Internet Engineering Task Force (IETF) R. Barnes
Request for Comments: 7165 Mozilla
Category: Informational April 2014
ISSN: 2070-1721
Use Cases and Requirements for
JSON Object Signing and Encryption (JOSE)
Abstract
Many Internet applications have a need for object-based security
mechanisms in addition to security mechanisms at the network layer or
transport layer. For many years, the Cryptographic Message Syntax
(CMS) has provided a binary secure object format based on ASN.1.
Over time, binary object encodings such as ASN.1 have become less
common than text-based encodings, such as the JavaScript Object
Notation (JSON). This document defines a set of use cases and
requirements for a secure object format encoded using JSON, drawn
from a variety of application security mechanisms currently in
development.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7165.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Basic Requirements . . . . . . . . . . . . . . . . . . . . . 5
4. Requirements on Application Protocols . . . . . . . . . . . . 6
5. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Security Tokens . . . . . . . . . . . . . . . . . . . . . 7
5.2. OAuth . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. OpenID Connect . . . . . . . . . . . . . . . . . . . . . 9
5.4. XMPP . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.5. ALTO . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.6. Emergency Alerting . . . . . . . . . . . . . . . . . . . 13
5.7. Web Cryptography . . . . . . . . . . . . . . . . . . . . 15
5.8. Constrained Devices . . . . . . . . . . . . . . . . . . . 16
5.8.1. Example: MAC Based on ECDH-Derived Key . . . . . . . 16
5.8.2. Object Security for CoAP . . . . . . . . . . . . . . 17
6. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 18
6.1. Functional Requirements . . . . . . . . . . . . . . . . . 18
6.2. Security Requirements . . . . . . . . . . . . . . . . . . 19
6.3. Desiderata . . . . . . . . . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. Normative References . . . . . . . . . . . . . . . . . . 21
8.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25
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1. Introduction
Internet applications rest on the layered architecture of the
Internet and take advantage of security mechanisms at all layers.
Many applications rely primarily on channel-based security
technologies such as IPsec and Transport Layer Security (TLS), which
create a secure channel at the IP layer or transport layer over which
application data can flow [RFC4301] [RFC5246]. These mechanisms,
however, cannot provide end-to-end security in some cases. For
example, in protocols with application-layer intermediaries, channel-
based security protocols would protect messages from attackers
between intermediaries, but not from the intermediaries themselves.
These cases require object-based security technologies, which embed
application data within a secure object that can be safely handled by
untrusted entities.
The most well-known example of such a protocol today is the use of
Secure/Multipurpose Internet Mail Extensions (S/MIME) protections
within the email system [RFC5751] [RFC5322]. An email message
typically passes through a series of intermediate Mail Transfer
Agents (MTAs) en route to its destination. While these MTAs often
apply channel-based security protections to their interactions (e.g.,
STARTTLS [RFC3207]), these protections do not prevent the MTAs from
interfering with the message. In order to provide end-to-end
security protections in the presence of untrusted MTAs, mail users
can use S/MIME to embed message bodies in a secure object format that
can provide confidentiality, integrity, and data origin
authentication.
S/MIME is based on the Cryptographic Message Syntax (CMS) for secure
objects [RFC5652]. CMS is defined using Abstract Syntax Notation 1
(ASN.1) and typically encoded using the ASN.1 Distinguished Encoding
Rules (DER), which define a binary encoding of the protected message
and associated parameters [ITU.X690.2002]. In recent years, usage of
ASN.1 has decreased (along with other binary encodings for general
objects), while more applications have come to rely on text-based
formats such as the Extensible Markup Language (XML) [W3C.REC-xml] or
the JavaScript Object Notation (JSON) [RFC7159].
Many current applications thus have much more robust support for
processing objects in these text-based formats than ASN.1 objects;
indeed, many lack the ability to process ASN.1 objects at all. To
simplify the addition of object-based security features to these
applications, the IETF JSON Object Signing and Encryption (JOSE)
working group has been chartered to develop a secure object format
based on JSON. While the basic requirements for this object format
are straightforward -- namely, confidentiality and integrity
mechanisms encoded in JSON -- discussions in the working group
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indicated that different applications hoping to use the formats
defined by JOSE have different requirements. This document
summarizes the use cases for JOSE envisioned by those potential
applications and the resulting requirements for security mechanisms
and object encodings.
Some systems that use XML have specified the use of XML-based
security mechanisms for object security, namely XML Digital
Signatures and XML Encryption [W3C.xmldsig-core] [W3C.xmlenc-core].
