JSON Web Algorithms (JWA)
draft-ietf-jose-json-web-algorithms-05

Abstract

The JSON Web Algorithms (JWA) specification enumerates cryptographic algorithms and identifiers to be used with the JSON Web Signature (JWS), JSON Web Encryption (JWE), and JSON Web Key (JWK) specifications.

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/⁠.

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This Internet-Draft will expire on January 29, 2013.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Notational Conventions
• 2. Terminology
• 2.1. Terms Incorporated from the JWS Specification
• 2.2. Terms Incorporated from the JWE Specification
• 2.3. Terms Incorporated from the JWK Specification
• 2.4. Defined Terms
• 3. Cryptographic Algorithms for JWS
• 3.1. "alg" (Algorithm) Header Parameter Values for JWS
• 3.2. MAC with HMAC SHA-256, HMAC SHA-384, or HMAC SHA-512
• 3.3. Digital Signature with RSA SHA-256, RSA SHA-384, or RSA SHA-512
• 3.4. Digital Signature with ECDSA P-256 SHA-256, ECDSA P-384 SHA-384, or ECDSA P-521 SHA-512
• 3.5. Using the Algorithm "none"
• 3.6. Additional Digital Signature/MAC Algorithms and Parameters
• 4. Cryptographic Algorithms for JWE
• 4.1. "alg" (Algorithm) Header Parameter Values for JWE
• 4.2. "enc" (Encryption Method) Header Parameter Values for JWE
• 4.3. "int" (Integrity Algorithm) Header Parameter Values for JWE
• 4.4. "kdf" (Key Derivation Function) Header Parameter Values for JWE
• 4.5. Key Encryption with RSAES-PKCS1-V1_5
• 4.6. Key Encryption with RSAES OAEP
• 4.7. Key Encryption with AES Key Wrap
• 4.8. Direct Encryption with a Shared Symmetric Key
• 4.9. Key Agreement with Elliptic Curve Diffie-Hellman Ephemeral Static (ECDH-ES)
• 4.10. Plaintext Encryption with AES CBC Mode
• 4.11. Plaintext Encryption with AES GCM
• 4.12. Integrity Calculation with HMAC SHA-256, HMAC SHA-384, or HMAC SHA-512
• 4.13. Key Derivation with Concat KDF and SHA-256, SHA-384, or SHA-512
• 4.14. Additional Encryption Algorithms and Parameters
• 5. Cryptographic Algorithms for JWK
• 5.1. "alg" (Algorithm Family) Parameter Values for JWK
• 5.2. JWK Parameters for Elliptic Curve Keys
• 5.2.1. "crv" (Curve) Parameter
• 5.2.2. "x" (X Coordinate) Parameter
• 5.2.3. "y" (Y Coordinate) Parameter
• 5.3. JWK Parameters for RSA Keys
• 5.3.1. "mod" (Modulus) Parameter
• 5.3.2. "exp" (Exponent) Parameter
• 5.4. Additional Key Algorithm Families and Parameters
• 6. IANA Considerations
• 6.1. JSON Web Signature and Encryption Algorithms Registry
• 6.1.1. Registration Template
• 6.1.2. Initial Registry Contents
• 6.2. JSON Web Key Algorithm Families Registry
• 6.2.1. Registration Template
• 6.2.2. Initial Registry Contents
• 6.3. JSON Web Key Parameters Registration
• 6.3.1. Registry Contents
• 7. Security Considerations
• 8. Open Issues
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Digital Signature/MAC Algorithm Identifier Cross-Reference
• Appendix B. Encryption Algorithm Identifier Cross-Reference
• Appendix C. Acknowledgements
• Appendix D. Document History
• Author's Address

1. Introduction

The JSON Web Algorithms (JWA) specification enumerates cryptographic algorithms and identifiers to be used with the JSON Web Signature (JWS) [JWS], JSON Web Encryption (JWE) [JWE], and JSON Web Key (JWK) [JWK] specifications. All these specifications utilize JavaScript Object Notation (JSON) [RFC4627] based data structures. This specification also describes the semantics and operations that are specific to these algorithms and algorithm families.

Enumerating the algorithms and identifiers for them in this specification, rather than in the JWS, JWE, and JWK specifications, is intended to allow them to remain unchanged in the face of changes in the set of required, recommended, optional, and deprecated algorithms over time.

1.1. Notational Conventions

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 Key words for use in RFCs to Indicate Requirement Levels [RFC2119].

2. Terminology

2.1. Terms Incorporated from the JWS Specification

These terms defined by the JSON Web Signature (JWS) [JWS] specification are incorporated into this specification:

JSON Web Signature (JWS)

A data structure cryptographically securing a JWS Header and a JWS Payload with a JWS Signature value.

JWS Header

A string representing a JavaScript Object Notation (JSON) [RFC4627] object that describes the digital signature or MAC operation applied to create the JWS Signature value.

JWS Payload

The bytes to be secured - a.k.a., the message. The payload can contain an arbitrary sequence of bytes.

JWS Signature

A byte array containing the cryptographic material that secures the contents of the JWS Header and the JWS Payload.

Base64url Encoding

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.)

Encoded JWS Header

Base64url encoding of the bytes of the UTF-8 [RFC3629] representation of the JWS Header.

Encoded JWS Payload

Base64url encoding of the JWS Payload.

Encoded JWS Signature

Base64url encoding of the JWS Signature.

JWS Secured Input

The concatenation of the Encoded JWS Header, a period ('.') character, and the Encoded JWS Payload.

