Origin-signed HTTP Responses
draft-yasskin-http-origin-signed-responses-01
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| Author | Jeffrey Yasskin | ||
| Last updated | 2017-12-05 | ||
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draft-yasskin-http-origin-signed-responses-01
http J. Yasskin
Internet-Draft Google
Intended status: Standards Track December 05, 2017
Expires: June 8, 2018
Origin-signed HTTP Responses
draft-yasskin-http-origin-signed-responses-01
Abstract
This document explores how a server can send particular responses
that are authoritative for an origin, when the server itself is not
authoritative for that origin. For now, the appendices containing
use cases and requirements should be treated as more confident than
the proposal itself.
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 June 8, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Straw proposal . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. The Signed-Headers Header . . . . . . . . . . . . . . . . 4
3.2. The Signature Header . . . . . . . . . . . . . . . . . . 5
3.2.1. Open Questions . . . . . . . . . . . . . . . . . . . 6
3.3. Significant parts of an exchange . . . . . . . . . . . . 6
3.3.1. Open Questions . . . . . . . . . . . . . . . . . . . 6
3.4. CBOR representation of an exchange . . . . . . . . . . . 7
3.4.1. Example . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Canonical CBOR serialization . . . . . . . . . . . . . . 8
3.6. Signature validity . . . . . . . . . . . . . . . . . . . 9
3.6.1. Validating a certificate chain for an authority . . . 12
3.6.2. Open Questions . . . . . . . . . . . . . . . . . . . 13
3.7. Updating signature validity . . . . . . . . . . . . . . . 13
3.7.1. Examples . . . . . . . . . . . . . . . . . . . . . . 14
4. Security considerations . . . . . . . . . . . . . . . . . . . 15
4.1. Aspects of the straw proposal . . . . . . . . . . . . . . 16
5. Privacy considerations . . . . . . . . . . . . . . . . . . . 16
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 17
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Normative References . . . . . . . . . . . . . . . . . . 17
7.2. Informative References . . . . . . . . . . . . . . . . . 19
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix A. Use cases . . . . . . . . . . . . . . . . . . . . . 20
A.1. PUSHed subresources . . . . . . . . . . . . . . . . . . . 20
A.2. Explicit use of a content distributor for subresources . 21
A.3. Subresource Integrity . . . . . . . . . . . . . . . . . . 21
A.4. Offline websites . . . . . . . . . . . . . . . . . . . . 22
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 22
B.1. Proof of origin . . . . . . . . . . . . . . . . . . . . . 22
B.1.1. Certificate constraints . . . . . . . . . . . . . . . 22
B.1.2. Signature constraints . . . . . . . . . . . . . . . . 23
B.1.3. Retrieving the certificate . . . . . . . . . . . . . 23
B.2. How much to sign . . . . . . . . . . . . . . . . . . . . 24
B.2.1. Conveying the signed headers . . . . . . . . . . . . 24
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B.3. Response lifespan . . . . . . . . . . . . . . . . . . . . 25
B.3.1. Certificate revocation . . . . . . . . . . . . . . . 25
B.3.2. Response downgrade attacks . . . . . . . . . . . . . 26
Appendix C. Determining validity using cache control . . . . . . 26
C.1. Example of updating cache control . . . . . . . . . . . . 27
C.2. Downsides of updating cache control . . . . . . . . . . . 28
Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 28
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
When I presented Web Packaging to DISPATCH [3], folks thought it
would make sense to split it into a way to sign individual HTTP
responses as coming from a particular origin, and separately a way to
bundle a collection of HTTP responses. This document explores the
constraints on any method of signing HTTP responses and sketches a
possible solution to the constraints.
2. Terminology
Author The entity that controls the server for a particular origin
[RFC6454]. The author 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 [RFC7540] section 8.
Intermediate An entity that fetches signed HTTP exchanges from an
author 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 author vouched for it as coming from its
claimed origin.
