draft-yasskin-http-origin-signed-responses.md

---

coding: utf-8

title: Signed HTTP Exchanges docname: draft-yasskin-http-origin-signed-responses-latest category: std

ipr: trust200902

stand_alone: yes pi: [comments, sortrefs, strict, symrefs, toc]

author:

name: Jeffrey Yasskin
organization: Google
email: [email protected]

normative: CDDL: RFC8610 FETCH: target: https://fetch.spec.whatwg.org/ title: Fetch author: org: WHATWG date: Living Standard POSIX: target: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/ title: The Open Group Base Specifications Issue 7 author: - org: IEEE - org: The Open Group seriesinfo: name: IEEE value: 1003.1-2008, 2016 Edition date: 2016 URL: target: https://url.spec.whatwg.org/ title: URL author: org: WHATWG date: Living Standard

informative: SRI: W3C.REC-SRI-20160623 BRs: target: https://cabforum.org/baseline-requirements-documents/ title: Baseline Requirements for the Issuance and Management of Publicly-Trusted Certificates author: - org: CA/Browser Forum date: 2018-12-10 CRLSets: target: https://www.imperialviolet.org/2012/02/05/crlsets.html title: Revocation checking and Chrome's CRL author: - name: Adam Langley org: Chromium date: 2012-02-05 OneCrl: target: https://blog.mozilla.org/security/2015/03/03/revoking-intermediate-certificates-introducing-onecrl/ title: "Revoking Intermediate Certificates: Introducing OneCRL" author: - name: Mark Goodwin org: Mozilla date: 2015-03-03

--- 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.

--- middle

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 {{uc-pushed-subresources}} and {{uc-explicit-distributor}}.
• 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 {{uc-sri}}.
• 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 {{uc-transparency}} and {{uc-static-analysis}}.

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.

Terminology {#terminology}

Absolute URL : A string for which the 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 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 {{application-signed-exchange}}, 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.

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.

Signing an exchange # {#proposal}

In the response of an HTTP exchange the server MAY include a Signature header field ({{signature-header}}) 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 ({{transfer}}).

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 ({{signature-validity}}). This validity then informs higher-level protocols.

Each signature is parameterised with information to let a client fetch assurance that a signed exchange is still valid, in the face of revoked certificates and newly-discovered vulnerabilities. This assurance can be bundled back into the signed exchange and forwarded to another client, which won't have to re-fetch this validity information for some period of time.

The Signature Header ## {#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 {{parameterised-binary}}). 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 {{!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 ({{terminology}}) 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}}).

{:#signature-validityurl} "validity-url"

: A string (Section 3.8 of {{!I-D.ietf-httpbis-header-structure}}) containing an absolute URL ({{terminology}}) 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.

Examples ### {#example-signature-header}

The following header is included in the response for an exchange with effective request URI https://example.com/resource.html. Newlines are added for readability.

Signature:
 sig1;
  sig=*MEUCIQDXlI2gN3RNBlgFiuRNFpZXcDIaUpX6HIEwcZEc0cZYLAIga9DsVOMM+g5YpwEBdGW3sS+bvnmAJJiSMwhuBdqp5UY=*;
  integrity="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 subjectAltNames 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 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.

Open Questions ### {#oq-signature-header}

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. {:#parameterised-binary}

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.

CBOR representation of exchange response headers ## {#cbor-representation}

To sign an exchange's response headers, they need to be serialized into a byte string. Since intermediaries and distributors might rearrange, add, or just reserialize headers, we can't use the literal bytes of the headers as this serialization. Instead, this section defines a CBOR representation that can be embedded into other CBOR, canonically serialized ({{canonical-cbor}}), 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.

Example ### {#example-cbor-representation}

Given the HTTP exchange:

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'
}

Loading a certificate chain ## {#cert-chain-format}

The resource at a signature's cert-url MUST have the application/cert-chain+cbor content type, MUST be canonically-encoded CBOR ({{canonical-cbor}}), 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.
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.

Canonical CBOR serialization ## {#canonical-cbor}

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.
  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.

Signature validity ## {#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 {{signature-header}}. 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 ({{canonical-cbor}}) of the CBOR representation ({{cbor-representation}}) 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.

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. {:#force-fetch}

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 ({{cert-chain-format}}). 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 {{!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.

   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 {{cbor-representation}}, 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 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.

Open Questions ### {#oq-signature-validity}

Should the signed message use the TLS format (with an initial 64 spaces) even though these certificates can't be used in TLS servers?

Updating signature validity ## {#updating-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" to get a chain including a newer OCSP response.

The "validity-url" parameter of the signatures provides a way to fetch new signatures or learn where to fetch a complete updated exchange.

Each version of a signed exchange SHOULD have its own validity URLs, since each version needs different signatures and becomes obsolete at different times.

The resource at a "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.

Examples ### {#examples-updating-validity}

For example, say a signed exchange whose URL is https://example.com/resource has the following Signature header field (with line breaks included and irrelevant fields omitted for ease of reading).

