . 54
B.3.2. Response downgrade attacks . . . . . . . . . . . . . 54
B.4. Low implementation complexity . . . . . . . . . . . . . . 55
B.4.1. Limited choices . . . . . . . . . . . . . . . . . . . 55
B.4.2. Bounded-buffering integrity checking . . . . . . . . 55
Appendix C. Determining validity using cache control . . . . . . 56
C.1. Example of updating cache control . . . . . . . . . . . . 56
C.2. Downsides of updating cache control . . . . . . . . . . . 57
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 57
Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 60
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 60
1. Introduction
Signed HTTP exchanges provide a way to prove the authenticity of a
resource in cases where the transport layer isn't sufficient. This
can be used in several ways:
* When signed by a certificate ([RFC5280]) that's trusted for an
origin, an exchange can be treated as authoritative for that
origin, even if it was transferred over a connection that isn't
authoritative (Section 9.1 of [RFC7230]) for that origin. See
Appendix A.1 and Appendix A.2.
* A top-level resource can use a public key to identify an expected
publisher for particular subresources, a system known as
Subresource Integrity ([SRI]). An exchange's signature provides
the matching proof of authorship. See Appendix A.3.
* A signature can vouch for the exchange in some way, for example
that it appears in a transparency log or that static analysis
indicates that it omits certain attacks. See Appendix A.4 and
Appendix A.5.
Subsequent work toward the use cases in [I-D.yasskin-wpack-use-cases]
will provide a way to group signed exchanges into bundles that can be
transmitted and stored together, but single signed exchanges are
useful enough to standardize on their own.
2. Terminology
Absolute URL A string for which the URL parser
(https://url.spec.whatwg.org/#concept-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
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absolute-URL string (https://url.spec.whatwg.org/#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 URL, content negotiation
information, and an HTTP response. This can be encoded into a
request message from a client with its matching response from a
server, into the request in a PUSH_PROMISE with its matching
response stream, or into the dedicated format in Section 5.3,
which uses [I-D.ietf-httpbis-variants] to encode the content
negotiation information. This is not quite the same meaning as
defined by Section 8 of [RFC7540], which assumes the content
negotiation information is embedded into HTTP request headers.
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
(http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
V1_chap04.html#tag_04_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).
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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 URL, response headers, and
content (Section 3.5). This validity then informs higher-level
protocols.
Each signature is parameterised with information to let a client
fetch assurance that a signed exchange is still valid, in the face of
revoked certificates and newly-discovered vulnerabilities. This
assurance can be bundled back into the signed exchange and forwarded
to another client, which won't have to re-fetch this validity
information for some period of time.
3.1. The Signature Header
The "Signature" header field conveys a list of signatures for an
exchange, each one accompanied by information about how to determine
the authority of and refresh that signature. Each signature directly
signs the exchange's URL and response 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 3.4 of
[I-D.ietf-httpbis-header-structure]). Its ABNF is:
Signature = sh-param-list
Each parameterised identifier in the list MUST have parameters named
"sig", "integrity", "validity-url", "date", and "expires". Each
parameterised identifier MUST also have either "cert-url" and "cert-
sha256" parameters or an "ed25519key" parameter. This specification
gives no meaning to the identifier itself, which can be used as a
human-readable identifier for the signature (however, this is likely
to change soon; see Section 3.1.2, Paragraph 1). The present
parameters MUST have the following values:
"sig" Byte sequence (Section 3.10 of
[I-D.ietf-httpbis-header-structure]) holding the signature of most
of these parameters and the exchange's URL and response headers.
"integrity" A string (Section 3.8 of
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[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 3.8 of
[I-D.ietf-httpbis-header-structure]) containing an absolute URL
(Section 2) with a scheme of "https" or "data".
"cert-sha256" Byte sequence (Section 3.10 of
[I-D.ietf-httpbis-header-structure]) holding the SHA-256 hash of
the first certificate found at "cert-url".
"ed25519key" Byte sequence (Section 3.10 of
[I-D.ietf-httpbis-header-structure]) holding an Ed25519 public key
([RFC8032]).
"validity-url" A string (Section 3.8 of
[I-D.ietf-httpbis-header-structure]) containing an absolute URL
(Section 2) with a scheme of "https".
"date" and "expires" An integer (Section 3.6 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.
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Signature:
sig1;
sig=*MEUCIQDXlI2gN3RNBlgFiuRNFpZXcDIaUpX6HIEwcZEc0cZYLAIga9DsVOMM+g5YpwEBdGW3sS+bvnmAJJiSMwhuBdqp5UY=*;
integrity="digest/mi-sha256";
validity-url="https://example.com/resource.validity.1511128380";
cert-url="https://example.com/oldcerts";
cert-sha256=*W7uB969dFW3Mb5ZefPS9Tq5ZbH5iSmOILpjv2qEArmI=*;
date=1511128380; expires=1511733180,
sig2;
sig=*MEQCIGjZRqTRf9iKNkGFyzRMTFgwf/BrY2ZNIP/dykhUV0aYAiBTXg+8wujoT4n/W+cNgb7pGqQvIUGYZ8u8HZJ5YH26Qg==*;
integrity="digest/mi-sha256";
validity-url="https://example.com/resource.validity.1511128380";
cert-url="https://example.com/newcerts";
cert-sha256=*J/lEm9kNRODdCmINbvitpvdYKNQ+YgBj99DlYp4fEXw=*;
date=1511128380; expires=1511733180,
srisig;
sig=*lGZVaJJM5f2oGczFlLmBdKTDL+QADza4BgeO494ggACYJOvrof6uh5OJCcwKrk7DK+LBch0jssDYPp5CLc1SDA==*;
integrity="digest/mi-sha256";
validity-url="https://example.com/resource.validity.1511128380";
ed25519key=*zsSevyFsxyZHiUluVBDd4eypdRLTqyWRVOJuuKUz+A8=*
date=1511128380; expires=1511733180,
thirdpartysig;
sig=*MEYCIQCNxJzn6Rh2fNxsobktir8TkiaJYQFhWTuWI1i4PewQaQIhAMs2TVjc4rTshDtXbgQEOwgj2mRXALhfXPztXgPupii+*;
integrity="digest/mi-sha256";
validity-url="https://thirdparty.example.com/resource.validity.1511161860";
cert-url="https://thirdparty.example.com/certs";
cert-sha256=*UeOwUPkvxlGRTyvHcsMUN0A2oNsZbU8EUvg8A9ZAnNc=*;
date=1511133060; expires=1511478660,
There are 4 signatures: 2 from different secp256r1 certificates
within "https://example.com/", one using a raw ed25519 public key
that's also controlled by "example.com", and a fourth using a
secp256r1 certificate owned by "thirdparty.example.com".
All 4 signatures rely on the "Digest" response header with the mi-
sha256 digest algorithm to guard the integrity of the response
payload.
The signatures include a "validity-url" that includes the first time
the resource was seen. This allows multiple versions of a resource
at the same URL to be updated with new signatures, which allows
clients to avoid transferring extra data while the old versions don't
have known security bugs.
The certificates at "https://example.com/oldcerts" and
"https://example.com/newcerts" have "subjectAltName"s of
"example.com", meaning that if they and their signatures validate,
the exchange can be trusted as having an origin of
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"https://example.com/". The publisher might be using two
certificates because their readers have disjoint sets of roots in
their trust stores.
The publisher signed with all three certificates at the same time, so
they share a validity range: 7 days starting at 2017-11-19 21:53 UTC.
The publisher then requested an additional signature from
"thirdparty.example.com", which did some validation or processing and
then signed the resource at 2017-11-19 23:11 UTC.
"thirdparty.example.com" only grants 4-day signatures, so clients
will need to re-validate more often.
3.1.2. Open Questions
The next revision of [I-D.ietf-httpbis-header-structure] will provide
a way to parameterise byte sequences, at which point the signature
itself is likely to become the main list item.
Should the cert-url and validity-url be lists so that intermediates
can offer a cache without losing the original URLs? Putting lists in
dictionary fields is more complex than
[I-D.ietf-httpbis-header-structure] allows, so they're single items
for now.
3.2. CBOR representation of exchange response headers
To sign an exchange's response headers, they need to be serialized
into a byte string. Since intermediaries and distributors
(Appendix A.2) might rearrange, add, or just reserialize headers, we
can't use the literal bytes of the headers as this serialization.
Instead, this section defines a CBOR representation that can be
embedded into other CBOR, canonically serialized (Section 3.4), and
then signed.
The CBOR representation of a set of response metadata and headers is
the CBOR ([RFC7049]) map with the following mappings:
* The byte string ':status' to the byte string containing the
response's 3-digit status code, and
* For each response header field, the header field's lowercase name
as a byte string to the header field's value as a byte string.
3.2.1. Example
Given the HTTP exchange:
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GET / HTTP/1.1
Host: example.com
Accept: */*
HTTP/1.1 200
Content-Type: text/html
Digest: mi-sha256=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 [CDDL] appendix G:
{
'digest': 'mi-sha256=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
+ augmented-certificate
]
augmented-certificate = {
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.
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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:
* Integers and the lengths of arrays, maps, and strings MUST use the
smallest possible encoding.