These mechanisms are used by several security token systems (e.g.,
Security Assertion Markup Language (SAML) [OASIS.saml-core-2.0-os],
Web Services Federation [WS-Federation]), and the Common Alerting
Protocol (CAP) emergency alerting format [CAP]. In practice,
however, XML-based secure object formats introduce similar levels of
complexity to ASN.1 (e.g., due to the need for XML canonicalization),
so developers that lack the tools or motivation to handle ASN.1
aren't likely to use XML security either. This situation motivates
the creation of a JSON-based secure object format that is simple
enough to implement and deploy that it can be easily adopted by
developers with minimal effort and tools.
2. Definitions
This document makes extensive use of standard security terminology
[RFC4949]. In addition, because the use cases for JOSE and CMS are
similar, we will sometimes make analogies to some CMS concepts
[RFC5652].
The JOSE working group charter calls for the group to define three
basic JSON object formats:
1. Integrity-protected object format
2. Confidentiality-protected object format
3. A format for expressing keys
In this document, we will refer to these as the "signed object
format", the "encrypted object format", and the "key format",
respectively. The JOSE working group items intended to describe
these formats are JSON Web Signature [JWS], JSON Web Encryption
[JWE], and JSON Web Key [JWK], respectively. Algorithms and
algorithm identifiers used by JWS, JWE, and JWK are defined in JSON
Web Algorithms [JWA].
In general, where there is no need to distinguish between asymmetric
and symmetric operations, we will use the terms "signing",
"signature", etc., to denote both true digital signatures involving
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asymmetric cryptography as well as Message Authentication Codes
(MACs) using symmetric keys.
In the lifespan of a secure object, there are two basic roles, an
entity that creates the object (e.g., encrypting or signing a
payload) and an entity that uses the object (decrypting and
verifying). We will refer to these roles as "sender" and
"recipient", respectively. Note that while some requirements and use
cases may refer to these as single entities, each object may have
multiple entities in each role. For example, a message may be signed
by multiple senders or decrypted by multiple recipients.
3. Basic Requirements
For the encrypted and signed object formats, the necessary
protections will be created using appropriate cryptographic
mechanisms: symmetric or asymmetric encryption for confidentiality
and MACs or digital signatures for integrity protection. In both
cases, it is necessary for the JOSE format to support both symmetric
and asymmetric operations.
o The JOSE encrypted object format must support object encryption in
the case where the sender and receiver share a symmetric key.
o The JOSE encrypted object format must support object encryption in
the case where the sender has only a public key for the receiver.
o The JOSE signed object format must support integrity protection
using MACs, for the case where the sender and receiver share only
a symmetric key.
o The JOSE signed object format must support integrity protection
using digital signatures, for the case where the receiver has only
a public key for the sender.
In some applications, the key used to process a JOSE object is
indicated by application context, instead of directly in the JOSE
object. However, in order to avoid confusion, endpoints that lack
the necessary context need to be able to recognize this and fail
cleanly. Other than keys, JOSE objects do not support pre-
negotiation; all cryptographic parameters must be expressed directly
in the JOSE object.
o The JOSE signed and encrypted object formats must define the
process by which an implementation recognizes whether it has the
key required to process a given object, whether the key is
specified by the object or by some out-of-band mechanism.
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o Each algorithm used for JOSE must define which parameters are
required to be present in a JOSE object using that algorithm.
In cases where two entities are going to be exchanging several JOSE
objects, it might be helpful to pre-negotiate some parameters so that
they do not have to be signaled in every JOSE object. However, so as
not to confuse endpoints that do not support pre-negotiation, it is
useful to signal when pre-negotiated parameters are in use in those
cases.
o It should be possible to extend the base JOSE signed and encrypted
object formats to indicate that pre-negotiated parameters are to
be used to process the object. This extension should also provide
a means of indicating which parameters are to be used.
The purpose of the key format is to provide the recipient with
sufficient information to use the encoded key to process
cryptographic messages. Thus, it is sometimes necessary to include
additional parameters along with the bare key.
o The JOSE key format must enable inclusion of all algorithm
parameters necessary to use the encoded key, including an
identifier for the algorithm with which the key is used as well as
any additional parameters required by the algorithm (e.g.,
elliptic curve parameters).
4. Requirements on Application Protocols
The JOSE secure object formats describe how cryptographic processing
is done on secured content, ensuring that the recipient of an object
is able to properly decrypt an encrypted object or verify a
signature. In order to make use of JOSE, however, applications will
need to specify several aspects of how JOSE is to be used:
o What application content is to be protected
o Which cryptographic algorithms are to be used
o How application protocol entities establish keys
o Whether keys are to be explicitly indicated in JOSE objects or
associated by application context
o Which serialization(s) of JOSE objects are to be used
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5. Use Cases
Several IETF working groups developing application-layer protocols
have expressed a desire to use the JOSE data formats in their designs
for end-to-end security features. In this section, we summarize the
use cases proposed by these groups and discuss the requirements that
they imply for the JOSE object formats.
