Network Working Group J. Yasskin
Internet-Draft Google
Intended status: Standards Track June 14, 2018
Expires: December 16, 2018
Signed HTTP Exchanges
draft-yasskin-http-origin-signed-responses-04
Abstract
This document specifies how a server can send an HTTP request/
response pair, known as an exchange, with signatures that vouch for
that exchange's authenticity. These signatures can be verified
against an origin's certificate to establish that the exchange is
authoritative for an origin even if it was transferred over a
connection that isn't. The signatures can also be used in other ways
described in the appendices.
These signatures contain countermeasures against downgrade and
protocol-confusion attacks.
Note to Readers
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
The source code and issues list for this draft can be found in
https://github.com/WICG/webpackage [2].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 16, 2018.
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Signing an exchange . . . . . . . . . . . . . . . . . . . . . 5
3.1. The Signature Header . . . . . . . . . . . . . . . . . . 5
3.1.1. Examples . . . . . . . . . . . . . . . . . . . . . . 6
3.1.2. Open Questions . . . . . . . . . . . . . . . . . . . 8
3.2. CBOR representation of exchange headers . . . . . . . . . 8
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 . . . . . . . . . . . . . . . . . . . 15
3.6. Updating signature validity . . . . . . . . . . . . . . . 15
3.6.1. Examples . . . . . . . . . . . . . . . . . . . . . . 16
3.7. The Accept-Signature header . . . . . . . . . . . . . . . 18
3.7.1. Integrity identifiers . . . . . . . . . . . . . . . . 19
3.7.2. Key type identifiers . . . . . . . . . . . . . . . . 19
3.7.3. Key value identifiers . . . . . . . . . . . . . . . . 19
3.7.4. Examples . . . . . . . . . . . . . . . . . . . . . . 20
3.7.5. Open Questions . . . . . . . . . . . . . . . . . . . 20
4. Cross-origin trust . . . . . . . . . . . . . . . . . . . . . 21
4.1. Stateful header fields . . . . . . . . . . . . . . . . . 22
4.2. Certificate Requirements . . . . . . . . . . . . . . . . 23
5. Transferring a signed exchange . . . . . . . . . . . . . . . 24
5.1. Same-origin response . . . . . . . . . . . . . . . . . . 24
5.1.1. Significant headers for a same-origin response . . . 24
5.1.2. The Signed-Headers Header . . . . . . . . . . . . . . 25
5.2. HTTP/2 extension for cross-origin Server Push . . . . . . 26
5.2.1. Indicating support for cross-origin Server Push . . . 26
5.2.2. NO_TRUSTED_EXCHANGE_SIGNATURE error code . . . . . . 26
5.2.3. Validating a cross-origin Push . . . . . . . . . . . 26
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5.3. application/signed-exchange format . . . . . . . . . . . 27
5.3.1. Cross-origin trust in application/signed-exchange . . 28
5.3.2. Example . . . . . . . . . . . . . . . . . . . . . . . 28
5.3.3. Open Questions . . . . . . . . . . . . . . . . . . . 29
6. Security considerations . . . . . . . . . . . . . . . . . . . 29
6.1. Over-signing . . . . . . . . . . . . . . . . . . . . . . 29
6.1.1. Session fixation . . . . . . . . . . . . . . . . . . 30
6.1.2. Misleading content . . . . . . . . . . . . . . . . . 30
6.2. Off-path attackers . . . . . . . . . . . . . . . . . . . 30
6.3. Downgrades . . . . . . . . . . . . . . . . . . . . . . . 30
6.4. Signing oracles are permanent . . . . . . . . . . . . . . 31
6.5. Unsigned headers . . . . . . . . . . . . . . . . . . . . 31
6.6. application/signed-exchange . . . . . . . . . . . . . . . 31
7. Privacy considerations . . . . . . . . . . . . . . . . . . . 32
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 32
8.1. Signature Header Field Registration . . . . . . . . . . . 32
8.2. Accept-Signature Header Field Registration . . . . . . . 33
8.3. Signed-Headers Header Field Registration . . . . . . . . 33
8.4. HTTP/2 Settings . . . . . . . . . . . . . . . . . . . . . 33
8.5. HTTP/2 Error code . . . . . . . . . . . . . . . . . . . . 33
8.6. Internet Media Type application/signed-exchange . . . . . 34
8.7. Internet Media Type application/cert-chain+cbor . . . . . 35
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
9.1. Normative References . . . . . . . . . . . . . . . . . . 36
9.2. Informative References . . . . . . . . . . . . . . . . . 38
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Appendix A. Use cases . . . . . . . . . . . . . . . . . . . . . 41
A.1. PUSHed subresources . . . . . . . . . . . . . . . . . . . 41
A.2. Explicit use of a content distributor for subresources . 41
A.3. Subresource Integrity . . . . . . . . . . . . . . . . . . 42
A.4. Binary Transparency . . . . . . . . . . . . . . . . . . . 42