* Items MUST NOT be encoded with indefinite length.
* 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.
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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 4.2.5 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
* "requestUrl", a byte sequence that can be parsed into the
exchange's effective request URI (Section 5.5 of [RFC7230]),
* "responseHeaders", a byte sequence holding the canonical
serialization (Section 3.4) of the CBOR representation
(Section 3.2) of the exchange's response metadata and headers, and
* "payload", a stream of bytes constituting the exchange's payload
body (Section 3.3 of [RFC7230]). Note that the payload body is
the message body with any transfer encodings removed.
Potentially-valid results include:
* The signed headers of the exchange so that higher-level protocols
can avoid relying on unsigned headers, and
* Either a certificate chain or a public key so that a higher-level
protocol can determine whether it's actually valid.
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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:
* "signature" be the signature (byte sequence 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.
* "ed25519key" be the signature's "ed25519key" parameter, if
any.
* "date" be the signature's "date" parameter, interpreted as a
Unix time.
* "expires" be the signature's "expires" parameter, interpreted
as a Unix time.
2. Set "publicKey" and "signing-alg" depending on which key fields
are present:
1. If "cert-url" is present:
1. Let "certificate-chain" be the result of loading the
certificate chain at "cert-url" passing the "forceFetch"
flag (Section 3.3). If this returns "invalid", return
"invalid".
2. Let "main-certificate" be the first certificate in
"certificate-chain".
3. Set "publicKey" to "main-certificate"'s public key.
4. If "publicKey" is an RSA key, return "invalid".
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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 [RFC8446].
6. Otherwise, either return "invalid" or set "signing-alg"
to a non-legacy signing algorithm defined by TLS 1.3 or
later ([RFC8446]). This choice MUST depend only on
"publicKey"'s type and not on any other context.
2. If "ed25519key" is present, set "publicKey" to "ed25519key"
and "signing-alg" to ed25519, as defined by [RFC8032]
3. If "expires" is more than 7 days (604800 seconds) after "date",
return "invalid".
4. If the current time is before "date" or after "expires", return
"invalid".
5. Let "message" be the concatenation of the following byte
strings. This matches the [RFC8446] format to avoid cross-
protocol attacks if anyone uses the same key in a TLS
certificate and an exchange-signing certificate.
1. A string that consists of octet 32 (0x20) repeated 64 times.
2. A context string: the ASCII encoding of "HTTP Exchange 1".
Note: RFC EDITOR PLEASE DELETE THIS NOTE; The implementation
of the final RFC MUST use this context string, but
implementations of drafts MUST NOT use it and MUST use
another draft-specific string beginning with "HTTP Exchange
1 " instead. This ensures that signers can predict how
their signatures will be used.
3. A single 0 byte which serves as a separator.
4. 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".
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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
"responseHeaders", followed by the bytes of
"responseHeaders".
6. 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 [RFC8446]), in order to allow updating the
stapled OCSP response without updating signatures at the same
time.
7. If "signature" is not a valid signature of "message" by
"publicKey" using "signing-alg", return "invalid".
8. If "headers", interpreted according to Section 3.2, does not
contain a "Content-Type" response header field (Section 3.1.1.5
of [RFC7231]), return "invalid".
Clients MUST interpret the signed payload as this specified
media type instead of trying to sniff a media type from the
bytes of the payload, for example by attaching an "X-Content-
Type-Options: nosniff" header field ([FETCH]) to the extracted
response.
9. If "integrity" names a header field and parameter that is not
present in "responseHeaders" or which the client cannot use to
check the integrity of "payload" (for example, the header field
is new and hasn't been implemented yet), then return "invalid".
If the selected header field provides integrity guarantees
weaker than SHA-256, return "invalid". If validating integrity
using the selected header field requires the client to process
records larger than 16384 bytes, return "invalid". Clients MUST
implement at least the "Digest" header field with its "mi-
sha256" digest algorithm (Section 3 of [I-D.thomson-http-mice]).
Note: RFC EDITOR PLEASE DELETE THIS NOTE; Implementations of
drafts of this RFC MUST recognize the draft spelling of the
content encoding and digest algorithm specified by
[I-D.thomson-http-mice] until that draft is published as an RFC.
For example, implementations of draft-thomson-http-mice-03 would
use "mi-sha256-03" and MUST NOT use "mi-sha256" itself. This
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ensures that final implementations don't need to handle
compatibility with implementations of early drafts of that
content encoding.
If "payload" doesn't match the integrity information in the
header described by "integrity", return "invalid".
10. Return "potentially-valid" with whichever is present of
"certificate-chain" or "ed25519key".
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" ([I-D.thomson-http-mice]) that can
incrementally validate the payload, early parts of the payload can be
determined to be potentially-valid before later parts of the payload.
Higher-level protocols MAY process parts of the exchange that have
been determined to be potentially-valid as soon as that determination
is made but MUST NOT process parts of the exchange that are not yet
potentially-valid. Similarly, as the higher-level protocol
determines that parts of the exchange are actually valid, the client
MAY process those parts of the exchange and MUST wait to process
other parts of the exchange until they too are determined to be
valid.
3.5.1. Open Questions
Should the signed message use the TLS format (with an initial 64
spaces) even though these certificates can't be used in TLS servers?
3.6. Updating signature validity
Both OCSP responses and signatures are designed to expire a short
time after they're signed, so that revoked certificates and signed
exchanges with known vulnerabilities are distrusted promptly.
This specification provides no way to update OCSP responses by
themselves. Instead, clients need to re-fetch the "cert-url"
(Section 3.5, Paragraph 6) to get a chain including a newer OCSP
response.
The "validity-url" parameter (Section 3.1) 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.
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The resource at a "validity-url" is "validity data", a CBOR map
matching the following CDDL ([CDDL]):
validity = {
? signatures: [ + bytes ]
? update: {
? size: uint,
}
]
The elements of the "signatures" array are parameterised identifiers
(Section 4.2.6 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,
sig2;
sig=*MEQCIG...*;
...
validity-url="https://example.com/resource.validity.1511157180";
cert-url="https://example.com/newcerts";
date=1511128380; expires=1511733180,
thirdpartysig;
sig=*MEYCIQ...*;
...
validity-url="https://thirdparty.example.com/resource.validity.1511161860";
cert-url="https://thirdparty.example.com/certs";
date=1511478660; expires=1511824260
At 2017-11-27 11:02 UTC, "sig1" and "sig2" have expired, but
"thirdpartysig" doesn't exipire until 23:11 that night, so the client
needs to fetch "https://example.com/resource.validity.1511157180"
(the "validity-url" of "sig1" and "sig2") if it wishes to update
those signatures. This URL might contain:
{
"signatures": [
'sig1; '
'sig=*MEQCIC/I9Q+7BZFP6cSDsWx43pBAL0ujTbON/+7RwKVk+ba5AiB3FSFLZqpzmDJ0NumNwN04pqgJZE99fcK86UjkPbj4jw==*; '
'validity-url="https://example.com/resource.validity.1511157180"; '
'integrity="digest/mi-sha256"; '
'cert-url="https://example.com/newcerts"; '
'cert-sha256=*J/lEm9kNRODdCmINbvitpvdYKNQ+YgBj99DlYp4fEXw=*; '
'date=1511733180; expires=1512337980'
],
"update": {
"size": 5557452
}
}
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This indicates that the client could fetch a newer version at
"https://example.com/resource" (the original URL of the exchange), or
that the validity period of the old version can be extended by
replacing the first two of the original signatures (the ones with a
validity-url of "https://example.com/resource.validity.1511157180")
with the single new signature provided. (This might happen at the
end of a migration to a new root certificate.) The signatures of the
updated signed exchange would be:
Signature:
sig1;
sig=*MEQCIC...*;
...
validity-url="https://example.com/resource.validity.1511157180";
cert-url="https://example.com/newcerts";
date=1511733180; expires=1512337980,
thirdpartysig;
sig=*MEYCIQ...*;
...
validity-url="https://thirdparty.example.com/resource.validity.1511161860";
cert-url="https://thirdparty.example.com/certs";
date=1511478660; expires=1511824260
"https://example.com/resource.validity.1511157180" could also expand
the set of signatures if its "signatures" array contained more than 2
elements.
3.7. The Accept-Signature header
"Signature" header fields cost on the order of 300 bytes for ECDSA
signatures, so servers might prefer to avoid sending them to clients
that don't intend to use them. A client can send the "Accept-
Signature" header field to indicate that it does intend to take
advantage of any available signatures and to indicate what kinds of
signatures it supports.
When a server receives an "Accept-Signature" header field in a client
request, it SHOULD reply with any available "Signature" header fields
for its response that the "Accept-Signature" header field indicates
the client supports. However, if the "Accept-Signature" value
violates a requirement in this section, the server MUST behave as if
it hadn't received any "Accept-Signature" header at all.
The "Accept-Signature" header field is a Structured Header as defined
by [I-D.ietf-httpbis-header-structure]. Its value MUST be a
parameterised list (Section 3.4 of
[I-D.ietf-httpbis-header-structure]). Its ABNF is:
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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
(https://www.iana.org/assignments/http-dig-alg/http-dig-alg.xhtml)
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" 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 a byte sequence (Section 3.10
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
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"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 a byte
sequence.
