JSON Web Encryption (JWE)
draft-jones-json-web-encryption-02

Abstract

JSON Web Encryption (JWE) is a means of representing encrypted content using JSON data structures. Related signature capabilities are described in the separate JSON Web Signature (JWS) specification.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on June 13, 2012.

Copyright Notice

Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. Terminology
• 3. JSON Web Encryption (JWE) Overview
• 3.1. Example JWE
• 4. JWE Header
• 4.1. Reserved Header Parameter Names
• 4.2. Public Header Parameter Names
• 4.3. Private Header Parameter Names
• 5. Message Encryption
• 6. Message Decryption
• 7. CEK Encryption
• 7.1. Asymmetric Encryption
• 7.2. Symmetric Encryption
• 8. Composition
• 9. Encrypting JWEs with Cryptographic Algorithms
• 9.1. Encrypting a JWE with TBD
• 9.2. Additional Algorithms
• 10. IANA Considerations
• 11. Security Considerations
• 11.1. Unicode Comparison Security Issues
• 12. Open Issues and Things To Be Done (TBD)
• 13. References
• 13.1. Normative References
• 13.2. Informative References
• Appendix A. JWE Examples
• Appendix A.1. JWE Example using TBD Algorithm
• Appendix A.1.1. Encrypting
• Appendix A.1.2. Decrypting
• Appendix B. Algorithm Identifier Cross-Reference
• Appendix C. Acknowledgements
• Appendix D. Document History
• Authors' Addresses

1. Introduction

JSON Web Encryption (JWE) is a compact encryption format intended for space constrained environments such as HTTP Authorization headers and URI query parameters. It provides a wrapper for encrypted content using JSON RFC 4627 [RFC4627] data structures. The JWE encryption mechanisms are independent of the type of content being encrypted. A related signature capability is described in a separate JSON Web Signature (JWS) [JWS] specification.

2. Terminology

JSON Web Encryption (JWE)

A data structure representing an encrypted version of a Plaintext. The structure consists of three parts: the JWE Header, the JWE Encrypted Key, and the JWE Ciphertext.

Plaintext

The bytes to be encrypted - a.k.a., the message.

Ciphertext

The encrypted version of the Plaintext.

Content Encryption Key (CEK)

A symmetric key generated to encrypt the Plaintext for the recipient to produce the Ciphertext, which is encrypted to the recipient as the JWE Encrypted Key.

JWE Header

A string representing a JSON object that describes the encryption operations applied to create the JWE Encrypted Key and the JWE Ciphertext.

JWE Encrypted Key

The Content Encryption Key (CEK) is encrypted with the intended recipient's key and the resulting encrypted content is recorded as a byte array, which is referred to as the JWE Encrypted Key.

JWE Ciphertext

A byte array containing the Ciphertext.

Encoded JWE Header

Base64url encoding of the bytes of the UTF-8 RFC 3629 [RFC3629] representation of the JWE Header.

Encoded JWE Encrypted Key

Base64url encoding of the JWE Encrypted Key.

Encoded JWE Ciphertext

Base64url encoding of the JWE Ciphertext.

Header Parameter Names

The names of the members within the JWE Header.

Header Parameter Values

The values of the members within the JWE Header.

Base64url Encoding

For the purposes of this specification, this term always refers to the URL- and filename-safe Base64 encoding described in RFC 4648 [RFC4648], Section 5, with the (non URL-safe) '=' padding characters omitted, as permitted by Section 3.2. (See Appendix C of [JWS] for notes on implementing base64url encoding without padding.)

3. JSON Web Encryption (JWE) Overview

JWE represents encrypted content using JSON data structures and base64url encoding. The representation consists of three parts: the JWE Header, the JWE Encrypted Key, and the JWE Ciphertext. The three parts are base64url-encoded for transmission, and typically represented as the concatenation of the encoded strings in that order, with the three strings being separated by period ('.') characters.

JWE utilizes encryption to ensure the confidentiality of the contents of the Plaintext. JWE does not add a content integrity check if not provided by the underlying encryption algorithm. If such a check is needed, an algorithm providing it such as AES-GCM [NIST-800-38D] can be used, or alternatively, it can be provided through composition by encrypting a representation of the signed content.

3.1. Example JWE

The following example JWE Header declares that:

• the Content Encryption Key is encrypted to the recipient using the RSA-PKCS1_1.5 algorithm to produce the JWE Encrypted Key,
• the Plaintext is encrypted using the AES-256-GCM algorithm to produce the JWE Ciphertext,
• the specified 64-bit Initialization Vector with the base64url encoding __79_Pv6-fg was used, and
• the thumbprint of the X.509 certificate that corresponds to the key used to encrypt the JWE has the base64url encoding 7noOPq-hJ1_hCnvWh6IeYI2w9Q0.

