Abstract

This specification describes a JavaScript API for discovering named, origin-specific pre-provisioned cryptographic keys for use with the Web Cryptography API. Pre-provisioned keys are keys which have been made available to the UA by means other than the generation, derivation, importation and unwrapping functions of the Web Cryptography API. Origin-specific keys are keys that are available only to a specified origin. Named keys are identified by a name assumed to be known to the origin in question and provisioned with the key itself. This specification concerns only keys which have all three of these properties.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document is an Editor's Draft of the WebCrypto Key Discovery recommendation. It defines an API that provides access to named origin-specific pre-provisioned keys. The privacy issues associated with such keys require special attention and the privacy related material in this document has not yet received thorough review by the community.

This document was published by the Web Cryptography WG as an Editor's Draft. If you wish to make comments regarding this document, please send them to public-webcrypto@w3.org (subscribe, archives). All comments are welcome.

Publication as an Editor's Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

1. Introduction

This section is non-normative.

The Web Cryptography API [WEBCRYPTO] describes a JavaScript API for performing basic cryptographic operations in web applications. Cryptographic keys in are represented in this API using Key objects. The API provides methods to generate, derive, import and unwrap cryptographic keying material, so creating Key objects.

This document concerns the discovery of cryptographic keys which are made available to the UA by other means. Specifically, this document provides an API for the discovery of cryptographic keys which have been pre-provisioned into a UA or device for use by a specific origin. Such keys are identified by names which are assumed to be known to the origin in question and thus they are referred to as named origin-specific pre-provisioned keys.

This enables web applications to establish secure proof that the UA has access to a particular pre-provisioned key. Depending on the nature of the key and its associated storage - for example within a Hardware Security Module or available only within a Trusted Execution Environment - further properties of the device on which the UA is running may be inferred.

The use of pre-provisioned keys requires the informed consent of the user, because such keys may be used for tracking and may reveal information about the users device. The privacy implications of origin-specific pre-provisioned keys are discussed further in Security and Privacy Considerations.

2. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this specification are to be interpreted as described in [RFC2119].

The following conformance classes are defined by this specification:

conforming user agent

A user agent is considered to be a conforming user agent if it satisfies all of the MUST-, REQUIRED- and SHALL-level criteria in this specification that apply to implementation. This specification uses both the terms "conforming user agent" and "user agent" to refer to this product class.

User agents MAY implement algorithms in this specification in any way desired, so long as the end result is indistinguishable from the result that would be obtained from the specification's algorithms.

User agents that use ECMAScript to implement the APIs defined in this specification MUST implement them in a manner consistent with the ECMAScript Bindings defined in the Web IDL specification [WEBIDL] as this specification uses that specification and terminology.

3. Scope

This section is non-normative.

The considerations in the Scope section of [WEBCRYPTO] apply to this specification as well. In particular, although this specification directly addresses the discovery of named origin-specific pre-provisioned keys, it does not address the processes used to provision those keys or the type and properties of the hardware or software components in which they are stored.

4. Privacy considerations

This section is non-normative.

The Privacy considerations of [WEBCRYPTO] apply to this specification.

4.1 Named origin-specific pre-provisioned keys

4.1.1 Comparison to cookies and persistent storage

The privacy considerations associated with origin-specific pre-provisioned keys are similar to those that apply to cookies [COOKIES] and persistent storage, such as [INDEXEDDB]. The principle difference is that pre-provisioned keys are pre-existing, whereas cookies and persistent store are not. Thus sites can be relied upon to work (in an 'out-of-the-box' fashion) when cookies have been deleted and persistent store cleared. By contrast, sites that rely on pre-provisioned keys may not operate or may operate in a different fashion if the expected pre-provisioned keys are not available. Such sites may require users to choose whether to trade a certain amount of privacy for whatever service the site offers, or not access the service at all.

4.1.2 User tracking

A third-party host (or any object capable of getting content distributed to multiple sites) could use a named origin-specific pre-provisioned key to track a user across multiple sessions, building a profile of the user's activities. In conjunction with a site that is aware of the user's real identity (for example an e-commerce site that requires authenticated credentials), this could allow oppressive groups to target individuals with greater accuracy than in a world with purely anonymous Web usage.