5.1. Security Tokens
Security tokens are a common use case for object-based security, for
example, SAML assertions [OASIS.saml-core-2.0-os]. Security tokens
are used to convey information about a subject entity ("claims" or
"assertions") from an issuer to a recipient. The security features
of a token format enable the recipient to verify that the claims came
from the issuer and, if the object is confidentiality protected, that
they were not visible to other parties.
Security tokens are used in federation protocols such as SAML 2.0
[OASIS.saml-core-2.0-os], WS-Federation [WS-Federation], Mozilla
Persona [Persona], and OpenID Connect [OpenID.Core], as well as in
resource authorization protocols such as OAuth 2.0 [RFC6749],
including for OAuth bearer tokens [RFC6750]. In some cases, security
tokens are used for client authentication and for access control
[JWT-BEARER] [SAML2].
JSON Web Token [JWT] is a security token format based on JSON and
JOSE. It is used with Mozilla Persona, OpenID Connect, and OAuth.
Because JWTs are often used in contexts with limited space (e.g.,
HTTP query parameters), it is a core requirement for JWTs, and thus
JOSE, to have a compact, URL-safe representation.
5.2. OAuth
The OAuth protocol defines a mechanism for distributing and using
authorization tokens using HTTP [RFC6749]. A client that wishes to
access a protected resource requests authorization from the resource
owner. If the resource owner allows this access, he directs an
authorization server to issue an access token to the client. When
the client wishes to access the protected resource, he presents the
token to the relevant resource server, which verifies the validity of
the token before providing access to the protected resource.
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+---------------+ +---------------+
| | | |
| Resource |<........>| Authorization |
| Server | | Server |
| | | |
+---------------+ +---------------+
^ |
| |
| |
| |
| |
+------------|--+ +--|------------+
| +----------------+ |
| | | Resource |
| Client | | Owner |
| | | |
+---------------+ +---------------+
Figure 1: The OAuth Process
In effect, this process moves the token from the authorization server
(as a sender of the object) to the resource server (recipient) via
the client as well as the resource owner (the latter because of the
HTTP mechanics underlying the protocol). As with email, we have a
case where an application object is transported via untrusted
intermediaries.
This application has two essential security requirements: integrity
and data origin authentication. Integrity protection is required so
that the resource owner and the client cannot modify the permission
encoded in the token. Although the resource owner is ultimately the
entity that grants authorization, it is not trusted to modify the
authorization token, since this could, for example, grant access to
resources not owned by the resource owner.
Data origin authentication is required so that the resource server
can verify that the token was issued by a trusted authorization
server.
Confidentiality protection may also be needed if the authorization
server is concerned about the visibility of permissions information
to the resource owner or client. For example, permissions related to
social networking might be considered private information. Note,
however, that OAuth already requires that the underlying HTTP
transactions be protected by TLS, so tokens are already
confidentiality protected from entities other than the resource owner
and client.
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The confidentiality and integrity needs are met by the basic
requirements for signed and encrypted object formats, whether the
signing and encryption are provided using asymmetric or symmetric
cryptography. The choice of which mechanism is applied will depend
on the relationship between the two servers, namely whether they
share a symmetric key or only public keys.
Authentication requirements will also depend on deployment
characteristics. Where there is a relatively strong binding between
the resource server and the authorization server, it may suffice for
the authorization server issuing a token to be identified by the key
used to sign the token. This requires that the protocol carry either
the public key of the authorization server or an identifier for the
public or symmetric key. In OAuth, the "client_id" parameter
(external to the token) identifies the key to be used.
There may also be more advanced cases where the authorization
server's key is not known in advance to the resource server. This
may happen, for instance, if an entity instantiated a collection of
authorization servers (say for load balancing), each of which has an
independent key pair. In these cases, it may be necessary to also
include a certificate or certificate chain for the authorization
server, so that the resource server can verify that the authorization
server is an entity that it trusts.
The HTTP transport for OAuth imposes a particular constraint on the
encoding. In the OAuth protocol, tokens frequently need to be passed
as query parameters in HTTP URIs [RFC2616] after having been
base64url encoded [RFC4648]. While there is no specified limit on
the length of URIs (and thus of query parameters), in practice, URIs
of more than 2,048 characters are rejected by some user agents. So
this use case requires that JOSE objects be sufficiently small, even
after being signed and possibly encrypted.
5.3. OpenID Connect
The OpenID Connect protocol [OpenID.Core] is a simple, REST/JSON-
based identity federation protocol layered on OAuth 2.0. It uses the
JWT and JOSE formats both to represent security tokens and to provide
security for other protocol messages (performing signing and
optionally encryption). OpenID Connect negotiates the algorithms to
be used and distributes information about the keys to be used using
protocol elements that are not part of the JWT and JOSE header
parameters.