Unix time Defined by [POSIX] section 4.16 [4].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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3. Straw proposal
As a response to an HTTP request or as a Server Push ([RFC7540],
section 8.2) the server MAY include a "Signed-Headers" header field
(Section 3.1) identifying significant (Section 3.3) header fields and
a "Signature" header field (Section 3.2) holding a list of one or
more parameterised signatures that vouch for the content of the
response.
The client categorizes each signature as "valid" or "invalid" by
validating that signature with its certificate or public key and
other metadata against the significant headers and content
(Section 3.6). 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 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 3.6
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 significant 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.
See Appendix B.2 for a discussion of why only the URL from the
request is included and not other request headers.
"Signed-Headers" is a Structured Header as defined by
[I-D.ietf-httpbis-header-structure]. Its value MUST be a list
([I-D.ietf-httpbis-header-structure], section 4.8) of lowercase
strings ([I-D.ietf-httpbis-header-structure], section 4.2) naming
HTTP response header fields. Pseudo-header field names ([RFC7540],
section 8.1.2.1) MUST not appear in this list.
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Higher-level protocols SHOULD place requirements on the minimum set
of headers to include in the "Signed-Headers" header field.
3.2. 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.
The "Signature" header is a Structured Header as defined by
[I-D.ietf-httpbis-header-structure]. Its value MUST be a list
([I-D.ietf-httpbis-header-structure], section 4.8) of parameterised
labels ([I-D.ietf-httpbis-header-structure], section 4.4).
Each parameterised label MUST have parameters named "sig",
"validityUrl", "date", and "expires", and either "certUrl" and
"certSha256" parameters or an "ed25519Key" parameter. This
specification gives no meaning to the label itself, which can be used
as a human-readable identifier for the signature (see
Section 3.2.1, Paragraph 1). The present parameters MUST have the
following values:
"sig" Binary content ([I-D.ietf-httpbis-header-structure], section
4.5) holding the signature of most of these parameters and the
significant parts of the exchange (Section 3.3).
"certUrl" A string ([I-D.ietf-httpbis-header-structure], section
4.2) containing a valid URL string [5].
"certSha256" Binary content ([I-D.ietf-httpbis-header-structure],
section 4.5) holding the SHA-256 hash of the first certificate
found at "certUrl".
"ed25519Key" Binary content ([I-D.ietf-httpbis-header-structure],
section 4.5) holding an Ed25519 public key ([RFC8032]).
"validityUrl" A string ([I-D.ietf-httpbis-header-structure], section
4.2) containing a valid URL string [6].
"date" and "expires" An unsigned integer
([I-D.ietf-httpbis-header-structure], section 4.1) representing a
Unix time.
The "certUrl" and "validityUrl" parameters are _not_ signed, so
intermediates can update them with pointers to cached versions.
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3.2.1. Open Questions
[I-D.ietf-httpbis-header-structure] provides a way to parameterise
labels but not other supported types like binary content. If the
"Signature" header field is notionally a list of parameterised
signatures, maybe we should add a "parameterised binary content"
type.
Should the certUrl and validityUrl be lists so that intermediates can
offer a cache without losing the original URLs? Putting lists in
dictionary fields is more complex than
[I-D.ietf-httpbis-header-structure] allows, so they're single items
for now.
Should "validityUrl" be signed or optionally signed so that an
exchange's author can prevent an intermediate from removing it, which
would prevent clients from sharing the exchange among themselves
without going back to the intermeidate?
3.3. Significant parts of an exchange
The significant parts of an exchange are:
o The method ([RFC7231], section 4) and effective request URI
([RFC7230], section 5.5) of the request.
o The response status code ([RFC7231], section 6) and the response
header fields whose names are listed in that exchange's "Signed-
Headers" header field (Section 3.1), in the order they appear in
that header field. If a response header field name from "Signed-
Headers" does not appear in the exchange's response header fields,
the exchange has no significant parts.
o The exchange's payload body ([RFC7230], section 3.3). Note that
the payload body is the message body with any transfer encodings
removed.