Note that "ed25519key; **" is an empty prefix, which matches all
public keys, so it's useful in subresource integrity (Appendix A.3)
cases like "<link rel=preload as=script href="...">" where the public
key isn't known until the matching "<script src="..."
integrity="...">" tag.
3.7.4. Examples
Accept-Signature: digest/mi-sha256
states that the client will accept signatures with payload integrity
assured by the "Digest" header and "mi-sha256" 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" header field.
3.7.5. Open Questions
Is an "Accept-Signature" header useful enough to pay for itself? If
clients wind up sending it on most requests, that may cost more than
the cost of sending "Signature"s unconditionally. On the other hand,
it gives servers an indication of which kinds of signatures are
supported, which can help us upgrade the ecosystem in the future.
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Is "Accept-Signature" the right spelling, or do we want to imitate
"Want-Digest" (Section 4.3.1 of [RFC3230]) instead?
Do I have the right structure for the identifiers indicating feature
support?
4. Cross-origin trust
To determine whether to trust a cross-origin exchange, the client
takes a "Signature" header field (Section 3.1) and the exchange's
* "requestUrl", a byte sequence that can be parsed into the
exchange's effective request URI (Section 5.5 of [RFC7230]),
* "responseHeaders", a byte sequence holding the canonical
serialization (Section 3.4) of the CBOR representation
(Section 3.2) of the exchange's response metadata and headers, and
* "payload", a stream of bytes constituting the exchange's payload
body (Section 3.3 of [RFC7230]).
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 (Section 3.1) is not
same-origin (https://html.spec.whatwg.org/multipage/
origin.html#same-origin) with "requestUrl", return "invalid".
2. Use Section 3.5 to determine the signature's validity for
"requestUrl", "responseHeaders", and "payload", getting
"certificate-chain" back. If this returned "invalid" or didn't
return a certificate chain, return "invalid".
3. Let "response" be the response metadata and headers parsed out of
"responseHeaders".
4. If Section 3 of [RFC7234] forbids a shared cache from storing
"response", return "invalid".
5. If "response"'s headers contain an uncached header field, as
defined in Section 4.1, return "invalid".
6. Let "authority" be the host component of "requestUrl".
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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. Uncached header fields
Hop-by-hop and other uncached headers MUST NOT appear in a signed
exchange. These will eventually be listed in
[I-D.ietf-httpbis-cache], but for now they're listed here:
* Hop-by-hop header fields listed in the Connection header field
(Section 6.1 of [RFC7230]).
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* Header fields listed in the no-cache response directive in the
Cache-Control header field (Section 5.2.2.2 of [RFC7234]).
* Header fields defined as hop-by-hop:
- Connection
- Keep-Alive
- Proxy-Connection
- Trailer
- Transfer-Encoding
- Upgrade
* Stateful headers as defined below.
4.1.1. Stateful header fields
As described in Section 6.1, a publisher can cause problems if they
sign an exchange that includes private information. There's no way
for a client to be sure an exchange does or does not include private
information, but header fields that store or convey stored state in
the client are a good sign.
A stateful 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:
* "Authentication-Control", [RFC8053]
* "Authentication-Info", [RFC7615]
* "Clear-Site-Data", [W3C.WD-clear-site-data-20171130]
* "Optional-WWW-Authenticate", [RFC8053]
* "Proxy-Authenticate", [RFC7235]
* "Proxy-Authentication-Info", [RFC7615]
* "Public-Key-Pins", [RFC7469]
* "Sec-WebSocket-Accept", [RFC6455]
* "Set-Cookie", [RFC6265]
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* "Set-Cookie2", [RFC2965]
* "SetProfile", [W3C.NOTE-OPS-OverHTTP]
* "Strict-Transport-Security", [RFC6797]
* "WWW-Authenticate", [RFC7235]
4.2. Certificate Requirements
We define a new X.509 extension, CanSignHttpExchanges to be used in
the certificate when the certificate permits the usage of signed
exchanges. When this extension is not present the client MUST NOT
accept a signature from the certificate as proof that a signed
exchange is authoritative for a domain covered by the certificate.
When it is present, the client MUST follow the validation procedure
in Section 4.
id-ce-canSignHttpExchanges OBJECT IDENTIFIER ::= { TBD }
CanSignHttpExchanges ::= NULL
Note that this extension contains an ASN.1 NULL (bytes "05 00")
because some implementations have bugs with empty extensions.
Leaf certificates without this extension need to be revoked if the
private key is exposed to an unauthorized entity, but they generally
don't need to be revoked if a signing oracle is exposed and then
removed.
CA certificates, by contrast, need to be revoked if an unauthorized
entity is able to make even one unauthorized signature.
Certificates with this extension MUST be revoked if an unauthorized
entity is able to make even one unauthorized signature.
Certificates with this extension MUST have a Validity Period no
greater than 90 days.
Conforming CAs MUST NOT mark this extension as critical.
A conforming CA MUST NOT issue certificates with this extension
unless, for each dNSName in the subjectAltName extension of the
certificate to be issued:
1. An "issue" or "issuewild" CAA property ([RFC6844]) exists that
authorizes the CA to issue the certificate; and
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2. The "cansignhttpexchanges" parameter (Section 4.2.1) is present
on the property and is equal to "yes"
Clients MUST NOT accept certificates with this extension in TLS
connections (Section 4.4.2.2 of [RFC8446]).
RFC EDITOR PLEASE DELETE THE REST OF THE PARAGRAPHS IN THIS SECTION
id-ce-google OBJECT IDENTIFIER ::= { 1 3 6 1 4 1 11129 }
id-ce-canSignHttpExchangesDraft OBJECT IDENTIFIER ::= { id-ce-google 2 1 22 }
Implementations of drafts of this specification MAY recognize the
"id-ce-canSignHttpExchangesDraft" OID as identifying the
CanSignHttpExchanges extension. This OID might or might not be used
as the final OID for the extension, so certificates including it
might need to be reissued once the final RFC is published.
Some certificates have already been issued with this extension and
with validity periods longer than 90 days. These certificates will
not immediately be treated as invalid. Instead:
* Clients MUST reject certificates with this extension that were
issued after 2019-05-01 and have a Validity Period longer than 90
days.
* After 2019-08-01, clients MUST reject all certificates with this
extension that have a Validity Period longer than 90 days.
The above requirements on CAs to limit the Validity Period and check
for a CAA parameter are effective starting 2019-05-01.
4.2.1. Extensions to the CAA Record: cansignhttpexchanges Parameter
A CAA parameter "cansignhttpexchanges" is defined for the "issue" and
"issuewild" properties defined by [RFC6844]. The value of this
parameter, if specified, MUST be "yes".
5. Transferring a signed exchange
A signed exchange can be transferred in several ways, of which three
are described here.
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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, clients don't know how the server's content negotiation
algorithm works, and intermediate servers add response header fields,
it can be impossible to have a signature for the exchange's exact
request, content negotiation, and response. 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:
* The "Signature" header field,
* The effective request URI (Section 5.5 of [RFC7230]) of the
request,
* The serialized headers defined by Section 5.1.1, and
* The response's payload.
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.
If the signed response includes a "Variants" header field, the client
MUST use the cache behavior algorithm in Section 4 of
[I-D.ietf-httpbis-variants] to check that the signed response is an
appropriate representation for the request the client is trying to
fulfil. If the response is not an appropriate representation, the
client MUST treat the signature as invalid.
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 response status code (Section 6
of [RFC7231]) and the response header fields whose names are listed
in that response's "Signed-Headers" header field (Section 5.1.2). If
a response header field name from "Signed-Headers" does not appear in
the response's header fields, the exchange has no serialized 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.
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5.1.1.1. Open Questions
Do the serialized headers of an exchange need to include the "Signed-
Headers" header field itself?
5.1.2. The Signed-Headers Header
The "Signed-Headers" header field identifies an ordered list of
response header fields to include in a signature. The request URL
and response status are included unconditionally. This allows a TLS-
terminating intermediate to reorder headers without breaking the
signature. This _can_ also allow the intermediate to add headers
that will be ignored by some higher-level protocols, but Section 3.5
provides a hook to let other higher-level protocols reject such
insecure headers.
This header field appears once instead of being incorporated into the
signatures' parameters because the signed header fields need to be
consistent across all signatures of an exchange, to avoid forcing
higher-level protocols to merge the header field lists of valid
signatures.
"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 3.8 of [I-D.ietf-httpbis-header-structure]) naming an HTTP
response header field. Pseudo-header field names (Section 8.1.2.1 of
[RFC7540]) MUST NOT appear in this list.
Higher-level protocols SHOULD place requirements on the minimum set
of headers to include in the "Signed-Headers" header field.
5.2. HTTP/2 extension for cross-origin Server Push
To allow servers to Server-Push (Section 8.2 of [RFC7540]) signed
exchanges (Section 3) signed by an authority for which the server is
not authoritative (Section 9.1 of [RFC7230]), this section defines an
HTTP/2 extension.
5.2.1. Indicating support for cross-origin Server Push
Clients that might accept signed Server Pushes with an authority for
which the server is not authoritative indicate this using the HTTP/2
SETTINGS parameter ENABLE_CROSS_ORIGIN_PUSH (0xSETTING-TBD).