In the OpenID Connect context, it is possible for the recipient of a
JWT to accept it without integrity protection in the JWT itself. In
such cases, the recipient chooses to rely on transport security
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rather than object security. For example, if the payload is
delivered over a TLS-protected channel, the recipient may regard the
protections provided by TLS as sufficient, so JOSE protection would
not be required.
However, even in this case, it is desirable to associate some
metadata with the JWT payload (claim set), such as the content type,
or other application-specific metadata. In a signed or encrypted
object, these metadata values could be carried in a header with other
metadata required for signing or encryption. It would thus simplify
the design of OpenID Connect if there could be a JOSE object format
that does not apply cryptographic protections to its payload, but
allows a header to be attached to the payload in the same way as a
signed or encrypted object.
5.4. XMPP
The Extensible Messaging and Presence Protocol (XMPP) routes messages
from one end client to another by way of XMPP servers [RFC6120].
There are typically two servers involved in delivering any given
message: The first client (Alice) sends a message for another client
(Bob) to her server (A). Server A uses Bob's identity and the DNS to
locate the server for Bob's domain (B) and then delivers the message
to that server. Server B then routes the message to Bob.
+-------+ +----------+ +----------+ +-----+
| Alice |-->| Server A |-->| Server B |-->| Bob |
+-------+ +----------+ +----------+ +-----+
Figure 2: Delivering an XMPP Message
The untrusted-intermediary problems are especially acute for XMPP
because in many current deployments, the holder of an XMPP domain
outsources the operation of the domain's servers to a different
entity. In this environment, there is a clear risk of exposing the
domain holder's private information to the domain operator. XMPP
already has a defined mechanism for end-to-end security using S/MIME,
but it has failed to gain widespread deployment [RFC3923], in part
because of key management challenges and in part because of the
difficulty of processing S/MIME objects.
The XMPP working group is in the process of developing a new
end-to-end encryption system with an encoding based on JOSE and a
clearer key management system [XMPP-E2E]. The process of sending an
encrypted message in this system involves two steps: First, the
sender generates a symmetric Session Master Key (SMK), encrypts the
message content (including a per-message Content Master Key), and
sends the encrypted message to the desired set of recipients.
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Second, each recipient "dials back" to the sender, providing his
public key. The sender then responds with the relevant SMK, wrapped
with the recipient's public key.
+-------+ +----------+ +----------+ +-----+
| Alice |<->| Server A |<->| Server B |<->| Bob |
+-------+ +----------+ +----------+ +-----+
| | | |
|------------Encrypted message---------->|
| | | |
|<---------------Public key--------------|
| | | |
|---------------Wrapped SMK------------->|
| | | |
Figure 3: Delivering a Secure XMPP Message
The main thing that this system requires from the JOSE formats is
confidentiality protection via content encryption, plus an integrity
check via a MAC derived from the same symmetric key. The separation
of the key exchange from the transmission of the encrypted content,
however, requires that the JOSE encrypted object format allow wrapped
symmetric keys to be carried separately from the encrypted payload.
In addition, the encrypted object will need to have a tag for the key
that was used to encrypt the content, so that the recipient (Bob) can
present the tag to the sender (Alice) when requesting the wrapped
key.
Another important feature of XMPP is that it allows for the
simultaneous delivery of a message to multiple recipients. In the
diagrams above, Server A could deliver the message not only to Server
B (for Bob) but also to Servers C, D, E, etc., for other users. In
such cases, to avoid the multiple "dial back" transactions implied by
the above mechanism, XMPP systems will likely reuse a given SMK for
multiple individual messages, refreshing the SMK on a periodic and/or
event-driven basis (e.g., when the recipient's presence changes).
They might also cache public keys for end recipients, so that wrapped
keys can be sent along with content on future messages. This implies
that the JOSE encrypted object format must support the provision of
multiple versions of the same wrapped SMK (much as a CMS
EnvelopedData structure can include multiple RecipientInfo
structures).
In the current draft of the XMPP end-to-end security system, each
party is authenticated by virtue of the other party's trust in the
XMPP message routing system. The sender is authenticated to the
receiver because he can receive messages for the identifier "Alice"
(in particular, the request for wrapped keys) and can originate
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messages for that identifier (the wrapped key). Likewise, the
receiver is authenticated to the sender because he received the
original encrypted message and originated the request for a wrapped
key. So, the authentication here requires not only that XMPP routing
be done properly, but also that TLS be used on every hop. Moreover,
it requires that the TLS channels have strong authentication, since a
man in the middle on any of the three hops can masquerade as Bob and
obtain the key material for an encrypted message.
Because this authentication is quite weak (depending on the use of
TLS on three hops) and unverifiable by the endpoints, it is possible
that the XMPP working group will integrate some sort of credentials
for end recipients, in which case there would need to be a way to
associate these credentials with JOSE objects.