{"alg":"RSA1_5",
 "enc":"A256GCM",
 "iv":"__79_Pv6-fg",
 "x5t":"7noOPq-hJ1_hCnvWh6IeYI2w9Q0"}

Base64url encoding the bytes of the UTF-8 representation of the JWE Header yields this Encoded JWE Header value (with line breaks for display purposes only):

eyJhbGciOiJSU0ExXzUiLA0KICJlbmMiOiJBMjU2R0NNIiwNCiAiaXYiOiJfXzc5
X1B2Ni1mZyIsDQogIng1dCI6Ijdub09QcS1oSjFfaENudldoNkllWUkydzlRMCJ9

TBD: Finish this example by showing generation of a Content Encryption Key (CEK), using the CEK to encrypt the Plaintext to produce the Ciphertext (and base64url encoding it), and using the recipient's key to encrypt the CEK to produce the JWE Encrypted Key (and base64url encoding it).

4. JWE Header

The members of the JSON object represented by the JWE Header describe the encryption applied to the Plaintext and optionally additional properties of the JWE. The Header Parameter Names within this object MUST be unique. Implementations MUST understand the entire contents of the header; otherwise, the JWE MUST be rejected for processing.

4.1. Reserved Header Parameter Names

The following header parameter names are reserved. All the names are short because a core goal of JWE is for the representations to be compact.

TBD: Describe the relationship between the JWS and JWE header parameters - especially the alg parameter, which can contain either signature algorithms (from JWS) or encryption algorithms (from JWE), and the key reference parameters jku, kid, x5u, and x5t.

Additional reserved header parameter names MAY be defined via the IANA JSON Web Encryption Header Parameters registry, as per Section 10. The syntax values used above are defined as follows:

4.2. Public Header Parameter Names

Additional header parameter names can be defined by those using JWE. However, in order to prevent collisions, any new header parameter name or algorithm value SHOULD either be defined in the IANA JSON Web Encryption Header Parameters registry or be defined as a URI that contains a collision resistant namespace. In each case, the definer of the name or value needs to take reasonable precautions to make sure they are in control of the part of the namespace they use to define the header parameter name.

New header parameters should be introduced sparingly, as they can result in non-interoperable JWEs.

4.3. Private Header Parameter Names

A producer and consumer of a JWE may agree to any header parameter name that is not a Reserved Name Section 4.1 or a Public Name Section 4.2. Unlike Public Names, these private names are subject to collision and should be used with caution.

New header parameters should be introduced sparingly, as they can result in non-interoperable JWEs.

5. Message Encryption

The message encryption process is as follows:

1. Generate a random Content Encryption Key (CEK). The CEK MUST have a length at least equal to that of the required encryption keys and MUST be generated randomly. See RFC 4086 [RFC4086] for considerations on generating random values.
2. Encrypt the CEK for the recipient (see Section 7).
3. Generate a random IV (if required for the algorithm).
4. Compress the Plaintext if a zip parameter was included.
5. Serialize the (compressed) Plaintext into a bitstring M.
6. Encrypt M using the CEK and IV to form the bitstring C.
7. Set the Encoded JWE Ciphertext equal to the base64url encoded representation of C.
8. Create a JWE Header containing the encryption parameters used. Note that white space is explicitly allowed in the representation and no canonicalization is performed before encoding.
9. Base64url encode the bytes of the UTF-8 representation of the JWE Header to create the Encoded JWE Header.
10. The three encoded parts, taken together, are the result of the encryption.

6. Message Decryption

The message decryption process is the reverse of the encryption process. If any of these steps fails, the JWE MUST be rejected.

1. The Encoded JWE Header, the Encoded JWE Encrypted Key, and the Encoded JWE Ciphertext MUST be successfully base64url decoded following the restriction that no padding characters have been used.
2. The resulting JWE Header MUST be completely valid JSON syntax conforming to RFC 4627 [RFC4627].
3. The resulting JWE Header MUST be validated to only include parameters and values whose syntax and semantics are both understood and supported.
4. Verify that the JWE Header appears to reference a key known to the recipient.
5. Decrypt the JWE Encrypted Key to produce the CEK.
6. Decrypt the binary representation of the JWE Ciphertext using the CEK.
7. Uncompress the result of the previous step, if a zip parameter was included.
8. Output the result.

7. CEK Encryption

JWE supports two forms of CEK encryption:

• Asymmetric encryption under the recipient's public key.
• Symmetric encryption under a shared key.