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 3.1, the exchange has
no significant parts.
3.3.1. Open Questions
Do the significant parts of an exchange need to include the "Signed-
Headers" header field itself?
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3.4. CBOR representation of an exchange
To sign an exchange, it needs to be serialized into a byte string.
Since intermediaries and distributors (Appendix A.2) might rearrange,
add, or just reserialize headers, and this can change the HPACK
encoding, we can't use the literal bytes of the header frames as this
serialization. Instead, this section defines a CBOR representation
that can be embedded into other CBOR, canonically serialized
(Section 3.5), and then signed.
The CBOR representation of an exchange is the result of the following
algorithm:
1. Let "exchange" be the exchange. This is expected to be the
significant parts (Section 3.3) of some other exchange.
2. Return a CBOR ([RFC7049]) array with the following content:
1. The text string "request".
2. The array consisting of the following items:
1. The byte string ':method'.
2. The byte string containing the request's method.
3. The byte string ':url'.
4. The byte string containing the request's effective
request URI.
3. The text string "response".
4. The array consisting of the initial two items
1. The byte string ':status'.
2. The byte string containing the response's 3-digit status
code.
Followed by the appended items from, for each response header
field in "exchange", in order:
1. Append the header field's name as a byte string.
2. Append the header field's value as a byte string.
5. The text string "payload".
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6. The byte string containing the response's payload body
([RFC7230], section 3.3). Note that the payload body is the
message body with any transfer encodings removed.
3.4.1. Example
Given the HTTP exchange:
GET https://example.com/ HTTP/1.1
accept = */*
HTTP/1.1 200
content-type = text/html
signed-headers = "content-type"
<!doctype html>
<html>
...
The cbor representation consists of the following item, represented
using the extended diagnostic notation from [I-D.ietf-cbor-cddl]
appendix G:
[
"request",
[
':method', 'GET',
':url', 'https://example.com/'
],
"response",
[
':status', '200',
'content-type', 'text/html'
],
"payload",
'<!doctype html>\n<html>...'
]
3.5. Canonical CBOR serialization
Within this specification, the canonical serialization of a CBOR item
uses the following rules derived from section 3.9 of [RFC7049]:
o Integers and the lengths of arrays and strings MUST use the
smallest possible encoding.
o Items MUST NOT be encoded with indefinite length.
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Note: this specification does not use CBOR maps, so the map ordering
rules aren't necessary. This specification also doesn't use floating
point, tags, or other more complex data types, so it doesn't need
rules to canonicalize those either.
3.6. Signature validity
The client MUST parse the "Signature" header field as the list of
parameterised values described in Section 3.2
([I-D.ietf-httpbis-header-structure], section 4.8.1). If an error is
thrown during this parsing, 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.
Potentially-valid results include:
o The signed parts 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 expired OCSP responses
MAY retry with "forceFetch" set to retrieve updated OCSPs from the
original server.
This algorithm also accepts an "allResponseHeaders" flag, which
insists that there are no non-significant response header fields in
the exchange.
1. Let "originalExchange" be the signature's exchange.
2. Let "exchange" be the significant parts (Section 3.3) of
"originalExchange". If "originalExchange" has no significant
parts, then return "invalid".
3. If "allResponseHeaders" is set and the response headers fields in
"originalExchange" are a proper superset of the response header
fields in "exchange", then return "invalid".
4. Let:
* "signature" be the signature (binary content in the
parameterised value's "sig" parameter).
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* "certUrl" be the signature's "certUrl" parameter, if any.
* "certSha256" be the signature's "certSha256" 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.