There are a number of techniques that can be used to mitigate this risk of tracking without user consent:

Site-specific white-listing of access to named origin-specific pre-provisioned keys

User agents should require the user to explicitly authorize access to named origin-specific pre-provisioned keys before a site can use the keys.

User agents should enable users to revoke this authorization either temporarily or permanently.

Blocking access to named origin-specific pre-provisioned keys

User agents may restrict access to named origin-specific pre-provisioned keys to scripts originating at the domain of the top-level document of the browsing context, for instance returning empty key search results for pages from other domains running in iframes.

Treating named origin-specific pre-provisioned keys as cookies

User agents should present the named origin-specific pre-provisioned keys feature to the user in a way that associates it strongly with HTTP session cookies. [COOKIES]

This might encourage users to view such keys with healthy suspicion.

Origin-tracking of named origin-specific pre-provisioned keys

User agents may record the origins of sites that contained content from third-party origins that used pre-provisioned keys.

If this information is then used to present a view of pre-provisioned keys to the user, it would allow the user to make informed decisions about authorizing sites to make use of keys. Combined with a blacklist ("delete this key" or "prevent this domain from ever accessing this key again"), the user can restrict the use of pre-provisioned keys to sites that he trusts.

Shared blacklists

User agents may allow users to share their pre-provisioned key domain blacklists.

This would allow communities to act together to protect their privacy.

While these suggestions prevent trivial use of this feature for user tracking, they do not block it altogether. Where a browser supports a mode of operation intended to preserve user anonymity, access to pre-provisioned keys should be disabled in this mode. Even so, within a single domain, a site can continue to track the user during a session, and can then pass all this information to the third party along with any identifying information (names, credit card numbers, addresses) obtained by the site. If a third party cooperates with multiple sites to obtain such information, a profile can still be created.

However, user tracking is to some extent possible even with no cooperation from the user agent whatsoever, for instance by using session identifiers in URLs, a technique already commonly used for innocuous purposes but easily repurposed for user tracking (even retroactively). This information can then be shared with other sites, using visitors' IP addresses and other user-specific data (e.g. user-agent headers and configuration settings) to combine separate sessions into coherent user profiles.

4.1.4 Sensitivity of data

User agents should treat pre-provisioned keys and material generated using such keys as potentially sensitive; it is quite possible for the user privacy to be compromised by the release of this information.

To this end, user agents should ensure that when deleting data, it is promptly deleted from the underlying storage.

5. Dependencies

This specification relies on several other underlying specifications.

HTML5
The terms and algorithms Window, Function, origin, same origin, structured clone, and structured clone algorithm are defined by the HTML 5 specification [HTML5].
Web Cryptography API

A conforming user agent MUST support the Web Cryptography API [WEBCRYPTO].

The term Key is defined in [WEBCRYPTO].

WebIDL
Many of the interface definitions and all of the IDL in this spec depends on [WEBIDL].
WebWorkers
The term WorkerGlobalScope is defined by the WebWorkers specification [WEBWORKERS].

6. API definition

6.1 Overview

This section is non-normative.

This specification defines a new cryptokeys attribute on the Window and WorkerGlobalScope objects. This attribute is an object supporting a method, getKeyByName which may be used to get the key with a given name. Keys are returned as NamedKey objects which are subclasses of the Key class.

6.2 NamedKey interface

interface NamedKey : Key {
    readonly    attribute DOMString  name;
    readonly    attribute DOMString? id;
};

6.2.1 Attributes

id of type DOMString, readonly , nullable

A global identifier associated with the key.

Origin-specific pre-provisioned keys are frequently provisioned with associated identifiers. Where an identifier exists that uniquely identifies the key amongst all keys pre-provisoned with the same origin and name and if this identifier can be canonically expressed as a sequence of no more than 256 bytes, then this identifier SHOULD be exposed, base64 encoded, as the id. If no identifier matching these conditions exists, id MUST be null.

name of type DOMString, readonly

A local identifier for the key.

6.2.2 Structured clone algorithm

When a user agent is required to obtain a structured clone of a NamedKey object, it must run the following steps.