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An ENABLE_CROSS_ORIGIN_PUSH value of 0 indicates that the client does
not support cross-origin Push. A value of 1 indicates that the
client does support cross-origin Push.
A client MUST NOT send a ENABLE_CROSS_ORIGIN_PUSH setting with a
value other than 0 or 1 or a value of 0 after previously sending a
value of 1. If a server receives a value that violates these rules,
it MUST treat it as a connection error (Section 5.4.1 of [RFC7540])
of type PROTOCOL_ERROR.
The use of a SETTINGS parameter to opt-in to an otherwise
incompatible protocol change is a use of "Extending HTTP/2" defined
by Section 5.5 of [RFC7540]. If a server were to send a cross-origin
Push without first receiving a ENABLE_CROSS_ORIGIN_PUSH setting with
the value of 1 it would be a protocol violation.
5.2.2. NO_TRUSTED_EXCHANGE_SIGNATURE error code
The signatures on a Pushed cross-origin exchange may be untrusted for
several reasons, for example that the certificate could not be
fetched, that the certificate does not chain to a trusted root, that
the signature itself doesn't validate, that the signature is expired,
etc. This draft conflates all of these possible failures into one
error code, NO_TRUSTED_EXCHANGE_SIGNATURE (0xERROR-TBD).
5.2.2.1. Open Questions
How fine-grained should this specification's error codes be?
5.2.3. Validating a cross-origin Push
If the client has set the ENABLE_CROSS_ORIGIN_PUSH setting to 1, the
server MAY Push a signed exchange for which it is not authoritative,
and the client MUST NOT treat a PUSH_PROMISE for which the server is
not authoritative as a stream error (Section 5.4.2 of [RFC7540]) of
type PROTOCOL_ERROR, as described in Section 8.2 of [RFC7540], unless
there is another error as described below.
Instead, the client MUST validate such a PUSH_PROMISE and its
response against the following list:
1. If the PUSH_PROMISE includes any non-pseudo request header
fields, the client MUST treat it as a stream error (Section 5.4.2
of [RFC7540]) of type PROTOCOL_ERROR.
2. If the PUSH_PROMISE's method is not "GET", the client MUST treat
it as a stream error (Section 5.4.2 of [RFC7540]) of type
PROTOCOL_ERROR.
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3. Run the algorithm in Section 4 over:
* The "Signature" header field from the response.
* The effective request URI from the PUSH_PROMISE.
* The canonical serialization (Section 3.4) of the CBOR
representation (Section 3.2) of the pushed response's status
and its headers except for the "Signature" header field.
* The response's payload.
If this returns "invalid", the client MUST treat the response as
a stream error (Section 5.4.2 of [RFC7540]) of type
NO_TRUSTED_EXCHANGE_SIGNATURE. Otherwise, the client MUST treat
the pushed response as if the server were authoritative for the
PUSH_PROMISE's authority.
5.2.3.1. Open Questions
Is it right that "validity-url" is required to be same-origin with
the exchange? This allows the mitigation against downgrades in
Section 6.3, but prohibits intermediates from providing a cache of
the validity information. We could do both with a list of URLs.
5.3. application/signed-exchange format
To allow signed exchanges to be the targets of "<link rel=prefetch>"
tags, we define the "application/signed-exchange" content type that
represents a signed HTTP exchange, including a request URL, response
metadata and header fields, and a response payload.
When served over HTTP, a response containing an "application/signed-
exchange" payload MUST include at least the following response header
fields, to reduce content sniffing vulnerabilities (Section 6.8):
* Content-Type: application/signed-exchange;v=_version_
* X-Content-Type-Options: nosniff
This content type consists of the concatenation of the following
items:
1. 8 bytes consisting of the ASCII characters "sxg1" followed by 4
0x00 bytes, 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.
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Note: RFC EDITOR PLEASE DELETE THIS NOTE; The implementation of
the final RFC MUST use this file signature, but implementations
of drafts MUST NOT use it and MUST use another implementation-
specific 8-byte string beginning with "sxg1-".
2. 2 bytes storing a big-endian integer "fallbackUrlLength".
3. "fallbackUrlLength" bytes holding a "fallbackUrl", which MUST
UTF-8 decode to 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
response headers of the exchange represented by the "application/
signed-exchange" resource (Section 3.2), excluding the
"Signature" header field.
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.
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5.3.2. Example
An example "application/signed-exchange" file representing a possible
signed exchange with https://example.com/ (https://example.com/)
follows, with lengths represented by descriptions in "<>"s, CBOR
represented in the extended diagnostic format defined in Appendix G
of [CDDL], and most of the "Signature" header field and payload
elided with a ...:
sxg1\0\0\0\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 map>sig1; sig=*...; integrity="digest/mi-sha256"; ...{
':status': '200',
'content-type': 'text/html'
}<!doctype html>\r\n<html>...
5.3.3. Open Questions
Should this be a CBOR format, or is the current mix of binary and
CBOR better?
Are the mime type, extension, and magic number right?
6. Security considerations
6.1. Over-signing
If a publisher blindly signs all responses as their origin, they can
cause at least two kinds of problems, described below. To avoid
this, publishers SHOULD design their systems to opt particular public
content that doesn't depend on authentication status into signatures
instead of signing by default.
Signing systems SHOULD also incorporate the following mitigations to
reduce the risk that private responses are signed:
1. Strip the "Cookie" request header field and other identifying
information like client authentication and TLS session IDs from
requests whose exchange is destined to be signed, before
forwarding the request to a backend.
2. Only sign exchanges where the response includes a "Cache-Control:
public" header. Clients are not required to fail signature-
checking for exchanges that omit this "Cache-Control" response
header field to reduce the risk that naive signing systems
blindly add it.
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6.1.1. Session fixation
Blind signing can sign responses that create session cookies or
otherwise change state on the client to identify a particular
session. This breaks certain kinds of CSRF defense and can allow an
attacker to force a user into the attacker's account, where the user
might unintentionally save private information, like credit card
numbers or addresses.
This specification defends against cookie-based attacks by blocking
the "Set-Cookie" response header, but it cannot prevent Javascript or
other response content from changing state.
6.1.2. Misleading content
If a site signs private information, an attacker might set up their
own account to show particular private information, forward that
signed information to a victim, and use that victim's confusion in a
more sophisticated attack.
Stripping authentication information from requests before sending
them to backends is likely to prevent the backend from showing
attacker-specific information in the signed response. It does not
prevent the attacker from showing their victim a signed-out page when
the victim is actually signed in, but while this is still misleading,
it seems less likely to be useful to the attacker.
6.2. Off-path attackers
Relaxing the requirement to consult DNS when determining authority
for an origin means that an attacker who possesses a valid
certificate no longer needs to be on-path to redirect traffic to
them; instead of modifying DNS or IP routing, they need only convince
the user to visit another Web site in order to serve responses signed
as the target. This consideration and mitigations for it are shared
by the combination of [RFC8336] and
[I-D.ietf-httpbis-http2-secondary-certs], and are discussed further
in [I-D.bishop-httpbis-origin-fed-up].
6.2.1. Mis-issued certificates
If a CA mis-issues a certificate for a domain, this specification
provides a way to detect the mis-issuance and mitigate harm within
approximately two weeks. Specifically, because all signed exchanges
must include a "SignedCertificateTimestampList" ([RFC6962], a CT log
has promised to publish the mis-issued certificate within that log's
Maximum Merge Delay, 1 day for many logs. The domain owner can then
detect the mis-issued certificate and notify the CA to revoke it,
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which the [BRs], section 4.9.1.1, say they must do within another 5
days.
Once the mis-issued certificate is revoked, existing OCSP responses
begin to expire. The [BRs], section 4.9.10, require that OCSP
responses have a maximum expiration time of 10 days, after which they
can't be used to validate a certificate chain (Section 3.3). This
leads to a total compromised time of 16 days after a mis-issuance.
However, CAs might future-date their OCSP responses, in which case
the mitigation doesn't work.
CAs are forbidden from future-dating their OCSP responses by the
[BRs] section 4.9.9, "OCSP responses MUST conform to RFC6960 and/or
RFC5019." [RFC6960] includes, "The time at which the status was
known to be correct SHALL be reflected in the thisUpdate field of the
response.", and [RFC5019] includes, "When pre-producing OCSPResponse
messages, the responder MUST set the thisUpdate, nextUpdate, and
producedAt times as follows: thisUpdate: The time at which the status
being indicated is known to be correct."
However, if a CA violates the [BRs] to sign future-dated OCSP
responses, attempts to keep the nonconformant OCSP responses private,
but then leaks them, it could cause clients to trust a hostile signed
exchange long after its certificate has been revoked.
Clients could use systems like [CRLSets] and [OneCrl] to revoke the
intermediate certificate that signed the future-dated OCSP responses.
6.2.2. Stolen private keys
If the private key for a CanSignHttpExchanges certificate is stolen,
it can be used at scale until the certificate expires or is revoked,
and unlike for a stolen key for a normal TLS-terminating certificate,
the rightful owner can't detect the problem by watching for attacks
on the DNS or routing infrastructure.