A.5. Static Analysis . . . . . . . . . . . . . . . . . . . . . 43
A.6. Offline websites . . . . . . . . . . . . . . . . . . . . 43
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 43
B.1. Proof of origin . . . . . . . . . . . . . . . . . . . . . 43
B.1.1. Certificate constraints . . . . . . . . . . . . . . . 43
B.1.2. Signature constraints . . . . . . . . . . . . . . . . 44
B.1.3. Retrieving the certificate . . . . . . . . . . . . . 44
B.2. How much to sign . . . . . . . . . . . . . . . . . . . . 45
B.2.1. Conveying the signed headers . . . . . . . . . . . . 45
B.3. Response lifespan . . . . . . . . . . . . . . . . . . . . 46
B.3.1. Certificate revocation . . . . . . . . . . . . . . . 46
B.3.2. Response downgrade attacks . . . . . . . . . . . . . 46
B.4. Low implementation complexity . . . . . . . . . . . . . . 47
B.4.1. Limited choices . . . . . . . . . . . . . . . . . . . 47
B.4.2. Bounded-buffering integrity checking . . . . . . . . 47
Appendix C. Determining validity using cache control . . . . . . 48
C.1. Example of updating cache control . . . . . . . . . . . . 48
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C.2. Downsides of updating cache control . . . . . . . . . . . 49
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 49
Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 51
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 51
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:
o 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.
o 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.
o 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-webpackage-use-cases] will provide a way to group signed
exchanges into bundles that can be transmitted and stored together,
but single signed exchanges are useful enough to standardize on their
own.
2. Terminology
Author The entity that wrote the content in a particular resource.
This specification deals with publishers rather than authors.
Publisher The entity that controls the server for a particular
origin [RFC6454]. The publisher can get a CA to issue
certificates for their private keys and can run a TLS server for
their origin.
Exchange (noun) An HTTP request/response pair. This can either be a
request from a client and the matching response from a server or
the request in a PUSH_PROMISE and its matching response stream.
Defined by Section 8 of [RFC7540].
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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 [3].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Signing an exchange
In the response of an HTTP exchange the server MAY include a
"Signature" header field (Section 3.1) holding a list of one or more
parameterised signatures that vouch for the content of the exchange.
Exactly which content the signature vouches for can depend on how the
exchange is transferred (Section 5).
The client categorizes each signature as "valid" or "invalid" by
validating that signature with its certificate or public key and
other metadata against the exchange's headers and content
(Section 3.5). This validity then informs higher-level protocols.
Each signature is parameterised with information to let a client
fetch assurance that a signed exchange is still valid, in the face of
revoked certificates and newly-discovered vulnerabilities. This
assurance can be bundled back into the signed exchange and forwarded
to another client, which won't have to re-fetch this validity
information for some period of time.
3.1. The Signature Header
The "Signature" header field conveys a list of signatures for an
exchange, each one accompanied by information about how to determine
the authority of and refresh that signature. Each signature directly
signs the exchange's headers and identifies one of those headers that
enforces the integrity of the exchange's payload.
The "Signature" header is a Structured Header as defined by
[I-D.ietf-httpbis-header-structure]. Its value MUST be a
parameterised list (Section 3.3 of
[I-D.ietf-httpbis-header-structure]). Its ABNF is:
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Signature = sh-param-list
Each parameterised identifier in the list MUST have parameters named
"sig", "integrity", "validity-url", "date", and "expires". Each
parameterised identifier MUST also have either "cert-url" and "cert-
sha256" parameters or an "ed25519key" parameter. This specification
gives no meaning to the identifier itself, which can be used as a
human-readable identifier for the signature (see
Section 3.1.2, Paragraph 1). The present parameters MUST have the
following values:
"sig" Binary content (Section 3.9 of
[I-D.ietf-httpbis-header-structure]) holding the signature of most
of these parameters and the exchange's headers.