7.1. Asymmetric Encryption

In the asymmetric encryption mode, the CEK is encrypted under the recipient's public key. The asymmetric encryption modes defined for use with this in this specification are listed in in Table 3.

7.2. Symmetric Encryption

In the symmetric encryption mode, the CEK is encrypted under a symmetric key shared between the sender and receiver. The symmetric encryption modes defined for use with this in this specification are listed in in Table 3. For GCM, the random 64-bit IV is prepended to the ciphertext.

8. Composition

This document does not specify a combination signed and encrypted mode. However, because the contents of a message can be arbitrary, encryption and data origin authentication can be provided by recursively encapsulating multiple JWE and JWS messages. In general, senders SHOULD sign the message and then encrypt the result (thus encrypting the signature). This prevents attacks in which the signature is stripped, leaving just an encrypted message, as well as providing privacy for the signer.

9. Encrypting JWEs with Cryptographic Algorithms

JWE uses cryptographic algorithms to encrypt the Content Encryption Key (CEK) and the Plaintext. This section specifies a set of specific algorithms for these purposes.

The table below Table 3 is the set of alg header parameter values that are defined by this specification. These algorithms are used to encrypt the CEK, which produces the JWE Encrypted Key.

The table below Table 4 is the set of enc header parameter values that are defined by this specification. These algorithms are used to encrypt the Plaintext, which produces the Ciphertext.

Of these algorithms, only RSA-PKCS1-1.5 with 2048 bit keys, AES-128-CBC, and AES-256-CBC MUST be implemented by conforming implementations. It is RECOMMENDED that implementations also support ECDH-ES with 256 bit keys, AES-128-GCM, and AES-256-GCM. Support for other algorithms and key sizes is OPTIONAL.

9.1. Encrypting a JWE with TBD

TBD: Descriptions of the particulars of using each specified algorithm go here.

9.2. Additional Algorithms

Additional algorithms MAY be used to protect JWEs with corresponding alg and enc header parameter values being defined to refer to them. New alg and enc header parameter values SHOULD either be defined in the IANA JSON Web Encryption Algorithms registry or be a URI that contains a collision resistant namespace. In particular, it is permissible to use the algorithm identifiers defined in XML Encryption [W3C.REC-xmlenc-core-20021210], XML Encryption 1.1 [W3C.CR-xmlenc-core1-20110303], and related specifications as alg and enc values.

10. IANA Considerations

This specification calls for:

• A new IANA registry entitled "JSON Web Encryption Header Parameters" for reserved header parameter names is defined in Section 4.1. Inclusion in the registry is RFC Required in the RFC 5226 [RFC5226] sense for reserved JWE header parameter names that are intended to be interoperable between implementations. The registry will just record the reserved header parameter name and a pointer to the RFC that defines it. This specification defines inclusion of the header parameter names defined in Table 1.
• A new IANA registry entitled "JSON Web Encryption Algorithms" for values used with the alg and enc header parameters is defined in Section 9.2. Inclusion in the registry is RFC Required in the RFC 5226 [RFC5226] sense. The registry will record the alg or enc value and a pointer to the RFC that defines it. This specification defines inclusion of the algorithm values defined in Table 3 and Table 4.

11. Security Considerations

TBD: Lots of work to do here. We need to remember to look into any issues relating to security and JSON parsing. One wonders just how secure most JSON parsing libraries are. Were they ever hardened for security scenarios? If not, what kind of holes does that open up? Also, we need to walk through the JSON standard and see what kind of issues we have especially around comparison of names. For instance, comparisons of header parameter names and other parameters must occur after they are unescaped. Need to also put in text about: Importance of keeping secrets secret. Rotating keys. Strengths and weaknesses of the different algorithms.

TBD: Need to put in text about why strict JSON validation is necessary. Basically, that if malformed JSON is received then the intent of the sender is impossible to reliably discern. One example of malformed JSON that MUST be rejected is an object in which the same member name occurs multiple times.

TBD: We need a section on generating randomness in browsers - it's easy to screw up.

When utilizing TLS to retrieve information, the authority providing the resource MUST be authenticated and the information retrieved MUST be free from modification.

11.1. Unicode Comparison Security Issues

Header parameter names in JWEs are Unicode strings. For security reasons, the representations of these names must be compared verbatim after performing any escape processing (as per RFC 4627 [RFC4627], Section 2.5).

This means, for instance, that these JSON strings must compare as being equal ("enc", "\u0065nc"), whereas these must all compare as being not equal to the first set or to each other ("ENC", "Enc", "en\u0043").