Finally, it's worth noting that XMPP is based on XML, not JSON. So
by using JOSE, XMPP will be carrying JSON objects within XML. It is
thus a desirable property for JOSE objects to be encoded in such a
way as to be safe for inclusion in XML. Otherwise, an explicit CDATA
indication must be given to the parser to indicate that it is not to
be parsed as XML. One way to meet this requirement would be to apply
base64url encoding, but for XMPP messages of medium-to-large size,
this could impose a fair degree of overhead.
5.5. ALTO
Application-Layer Traffic Optimization (ALTO) is a system for
distributing network topology information to end devices, so that
those devices can modify their behavior to have a lower impact on the
network [RFC6708]. The ALTO protocol distributes topology
information in the form of JSON objects carried in HTTP [RFC2616]
[ALTO]. The basic version of ALTO is simply a client-server
protocol, so simple use of HTTPS suffices for this case [RFC2818].
However, there is beginning to be some discussion of use cases for
ALTO in which these JSON objects will be distributed through a
collection of intermediate servers before reaching the client, while
still preserving the ability of the client to authenticate the
original source of the object. Even the base ALTO protocol notes
that "ALTO Clients obtaining ALTO information through redistribution
must be able to validate the received ALTO information" to ensure
that it was generated by an appropriate ALTO server.
In this case, the security requirements are straightforward. JOSE
objects carrying ALTO payloads will need to bear digital signatures
from the originating servers, which will be bound to certificates
attesting to the identities of the servers. There is no requirement
for confidentiality in this case, since ALTO information is generally
public.
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The more interesting questions are encoding questions. ALTO objects
are likely to be much larger than payloads in the two cases above,
with sizes of up to several megabytes. Processing of such large
objects can be done more quickly if it can be done in a single pass,
which may be possible if JOSE objects require specific orderings of
fields within the JSON structure.
In addition, because ALTO objects are also encoded as JSON, they are
already safe for inclusion in a JOSE object. Signed JOSE objects
will likely carry the signed data in a string alongside the
signature. JSON objects have the property that they can be safely
encoded in JSON strings. All they require is that unnecessary white
space be removed, a much simpler transformation than, say, base64url
encoding. This raises the question of whether it might be possible
to optimize the JOSE encoding for certain "JSON-safe" cases.
Finally, it may be desirable for ALTO to have a "detached signature"
mechanism, that is, a way to encode signature information separate
from the protected content. This would allow the ALTO protocol to
include the signature in an HTTPS header, with the signed content as
the HTTPS entity body.
5.6. Emergency Alerting
Emergency alerting is an emerging use case for IP networks
[ALERT-REQ]. Alerting systems allow authorities to warn users of
impending danger by sending alert messages to connected devices. For
example, in the event of a hurricane or tornado, alerts might be sent
to all devices in the path of the storm.
The most critical security requirement for alerting systems is that
it must not be possible for an attacker to send false alerts to
devices. Such a capability would potentially allow an attacker to
create wide-spread panic. In practice, alert systems prevent these
attacks both by controls on sending messages at points where alerts
are originated, and by having recipients of alerts verify that the
alert was sent by an authorized source. The former type of control
is implemented with local security on hosts from which alerts can be
originated. The latter type is implemented by digital signatures on
alert messages (using channel-based or object-based mechanisms).
With an object-based mechanism, the signature value is encoded in a
secure object. With a channel-based mechanism, the alert is "signed"
by virtue of being sent over an authenticated, integrity-protected
channel.
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Alerts typically reach end recipients via a series of intermediaries.
For example, while a national weather service might originate a
hurricane alert, it might first be delivered to a national gateway
and then to network operators, who broadcast it to end subscribers.
+------------+ +------------+ +------------+
| Originator | | Originator | | Originator |
+------------+ +------------+ +------------+
| . .
+-----------------+..................
|
V
+---------+
| Gateway |
+---------+
|
+------------+------------+
| |
V V
+---------+ +---------+
| Network | | Network |
+---------+ +---------+
| |
+------+-----+ +------+-----+
| | | |
V V V V
+--------+ +--------+ +--------+ +--------+
| Device | | Device | | Device | | Device |
+--------+ +--------+ +--------+ +--------+
Figure 4: Delivering an Emergency Alert
In order to verify alert signatures, recipients must be provisioned
with the proper public keys for trusted alert authorities. This
trust may be "piece-wise" along the path the alert takes. For
example, the alert relays operated by networks might have a full set
of certificates for all alert originators, while end devices may only
trust their local alert relay. Or, devices might require that a
device be signed by an authorized originator and by its local
network's relay.
This scenario creates a need for multiple signatures on alert
documents, so that an alert can bear signatures from any or all of
the entities that processed it along the path. In order to minimize
complexity, these signatures should be "modular" in the sense that a
new signature can be added without a need to alter or recompute
previous signatures.