This specification does not currently propose a way for the rightful
owner to detect that their keys are being used by an attacker, after
they've opted into the risk by requesting a CanSignHttpExchanges
certificate in the first place. Clients can fetch a signature's
"validity-url" (Section 3.1) to help owners detect key compromise,
but that compromises some of the privacy properties of this
specification.
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6.3. Downgrades
Signing a bad response can affect more users than simply serving a
bad response, since a served response will only affect users who make
a request while the bad version is live, while an attacker can
forward a signed response until its signature expires. Publishers
should consider shorter signature expiration times than they use for
cache expiration times.
Clients MAY also check the "validity-url" (Section 3.1) of an
exchange more often than the signature's expiration would require.
Doing so for an exchange with an HTTPS request URI provides a TLS
guarantee that the exchange isn't out of date (as long as
Section 5.2.3.1 is resolved to keep the same-origin requirement).
6.4. Signing oracles are permanent
An attacker with temporary access to a signing oracle can sign "still
valid" assertions with arbitrary timestamps and expiration times. As
a result, when a signing oracle is removed, the keys it provided
access to MUST be revoked so that, even if the attacker used them to
sign future-dated exchange validity assertions, the key's OCSP
assertion will expire, causing the exchange as a whole to become
untrusted.
6.5. Unsigned headers
The use of a single "Signed-Headers" header field prevents us from
signing aspects of the request other than its effective request URI
(Section 5.5 of [RFC7230]). For example, if a publisher signs both
"Content-Encoding: br" and "Content-Encoding: gzip" variants of a
response, what's the impact if an attacker serves the brotli one for
a request with "Accept-Encoding: gzip"? This is mitigated by using
[I-D.ietf-httpbis-variants] instead of request headers to describe
how the client should run content negotiation.
The simple form of "Signed-Headers" also prevents us from signing
less than the full request URL. The SRI use case (Appendix A.3) may
benefit from being able to leave the authority less constrained.
Section 3.5 can succeed when some delivered headers aren't included
in the signed set. This accommodates current TLS-terminating
intermediates and may be useful for SRI (Appendix A.3), but is risky
for trusting cross-origin responses (Appendix A.1, Appendix A.2, and
Appendix A.6). Section 5.2 requires all headers to be included in
the signature before trusting cross-origin pushed resources, at Ryan
Sleevi's recommendation.
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6.6. application/signed-exchange
Clients MUST NOT trust an effective request URI claimed by an
"application/signed-exchange" resource (Section 5.3) without either
ensuring the resource was transferred from a server that was
authoritative (Section 9.1 of [RFC7230]) for that URI's origin, or
calling the algorithm in Section 5.3.1 and getting "valid" back.
6.7. Key re-use with TLS
In general, key re-use across multiple protocols is a bad idea.
Using an exchange-signing key in a TLS (or other directly-internet-
facing) server increases the risk that an attacker can steal the
private key, which will allow them to mint packages (similar to
Section 6.4) until their theft is discovered.
Using a TLS key in a CanSignHttpExchanges certificate makes it less
likely that the server operator will discover key theft, due to the
considerations in Section 6.2.
This specification uses the CanSignHttpExchanges X.509 extension
(Section 4.2) to discourage re-use of TLS keys to sign exchanges or
vice-versa.
We require that clients reject certificates with the
CanSignHttpExchanges extension when making TLS connections to
minimize the chance that servers will re-use keys like this.
Ideally, we would make the extension critical so that even clients
that don't understand it would reject such TLS connections, but this
proved impossible because certificate-validating libraries ship on
significantly different schedules from the clients that use them.
Even once all clients reject these certificates in TLS connections,
this will still just discourage and not prevent key re-use, since a
server operator can unwisely request two different certificates with
the same private key.
6.8. Content sniffing
While modern browsers tend to trust the "Content-Type" header sent
with a resource, especially when accompanied by "X-Content-Type-
Options: nosniff", plugins will sometimes search for executable
content buried inside a resource and execute it in the context of the
origin that served the resource, leading to XSS vulnerabilities. For
example, some PDF reader plugins look for "%PDF" anywhere in the
first 1kB and execute the code that follows it.
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The "application/signed-exchange" format (Section 5.3) includes a URL
and response headers early in the format, which an attacker could use
to cause these plugins to sniff a bad content type.
To avoid vulnerabilities, in addition to the response header
requirements in Section 5.3, servers are advised to only serve an
"application/signed-exchange" resource (SXG) from a domain if it
would also be safe for that domain to serve the SXG's content
directly, and to follow at least one of the following strategies:
1. Only serve signed exchanges from dedicated domains that don't
have access to sensitive cookies or user storage.
2. Generate signed exchanges "offline", that is, in response to a
trusted author submitting content or existing signatures reaching
a certain age, rather than in response to untrusted-reader
queries.
3. Do all of:
1. If the SXG's fallback URL (Section 5.3) is derived from the
request URL, percent-encode
(https://url.spec.whatwg.org/#percent-encode) ([URL]) any
bytes that are greater than 0x7E or are not URL code points
(https://url.spec.whatwg.org/#url-code-points) ([URL]) in the
fallback URL . It is particularly important to make sure no
unescaped nulls (0x00) or angle brackets (0x3C and 0x3E)
appear.
2. Do not reflect request header fields into the set of response
headers.
There are still a few binary length fields that an attacker may
influence to contain sensitive bytes, but they're always followed by
lowercase alphabetic strings from a small set of possibilities, which
reduces the chance that a client will sniff them as indicating a
particular content type.
To encourage servers to include the "X-Content-Type-Options: nosniff"
header field, clients SHOULD reject signed exchanges served without
it.
7. Privacy considerations
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7.1. Visibility of resource requests
Normally, when a client follows a link from https://source.example/
page.html to "https://publisher.example/page.html",
"publisher.example" learns that the client is interested in the
resource. "source.example" also has several ways of discovering that
the client has clicked the link, including the use of Javascript to
record the click or having the link point to a URL that serves a 302
redirect to the real target.
If "publisher.example" signs "page.html" into "page.sxg",
"distributor.example" serves it as
"https://distributor.example/publisher/page.sxg", and the client
fetches it from there, then "distributor.example" learns that the
client is interested, and if the client executes some Javascript on
the page or makes subresource requests, that could also report the
client's interest back to "publisher.example".
To prevent network operators other than "distributor.example" or
"publisher.example" 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
([RFC8446]) or a successor transport layer, and MUST NOT accept
exchanges transferred over plain HTTP without TLS.
If "source.example" and "distributor.example" are controlled by the
same entity, no extra information escapes here. If they are run by
different entities, a similar amount of information escapes as if
"source.example" had implemented its click tracking by outsourcing to
a service like https://bit.ly/ (https://bit.ly/).
There has been discussion of allowing a publisher to restrict the set
of distributors that can host its signed content. If that's added,
then the privacy situation becomes more similar to the situation with
CDNs, where a publisher chooses a CDN to serve their content, and the
CDN learns about all requests for that content. Here the publisher
would choose one or more distributors, and the distributor(s) would
learn about requests for the content.
For non-executable resource types, a signed response can improve the
privacy situation by hiding the client's interest from the original
publisher.
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7.2. User ID transfer
If a request for "https://distributor.example/publisher/page.sxg"
comes with the source's or distributor's user ID for the user, either
because it's sent with the distributor's cookies or because the
source stashes an encoded user ID into either the request's path or a
subdomain, the distributor has a few ways to pass that user ID on to
the publisher that signed the page:
1. If the distributor has the publisher's signing keys, it can sign
a new page with its user ID directly embedded.
2. Otherwise, the publisher can sign lots of copies of their
package, and the distributor can choose a particular copy to send
a subset of the bits in its user ID to the publisher on each
click, which will eventually transfer the whole thing.
To prevent this, the request for a signed exchange needs to omit
credentials and block them from appearing in the URL in the same way
it would block them from appearing in a cross-origin URL. We're
exploring ways the link can mark the request so user agents can take
the right counter-measures.
8. IANA considerations
TODO: possibly register the validity-url format.
8.1. Signature Header Field Registration
This section registers the "Signature" header field in the "Permanent
Message Header Field Names" registry ([RFC3864]).
Header field name: "Signature"
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document(s): Section 3.1 of this document
8.2. Accept-Signature Header Field Registration
This section registers the "Accept-Signature" header field in the
"Permanent Message Header Field Names" registry ([RFC3864]).
Header field name: "Accept-Signature"
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Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document(s): Section 3.7 of this document
8.3. Signed-Headers Header Field Registration
This section registers the "Signed-Headers" header field in the
"Permanent Message Header Field Names" registry ([RFC3864]).
Header field name: "Signed-Headers"
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document(s): Section 5.1.2 of this document
8.4. HTTP/2 Settings
This section establishes an entry for the HTTP/2 Settings Registry
that was established by Section 11.3 of [RFC7540]
Name: ENABLE_CROSS_ORIGIN_PUSH
Code: 0xSETTING-TBD
Initial Value: 0
Specification: This document
8.5. HTTP/2 Error code
This section establishes an entry for the HTTP/2 Error Code Registry
that was established by Section 11.4 of [RFC7540]
Name: NO_TRUSTED_EXCHANGE_SIGNATURE
Code: 0xERROR-TBD
Description: The client does not trust the signature for a cross-
origin Pushed signed exchange.