JSON strings MAY contain characters outside the Unicode Basic Multilingual Plane. For instance, the G clef character (U+1D11E) may be represented in a JSON string as "\uD834\uDD1E". Ideally, JWE implementations SHOULD ensure that characters outside the Basic Multilingual Plane are preserved and compared correctly; alternatively, if this is not possible due to these characters exercising limitations present in the underlying JSON implementation, then input containing them MUST be rejected.

12. Open Issues and Things To Be Done (TBD)

The following items remain to be done in this draft:

• Describe the relationship between the JWE, JWS, and JWT header parameters. In particular, point out that the set of "alg" values defined by each must be compatible and non-overlapping.
• Consider whether we want to define composite signing/encryption operations (as was the consensus to do at IIW, as documented at http://self-issued.info/?p=378). This would provide both confidentiality and integrity.
• Consider whether reusing the JWS jku, kid, x5u, and x5t parameters is the right thing to do, particularly as it effectively precludes specifying composite operations.
• Consider whether to add parameters for directly including keys in the header, either as JWK Key Objects, or X.509 cert values, or both.
• Consider whether to add version numbers.
• Consider which of the open issues from the JWS and JWT specs also apply here.
• Think about how to best describe the concept currently described as "the bytes of the UTF-8 representation of". Possible terms to use instead of "bytes of" include "byte sequence", "octet series", and "octet sequence". Also consider whether we want to add an overall clarifying statement somewhere in each spec something like "every place we say 'the UTF-8 representation of X', we mean 'the bytes of the UTF-8 representation of X'". That would potentially allow us to omit the "the bytes of" part everywhere else.
• Finish the Security Considerations section.
• Write a note in the Security Considerations section about how x5t (x.509 certificate thumbprint) should be deprecated because of known problems with SHA-1.
• Should StringOrURI use IRIs rather than RFC 3986 URIs?
• Provide a more robust description of the use of the IV. The current statement "For GCM, the random 64-bit IV is prepended to the ciphertext" in the Symmetric Encryption section is almost certainly out of place.
• It would be good to say somewhere, in normative language, that eventually the algorithms and/or key sizes currently specified will no longer be considered sufficiently secure and will be removed. Therefore, implementers MUST be prepared for this eventuality.
• Should we define the use of RFC 5649 key wrapping functions, which allow arbitrary key sizes, in addition to the current use of RFC 3394 key wrapping functions, which require that keys be multiples of 64 bits? Is this needed in practice?

13. References

13.1. Normative References

13.2. Informative References

Appendix A. JWE Examples

This section provides several examples of JWEs.

Appendix A.1. JWE Example using TBD Algorithm

Appendix A.1.1. Encrypting

TBD: Demonstrate encryption steps with this algorithm

Appendix A.1.2. Decrypting

TBD: Demonstrate decryption steps with this algorithm

Appendix B. Algorithm Identifier Cross-Reference

This appendix contains a table cross-referencing the alg and enc values used in this specification with the equivalent identifiers used by other standards and software packages. See XML Encryption [W3C.REC-xmlenc-core-20021210], XML Encryption 1.1 [W3C.CR-xmlenc-core1-20110303], and Java Cryptography Architecture [JCA] for more information about the names defined by those documents.

Appendix C. Acknowledgements

Solutions for encrypting JSON content were also explored by [JSS] and [I-D.rescorla-jsms], both of which significantly influenced this draft. This draft attempts to explicitly reuse as much from XML Encryption 1.1 [W3C.CR-xmlenc-core1-20110303] and RFC 5652 [RFC5652] as possible, while utilizing simple compact JSON-based data structures.

Special thanks are due to John Bradley and Nat Sakimura for the discussions that helped inform the content of this specification and to Eric Rescorla and Joe Hildebrand for allowing the reuse of some of the text from [I-D.rescorla-jsms] in this document.

Appendix D. Document History

-02

• Update to use short JWK Key Object names in Ephemeral Public Keys.
• Moved "MUST" requirements from the Overview to later in the spec.
• Respect line length restrictions in examples.
• Applied other editorial improvements.

-01

• Changed type of Ephemeral Public Key (epk) from string to JSON object, so that a JWK Key Object value can be used directly.
• Specified that the Digest Method for ECDH-ES is SHA-256. (The specification was previously silent about the choice of digest method.)
• The jku and x5u URLs are now required to be absolute URLs.
• Removed this unnecessary language from the kid description: "Omitting this parameter is equivalent to setting it to an empty string".
• Use the same language as RFC 2616 does when describing GZIP message compression.

-00

• First encryption draft based upon consensus decisions at IIW documented at http://self-issued.info/?p=378. The ability to provide encryption for JSON Web Tokens (JWTs) [JWT] is a primary use case.

Authors' Addresses