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5.7. Web Cryptography
The W3C Web Cryptography API defines a standard cryptographic API for
the Web [WebCrypto]. If a browser exposes this API, then JavaScript
provided as part of a Web page can ask the browser to perform
cryptographic operations, such as digest, MAC, encryption, or digital
signing.
One of the key reasons to have the browser perform cryptographic
operations is to avoid allowing JavaScript code to access the keying
material used for these operations. For example, this separation
would prevent code injected through a cross-site scripting (XSS)
attack from reading and exfiltrating keys stored within a browser.
While the malicious code could still use the key while running in the
browser, this vulnerability can only be exercised while the malicious
code is active in a user's browser.
However, the Web Cryptography API also provides a key export
functionality, which can allow JavaScript to extract a key from the
API in wrapped form. For example, JavaScript code might provide a
public key for which the corresponding private key is held by another
device. The wrapped key provided by the API could then be used to
safely transport the key to the new device. While this could
potentially allow malicious code to export a key, the need for an
explicit export operation provides a control point, allowing for user
notification or consent verification.
The Web Cryptography API also allows browsers to impose limitations
on the usage of the keys it handles. For example, a symmetric key
might be marked as usable only for encryption, and not for MAC. When
a key is exported in wrapped form, these attributes should be carried
along with it.
The Web Cryptography API thus requires formats to express several
forms of keys. Obviously, the public key from an asymmetric key pair
can be freely imported to and exported from the browser, so there
needs to be a format for public keys. There is also a need for a
format to express private keys and symmetric keys. For non-public
keys, the primary need is for a wrapped form, where the
confidentiality and integrity of the key is assured
cryptographically; these protections should also apply to any
attributes of the key. It may also be useful to define a direct,
unwrapped format for use within a security boundary.
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5.8. Constrained Devices
This section describes use cases for constrained devices as defined
in [CONSTRAINED]. Typical issues with this type of device are
limited memory, limited power supply, low processing power, and
severe message size limitations for the communication protocols.
5.8.1. Example: MAC Based on ECDH-Derived Key
Suppose a small, low power device maker has decided on using the
output of the JOSE working group as their encryption and
authentication framework. The device maker has a limited budget for
both gates and power. For this reason there are a number of short
cuts and design decisions that have been made in order to minimize
these needs.
The design team has determined that the use of MACs is going to be
sufficient to provide the necessary authentication. However,
although a MAC is going to be used, they do not want to use a single
long-term shared secret. Instead, they have adopted the following
proposal for computing a shared secret that can be validated:
o An Elliptic-Curve Diffie-Hellman (ECDH) key pair is generated for
the device at the time of manufacturing. (Or, as part of the
configuration process during installation.)
o An ECDH public key for the controller is configured at the time of
configuration.
o The configuration system performs the ECDH computation and
configures the device with the resulting shared secret. This
process eliminates the need for the device to be able to perform
the required ECDH processing. The security requirements on
protecting this computed shared secret are the same as the
requirements on protecting the private ECDH key.
o A counter and an increment value are configured onto the device.
o When a message is to be sent by the device, the counter is
incremented and a new MAC key is computed from the ECDH secret and
the counter value. A custom Key Derivation Function (KDF) based
on AES-CBC is used to derive the required MAC key. The MAC key is
then used to compute the MAC value for the message.
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In a similar manner, the KDF function can be used to compute an
Authenticated Encryption with Associated Data (AEAD) algorithm key
when the system needs to provide confidentiality for the message.
The controller, being a larger device, will perform the ECDH step and
use a random number generator to generate the sender nonce value.
5.8.2. Object Security for CoAP
This use case deals with constrained devices of class C0/C1 (see
[CONSTRAINED]). These devices communicate using RESTful requests and
responses transferred using the Constrained Application Protocol
[CoAP]. To simplify matters, all communication is assumed to be
unicast; i.e., these security measures don't cover multicast or
broadcast.
In this type of setting, it may be too costly to use session-based
security (e.g., to run a 4-pass authentication protocol) since
receiving and in particular sending consumes a lot of power,
especially for wireless devices. Therefore, to just secure the CoAP
payload by replacing a plaintext payload of a request or response
with a JWE object is an important alternative solution, which allows
a trade-off between protection (the CoAP headers are not protected)
and performance.
In a simple setting, consider the payload of a CoAP GET response from
a sensor type device. The information in a sensor reading may be
privacy or business sensitive and needs both integrity protection and
encryption.
However, some sensor readings are very short, say, a few bytes, and
in this case, default encryption and integrity protection algorithms
(such as 128-bit AES-CBC with HMAC_SHA256) may cause a dramatic
expansion of the payload, even disregarding JWE headers.
Also, the value of certain sensor readings may decline rapidly, e.g.,
traffic or environmental measurements, so it must be possible to
reduce the security overhead.