"integrity" A string (Section 3.7 of
[I-D.ietf-httpbis-header-structure]) containing the lowercase name
of the response header field that guards the response payload's
integrity.
"cert-url" A string (Section 3.7 of
[I-D.ietf-httpbis-header-structure]) containing an absolute-URL
string [4] ([URL]).
"cert-sha256" Binary content (Section 3.9 of
[I-D.ietf-httpbis-header-structure]) holding the SHA-256 hash of
the first certificate found at "cert-url".
"ed25519key" Binary content (Section 3.9 of
[I-D.ietf-httpbis-header-structure]) holding an Ed25519 public key
([RFC8032]).
"validity-url" A string (Section 3.7 of
[I-D.ietf-httpbis-header-structure]) containing an absolute-URL
string [5] ([URL]).
"date" and "expires" An integer (Section 3.5 of
[I-D.ietf-httpbis-header-structure]) representing a Unix time.
The "cert-url" parameter is _not_ signed, so intermediates can update
it with a pointer to a cached version.
3.1.1. Examples
The following header is included in the response for an exchange with
effective request URI "https://example.com/resource.html". Newlines
are added for readability.
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Signature:
sig1;
sig=*MEUCIQDXlI2gN3RNBlgFiuRNFpZXcDIaUpX6HIEwcZEc0cZYLAIga9DsVOMM+g5YpwEBdGW3sS+bvnmAJJiSMwhuBdqp5UY=*;
integrity="mi";
validity-url="https://example.com/resource.validity.1511128380";
cert-url="https://example.com/oldcerts";
cert-sha256=*W7uB969dFW3Mb5ZefPS9Tq5ZbH5iSmOILpjv2qEArmI=*;
date=1511128380; expires=1511733180,
sig2;
sig=*MEQCIGjZRqTRf9iKNkGFyzRMTFgwf/BrY2ZNIP/dykhUV0aYAiBTXg+8wujoT4n/W+cNgb7pGqQvIUGYZ8u8HZJ5YH26Qg=*;
integrity="mi";
validity-url="https://example.com/resource.validity.1511128380";
cert-url="https://example.com/newcerts";
cert-sha256=*J/lEm9kNRODdCmINbvitpvdYKNQ+YgBj99DlYp4fEXw=*;
date=1511128380; expires=1511733180,
srisig;
sig=*lGZVaJJM5f2oGczFlLmBdKTDL+QADza4BgeO494ggACYJOvrof6uh5OJCcwKrk7DK+LBch0jssDYPp5CLc1SDA=*
integrity="mi";
validity-url="https://example.com/resource.validity.1511128380";
ed25519key=*zsSevyFsxyZHiUluVBDd4eypdRLTqyWRVOJuuKUz+A8=*
date=1511128380; expires=1511733180,
thirdpartysig;
sig=*MEYCIQCNxJzn6Rh2fNxsobktir8TkiaJYQFhWTuWI1i4PewQaQIhAMs2TVjc4rTshDtXbgQEOwgj2mRXALhfXPztXgPupii+=*;
integrity="mi";
validity-url="https://thirdparty.example.com/resource.validity.1511161860";
cert-url="https://thirdparty.example.com/certs";
cert-sha256=*UeOwUPkvxlGRTyvHcsMUN0A2oNsZbU8EUvg8A9ZAnNc=*;
date=1511133060; expires=1511478660,
There are 4 signatures: 2 from different secp256r1 certificates
within "https://example.com/", one using a raw ed25519 public key
that's also controlled by "example.com", and a fourth using a
secp256r1 certificate owned by "thirdparty.example.com".
All 4 signatures rely on the "MI" response header to guard the
integrity of the response payload.
The signatures include a "validity-url" that includes the first time
the resource was seen. This allows multiple versions of a resource
at the same URL to be updated with new signatures, which allows
clients to avoid transferring extra data while the old versions don't
have known security bugs.
The certificates at "https://example.com/oldcerts" and
"https://example.com/newcerts" have "subjectAltName"s of
"example.com", meaning that if they and their signatures validate,
the exchange can be trusted as having an origin of
"https://example.com/". The publisher might be using two
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certificates because their readers have disjoint sets of roots in
their trust stores.
The publisher signed with all three certificates at the same time, so
they share a validity range: 7 days starting at 2017-11-19 21:53 UTC.
