JSON Web Encryption (JWE)
draft-ietf-jose-json-web-encryption-01

Abstract

JSON Web Encryption (JWE) is a means of representing encrypted content using JSON data structures. Cryptographic algorithms and identifiers used with this specification are enumerated in the separate JSON Web Algorithms (JWA) specification. Related digital signature and HMAC 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 September 11, 2012.

Copyright Notice

Copyright (c) 2012 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 with an Integrated Integrity Check
• 3.2. Example JWE with a Separate Integrity Check
• 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. Key Derivation
• 8. CMK Encryption
• 8.1. Asymmetric Encryption
• 8.2. Symmetric Encryption
• 9. Integrity Value Calculation
• 10. Encrypting JWEs with Cryptographic Algorithms
• 11. IANA Considerations
• 12. Security Considerations
• 12.1. Unicode Comparison Security Issues
• 13. Open Issues and Things To Be Done (TBD)
• 14. References
• 14.1. Normative References
• 14.2. 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. Acknowledgements
• Appendix C. 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. Cryptographic algorithms and identifiers used with this specification are enumerated in the separate JSON Web Algorithms (JWA) [JWA] specification. Related digital signature and HMAC capabilities are described in the 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 four parts: the JWE Header, the JWE Encrypted Key, the JWE Ciphertext, and the JWE Integrity Value.

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 used to encrypt the Plaintext for the recipient to produce the Ciphertext.

Content Integrity Key (CIK)

A key used with an HMAC function to ensure the integrity of the Ciphertext and the parameters used to create it.

Content Master Key (CMK)

A randomly generated key from which the CEK and CIK are derived, 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.

JWE Integrity Value

A byte array containing a HMAC value that ensures the integrity of the Ciphertext and the parameters used to create it.

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.

Encoded JWE Integrity Value

Base64url encoding of the JWE Integrity Value.

Header Parameter Names

The names of the members within the JWE Header.

Header Parameter Values

The values of the members within the JWE Header.

JWE Compact Serialization

A representation of the JWE as the concatenation of the Encoded JWE Header, the Encoded JWE Encrypted Key, the Encoded JWE Ciphertext, and the Encoded JWE Integrity Value in that order, with the four strings being separated by period ('.') characters.

AEAD Algorithm

An Authenticated Encryption with Associated Data (AEAD) [RFC5116] encryption algorithm is one that provides an integrated content integrity check. AES Galois/Counter Mode (GCM) is one such algorithm.

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 B 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 four parts: the JWE Header, the JWE Encrypted Key, the JWE Ciphertext, and the JWE Integrity Value. In the Compact Serialization, the four parts are base64url-encoded for transmission, and represented as the concatenation of the encoded strings in that order, with the four strings being separated by period ('.') characters. (A JSON Serialization for this information is defined in the separate JSON Web Encryption JSON Serialization (JWE-JS) [JWE-JS] specification.)

JWE utilizes encryption to ensure the confidentiality of the contents of the Plaintext. JWE adds a content integrity check if not provided by the underlying encryption algorithm.

3.1. Example JWE with an Integrated Integrity Check

The following example JWE Header declares that:

• the Content Master 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
• a JSON Web Key (JWK) representation of the public key used to encrypt the JWE is located at https://example.com/public_key.jwk.

{"alg":"RSA1_5",
 "enc":"A256GCM",
 "iv":"__79_Pv6-fg",
 "jku":"https://example.com/public_key.jwk"}

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
X1B2Ni1mZyIsDQogImprdSI6Imh0dHBzOi8vZXhhbXBsZS5jb20vcHVibGljX2tl
eS5qd2sifQ

TBD: Finish this example by showing generation of a Content Master Key (CMK), saying that the CMK is used as the CEK and there is no separate integrity check since AES GCM is an AEAD algorithm, using the CEK to encrypt the Plaintext to produce the Ciphertext, using the recipient's key to encrypt the CMK to produce the JWE Encrypted Key, base64url encoding these values, and assembling the result.

Concatenating these parts in the order Header.EncryptedKey.Ciphertext.IntegrityValue with period characters between the parts yields this complete JWE representation (with line breaks for display purposes only):

eyJhbGciOiJSU0ExXzUiLA0KICJlbmMiOiJBMjU2R0NNIiwNCiAiaXYiOiJfXzc5
X1B2Ni1mZyIsDQogImprdSI6Imh0dHBzOi8vZXhhbXBsZS5jb20vcHVibGljX2tl
eS5qd2sifQ
.
TBD_encrypted_key_value_TBD
.
TBD_ciphertext_value_TBD
.

3.2. Example JWE with a Separate Integrity Check

The following example JWE Header declares that:

• the Content Master 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-CBC algorithm to produce the JWE Ciphertext,
• the JWE Integrity Value safeguarding the integrity of the Ciphertext and the parameters used to create it was computed with the HMAC SHA-256 algorithm,
• the specified 64-bit Initialization Vector with the base64url encoding Mz-mW_4JHfg 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":"A256CBC",
 "int":"HS256",
 "iv":"Mz-mW_4JHfg",
 "x5t":"7noOPq-hJ1_hCnvWh6IeYI2w9Q0"}

Because AES CBC is not an AEAD algorithm (and so provides no integrated content integrity check), a separate integrity check value is used.