This leads to the following requirements that could be covered by
specific JWE/JWS profiles:
o The size of the secure object shall be as small as possible.
Receiving an object may cost orders of magnitude more in terms of
power than performing, say, public key cryptography on the object,
in particular in a wireless setting.
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o Integrity protection: The object shall be able to support
integrity protection, i.e., have a field containing a digital
signature, both public key signatures and keyed MACs shall be
supported.
o Encryption: The object shall be able to support encryption as an
optional addition to integrity protection. It shall be possible
to exclude certain fields from encryption, which are needed before
verifying integrity or decrypting the object.
o Cipher suites: It should be possible to support a variety of
cipher suites to support the constrained devices' use cases. For
example:
* Block ciphers with block sizes of, e.g., 96 bits, in addition
to the standard 128 bits.
* Modes of operation for block ciphers that do not expand the
message size to a block boundary, such as AES-GCM.
* Cipher suites that support combined encryption and MAC
calculation (i.e., AEAD modes for block ciphers).
6. Requirements
This section summarizes the requirements from the above use cases and
lists further requirements not directly derived from the above use
cases. There are also some constraints that are not hard
requirements but that are still desirable properties for the JOSE
system to have.
6.1. Functional Requirements
F1 Define formats for secure objects that provide the following
security properties:
* Digital signature (integrity/authentication under an asymmetric
key pair)
* Message authentication (integrity/authentication under a
symmetric key)
* Authenticated encryption
F2 Define a format for public keys and private keys for asymmetric
cryptographic algorithms, with associated attributes, including a
wrapped form for private keys.
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F3 Define a format for symmetric keys with associated attributes,
allowing for both wrapped and unwrapped keys.
F4 Define a JSON serialization for each of the above objects. An
object in this encoding must be valid according to the JSON ABNF
syntax [RFC7159].
F5 Define a compact, URL-safe text serialization for the encrypted
and signed object formats.
F6 Allow for attributes associated to wrapped keys to be bound to
them cryptographically.
F7 Allow for wrapped keys to be separated from a secure object that
uses a symmetric key. In such cases, cryptographic components of
the secure object other than the wrapped key (e.g., ciphertext,
MAC values) must be independent of the wrapped form of the key.
For example, if an encrypted object is prepared for multiple
recipients, then only the wrapped key may vary, not the
ciphertext.
F8 Do not impose more overhead than is required to meet the
requirements in this document, especially when a large amount of
application content is being protected.
6.2. Security Requirements
S1 Provide mechanisms to avoid repeated use of the same symmetric key
for encryption or MAC computation. Instead, long-lived keys
should be used only for key wrapping, not for direct encryption/
MAC. It should be possible to use any of the key management
techniques provided in CMS [RFC5652]:
* Key transport (wrapping for a public key)
* Key encipherment (wrapping for a symmetric key)
* Key agreement (wrapping for a Diffie-Hellman (DH) public key)
* Password-based encryption (wrapping under a key derived from a
password)
S2 Where long-lived symmetric keys are used directly for
cryptographic operations (i.e., where requirement S1 is not met),
provide deployment guidance on key management practices, such as
the need to limit key lifetimes.
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S3 Use cryptographic algorithms in a manner compatible with major
validation processes. For example, if typical validation
standards allow algorithm A to be used for purpose X but not
purpose Y, then JOSE should not recommend using algorithm A for
purpose Y.
S4 Support operation with or without pre-negotiation. It must be
possible to create or process secure objects without any
configuration beyond key provisioning. If it is possible for keys
to be derived from application context, it must be possible for a
recipient to recognize when it does not have the appropriate key.
6.3. Desiderata
D1 Maximize compatibility with the W3C Web Crypto specifications,
e.g., by coordinating with the Web Crypto working group to
encourage alignment of algorithms and algorithm identifiers.
D2 Avoid JSON canonicalization to the extent possible. That is, all
other things being equal, techniques that rely on fixing a
serialization of an object (e.g., by encoding it with base64url)
are preferred over those that require converting an object to a
canonical form.
D3 Maximize the extent to which the inputs and outputs of JOSE
cryptographic operations can be controlled by the applications, as
opposed to involving processing specific to JOSE. This allows
JOSE the flexibility to address the needs of many cryptographic
protocols. For example, in some cases, it might allow JOSE
objects to be translated to legacy formats such as CMS without the
need for re-encryption or re-signing.
7. Security Considerations
The primary focus of this document is the requirements for a JSON-
based secure object format. At the level of general security
considerations for object-based security technologies, the security
considerations for this format are the same as for CMS [RFC5652].
The primary difference between the JOSE format and CMS is that JOSE
is based on JSON, which does not have a canonical representation.
The lack of a canonical form means that it is difficult to determine
whether two JSON objects represent the same information, which could
lead to vulnerabilities in some usages of JOSE.