The publisher then requested an additional signature from
"thirdparty.example.com", which did some validation or processing and
then signed the resource at 2017-11-19 23:11 UTC.
"thirdparty.example.com" only grants 4-day signatures, so clients
will need to re-validate more often.
3.1.2. Open Questions
[I-D.ietf-httpbis-header-structure] provides a way to parameterise
identifiers but not other supported types like binary content. If
the "Signature" header field is notionally a list of parameterised
signatures, maybe we should add a "parameterised binary content"
type.
Should the cert-url and validity-url be lists so that intermediates
can offer a cache without losing the original URLs? Putting lists in
dictionary fields is more complex than
[I-D.ietf-httpbis-header-structure] allows, so they're single items
for now.
3.2. CBOR representation of exchange headers
To sign an exchange's headers, they need to be serialized into a byte
string. Since intermediaries and distributors (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 an exchange "exchange"'s headers is the
CBOR ([RFC7049]) array with the following content:
1. The map mapping:
* The byte string ':method' to the byte string containing
"exchange"'s request's method.
* The byte string ':url' to the byte string containing
"exchange"'s request's effective request URI, which MUST be an
absolute-URL string [6] ([URL]).
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* For each request header field in "exchange" except for the
"Host" header field, the header field's lowercase name as a
byte string to the header field's value as a byte string.
Note: "Host" is excluded because it is already part of the
effective request URI.
2. The map mapping:
* the byte string ':status' to the byte string containing
"exchange"'s response's 3-digit status code, and
* for each response header field in "exchange", the header
field's lowercase name as a byte string to the header field's
value as a byte string.
3.2.1. Example
Given the HTTP exchange:
GET / HTTP/1.1
Host: example.com
Accept: */*
HTTP/1.1 200
Content-Type: text/html
MI: mi-sha256=20addcf7368837f616d549f035bf6784ea6d4bf4817a3736cd2fc7a763897fe3
Signed-Headers: "content-type", "mi"
...
The cbor representation consists of the following item, represented
using the extended diagnostic notation from [I-D.ietf-cbor-cddl]
appendix G:
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[
{
':url': 'https://example.com/',
'accept': '*/*',
':method': 'GET',
},
{
'mi': 'mi-sha256=20addcf7368837f616d549f035bf6784ea6d4bf4817a3736cd2fc7a763897fe3',
':status': '200',
'content-type': 'text/html'
}
]
3.3. Loading a certificate chain
The resource at a signature's "cert-url" MUST have the "application/
cert-chain+cbor" content type, MUST be canonically-encoded CBOR
(Section 3.4), and MUST match the following CDDL:
cert-chain = [
"📜⛓", ; U+1F4DC U+26D3
+ {
cert: bytes,
? ocsp: bytes,
? sct: bytes,
* tstr => any,
}
]
The first item in the CBOR array is treated as the end-entity
certificate, and the client will attempt to build a path ([RFC5280])
to it from a trusted root using the other certificates in the chain.
1. Each "cert" value MUST be a DER-encoded X.509v3 certificate
([RFC5280]). Other key/value pairs in the same array item define
properties of this certificate.
2. The first certificate's "ocsp" value if any MUST be a complete,
DER-encoded OCSP response for that certificate (using the ASN.1
type "OCSPResponse" defined in [RFC6960]). Subsequent
certificates MUST NOT have an "ocsp" value.
3. Each certificate's "sct" value MUST be a
"SignedCertificateTimestampList" for that certificate as defined
by Section 3.3 of [RFC6962].
Loading a "cert-url" takes a "forceFetch" flag. The client MUST:
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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:
o Integers and the lengths of arrays, maps, and strings MUST use the
smallest possible encoding.
o Items MUST NOT be encoded with indefinite length.
o The keys in every map MUST be sorted in the bytewise lexicographic
order of their canonical encodings. For example, the following
keys are correctly sorted:
1. 10, encoded as 0A.
2. 100, encoded as 18 64.
3. -1, encoded as 20.
4. "z", encoded as 61 7A.
5. "aa", encoded as 62 61 61.
6. [100], encoded as 81 18 64.
7. [-1], encoded as 81 20.
8. false, encoded as F4.
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Note: this specification does not use floating point, tags, or other
more complex data types, so it doesn't need rules to canonicalize
those.