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):

eyJhbGciOiJSU0ExXzUiLA0KICJlbmMiOiJBMjU2Q0JDIiwNCiAiaW50IjoiSFMy
NTYiLA0KICJpdiI6Ik16LW1XXzRKSGZnIiwNCiAieDV0IjoiN25vT1BxLWhKMV9o
Q252V2g2SWVZSTJ3OVEwIn0

TBD: Finish this example by showing generation of a Content Master Key (CMK), showing the derivation of the CEK and the CEK from the CMK, using the CEK to encrypt the Plaintext to produce the Ciphertext, using the recipient's key to encrypt the CMK to produce the JWE Encrypted Key, showing the computation of the JWE Integrity Value, base64url encoding these values, and assembling the result.

eyJhbGciOiJSU0ExXzUiLA0KICJlbmMiOiJBMjU2Q0JDIiwNCiAiaW50IjoiSFMy
NTYiLA0KICJpdiI6Ik16LW1XXzRKSGZnIiwNCiAieDV0IjoiN25vT1BxLWhKMV9o
Q252V2g2SWVZSTJ3OVEwIn0
.
TBD_encrypted_key_value_TBD
.
TBD_ciphertext_value_TBD
.
TBD_integrity_value_TBD

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.

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.

Additional reserved header parameter names MAY be defined via the IANA JSON Web Encryption Header Parameters registry, as per Section 11. 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 since an implementation that does not understand a parameter MUST reject the JWE.

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. The order of the steps is not significant in cases where there are no dependencies between the inputs and outputs of the steps.

1. Generate a random Content Master Key (CMK). The CMK MUST have a length at least equal to that of the larger of the required encryption or integrity keys and MUST be generated randomly. See RFC 4086 [RFC4086] for considerations on generating random values.
2. Encrypt the CMK for the recipient (see Section 8) and let the result be the JWE Encrypted Key.
3. Base64url encode the JWE Encrypted Key to create the Encoded JWE Encrypted Key.
4. Generate a random Initialization Vector (IV) (if required for the algorithm).
5. If not using an AEAD algorithm, run the key derivation algorithm (see Section 7) to generate the Content Encryption Key (CEK) and the Content Integrity Key (CIK); otherwise (when using an AEAD algorithm), set the CEK to be the CMK.
6. Compress the Plaintext if a zip parameter was included.
7. Serialize the (compressed) Plaintext into a bitstring M.
8. Encrypt M using the CEK and IV to form the bitstring C.
9. Base64url encode C to create the Encoded JWE Ciphertext.
10. Create a JWE Header containing the encryption parameters used. Note that white space is explicitly allowed in the representation and no canonicalization need be performed before encoding.
11. Base64url encode the bytes of the UTF-8 representation of the JWE Header to create the Encoded JWE Header.
12. If not using an AEAD algorithm, run the integrity algorithm (see Section 9) using the CIK to compute the JWE Integrity Value; otherwise (when using an AEAD algorithm), set the JWE Integrity Value to be the empty byte string.
13. Base64url encode the JWE Integrity Value to create the Encoded JWE Integrity Value.
14. The four encoded parts, taken together, are the result. The Compact Serialization of this result is the concatenation of the Encoded JWE Header, the Encoded JWE Encrypted Key, the Encoded JWE Ciphertext, and the Encoded JWE Integrity Value in that order, with the four strings being separated by period ('.') characters.

6. Message Decryption

The message decryption process is the reverse of the encryption process. The order of the steps is not significant in cases where there are no dependencies between the inputs and outputs of the steps. If any of these steps fails, the JWE MUST be rejected.

1. Parse the four parts of the input (which are separated by period characters when using the JWE Compact Serialization) into the Encoded JWE Header, the Encoded JWE Encrypted Key, the Encoded JWE Ciphertext, and the Encoded JWE Integrity Value.
2. The Encoded JWE Header, the Encoded JWE Encrypted Key, the Encoded JWE Ciphertext, and the Encoded JWE Integrity Value MUST be successfully base64url decoded following the restriction that no padding characters have been used.
3. The resulting JWE Header MUST be completely valid JSON syntax conforming to RFC 4627 [RFC4627].
4. The resulting JWE Header MUST be validated to only include parameters and values whose syntax and semantics are both understood and supported.
5. Verify that the JWE Header references a key known to the recipient.
6. Decrypt the JWE Encrypted Key to produce the Content Master Key (CMK).
7. If not using an AEAD algorithm, run the key derivation algorithm (see Section 7) to generate the Content Encryption Key (CEK) and the Content Integrity Key (CIK); otherwise (when using an AEAD algorithm), set the CEK to be the CMK.
8. If not using an AEAD algorithm, run the integrity algorithm (see Section 9) using the CIK to compute an integrity value for the input received. This computed value MUST match the received JWE Integrity Value; otherwise (when using an AEAD algorithm), the received JWE Integrity Value MUST be empty.
9. Decrypt the binary representation of the JWE Ciphertext using the CEK.
10. Remove the Initialization Vector (IV) value from the decrypted result (if an IV was used).
11. Uncompress the result of the previous step, if a zip parameter was included.
12. Output the resulting Plaintext.