5. Set "publicKey" and "signing-alg" depending on which key fields
are present:
1. If "certUrl" is present:
1. Let "certificate-chain" be the result of fetching
([FETCH]) "certUrl" and parsing it as a TLS 1.3
Certificate message ([I-D.ietf-tls-tls13], section 4.4.2)
containing X.509v3 certificates. If "forceFetch" is
_not_ set, the fetch can be fulfilled from a cache using
normal HTTP semantics [RFC7234]. If this fetch or parse
fails, return "invalid".
2. Let "main-certificate" be the first certificate in
"certificate-chain".
3. If the SHA-256 hash of "main-certificate"'s "cert_data"
is not equal to "certSha256", return "invalid". See the
open questions (Section 3.6.2, Paragraph 1).
4. Set "publicKey" to "main-certificate"'s public key
5. The client MUST define a partial function from public key
types to signing algorithms, and this function must at
the minimum include the following mappings:
RSA, 2048 bits: rsa_pss_sha256 as defined in
Section 4.2.3 of [I-D.ietf-tls-tls13].
EC, with the secp256r1 curve: ecdsa_secp256r1_sha256 as
defined in Section 4.2.3 of [I-D.ietf-tls-tls13].
EC, with the secp384r1 curve: ecdsa_secp384r1_sha384 as
defined in Section 4.2.3 of [I-D.ietf-tls-tls13].
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Set "signing-alg" to the result of applying this function
to type of "main-certificate"'s public key. If the
function is undefined on this input, return "invalid".
2. If "ed25519Key" is present, set "publicKey" to "ed25519Key"
and "signing-alg" to ed25519, as defined by [RFC8032]
6. If "expires" is more than 7 days (604800 seconds) after "date",
return "invalid".
7. If the current time is before "date" or after "expires", return
"invalid".
8. Let "message" be the concatenation of the following byte strings.
This matches the [I-D.ietf-tls-tls13] format to avoid cross-
protocol attacks when TLS certificates are used to sign
manifests.
1. A string that consists of octet 32 (0x20) repeated 64 times.
2. A context string: the ASCII encoding of "HTTP Exchange".
3. A single 0 byte which serves as a separator.
4. The bytes of the canonical CBOR serialization (Section 3.5)
of a CBOR array consisting of:
1. The text string "certSha256".
2. The byte string "certSha256".
3. The text string "date".
4. The integer value of "date".
5. The text string "expires".
6. The integer value of "expires".
7. The text string "exchange".
8. The CBOR representation (Section 3.4) of "exchange". See
the open questions (Section 3.6.2, Paragraph 2).
9. If "signature" is "message"'s signature by "main-certificate"'s
public key using "signing-alg", return "potentially-valid" with
"exchange" and whichever is present of "certificate-chain" or
"ed25519Key". Otherwise, return "invalid".
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3.6.1. Validating a certificate chain for an authority
[RFC7540] section 8.2 includes the rule:
The server MUST include a value in the :authority pseudo-header
field for which the server is authoritative (see Section 10.1). A
client MUST treat a PUSH_PROMISE for which the server is not
authoritative as a stream error (Section 5.4.2) of type
PROTOCOL_ERROR.
If the Server Push contains a signed exchange for which the server is
not authoritative, instead of treating it as a stream error, the
client MAY search for a signature for which the following algorithm
returns "valid". If such a signature is found, the client MAY treat
the server as authoritative for this particular exchange and store
the exchange as described by [RFC7540]. If not, the client MUST
treat the exchange as a stream error as described by [RFC7540].
1. Run Section 3.6 over the signature with the "allResponseHeaders"
flag set, getting "exchange" and "certificate-chain" back. If
this returned "invalid" or didn't return a certificate chain,
return "invalid".
2. Let "authority" be the host component of "exchange"'s effective
request URI.
3. Validate the "certificate-chain" using the following substeps.
If any of them fail, re-run Section 3.6 once over the signature
with both the "forceFetch" flag and the "allResponseHeaders" 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 all certificates in "path" include
"status_request" extensions with valid OCSP responses.