3.5. Signature validity
The client MUST parse the "Signature" header field as the
parameterised list (Section 4.2.3 of
[I-D.ietf-httpbis-header-structure]) described in Section 3.1. If an
error is thrown during this parsing or any of the requirements
described there aren't satisfied, the exchange has no valid
signatures. Otherwise, each member of this list represents a
signature with parameters.
The client MUST use the following algorithm to determine whether each
signature with parameters is invalid or potentially-valid for an
"exchange". Potentially-valid results include:
o The signed headers of the exchange so that higher-level protocols
can avoid relying on unsigned headers, and
o Either a certificate chain or a public key so that a higher-level
protocol can determine whether it's actually valid.
This algorithm accepts a "forceFetch" flag that avoids the cache when
fetching URLs. A client that determines that a potentially-valid
certificate chain is actually invalid due to an expired OCSP response
MAY retry with "forceFetch" set to retrieve an updated OCSP from the
original server.
1. Let "payload" be the payload body (Section 3.3 of [RFC7230]) of
"exchange". Note that the payload body is the message body with
any transfer encodings removed.
2. Let:
* "signature" be the signature (binary content in the
parameterised identifier's "sig" parameter).
* "integrity" be the signature's "integrity" parameter.
* "validity-url" be the signature's "validity-url" parameter.
* "cert-url" be the signature's "cert-url" parameter, if any.
* "cert-sha256" be the signature's "cert-sha256" parameter, if
any.
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* "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.
3. If "integrity" names a header field that is not present in
"exchange"'s response headers or which the client cannot use to
check the integrity of "payload" (for example, the header field
is new and hasn't been implemented yet), then return "invalid".
If the selected header field provides integrity guarantees weaker
than SHA-256, return "invalid". If validating integrity using
the selected header field requires the client to process records
larger than TBD bytes, return "invalid". Clients MUST implement
at least the "MI" ([I-D.thomson-http-mice]) header field with its
"mi-sha256" content encoding.
4. Set "publicKey" and "signing-alg" depending on which key fields
are present:
1. If "cert-url" is present:
1. Let "certificate-chain" be the result of loading the
certificate chain at "cert-url" passing the "forceFetch"
flag (Section 3.3). If this returns "invalid", return
"invalid".
2. Let "main-certificate" be the first certificate in
"certificate-chain".
3. Set "publicKey" to "main-certificate"'s public key.
4. If "publicKey" is an RSA key, return "invalid".
5. If "publicKey" is a key using the secp256r1 elliptic
curve, set "signing-alg" to ecdsa_secp256r1_sha256 as
defined in Section 4.2.3 of [I-D.ietf-tls-tls13].
6. Otherwise, either return "invalid" or set "signing-alg"
to a non-legacy signing algorithm defined by TLS 1.3 or
later ([I-D.ietf-tls-tls13]). This choice MUST depend
only on "publicKey"'s type and not on any other context.
2. If "ed25519key" is present, set "publicKey" to "ed25519key"
and "signing-alg" to ed25519, as defined by [RFC8032]
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5. If "expires" is more than 7 days (604800 seconds) after "date",
return "invalid".
6. If the current time is before "date" or after "expires", return
"invalid".
7. Let "message" be the concatenation of the following byte strings.
This matches the [I-D.ietf-tls-tls13] format to avoid cross-
protocol attacks if anyone uses the same key in a TLS certificate
and an exchange-signing certificate.
1. A string that consists of octet 32 (0x20) repeated 64 times.
2. A context string: the ASCII encoding of "HTTP Exchange 1".
Note: RFC EDITOR PLEASE DELETE THIS NOTE; The implementation
of the final RFC MUST use this context string, but
implementations of drafts MUST NOT use it and MUST use
another draft-specific string beginning with "HTTP Exchange 1
" instead. This ensures that signers can predict how their
signatures will be used.
3. A single 0 byte which serves as a separator.
4. The bytes of the canonical CBOR serialization (Section 3.4)
of a CBOR map mapping:
1. If "cert-sha256" is set:
1. The text string "cert-sha256" to the byte string
value of "cert-sha256".
2. The text string "validity-url" to the byte string value
of "validity-url".
3. The text string "date" to the integer value of "date".
4. The text string "expires" to the integer value of
"expires".
5. The text string "headers" to the CBOR representation
(Section 3.2) of "exchange"'s headers.
8. If "cert-url" is present and the SHA-256 hash of "main-
certificate"'s "cert_data" is not equal to "cert-sha256" (whose
presence was checked when the "Signature" header field was
parsed), return "invalid".