Signature:
 sig1;
  sig=*MEUCIQ...*;
  ...
  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
  }
}

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.

The Accept-Signature header ## {#accept-signature}

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:

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 ({{signature-header}}) 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.

Integrity identifiers ### {#accept-signature-integrity}

Identifiers starting with "digest/" indicate that the client supports the Digest header field ({{!RFC3230}}) with the parameter from the HTTP Digest Algorithm Values Registry 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.

Key type identifiers ### {#accept-signature-key-types}

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.

Key value identifiers ### {#accept-signature-key-values}

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 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 ({{uc-sri}}) cases like <link rel=preload as=script href="..."> where the public key isn't known until the matching <script src="..." integrity="..."> tag.

Examples ### {#accept-signature-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.

Open Questions ### {#oq-accept-signature}

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 Signatures 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.

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?

Cross-origin trust {#cross-origin-trust}

To determine whether to trust a cross-origin exchange, the client takes a Signature header field ({{signature-header}}) 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 ({{canonical-cbor}}) of the CBOR representation ({{cbor-representation}}) 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 {{signature-validity}}, 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 is not same-origin with requestUrl, return "invalid".
2. Use {{signature-validity}} 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 {{uncached-headers}}, return "invalid".
6. Let authority be the host component of requestUrl.
7. Validate the certificate-chain using the following substeps. If any of them fail, re-run {{signature-validity}} 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 ({{cross-origin-cert-req}}).

   3. Validate that main-certificate has an ocsp property ({{cert-chain-format}}) 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 ({{cert-chain-format}}),
      • 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".

Uncached header fields {#uncached-headers}

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}}).
• 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.

Stateful header fields {#stateful-headers}

As described in {{seccons-over-signing}}, 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}}
• Set-Cookie2, {{?RFC2965}}
• SetProfile, {{?W3C.NOTE-OPS-OverHTTP}}
• Strict-Transport-Security, {{?RFC6797}}
• WWW-Authenticate, {{?RFC7235}}

Certificate Requirements {#cross-origin-cert-req}

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 {{cross-origin-trust}}.

   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
2. The "cansignhttpexchanges" parameter ({{caa-cansignhttpexchanges}}) 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.

Extensions to the CAA Record: cansignhttpexchanges Parameter {#caa-cansignhttpexchanges}

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".

Transferring a signed exchange {#transfer}

A signed exchange can be transferred in several ways, of which three are described here.

Same-origin response {#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 {{signature-validity}}) 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 {{serialized-headers}}, 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.

Serialized headers for a same-origin response {#serialized-headers}

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 ({{canonical-cbor}}) of the CBOR representation ({{cbor-representation}}) 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 ({{signed-headers}}). 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 {{signed-headers}}, the exchange has no serialized headers.

Open Questions {#oq-serialized-headers}

Do the serialized headers of an exchange need to include the Signed-Headers header field itself?

The Signed-Headers Header {#signed-headers}

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 {{signature-validity}} 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.

HTTP/2 extension for cross-origin Server Push # {#cross-origin-push}

To allow servers to Server-Push (Section 8.2 of {{?RFC7540}}) signed exchanges ({{proposal}}) signed by an authority for which the server is not authoritative (Section 9.1 of {{?RFC7230}}), this section defines an HTTP/2 extension.

Indicating support for cross-origin Server Push # {#setting}

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).

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.

NO_TRUSTED_EXCHANGE_SIGNATURE error code {#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).

Open Questions ### {#oq-error-code}

How fine-grained should this specification's error codes be?

Validating a cross-origin Push ## {#validating-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.

3. Run the algorithm in {{cross-origin-trust}} over:

   • The Signature header field from the response.
   • The effective request URI from the PUSH_PROMISE.
   • The canonical serialization ({{canonical-cbor}}) of the CBOR representation ({{cbor-representation}}) 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.

Open Questions ### {#oq-cross-origin-push}

Is it right that "validity-url" is required to be same-origin with the exchange? This allows the mitigation against downgrades in {{seccons-downgrades}}, but prohibits intermediates from providing a cache of the validity information. We could do both with a list of URLs.

application/signed-exchange format # {#application-signed-exchange}

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 ({{seccons-content-sniffing}}):

• 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.

   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 ({{signature-header}}).

7. headerLength bytes holding signedHeaders, the canonical serialization ({{canonical-cbor}}) of the CBOR representation of the response headers of the exchange represented by the application/signed-exchange resource ({{cbor-representation}}), 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.

Cross-origin trust in application/signed-exchange {#co-trust-app-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 {{cross-origin-trust}}.

Example ## {#example-application-signed-exchange}

An example application/signed-exchange file representing a possible signed exchange with 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>...

Open Questions ## {#oq-application-signed-exchange}

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?

Security considerations

Over-signing ## {#seccons-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 naïve signing systems blindly add it.