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Specification: This document
8.6. Internet Media Type application/signed-exchange
IANA is requested to register the MIME media type
([IANA.media-types]) for signed exchanges, application/signed-
exchange, as follows:
Type name: application
Subtype name: signed-exchange
Required parameters:
* v: A string denoting the version of the file format. ([RFC5234]
ABNF: "version = DIGIT/%x61-7A") The version defined in this
specification is "1". When used with the "Accept" header field
(Section 5.3.2 of [RFC7231]), this parameter can be a comma
(,)-separated list of version strings. ([RFC5234] ABNF: "version-
list = version *( "," version )") The server is then expected to
reply with a resource using a particular version from that list.
Note: RFC EDITOR PLEASE DELETE THIS NOTE; Implementations of
drafts of this specification MUST NOT use simple integers to
describe their versions, and MUST instead define implementation-
specific strings to identify which draft is implemented. The
newest version of
[I-D.yasskin-httpbis-origin-signed-exchanges-impl] describes the
meaning of one such string.
Optional parameters: N/A
Encoding considerations: binary
Security considerations: see Section 6.6
Interoperability considerations: N/A
Published specification: This specification (see Section 5.3).
Applications that use this media type: N/A
Fragment identifier considerations: N/A
Additional information:
Deprecated alias names for this type: N/A
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Magic number(s): 73 78 67 31 00
File extension(s): .sxg
Macintosh file type code(s): N/A
Person and email address to contact for further information: See
Authors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section.
Change controller: IESG
Provisional registration? Yes
8.7. Internet Media Type application/cert-chain+cbor
IANA is requested to register the MIME media type
([IANA.media-types]) for CBOR-format certificate chains, application/
cert-chain+cbor, as follows:
Type name: application
Subtype name: cert-chain+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: N/A
Interoperability considerations: N/A
Published specification: This specification (see Section 3.3).
Applications that use this media type: N/A
Fragment identifier considerations: N/A
Additional information:
Deprecated alias names for this type: N/A
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Magic number(s): 1*9(??) 67 F0 9F 93 9C E2 9B 93
File extension(s): N/A
Macintosh file type code(s): N/A
Person and email address to contact for further information: See
Authors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section.
Change controller: IESG
Provisional registration? Yes
8.8. The cansignhttpexchanges CAA Parameter
There are no IANA considerations for this parameter.
9. References
9.1. Normative References
[CDDL] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[FETCH] WHATWG, "Fetch", July 2020,
<https://fetch.spec.whatwg.org/>.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", Work in Progress, Internet-Draft, draft-ietf-
httpbis-header-structure-19, 3 June 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-httpbis-
header-structure-19.txt>.
[I-D.ietf-httpbis-variants]
Nottingham, M., "HTTP Representation Variants", Work in
Progress, Internet-Draft, draft-ietf-httpbis-variants-06,
3 November 2019, <http://www.ietf.org/internet-drafts/
draft-ietf-httpbis-variants-06.txt>.
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[I-D.thomson-http-mice]
Thomson, M. and J. Yasskin, "Merkle Integrity Content
Encoding", Work in Progress, Internet-Draft, draft-
thomson-http-mice-03, 13 August 2018,
<http://www.ietf.org/internet-drafts/draft-thomson-http-
mice-03.txt>.
[IANA.media-types]
IANA, "Media Types",
<http://www.iana.org/assignments/media-types>.
[POSIX] IEEE and The Open Group, "The Open Group Base
Specifications Issue 7", value 1003.1-2008, 2016 Edition,
name IEEE, 2016,
<http://pubs.opengroup.org/onlinepubs/9699919799/
basedefs/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3230] Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
RFC 3230, DOI 10.17487/RFC3230, January 2002,
<https://www.rfc-editor.org/info/rfc3230>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
DOI 10.17487/RFC6844, January 2013,
<https://www.rfc-editor.org/info/rfc6844>.
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[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>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[URL] WHATWG, "URL", July 2020, <https://url.spec.whatwg.org/>.
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9.2. Informative References
[BRs] CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates", 10
December 2018,
<https://cabforum.org/baseline-requirements-documents/>.
[CRLSets] Langley, A., "Revocation checking and Chrome's CRL", 5
February 2012,
<https://www.imperialviolet.org/2012/02/05/crlsets.html>.
[I-D.bishop-httpbis-origin-fed-up]
Bishop, M. and E. Nygren, "DNS Security with HTTP/2
ORIGIN", Work in Progress, Internet-Draft, draft-bishop-
httpbis-origin-fed-up-00, 8 January 2019,
<http://www.ietf.org/internet-drafts/draft-bishop-httpbis-
origin-fed-up-00.txt>.
[I-D.burke-content-signature]
Burke, B., "HTTP Header for digital signatures", Work in
Progress, Internet-Draft, draft-burke-content-signature-
00, 7 March 2011, <http://www.ietf.org/internet-drafts/
draft-burke-content-signature-00.txt>.
[I-D.cavage-http-signatures]
Cavage, M. and M. Sporny, "Signing HTTP Messages", Work in
Progress, Internet-Draft, draft-cavage-http-signatures-12,
21 October 2019, <http://www.ietf.org/internet-drafts/
draft-cavage-http-signatures-12.txt>.
[I-D.ietf-httpbis-cache]
Fielding, R., Nottingham, M., and J. Reschke, "HTTP
Caching", Work in Progress, Internet-Draft, draft-ietf-
httpbis-cache-10, 12 July 2020, <http://www.ietf.org/
internet-drafts/draft-ietf-httpbis-cache-10.txt>.
[I-D.ietf-httpbis-http2-secondary-certs]
Bishop, M., Sullivan, N., and M. Thomson, "Secondary
Certificate Authentication in HTTP/2", Work in Progress,
Internet-Draft, draft-ietf-httpbis-http2-secondary-certs-
06, 14 May 2020, <http://www.ietf.org/internet-drafts/
draft-ietf-httpbis-http2-secondary-certs-06.txt>.
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[I-D.thomson-http-content-signature]
Thomson, M., "Content-Signature Header Field for HTTP",
Work in Progress, Internet-Draft, draft-thomson-http-
content-signature-00, 2 July 2015, <http://www.ietf.org/
internet-drafts/draft-thomson-http-content-signature-
00.txt>.
[I-D.yasskin-httpbis-origin-signed-exchanges-impl]
Yasskin, J. and K. Ueno, "Signed HTTP Exchanges
Implementation Checkpoints", Work in Progress, Internet-
Draft, draft-yasskin-httpbis-origin-signed-exchanges-impl-
03, 25 July 2019, <http://www.ietf.org/internet-drafts/
draft-yasskin-httpbis-origin-signed-exchanges-impl-
03.txt>.
[I-D.yasskin-wpack-use-cases]
Yasskin, J., "Use Cases and Requirements for Web
Packages", Work in Progress, Internet-Draft, draft-
yasskin-wpack-use-cases-00, 30 October 2019,
<http://www.ietf.org/internet-drafts/draft-yasskin-wpack-
use-cases-00.txt>.
[OneCrl] Goodwin, M., "Revoking Intermediate Certificates:
Introducing OneCRL", 3 March 2015,
<https://blog.mozilla.org/security/2015/03/03/revoking-
intermediate-certificates-introducing-onecrl/>.
[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>.
[RFC5019] Deacon, A. and R. Hurst, "The Lightweight Online
Certificate Status Protocol (OCSP) Profile for High-Volume
Environments", RFC 5019, DOI 10.17487/RFC5019, September
2007, <https://www.rfc-editor.org/info/rfc5019>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
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[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, DOI 10.17487/RFC6455, December 2011,
<https://www.rfc-editor.org/info/rfc6455>.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797,
DOI 10.17487/RFC6797, November 2012,
<https://www.rfc-editor.org/info/rfc6797>.
[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>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <https://www.rfc-editor.org/info/rfc7469>.
[RFC7615] Reschke, J., "HTTP Authentication-Info and Proxy-
Authentication-Info Response Header Fields", RFC 7615,
DOI 10.17487/RFC7615, September 2015,
<https://www.rfc-editor.org/info/rfc7615>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<https://www.rfc-editor.org/info/rfc8017>.
[RFC8053] Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
T., and Y. Ioku, "HTTP Authentication Extensions for
Interactive Clients", RFC 8053, DOI 10.17487/RFC8053,
January 2017, <https://www.rfc-editor.org/info/rfc8053>.
[RFC8336] Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
RFC 8336, DOI 10.17487/RFC8336, March 2018,
<https://www.rfc-editor.org/info/rfc8336>.
[SRI] Akhawe, D., Braun, F., Marier, F., and J. Weinberger,
"Subresource Integrity", World Wide Web Consortium
Recommendation REC-SRI-20160623, 23 June 2016,
<http://www.w3.org/TR/2016/REC-SRI-20160623>.