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Note that this intentionally differs from TLS 1.3, which signs
the entire certificate chain in its Certificate Verify
(Section 4.4.3 of [I-D.ietf-tls-tls13]), in order to allow
updating the stapled OCSP response without updating signatures at
the same time.
9. If "signature" is a valid signature of "message" by "publicKey"
using "signing-alg", return "potentially-valid" with whichever is
present of "certificate-chain" or "ed25519key". Otherwise,
return "invalid".
Note that the above algorithm can determine that an exchange's
headers are potentially-valid before the exchange's payload is
received. Similarly, if "integrity" identifies a header field like
"MI" ([I-D.thomson-http-mice]) that can incrementally validate the
payload, early parts of the payload can be determined to be
potentially-valid before later parts of the payload. Higher-level
protocols MAY process parts of the exchange that have been determined
to be potentially-valid as soon as that determination is made but
MUST NOT process parts of the exchange that are not yet potentially-
valid. Similarly, as the higher-level protocol determines that parts
of the exchange are actually valid, the client MAY process those
parts of the exchange and MUST wait to process other parts of the
exchange until they too are determined to be valid.
3.5.1. Open Questions
Should the signed message use the TLS format (with an initial 64
spaces) even though these certificates can't be used in TLS servers?
3.6. Updating signature validity
Both OCSP responses and signatures are designed to expire a short
time after they're signed, so that revoked certificates and signed
exchanges with known vulnerabilities are distrusted promptly.
This specification provides no way to update OCSP responses by
themselves. Instead, clients need to re-fetch the "cert-url"
(Section 3.5, Paragraph 4) to get a chain including a newer OCSP
response.
The "validity-url" parameter (Paragraph 6) 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 ([I-D.ietf-cbor-cddl]):
validity = {
? signatures: [ + bytes ]
? update: {
? size: uint,
}
]
The elements of the "signatures" array are parameterised identifiers
(Section 4.2.4 of [I-D.ietf-httpbis-header-structure]) meant to
replace the signatures within the "Signature" header field pointing
to this validity data. If the signed exchange contains a bug severe
enough that clients need to stop using the content, the "signatures"
array MUST NOT be present.
If the the "update" map is present, that indicates that a new version
of the signed exchange is available at its effective request URI
(Section 5.5 of [RFC7230]) and can give an estimate of the size of
the updated exchange ("update.size"). If the signed exchange is
currently the most recent version, the "update" SHOULD NOT be
present.
If both the "signatures" and "update" fields are present, clients can
use the estimated size to decide whether to update the whole resource
or just its signatures.
3.6.1. Examples
For example, say a signed exchange whose URL is "https://example.com/
resource" has the following "Signature" header field (with line
breaks included and irrelevant fields omitted for ease of reading).
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Signature:
sig1;
sig=*MEUCIQ...*;
...
validity-url="https://example.com/resource.validity.1511157180";
cert-url="https://example.com/oldcerts";
date=1511128380; expires=1511733180,
sig2;
sig=*MEQCIG...*;
...
validity-url="https://example.com/resource.validity.1511157180";
cert-url="https://example.com/newcerts";
date=1511128380; expires=1511733180,
thirdpartysig;
sig=*MEYCIQ...*;
...
validity-url="https://thirdparty.example.com/resource.validity.1511161860";
cert-url="https://thirdparty.example.com/certs";
date=1511478660; expires=1511824260
At 2017-11-27 11:02 UTC, "sig1" and "sig2" have expired, but
"thirdpartysig" doesn't exipire until 23:11 that night, so the client
needs to fetch "https://example.com/resource.validity.1511157180"
(the "validity-url" of "sig1" and "sig2") to update those signatures.
This URL might contain:
{
"signatures": [
'sig1; '
'sig=*MEQCIC/I9Q+7BZFP6cSDsWx43pBAL0ujTbON/+7RwKVk+ba5AiB3FSFLZqpzmDJ0NumNwN04pqgJZE99fcK86UjkPbj4jw==*; '
'validity-url="https://example.com/resource.validity.1511157180"; '
'integrity="mi"; '
'cert-url="https://example.com/newcerts"; '
'cert-sha256=*J/lEm9kNRODdCmINbvitpvdYKNQ+YgBj99DlYp4fEXw=*; '
'date=1511733180; expires=1512337980'
],
"update": {
"size": 5557452
}
}
This indicates that the client could fetch a newer version at
"https://example.com/resource" (the original URL of the exchange), or
that the validity period of the old version can be extended by
replacing the first two of the original signatures (the ones with a
validity-url of "https://example.com/resource.validity.1511157180")
with the single new signature provided. (This might happen at the
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end of a migration to a new root certificate.) The signatures of the
updated signed exchange would be:
Signature:
sig1;
sig=*MEQCIC...*;
...