Session fixation ### {#seccons-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.

Misleading content ### {#seccons-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.

Off-path attackers ## {#seccons-off-path}

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}}.

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, 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 ({{cert-chain-format}}). 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.

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" ({{signature-header}}) to help owners detect key compromise, but that compromises some of the privacy properties of this specification.

Downgrades ## {#seccons-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" 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 {{oq-cross-origin-push}} is resolved to keep the same-origin requirement).

Signing oracles are permanent ## {#seccons-signing-oracles}

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.

Unsigned headers ## {#seccons-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 ({{uc-sri}}) may benefit from being able to leave the authority less constrained.

{{signature-validity}} 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 ({{uc-sri}}), but is risky for trusting cross-origin responses ({{uc-pushed-subresources}}, {{uc-explicit-distributor}}, and {{uc-offline-websites}}). {{cross-origin-push}} requires all headers to be included in the signature before trusting cross-origin pushed resources, at Ryan Sleevi's recommendation.

application/signed-exchange ## {#security-application-signed-exchange}

Clients MUST NOT trust an effective request URI claimed by an application/signed-exchange resource ({{application-signed-exchange}}) 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 {{co-trust-app-signed-exchange}} and getting "valid" back.

Key re-use with TLS ## {#seccons-key-re-use}

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 {{seccons-signing-oracles}}) 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 {{seccons-off-path}}.

This specification uses the CanSignHttpExchanges X.509 extension ({{cross-origin-cert-req}}) 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.

Content sniffing ## {#seccons-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.

The application/signed-exchange format ({{application-signed-exchange}}) 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 {{application-signed-exchange}}, 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 ({{application-signed-exchange}}) is derived from the request URL, percent-encode ({{URL}}) any bytes that are greater than 0x7E or are not 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.

Privacy considerations

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/.

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.

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.

IANA considerations

TODO: possibly register the validity-url format.

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): {{signature-header}} of this document

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

Applicable protocol: http

Status: standard

Author/Change controller: IETF

Specification document(s): {{accept-signature}} of this document

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): {{signed-headers}} of this document

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

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.

Specification: This document

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 {{security-application-signed-exchange}}

Interoperability considerations: N/A

Published specification: This specification (see {{application-signed-exchange}}).

Applications that use this media type: N/A

Fragment identifier considerations: N/A

Additional information:

Deprecated alias names for this type: N/A

Magic number(s): 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

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 {{cert-chain-format}}).

Applications that use this media type: N/A

Fragment identifier considerations: N/A

Additional information:

Deprecated alias names for this type: N/A

Magic number(s): 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

The cansignhttpexchanges CAA Parameter {#iana-caa-cansignhttpexchanges}

There are no IANA considerations for this parameter.

--- back

Use cases

PUSHed subresources {#uc-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.

Explicit use of a content distributor for subresources {#uc-explicit-distributor}

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. However, this has the downside that the 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 ({{uc-sri}}).

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.

Subresource Integrity {#uc-sri}

The W3C WebAppSec group is investigating using signatures 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 WICG/signature-based-sri#5. The details of what they need to sign will affect whether and how they can use this proposal.

Binary Transparency {#uc-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}}.

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.

Static Analysis {#uc-static-analysis}

Native app stores like the Apple App Store and the Android Play 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.

Offline websites {#uc-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 repository.

Requirements

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.

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 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 {{cross-origin-cert-req}} 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.

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 {{cross-origin-cert-req}} 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.

Retrieving the certificate {#certificate-chain}

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 with the signed content or PUSHed 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}}).

How much to sign ## {#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. {{serialized-headers}} 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 ({{proposal}}) assumes that is in use and doesn't sign request headers.

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}}.

Since proxies are unlikely to modify unknown content types, we can wrap the original exchange into an application/signed-exchange format ({{application-signed-exchange}}) 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 ({{uc-pushed-subresources}}), 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.

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.

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, 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 ({{certificate-chain}}) to include the OCSP response too, perhaps in the TLS 1.3 CertificateEntry format.

Response downgrade attacks {#downgrade}

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 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 {{proposal}}.
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 {{validity-with-cache-control}}.

The signature also needs to include instructions to intermediates for how to fetch updated validity assertions.

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.

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.

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, {{application-signed-exchange}} requires the signature to be less than 16kB and the headers to be less than 512kB, and {{signature-validity}} requires that the MI record size be less than 16kB. This allows the network 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.

Determining validity using cache control # {#validity-with-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 ({{signature-header}}) would no longer contain "date" or "expires" fields.

The validity-checking algorithm ({{signature-validity}}) 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 ({{updating-validity}}) 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.

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>
...

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>
...

Downsides of updating cache control ## {#downsides-of-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.

Change Log

RFC EDITOR PLEASE DELETE THIS SECTION.

draft-09

• No change

draft-08

• Improve the privacy considerations.

draft-07

• 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.
• 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.
• 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.

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.