7. Key Derivation

The key derivation process converts the CMK into a CEK and a CIK. It assumes as a primitive a Key Derivation Function (KDF) which notionally takes three arguments: [NIST-800-56A], where the Digest Method is SHA-256, the SuppPubInfo parameter is the Label, and the remaining OtherInfo parameters are the empty bit string.

MasterKey:

The master key used to compute the individual use keys

Label:

The use key label, used to differentiate individual use keys

Length:

The length of the desired use key

The only KDF used in this document is the Concat KDF, as defined in

To compute the CEK from the CMK, the ASCII label "Encryption" is used.

To compute the CIK from the CMK, the ASCII label "Integrity" is used.

When AEAD algorithms are used the KDF element MUST NOT be present. When they are not used, it MUST be present.

8. CMK Encryption

JWE supports two forms of CMK encryption:

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

8.1. Asymmetric Encryption

In the asymmetric encryption mode, the CMK is encrypted under the recipient's public key. The asymmetric encryption modes defined for use with this in this specification are listed in Section 4, Table 2 of the JSON Web Algorithms (JWA) [JWA] specification.

8.2. Symmetric Encryption

In the symmetric encryption mode, the CMK 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 Section 4, Table 2 of the JSON Web Algorithms (JWA) [JWA] specification. For GCM, the random 64-bit IV is prepended to the ciphertext.

9. Integrity Value Calculation

When a non-AEAD algorithm is used (an algorithm without an integrated content check), JWE adds an explicit integrity check value to the representation. This value is computed in the manner described in the JSON Web Signature (JWS) [JWS] specification, with these modifications:

• The algorithm used is taken from the int (integrity algorithm) header parameter rather than the alg header parameter.
• The algorithm MUST be an HMAC algorithm (normally HMAC SHA-256).
• The JWS Secured Input used is the concatenation of the Encoded JWE Header, a period ('.') character, the Encoded JWE Encrypted Key, a period ('.') character, and the Encoded JWE Ciphertext.
• The CIK is used as the HMAC key.

The computed JWS Signature value is the resulting integrity value.

10. Encrypting JWEs with Cryptographic Algorithms

JWE uses cryptographic algorithms to encrypt the Content Encryption Key (CMK) and the Plaintext. The JSON Web Algorithms (JWA) [JWA] specification enumerates a set of cryptographic algorithms and identifiers to be used with this specification. Specifically, Section 4, Table 2 enumerates a set of alg (algorithm) header parameter values and Section 4, Table 3 enumerates a set of enc (encryption method) header parameter values intended for use this specification. It also describes the semantics and operations that are specific to these algorithms and algorithm families.

Public keys employed for encryption can be identified using the Header Parameter methods described in Section 4.1 or can be distributed using methods that are outside the scope of this specification.

11. 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.

12. 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.

12.1. 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.

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

The following items remain to be done in this draft:

• EDITORIAL: Give each header parameter definition its own section. This will let them appear in the index, will give space for examples when needed, and will get rid of the way-too-cramped tables.
• Consider adding the DEFLATE compression algorithm (which omits the ZLIB header and checksum fields) and so produces smaller results than GZIP.
• Provide a more robust description of the use of the Initialization Vector (IV), including listing which algorithms require an IV. (This list may belong in the JWA spec.) 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 and insufficiently general.
• Finish the Security Considerations section.
• Consider which of the open issues from the JWS and JWT specs also apply here.
• Should the JWE Encrypted Key be moved to the header (which would add about 20 bytes to every JWE) or left in a separate period-separated segment to prevent double base64 encoding?

14. References

14.1. Normative References

14.2. 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. Acknowledgements

Solutions for encrypting JSON content were also explored by JSON Simple Encryption [JSE] and JavaScript Message Security Format [I-D.rescorla-jsms], both of which significantly influenced this draft. This draft attempts to explicitly reuse as many of the relevant concepts 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 text from [I-D.rescorla-jsms] in this document.

Appendix C. Document History

-01

• Added an integrity check for non-AEAD algorithms.
• Added jpk and x5c header parameters for including JWK public keys and X.509 certificate chains directly in the header.
• Clarified that this specification is defining the JWE Compact Serialization. Referenced the new JWE-JS spec, which defines the JWE JSON Serialization.
• Added text "New header parameters should be introduced sparingly since an implementation that does not understand a parameter MUST reject the JWE".
• Clarified that the order of the encryption and decryption steps is not significant in cases where there are no dependencies between the inputs and outputs of the steps.
• Made other editorial improvements suggested by JOSE working group participants.

-00

• Created the initial IETF draft based upon draft-jones-json-web-encryption-02 with no normative changes.
• Changed terminology to no longer call both digital signatures and HMACs "signatures".

Authors' Addresses