([RFC6960])
3. Validate that all certificates in "path" include
"signed_certificate_timestamp" extensions containing valid
SCTs from trusted logs. ([RFC6962])
4. Return "valid".
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3.6.2. Open Questions
TLS 1.3 signs the entire certificate chain, but doing that here would
preclude updating the OCSP signatures without replacing all
signatures using that chain at the same time. What attack do I allow
by hashing only the end-entity certificate?
Including the entire exchange in the signed data forces a client to
download the whole thing before trusting any of it.
[I-D.thomson-http-mice] is designed to let us check the validity of
just the "MI" header up front and then incrementally check blocks of
the payload as they arrive. What's the best way to integrate that?
Maybe add a flag to the "Signature" header field or its signatures
saying that the payload is guarded by some other header field, so
isn't included in the significant parts (Section 3.3).
3.7. 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 "certUrl"
(Section 3.6, Paragraph 4) to get a chain including newer OCSPs.
The "validityUrl" parameter (Paragraph 5) of the signatures provides
a way to fetch new signatures or learn where to fetch a complete
updated package.
Each version of a signed exchange SHOULD have its own validity URLs,
since each version needs different signatures and becomes obsolete at
different times.
The resource at a "validityUrl" is "validity data", a CBOR map
matching the following CDDL ([I-D.ietf-cbor-cddl]):
validity = {
? signatures: [ + bytes ]
? update: {
url: text,
? size: uint,
}
]
The elements of the "signatures" array are header field values meant
to replace the signatures within the "Signature" header field
pointing to this validity data. If the signed exchange contains a
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bug severe enough that clients need to stop using the content, the
"signatures" array MUST NOT be present.
The "update" map gives a location to update the entire signed
exchange and an estimate of the size of the resource at that URL. 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.
3.7.1. Examples
For example, if a signed exchange has the following "Signature"
header field (written as multiple fields for convenience):
Signature: sig1;
sig=*MEUCIQDXlI2gN3RNBlgFiuRNFpZXcDIaUpX6HIEwcZEc0cZYLAIga9DsVOMM+g5YpwEBdGW3sS+bvnmAJJiSMwhuBdqp5UY;
validityUrl="https://example.com/resource.validity";
certUrl="https://example.com/certs";
certSha256=*W7uB969dFW3Mb5ZefPS9Tq5ZbH5iSmOILpjv2qEArmI;
date=1511128380; expires=1511560380
Signature: sig2;
sig=*MEQCIGjZRqTRf9iKNkGFyzRMTFgwf/BrY2ZNIP/dykhUV0aYAiBTXg+8wujoT4n/W+cNgb7pGqQvIUGYZ8u8HZJ5YH26Qg;
validityUrl="https://example.com/resource.validity";
certUrl="https://example.com/certs";
certSha256=*kQAA8u33cZRTy7RHMO4+dv57baZL48SYA2PqmYvPPbg;
date=1511301183; expires=1511905983
Signature: sig3;
sig=*MEYCIQCNxJzn6Rh2fNxsobktir8TkiaJYQFhWTuWI1i4PewQaQIhAMs2TVjc4rTshDtXbgQEOwgj2mRXALhfXPztXgPupii+;
validityUrl="https://thirdparty.example.com/resource.validity";
certUrl="https://thirdparty.example.com/certs";
certSha256=*UeOwUPkvxlGRTyvHcsMUN0A2oNsZbU8EUvg8A9ZAnNc;
date=1511301183; expires=1511905983
https://example.com/resource.validity might contain:
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{
"signatures": [
'sig4; '
'sig=*MEQCIC/I9Q+7BZFP6cSDsWx43pBAL0ujTbON/+7RwKVk+ba5AiB3FSFLZqpzmDJ0NumNwN04pqgJZE99fcK86UjkPbj4jw; '
'validityUrl="https://example.com/resource.validity"; '
'certUrl="https://example.com/certs"; '
'certSha256=*W7uB969dFW3Mb5ZefPS9Tq5ZbH5iSmOILpjv2qEArmI; '
'date=1511467200; expires=1511985600'
],
"update": {
"url": "https://example.com/resource",
"size": 5557452
}
}
This indicates that the first two of the original signatures (the
ones with a validityUrl of "https://example.com/resource.validity")
can be replaced with a single new signature. The signatures of the
updated signed exchange would be:
Signature: sig4;
sig=*MEQCIC/I9Q+7BZFP6cSDsWx43pBAL0ujTbON/+7RwKVk+ba5AiB3FSFLZqpzmDJ0NumNwN04pqgJZE99fcK86UjkPbj4jw;