  1. Let input and memory be the corresponding inputs defined by the internal structured clone algorithm, where input is a NamedKey object to be cloned.
  2. Let output be a newly constructed NamedKey object, where the structured clone algorithm is followed for the base Key object.
  3. Let the following attributes of output be equal to the value obtained by invoking the internal structured clone algorithm recursively, using the corresponding attribute on input as the new "input" argument and memory as the new "memory" argument:
    • name
    • id

6.2.3 Immutability of NamedKey objects

The name and id attributes of a NamedKey object SHALL NOT change. The underlying cryptographic key SHALL NOT change, except that it MAY be removed altogether. In this case any attempt to use the NamedKey object shall return an error.

6.3 CryptoKeys interface

[NoInterfaceObject]
interface CryptoKeys {
    Promise getKeyByName (DOMString name);
};

6.3.1 Methods

getKeyByName

When invoked, this method must perform the following steps:

  1. Let promise be a new Promise
  2. Make an asynchronous request to search for a key matching the name specifier provided in name. A name specifier matches the name of a key if they are identical when expressed as a string of unicode characters.
    If a key is found
    1. Let the result be an object of type NamedKey representing the key.
    2. Resolve promise with result.
    Otherwise
    Resolve promise with null.
  3. Return promise.

ParameterTypeNullableOptionalDescription
nameDOMString
Return type: Promise

6.4 Extension of Window interface

partial interface Window {
    readonly    attribute CryptoKeys cryptokeys;
};

6.4.1 Attributes

cryptokeys of type CryptoKeys, readonly
The object that exposes the key discovery methods

6.5 Extension of WorkerGlobalScope interface

partial interface WorkerGlobalScope {
    readonly    attribute CryptoKeys cryptokeys;
};

6.5.1 Attributes

cryptokeys of type CryptoKeys, readonly
The object that exposes the key discovery methods

7. Example

7.1 Using a pre-provisioned AES-KW key to unwrap a session key

Example 1
window.cryptokeys.getKeyByName( "Kunwrap" ).then( function( key ) {

  if ( key === null ) {
    console.error( "Unwrapping key, 'Kunwrap', not found.");
  } else {
    // Call a function to get a wrapped session key from the server, based on key.id
    getSessionKeyForId( key.id ).then( function( wrappedJwkKeyData ) {
      
      window.crypto.subtle.unwrapKey( 'jwk',
                wrappedJwkKeyData,            // the wrapped JWK session key
                key,                          // the pre-provisoned unwrapping key
                { name: 'AES-KW' },           // the key wrapping algorithm
                { name: 'AES-GCM' },          // the algorithm the unwrapped key will be used with
                false,                        // the result must not be extractable
                [ 'encrypt', 'decrypt' ]      // we'll use the result for encrypt and decrypt
                ).then( function( sessionKey ) {
                
          // use session key here for secure communication bound to the pre-provisioned key
                
       }, console.error.bind( console, "Unwrapping returned an error" ) );
    }, console.error.bind( console, "Could not get session key from server" ) );
  }
});

A. References

A.1 Normative references

[HTML5]
Robin Berjon; Steve Faulkner; Travis Leithead; Erika Doyle Navara; Edward O'Connor; Silvia Pfeiffer. HTML5. 29 April 2014. W3C Candidate Recommendation. URL: http://www.w3.org/TR/html5/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt
[WEBCRYPTO]
Ryan Sleevi, Mark Watson Web Cryptography API. W3C Working Draft (Work in progress.) URL: http://www.w3.org/TR/WebCryptoAPI/
[WEBIDL]
Cameron McCormack. Web IDL. 19 April 2012. W3C Candidate Recommendation. URL: http://www.w3.org/TR/WebIDL/
[WEBWORKERS]
Ian Hickson. Web Workers. 1 May 2012. W3C Candidate Recommendation. URL: http://www.w3.org/TR/workers/

A.2 Informative references

[COOKIES]
A. Barth. HTTP State Management Mechanism. April 2011. RFC. URL: http://www.ietf.org/rfc/rfc6265.txt
[INDEXEDDB]
Nikunj Mehta; Jonas Sicking; Eliot Graff; Andrei Popescu; Jeremy Orlow; Joshua Bell. Indexed Database API. 4 July 2013. W3C Candidate Recommendation. URL: http://www.w3.org/TR/IndexedDB/