[W3C.NOTE-OPS-OverHTTP]
Hensley, P., Metral, M., Shardanand, U., Converse, D., and
M. Myers, "Implementation of OPS Over HTTP", W3C NOTE
NOTE-OPS-OverHTTP, 2 June 1997,
<http://www.w3.org/TR/NOTE-OPS-OverHTTP>.
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[W3C.WD-clear-site-data-20171130]
West, M., "Clear Site Data", World Wide Web Consortium WD
WD-clear-site-data-20171130, 30 November 2017,
<https://www.w3.org/TR/2017/WD-clear-site-data-20171130>.
Appendix A. Use cases
A.1. PUSHed subresources
To reduce round trips, a server might use HTTP/2 Push (Section 8.2 of
[RFC7540]) to inject a subresource from another server into the
client's cache. If anything about the subresource is expired or
can't be verified, the client would fetch it from the original
server.
For example, if "https://example.com/index.html" includes
<script src="https://jquery.com/jquery-1.2.3.min.js">
Then to avoid the need to look up and connect to "jquery.com" in the
critical path, "example.com" might push that resource signed by
"jquery.com".
A.2. Explicit use of a content distributor for subresources
In order to speed up loading but still maintain control over its
content, an HTML page in a particular origin "O.com" could tell
clients to load its subresources from an intermediate content
distributor that's not authoritative, but require that those
resources be signed by "O.com" so that the distributor couldn't
modify the resources. This is more constrained than the common CDN
case where "O.com" has a CNAME granting the CDN the right to serve
arbitrary content as "O.com".
<img logicalsrc="https://O.com/img.png"
physicalsrc="https://distributor.com/O.com/img.png">
To make it easier to configure the right distributor for a given
request, computation of the "physicalsrc" could be encapsulated in a
custom element:
<dist-img src="https://O.com/img.png"></dist-img>
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where the "<dist-img>" implementation generates an appropriate
"<img>" based on, for example, a "<meta name="dist-base">" tag
elsewhere in the page. However, this has the downside that the
preloader (https://calendar.perfplanet.com/2013/big-bad-preloader/)
can no longer see the physical source to download it. The resulting
delay might cancel out the benefit of using a distributor.
This could be used for some of the same purposes as SRI
(Appendix A.3).
To implement this with the current proposal, the distributor would
respond to the physical request to "https://distributor.com/O.com/
img.png" with first a signed PUSH_PROMISE for "https://O.com/img.png"
and then a redirect to "https://O.com/img.png".
A.3. Subresource Integrity
The W3C WebAppSec group is investigating using signatures
(https://github.com/mikewest/signature-based-sri) in [SRI]. They
need a way to transmit the signature with the response, which this
proposal provides.
Their needs are simpler than most other use cases in that the
"integrity="ed25519-[public-key]"" attribute and CSP-based ways of
expressing a public key don't need that key to be wrapped into a
certificate.
The "ed25519key" signature parameter supports this simpler way of
attaching a key.
The current proposal for signature-based SRI describes signing only
the content of a resource, while this specification requires them to
sign the request URI as well. This issue is tracked in
https://github.com/mikewest/signature-based-sri/issues/5
(https://github.com/mikewest/signature-based-sri/issues/5). The
details of what they need to sign will affect whether and how they
can use this proposal.
A.4. Binary Transparency
So-called "Binary Transparency" may eventually allow users to verify
that a program they've been delivered is one that's available to the
public, and not a specially-built version intended to attack just
them. Binary transparency systems don't exist yet, but they're
likely to work similarly to the successful Certificate Transparency
logs described by [RFC6962].
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Certificate Transparency depends on Signed Certificate Timestamps
that prove a log contained a particular certificate at a particular
time. To build the same thing for Binary Transparency logs
containing HTTP resources or full websites, we'll need a way to
provide signatures of those resources, which signed exchanges
provides.
A.5. Static Analysis
Native app stores like the Apple App Store
(https://www.apple.com/ios/app-store/) and the Android Play Store
(https://play.google.com/store) grant their contents powerful
abilities, which they attempt to make safe by analyzing the
applications before offering them to people. The web has no
equivalent way for people to wait to run an update of a web
application until a trusted authority has vouched for it.
While full application analysis probably needs to wait until the
authority can sign bundles of exchanges, authorities may be able to
guarantee certain properties by just checking a top-level resource
and its [SRI]-constrained sub-resources.
A.6. Offline websites
Fully-offline websites can be represented as bundles of signed
exchanges, although an optimization to reduce the number of signature
verifications may be needed. Work on this is in progress in the
https://github.com/WICG/webpackage (https://github.com/WICG/
webpackage) repository.
Appendix B. Requirements
B.1. Proof of origin
To verify that a thing came from a particular origin, for use in the
same context as a TLS connection, we need someone to vouch for the
signing key with as much verification as the signing keys used in
TLS. The obvious way to do this is to re-use the web PKI and CA
ecosystem.
B.1.1. Certificate constraints
If we re-use existing TLS server certificates, we incur the risks
that:
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1. TLS server certificates must be accessible from online servers,
so they're easier to steal or use as signing oracles than an
offline key. An exchange's signing key doesn't need to be
online.
2. A server using an origin-trusted key for one purpose (e.g. TLS)
might accidentally sign something that looks like an exchange, or
vice versa.
These risks are considered too high, so we define a new X.509
certificate extension in Section 4.2 that requires CAs to issue new
certificates for this purpose. We expect at least one low-cost CA to
be willing to sign certificates with this extension.
B.1.2. Signature constraints
In order to prevent an attacker who can convince the server to sign
some resource from causing those signed bytes to be interpreted as
something else the new X.509 extension here is forbidden from being
used in TLS servers. If Section 4.2 changes to allow re-use in TLS
servers, we would need to:
1. Avoid key types that are used for non-TLS protocols whose output
could be confused with a signature. That may be just the
"rsaEncryption" OID from [RFC8017].
2. Use the same format as TLS's signatures, specified in
Section 4.4.3 of [RFC8446], with a context string that's specific
to this use.
The specification also needs to define which signing algorithm to
use. It currently specifies that as a function from the key type,
instead of allowing attacker-controlled data to specify it.
B.1.3. Retrieving the certificate
The client needs to be able to find the certificate vouching for the
signing key, a chain from that certificate to a trusted root, and
possibly other trust information like SCTs ([RFC6962]). One approach
would be to include the certificate and its chain in the signature
metadata itself, but this wastes bytes when the same certificate is
used for multiple HTTP responses. If we decide to put the signature
in an HTTP header, certificates are also unusually large for that
context.
Another option is to pass a URL that the client can fetch to retrieve
the certificate and chain. To avoid extra round trips in fetching
that URL, it could be bundled (Appendix A.6) with the signed content
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or PUSHed (Appendix A.1) with it. The risks from the
"client_certificate_url" extension (Section 11.3 of [RFC6066]) don't
seem to apply here, since an attacker who can get a client to load an
exchange and fetch the certificates it references, can also get the
client to perform those fetches by loading other HTML.
To avoid using an unintended certificate with the same public key as
the intended one, the content of the leaf certificate or the chain
should be included in the signed data, like TLS does (Section 4.4.3
of [RFC8446]).
B.2. How much to sign
The previous [I-D.thomson-http-content-signature] and
[I-D.burke-content-signature] schemes signed just the content, while
([I-D.cavage-http-signatures] could also sign the response headers
and the request method and path. However, the same path, response
headers, and content may mean something very different when retrieved
from a different server. Section 5.1.1 currently includes the whole
request URL in the signature, but it's possible we need a more
flexible scheme to allow some higher-level protocols to accept a
less-signed URL.
Servers might want to sign other request headers in order to capture
their effects on content negotiation. However, there's no standard
algorithm to check that a client's actual request headers match
request headers sent by a server. The most promising attempt at this
is [I-D.ietf-httpbis-variants], which encodes the content negotiation
algorithm into the "Variants" and "Variant-Key" response headers.
The proposal here (Section 3) assumes that is in use and doesn't sign
request headers.
B.2.1. Conveying the signed headers
HTTP headers are traditionally munged by proxies, making it
impossible to guarantee that the client will see the same sequence of
bytes as the publisher published. In the HTTPS world, we have more
end-to-end header integrity, but it's still likely that there are
enough TLS-terminating proxies that the publisher's signatures would
tend to break before getting to the client.
There's no way in current HTTP for the response to a client-initiated
request (Section 8.1 of [RFC7540]) to convey the request headers it
expected to respond to, but we sidestep that by conveying content
negotiation information in response headers, per
[I-D.ietf-httpbis-variants].
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Since proxies are unlikely to modify unknown content types, we can
wrap the original exchange into an "application/signed-exchange"
format (Section 5.3) and include the "Cache-Control: no-transform"
header when sending it.
To reduce the likelihood of accidental modification by proxies, the
"application/signed-exchange" format includes a file signature that
doesn't collide with other known signatures.
To help the PUSHed subresources use case (Appendix A.1), we might
also want to extend the "PUSH_PROMISE" frame type to include a
signature, and that could tell intermediates not to change the
ensuing headers.
B.3. Response lifespan
A normal HTTPS response is authoritative only for one client, for as
long as its cache headers say it should live. A signed exchange can
be re-used for many clients, and if it was generated while a server
was compromised, it can continue compromising clients even if their
requests happen after the server recovers. This signing scheme needs
to mitigate that risk.