validityUrl="https://example.com/resource.validity";
certUrl="https://example.com/certs";
certSha256=*W7uB969dFW3Mb5ZefPS9Tq5ZbH5iSmOILpjv2qEArmI;
date=1511467200; expires=1511985600
Signature: sig3;
sig=*MEYCIQCNxJzn6Rh2fNxsobktir8TkiaJYQFhWTuWI1i4PewQaQIhAMs2TVjc4rTshDtXbgQEOwgj2mRXALhfXPztXgPupii+;
validityUrl="https://thirdparty.example.com/resource.validity";
certUrl="https://thirdparty.example.com/certs";
certSha256=*UeOwUPkvxlGRTyvHcsMUN0A2oNsZbU8EUvg8A9ZAnNc;
date=1511301183; expires=1511905983
https://example.com/resource.validity could also expand the set of
signatures if its "signatures" array contained more than 2 elements.
4. Security considerations
Authors MUST NOT include confidential information in a signed
response that an untrusted intermediate could forward, since the
response is only signed and not encrypted. Intermediates can read
the content.
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, they need only convince the user to
visit another Web site in order to serve responses signed as the
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target. This consideration and mitigations for it are shared by
[I-D.ietf-httpbis-origin-frame].
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. Authors
should consider shorter signature expiration times than they use for
cache expiration times.
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 SHOULD be revoked so that, even if the attacker used them
to sign future-dated package validity assertions, the key's OCSP
assertions will expire, causing the package as a whole to become
untrusted.
4.1. Aspects of the straw proposal
The use of a single "Signed-Headers" header field prevents us from
signing aspects of the request other than its effective request URI
([RFC7230], section 5.5). For example, if an author 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"?
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 3.6 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.4). Section 3.6.1 requires all headers to be included in
the signature before trusting cross-origin pushed resources, at Ryan
Sleevi's recommendation.
5. Privacy considerations
Normally, when a client fetches "https://o1.com/resource.js",
"o1.com" learns that the client is interested in the resource. If
"o1.com" signs "resource.js", "o2.com" serves it as "https://o2.com/
o1resource.js", and the client fetches it from there, then "o2.com"
learns that the client is interested, and if the client executes the
Javascript, that could also report the client's interest back to
"o1.com".
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Often, "o2.com" already knew about the client's interest, because
it's the entity that directed the client to "o1resource.js", but
there may be cases where this leaks extra information.
For non-executable resource types, a signed response can improve the
privacy situation by hiding the client's interest from the original
author.
6. IANA considerations
TODO: possibly register the validityUrl format.
7. References
7.1. Normative References
[FETCH] WHATWG, "Fetch", December 2017,
<https://fetch.spec.whatwg.org/>.
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-ietf-cbor-cddl-00
(work in progress), July 2017.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-02 (work in progress),
November 2017.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-22 (work in progress),
November 2017.
[POSIX] IEEE and The Open Group, "The Open Group Base
Specifications Issue 7", name IEEE, value 1003.1-2008,
2016 Edition, 2016,
<http://pubs.opengroup.org/onlinepubs/9699919799/
basedefs/>.
[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>.
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[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[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>.
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7.2. Informative References
[I-D.burke-content-signature]
Burke, B., "HTTP Header for digital signatures", draft-
burke-content-signature-00 (work in progress), March 2011.