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8. References
8.1. Normative References
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC6708] Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
Y. Yang, "Application-Layer Traffic Optimization (ALTO)
Requirements", RFC 6708, September 2012.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
6749, October 2012.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
[W3C.REC-xml]
Bray, T., Maler, E., Paoli, J., and C. Sperberg-McQueen,
"Extensible Markup Language (XML) 1.0 (Fifth Edition)",
W3C Recommendation, November 2008,
<
http://www.w3.org/TR/2008/REC-xml-20081126/>.
[WebCrypto]
Dahl, D. and R. Sleevi, "Web Cryptography API", W3C
Working Draft, January 2013,
<
http://www.w3.org/TR/2013/WD-WebCryptoAPI-20130108/>.
8.2. Informative References
[ALERT-REQ]
Schulzrinne, H., Norreys, S., Rosen, B., and H.
Tschofenig, "Requirements, Terminology and Framework for
Exigent Communications", Work in Progress, March 2012.
[ALTO] Alimi, R., Ed., Penno, R., Ed., and Y. Yang, Ed., "ALTO
Protocol", Work in Progress, March 2014.
[CAP] Botterell, A. and E. Jones, "Common Alerting Protocol,
v1.1", OASIS Standard CAP-V1.1, October 2005,
<
http://www.oasis-open.org/committees/download.php/15135/
emergency-CAPv1.1-Corrected_DOM.pdf>.
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[CONSTRAINED]
Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained Node Networks", Work in Progress, March 2014.
[CoAP] Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", Work in Progress, June 2013.
[ITU.X690.2002]
International Telecommunications Union, "Information
Technology - ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", ITU-T Recommendation
X.690, July 2002.
[JWA] Jones, M., "JSON Web Algorithms (JWA)", Work in Progress,
March 2014.
[JWE] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
Work in Progress, March 2014.
[JWK] Jones, M., "JSON Web Key (JWK)", Work in Progress, March
2014.
[JWS] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", Work in Progress, March 2014.
[JWT-BEARER]
Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
(JWT) Profile for OAuth 2.0 Client Authentication and
Authorization Grants", Work in Progress, March 2014.
[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", Work in Progress, March 2014.
[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Maler, E., and R. Philpott,
"Assertions and Protocols for the OASIS Security Assertion
Markup Language (SAML) V2.0", Oasis Standard, March 2005,
<
http://docs.oasis-open.org/security/saml/v2.0/
saml-core-2.0-os.pdf>.
[OpenID.Core]
Bradley, J., de Medeiros, B., Jones, M., Mortimore, C.,
and N. Sakimura, "OpenID Connect Core 1.0", December 2013,
<
http://openid.net/specs/openid-connect-core-1_0.html>.
[Persona] Mozilla Developer Network, "Mozilla Persona", April 2013,
<
https://developer.mozilla.org/en-US/docs/Persona>.
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[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC3923] Saint-Andre, P., "End-to-End Signing and Object Encryption
for the Extensible Messaging and Presence Protocol
(XMPP)", RFC 3923, October 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750, October 2012.
[SAML2] Campbell, B., Mortimore, C., and M. Jones, "SAML 2.0
Profile for OAuth 2.0 Client Authentication and
Authorization Grants", Work in Progress, March 2014.
[W3C.xmldsig-core]
Eastlake, D., Reagle, J., and D. Solo, "XML-Signature
Syntax and Processing", W3C Recommendation, June 2008,
<
http://www.w3.org/TR/2008/REC-xmldsig-core-20080610/>.
[W3C.xmlenc-core]
Eastlake, D. and J. Reagle, "XML Encryption Syntax and
Processing", W3C Candidate Recommendation, December 2002,
<
http://www.w3.org/TR/2002/REC-xmlenc-core-20021210/>.
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[WS-Federation]
Goodner, M., Kaler, C., McIntosh, M., and A. Nadalin, "Web
Services Federation Language (WS-Federation) Version 1.2",
Oasis Standard, May 2009, <
http://docs.oasis-open.org/
wsfed/federation/v1.2/os/ws-federation-1.2-spec-os.html>.
[XMPP-E2E] Miller, M., "End-to-End Object Encryption and Signatures
for the Extensible Messaging and Presence Protocol
(XMPP)", Work in Progress, June 2013.
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Appendix A. Acknowledgements
Thanks to Matt Miller for discussions related to the XMPP end-to-end
security model and to Mike Jones for considerations related to
security tokens and XML security. Thanks to Mark Watson for raising
the need for representing symmetric keys and binding attributes to
them. Thanks to Ludwig Seitz for contributing the constrained device
use case.
Author's Address
Richard Barnes
Mozilla
331 E. Evelyn Ave.
Mountain View, CA 94041
US
EMail:
[email protected]
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