validity-url="https://example.com/resource.validity.1511157180";
cert-url="https://example.com/newcerts";
date=1511733180; expires=1512337980,
thirdpartysig;
sig=*MEYCIQ...*;
...
validity-url="https://thirdparty.example.com/resource.validity.1511161860";
cert-url="https://thirdparty.example.com/certs";
date=1511478660; expires=1511824260
"https://example.com/resource.validity.1511157180" could also expand
the set of signatures if its "signatures" array contained more than 2
elements.
3.7. The Accept-Signature header
"Signature" header fields cost on the order of 300 bytes for ECDSA
signatures, so servers might prefer to avoid sending them to clients
that don't intend to use them. A client can send the "Accept-
Signature" header field to indicate that it does intend to take
advantage of any available signatures and to indicate what kinds of
signatures it supports.
When a server receives an "Accept-Signature" header field in a client
request, it SHOULD reply with any available "Signature" header fields
for its response that the "Accept-Signature" header field indicates
the client supports. However, if the "Accept-Signature" value
violates a requirement in this section, the server MUST behave as if
it hadn't received any "Accept-Signature" header at all.
The "Accept-Signature" header field is a Structured Header as defined
by [I-D.ietf-httpbis-header-structure]. Its value MUST be a
parameterised list (Section 3.3 of
[I-D.ietf-httpbis-header-structure]). Its ABNF is:
Accept-Signature = sh-param-list
The order of identifiers in the "Accept-Signature" list is not
significant. Identifiers, ignoring any initial "-" character, MUST
NOT be duplicated.
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Each identifier in the "Accept-Signature" header field's value
indicates that a feature of the "Signature" header field
(Section 3.1) is supported. If the identifier begins with a "-"
character, it instead indicates that the feature named by the rest of
the identifier is not supported. Unknown identifiers and parameters
MUST be ignored because new identifiers and new parameters on
existing identifiers may be defined by future specifications.
3.7.1. Integrity identifiers
Identifiers starting with "mi/" indicate that the client supports the
"MI" header field ([I-D.thomson-http-mice]) with the parameter from
the HTTP MI Parameter Registry registry named in lower-case by the
rest of the identifier. For example, "mi/mi-blake2" indicates
support for Merkle integrity with the as-yet-unspecified mi-blake2
parameter, and "-digest/mi-sha256" indicates non-support for Merkle
integrity with the mi-sha256 content encoding.
If the "Accept-Signature" header field is present, servers SHOULD
assume support for "mi/mi-sha256" unless the header field states
otherwise.
3.7.2. Key type identifiers
Identifiers starting with "ecdsa/" indicate that the client supports
certificates holding ECDSA public keys on the curve named in lower-
case by the rest of the identifier.
If the "Accept-Signature" header field is present, servers SHOULD
assume support for "ecdsa/secp256r1" unless the header field states
otherwise.
3.7.3. Key value identifiers
The "ed25519key" identifier has parameters indicating the public keys
that will be used to validate the returned signature. Each
parameter's name is re-interpreted as binary content (Section 3.9 of
[I-D.ietf-httpbis-header-structure]) encoding a prefix of the public
key. For example, if the client will validate signatures using the
public key whose base64 encoding is
"11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo=", valid "Accept-
Signature" header fields include:
Accept-Signature: ..., ed25519key; *11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo=*
Accept-Signature: ..., ed25519key; *11qYAYKxCrfVS/7TyWQHOg==*
Accept-Signature: ..., ed25519key; *11qYAQ==*
Accept-Signature: ..., ed25519key; **
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but not
Accept-Signature: ..., ed25519key; *11qYA===*
because 5 bytes isn't a valid length for encoded base64, and not
Accept-Signature: ..., ed25519key; 11qYAQ
because it doesn't start or end with the "*"s that indicate binary
content.
Note that "ed25519key; **" is an empty prefix, which matches all
public keys, so it's useful in subresource integrity (Appendix A.3)
cases like "" where the public
key isn't known until the matching "