[I-D.cavage-http-signatures]
Cavage, M. and M. Sporny, "Signing HTTP Messages", draft-
cavage-http-signatures-09 (work in progress), November
2017.
[I-D.ietf-httpbis-origin-frame]
Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
draft-ietf-httpbis-origin-frame-04 (work in progress),
August 2017.
[I-D.thomson-http-content-signature]
Thomson, M., "Content-Signature Header Field for HTTP",
draft-thomson-http-content-signature-00 (work in
progress), July 2015.
[I-D.thomson-http-mice]
Thomson, M., "Merkle Integrity Content Encoding", draft-
thomson-http-mice-02 (work in progress), October 2016.
[I-D.vkrasnov-h2-compression-dictionaries]
Krasnov, V., "Compression Dictionaries for HTTP/2", draft-
vkrasnov-h2-compression-dictionaries-02 (work in
progress), March 2017.
[I-D.yasskin-dispatch-web-packaging]
Yasskin, J., "Web Packaging", draft-yasskin-dispatch-web-
packaging-00 (work in progress), June 2017.
[RFC2437] Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography
Specifications Version 2.0", RFC 2437,
DOI 10.17487/RFC2437, October 1998,
<https://www.rfc-editor.org/info/rfc2437>.
[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>.
[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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[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/info/rfc7541>.
[SRI] Akhawe, D., Braun, F., Marier, F., and J. Weinberger,
"Subresource Integrity", World Wide Web Consortium
Recommendation REC-SRI-20160623, June 2016,
<http://www.w3.org/TR/2016/REC-SRI-20160623>.
7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] https://github.com/WICG/webpackage
[3] https://datatracker.ietf.org/doc/minutes-99-dispatch/
[4] http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
V1_chap04.html#tag_04_16
[5] https://url.spec.whatwg.org/#valid-url-string
[6] https://url.spec.whatwg.org/#valid-url-string
[7] https://github.com/mikewest/signature-based-sri
[8] https://github.com/mikewest/signature-based-sri/issues/5
[9] https://github.com/WICG/webpackage
[10] https://tools.ietf.org/html/rfc7540#section-8.2
[11] https://tools.ietf.org/html/rfc7540#section-4.2
[12] https://www.imperialviolet.org/2012/02/05/crlsets.html
[13] https://tlswg.github.io/tls13-spec/draft-ietf-tls-
tls13.html#ocsp-and-sct
Appendix A. Use cases
A.1. PUSHed subresources
To reduce round trips, a server might use HTTP/2 PUSH 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.
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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 ([RFC7540],
section 8.2), 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>
where the "<dist-img>" implementation generates an appropriate
"<img>" based on, for example, a "<meta name="dist-base">" tag
elsewhere in the page.
This could be used for some of the same purposes as SRI
(Appendix A.3).
Note that the current proposal doesn't support this use case because
there's no way aside from a Server Push to override the physical
request URL.
A.3. Subresource Integrity
The W3C WebAppSec group is investigating using signatures [7] in
[SRI]. They need a way to transmit the signature with the response,
which this proposal could provide.
However, their needs also differ in some significant ways:
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1. The "integrity="ed25519-[public-key]"" attribute and CSP-based
ways of expressing a public key don't need the signing key to be
also trusted to sign arbitrary content for an origin.
2. Some uses of SRI want to constrain subresources to be vouched for
by a third-party, rather than just being signed by the
subresource's author.
While we can design this system to cover both origin-trusted and
simple-key signatures, we should check that this is better than
having two separate systems for the two kinds of signatures.
Note that while the current proposal for SRI describes signing only
the content of a resource, they may need to sign its name as well, to
prevent security vulnerabilities [8]. The details of what they need
to sign will affect whether and how they can use this proposal.