B.3.1. Certificate revocation
Certificates are mis-issued and private keys are stolen, and in
response clients need to be able to stop trusting these certificates
as promptly as possible. Online revocation checks don't work
(https://www.imperialviolet.org/2012/02/05/crlsets.html), so the
industry has moved to pushed revocation lists and stapled OCSP
responses [RFC6066].
Pushed revocation lists work as-is to block trust in the certificate
signing an exchange, but the signatures need an explicit strategy to
staple OCSP responses. One option is to extend the certificate
download (Appendix B.1.3) to include the OCSP response too, perhaps
in the TLS 1.3 CertificateEntry (https://tlswg.github.io/tls13-spec/
draft-ietf-tls-tls13.html#ocsp-and-sct) format.
B.3.2. Response downgrade attacks
The signed content in a response might be vulnerable to attacks, such
as XSS, or might simply be discovered to be incorrect after
publication. Once the author fixes those vulnerabilities or
mistakes, clients should stop trusting the old signed content in a
reasonable amount of time. Similar to certificate revocation, I
expect the best option to be stapled "this version is still valid"
assertions with short expiration times.
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These assertions could be structured as:
1. A signed minimum version number or timestamp for a set of request
headers: This requires that signed responses need to include a
version number or timestamp, but allows a server to provide a
single signature covering all valid versions.
2. A replacement for the whole exchange's signature. This requires
the publisher to separately re-sign each valid version and
requires each version to include a different update URL, but
allows intermediates to serve less data. This is the approach
taken in Section 3.
3. A replacement for the exchange's signature and an update for the
embedded "expires" and related cache-control HTTP headers
[RFC7234]. This naturally extends publishers' intuitions about
cache expiration and the existing cache revalidation behavior to
signed exchanges. This is sketched and its downsides explored in
Appendix C.
The signature also needs to include instructions to intermediates for
how to fetch updated validity assertions.
B.4. Low implementation complexity
Simpler implementations are, all things equal, less likely to include
bugs. This section describes decisions that were made in the rest of
the specification to reduce complexity.
B.4.1. Limited choices
In general, we're trying to eliminate unnecessary choices in the
specification. For example, instead of requiring clients to support
two methods for verifying payload integrity, we only require one.
B.4.2. Bounded-buffering integrity checking
Clients can be designed with a more-trusted network layer that
decides how to trust resources and then provides those resources to
less-trusted rendering processes along with handles to the storage
and other resources they're allowed to access. If the network layer
can enforce that it only operates on chunks of data up to a certain
size, it can avoid the complexity of spooling large files to disk.
To allow the network layer to verify signed exchanges using a bounded
amount of memory, Section 5.3 requires the signature to be less than
16kB and the headers to be less than 512kB, and Section 3.5 requires
that the MI record size be less than 16kB. This allows the network
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layer to validate a bounded chunk at a time, and pass that chunk on
to a renderer, and then forget about that chunk before processing the
next one.
The "Digest" header field from [RFC3230] requires the network layer
to buffer the entire response body, so it's disallowed.
Appendix C. Determining validity using cache control
This draft could expire signature validity using the normal HTTP
cache control headers ([RFC7234]) instead of embedding an expiration
date in the signature itself. This section specifies how that would
work, and describes why I haven't chosen that option.
The signatures in the "Signature" header field (Section 3.1) would no
longer contain "date" or "expires" fields.
The validity-checking algorithm (Section 3.5) would initialize "date"
from the resource's "Date" header field (Section 7.1.1.2 of
[RFC7231]) and initialize "expires" from either the "Expires" header
field (Section 5.3 of [RFC7234]) or the "Cache-Control" header
field's "max-age" directive (Section 5.2.2.8 of [RFC7234]) (added to
"date"), whichever is present, preferring "max-age" (or failing) if
both are present.
Validity updates (Section 3.6) would include a list of replacement
response header fields. For each header field name in this list, the
client would remove matching header fields from the stored exchange's
response header fields. Then the client would append the replacement
header fields to the stored exchange's response header fields.
C.1. Example of updating cache control
For example, given a stored exchange of:
GET / HTTP/1.1
Host: example.com
Accept: */*
HTTP/1.1 200
Date: Mon, 20 Nov 2017 10:00:00 UTC
Content-Type: text/html
Date: Tue, 21 Nov 2017 10:00:00 UTC
Expires: Sun, 26 Nov 2017 10:00:00 UTC
<!doctype html>
<html>
...
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And an update listing the following headers:
Expires: Fri, 1 Dec 2017 10:00:00 UTC
Date: Sat, 25 Nov 2017 10:00:00 UTC
The resulting stored exchange would be:
GET / HTTP/1.1
Host: example.com
Accept: */*
HTTP/1.1 200
Content-Type: text/html
Expires: Fri, 1 Dec 2017 10:00:00 UTC
Date: Sat, 25 Nov 2017 10:00:00 UTC
<!doctype html>
<html>
...
C.2. Downsides of updating cache control
In an exchange with multiple signatures, using cache control to
expire signatures forces all signatures to initially live for the
same period. Worse, the update from one signature's "validity-url"
might not match the update for another signature. Clients would need
to maintain a current set of headers for each signature, and then
decide which set to use when actually parsing the resource itself.
This need to store and reconcile multiple sets of headers for a
single signed exchange argues for embedding a signature's lifetime
into the signature.
Appendix D. Change Log
RFC EDITOR PLEASE DELETE THIS SECTION.
draft-09
* No change
draft-08
* Improve the privacy considerations.
draft-07
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* Provisionally register application/signed-exchange and
application/cert-chain+cbor.
draft-06
* Add a security consideration for future-dated OCSP responses and
for stolen private keys.
* Define a CAA parameter to opt into certificate issuance.
* Limit certificate lifetimes to 90 days.
* UTF-8 decode the fallback URL.
draft-05
* Define absolute URLs, and limit the schemes each instance can use.
* Fill in TBD size limits.
* Update to mice-03 including the Digest header.
* Refer to draft-yasskin-httpbis-origin-signed-exchanges-impl for
draft version numbers.
* Require "exchange"'s response to be cachable by a shared cache.
* Define the "integrity" field of the Signature header to include
subfields of the main integrity-protecting header, including the
digest algorithm.
* Put a fallback URL at the beginning of the "application/signed-
exchange" format, which replaces the ':url' key from the CBOR
representation of the exchange's request and response metadata and
headers.
* Remove the rest of the request headers from the signed data, in
favor of representing content negotiation with the "Variants"
response header.
* Make the signed message format a concatenation of byte sequences,
which helps implementations avoid re-serializing the exchange's
request and response metadata and headers.
* Explicitly check the response payload's integrity instead of
assuming the client did it elsewhere in processing the response.
* Reject uncached header fields.
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* Update to draft-ietf-httpbis-header-structure-09.
* Update to the final TLS 1.3 RFC.
draft-04
* Update to draft-ietf-httpbis-header-structure-06.
* Replace the application/http-exchange+cbor format with a simpler
application/signed-exchange format that:
- Doesn't require a streaming CBOR parser parse it from a network
stream.
- Doesn't allow request payloads or response trailers, which
don't fit into the signature model.
- Allows checking the signature before parsing the exchange
headers.
* Require absolute URLs.
* Make all identifiers in headers lower-case, as required by
Structured Headers.
* Switch back to the TLS 1.3 signature format.
* Include the version and draft number in the signature context
string.
* Remove support for integrity protection using the Digest header
field.
* Limit the record size in the mi-sha256 encoding.
* Forbid RSA keys, and only require clients to support secp256r1
keys.
* Add a test OID for the CanSignHttpExchanges X.509 extension.
draft-03
* Allow each method of transferring an exchange to define which
headers are signed, have the cross-origin methods use all headers,
and remove the "allResponseHeaders" flag.
* Describe footguns around signing private content, and block
certain headers to make it less likely.
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* Define a CBOR structure to hold the certificate chain instead of
re-using the TLS1.3 message. The TLS 1.3 parser fails on
unexpected extensions while this format should ignore them, and
apparently TLS implementations don't expose their message parsers
enough to allow passing a message to a certificate verifier.
* Require an X.509 extension for the signing certificate.
draft-02
* Signatures identify a header (e.g. Digest or MI) to guard the
payload's integrity instead of directly signing over the payload.
* The validityUrl is signed.
* Use CBOR maps where appropriate, and define how they're
canonicalized.
* Remove the update.url field from signature validity updates, in
favor of just re-fetching the original request URL.
* Define an HTTP/2 extension to use a setting to enable cross-origin
Server Push.
* Define an "Accept-Signature" header to negotiate whether to send
Signatures and which ones.
* Define an "application/http-exchange+cbor" format to fetch signed
exchanges without HTTP/2 Push.
* 2 new use cases.
Appendix E. Acknowledgements
Thanks to Andrew Ayer, Devin Mullins, Ilari Liusvaara, John Wilander,
Justin Schuh, Mark Nottingham, Mike Bishop, Ryan Sleevi, and Yoav
Weiss for comments that improved this draft.
Author's Address
Jeffrey Yasskin
Google
Email: jyasskin@chromium.org
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