A.4. Offline websites
See https://github.com/WICG/webpackage [9] and
[I-D.yasskin-dispatch-web-packaging]. This use requires origin-
signed resources to be bundled.
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:
1. TLS server certificates must be accessible from online servers,
so they're easier to steal than an offline key. A package'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 a package, or
vice versa.
If these risks are too high, we could define a new Extended Key Usage
([RFC5280], section 4.2.1.12) that requires CAs to issue new keys for
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this purpose or a new certificate extension to do the same. A new
EKU would probably require CAs to also issue new intermediate
certificates because of how browsers trust EKUs. Both an EKU and a
new extension take a long time to deploy and allow CAs to charge
package-signers more than normal server operators, which will reduce
adoption.
The rest of this document will assume we can re-use existing TLS
server certificates.
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, signatures here 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 [RFC2437].
2. Use the same format as TLS's signatures, specified in
[I-D.ietf-tls-tls13] section 4.4.3, with a context string that's
specific to this use.
The specification also needs to define which signing algorithm to
use. I expect to define 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.4) with the signed content
or PUSHed (Appendix A.1) with it. The risks from the
"client_certificate_url" extension ([RFC6066] section 11.3) don't
seem to apply here, since an attacker who can get a client to load a
package and fetch the certificates it references, can also get the
client to perform those fetches by loading other HTML.
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To avoid using an unintended certificate with the same public key as
the intended one, the content of the certificate chain should be
included in the signed data, like TLS does ([I-D.ietf-tls-tls13],
section 4.4.3).
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 3.3 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.
The question of whether to include other request headers--primarily
the "accept*" family--is still open. These headers need to be
represented so that clients wanting a different language, say, can
avoid using the wrong-language response, but it's not obvious that
there's a security vulnerability if an attacker can spoof them. For
now, the proposal (Section 3) omits other request headers.
In order to allow multiple clients to consume the same signed
exchange, the exchange shouldn't include the exact request headers
that any particular client sends. For example, a Japanese resource
wouldn't include
accept-language: ja-JP, ja;q=0.9, en;q=0.8, zh;q=0.7, *;q=0.5
Instead, it would probably include just
accept-language: ja-JP, ja
and clients would use the same matching logic as for PUSH_PROMISE
[10] frame 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 author wrote. 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 author's signatures would tend to
break before getting to the client.
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There's also no way in current HTTP for the response to a client-
initiated request ([RFC7540], section 8.1) to convey the request
headers it expected to respond to. A PUSH_PROMISE ([RFC7540],
section 8.2) does not have this problem, and it would be possible to
introduce a response header to convey the expected request headers.
Since proxies don't modify unknown content types, we could wrap the
original exchange into an "application/http2" format. This could be
as simple as a series of HTTP/2 frames, or could
1. Allow longer contiguous bodies than HTTP/2's 16MB frame limit
[11], and
2. Use better compression than [RFC7541] for the non-confidential
headers. Note that header compression can probably share a
compression state across a single signed exchange, but needs a
mechanism like [I-D.vkrasnov-h2-compression-dictionaries] to use
any compression state from other responses.
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 [12],
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 [13] format.
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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.
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 author 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 authors' 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.
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 3.2) would no
longer contain "date" or "expires" fields.
The validity-checking algorithm (Section 3.6) would initialize "date"
from the resource's "Date" header field ([RFC7231], section 7.1.1.2)
and initialize "expires" from either the "Expires" header field
([RFC7234] section 5.3) or the "Cache-Control" header field's "max-
age" directive ([RFC7234] section 5.2.2.8) (added to "date"),
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whichever is present, preferring "max-age" (or failing) if both are
present.
Validity updates (Section 3.7) 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 https://example.com/ HTTP/1.1
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>
...
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 https://example.com/ HTTP/1.1
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>
...
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Internet-Draft Origin-signed HTTP Responses December 2017
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 "validityUrl"
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. Acknowledgements
Thanks to Ilari Liusvaara, Mark Nottingham, 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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