Collision Resistant Namespace

A namespace that allows names to be allocated in a manner such that they are highly unlikely to collide with other names. For instance, collision resistance can be achieved through administrative delegation of portions of the namespace or through use of collision-resistant name allocation functions. Examples of Collision Resistant Namespaces include: Domain Names, Object Identifiers (OIDs) as defined in the ITU-T X.660 and X.670 Recommendation series, and Universally Unique IDentifiers (UUIDs) [RFC4122]. When using an administratively delegated namespace, the definer of a name needs to take reasonable precautions to ensure they are in control of the portion of the namespace they use to define the name.

2.2. Terms Incorporated from the JWE Specification

These terms defined by the JSON Web Encryption (JWE) [JWE] specification are incorporated into this specification:

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. The plaintext can contain an arbitrary sequence of bytes.

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 a MAC function to ensure the integrity of the Ciphertext and the parameters used to create it.

Content Master Key (CMK)

A key from which the CEK and CIK are derived. When key wrapping or key encryption are employed, the CMK is randomly generated and encrypted to the recipient as the JWE Encrypted Key. When key agreement is employed, the CMK is the result of the key agreement algorithm.

JWE Header

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

JWE Encrypted Key

When key wrapping or key encryption are employed, the Content Master Key (CMK) 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. Otherwise, when key agreement is employed, the JWE Encrypted Key is the empty byte array.

JWE Ciphertext

A byte array containing the Ciphertext.

JWE Integrity Value

A byte array containing a MAC 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 [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.

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.

2.3. Terms Incorporated from the JWK Specification

These terms defined by the JSON Web Key (JWK) [JWK] specification are incorporated into this specification:

JSON Web Key (JWK)

A JSON data structure that represents a public key.

JSON Web Key Set (JWK Set)

A JSON object that contains an array of JWKs as a member.

2.4. Defined Terms

These terms are defined for use by this specification:

Header Parameter Name

The name of a member of the JSON object representing a JWS Header or JWE Header.

Header Parameter Value

The value of a member of the JSON object representing a JWS Header or JWE Header.

3. Cryptographic Algorithms for JWS

JWS uses cryptographic algorithms to digitally sign or create a Message Authentication Codes (MAC) of the contents of the JWS Header and the JWS Payload. The use of the following algorithms for producing JWSs is defined in this section.

3.1. "alg" (Algorithm) Header Parameter Values for JWS

The table below is the set of alg (algorithm) header parameter values defined by this specification for use with JWS, each of which is explained in more detail in the following sections:

All the names are short because a core goal of JWS is for the representations to be compact. However, there is no a priori length restriction on alg values.

The use of "+" in the Implementation Requirements indicates that the requirement strength is likely to be increased in a future version of the specification.

See Appendix Appendix A for a table cross-referencing the digital signature and MAC alg (algorithm) values used in this specification with the equivalent identifiers used by other standards and software packages.

3.2. MAC with HMAC SHA-256, HMAC SHA-384, or HMAC SHA-512

Hash-based Message Authentication Codes (HMACs) enable one to use a secret plus a cryptographic hash function to generate a Message Authentication Code (MAC). This can be used to demonstrate that the MAC matches the hashed content, in this case the JWS Secured Input, which therefore demonstrates that whoever generated the MAC was in possession of the secret. The means of exchanging the shared key is outside the scope of this specification.

The algorithm for implementing and validating HMACs is provided in RFC 2104 [RFC2104]. This section defines the use of the HMAC SHA-256, HMAC SHA-384, and HMAC SHA-512 functions [SHS]. The alg (algorithm) header parameter values HS256, HS384, and HS512 are used in the JWS Header to indicate that the Encoded JWS Signature contains a base64url encoded HMAC value using the respective hash function.

A key of the same size as the hash output (for instance, 256 bits for HS256) or larger MUST be used with this algorithm.

The HMAC SHA-256 MAC is generated per RFC 2104, using SHA-256 as the hash algorithm "H", using the bytes of the ASCII [USASCII] representation of the JWS Secured Input as the "text" value, and using the shared key. The HMAC output value is the JWS Signature. The JWS signature is base64url encoded to produce the Encoded JWS Signature.

The HMAC SHA-256 MAC for a JWS is validated by computing an HMAC value per RFC 2104, using SHA-256 as the hash algorithm "H", using the bytes of the ASCII representation of the received JWS Secured input as the "text" value, and using the shared key. This computed HMAC value is then compared to the result of base64url decoding the received Encoded JWS signature. Alternatively, the computed HMAC value can be base64url encoded and compared to the received Encoded JWS Signature, as this comparison produces the same result as comparing the unencoded values. In either case, if the values match, the HMAC has been validated. If the validation fails, the JWS MUST be rejected.

Securing content with the HMAC SHA-384 and HMAC SHA-512 algorithms is performed identically to the procedure for HMAC SHA-256 - just using the corresponding hash algorithm with correspondingly larger minimum key sizes and result values: 384 bits each for HMAC SHA-384 and 512 bits each for HMAC SHA-512.

3.3. Digital Signature with RSA SHA-256, RSA SHA-384, or RSA SHA-512

This section defines the use of the RSASSA-PKCS1-V1_5 digital signature algorithm as defined in Section 8.2 of RFC 3447 [RFC3447], (commonly known as PKCS #1), using SHA-256, SHA-384, or SHA-512 [SHS] as the hash functions. The alg (algorithm) header parameter values RS256, RS384, and RS512 are used in the JWS Header to indicate that the Encoded JWS Signature contains a base64url encoded RSA digital signature using the respective hash function.

A key of size 2048 bits or larger MUST be used with these algorithms.

The RSA SHA-256 digital signature is generated as follows:

1. Generate a digital signature of the bytes of the ASCII representation of the JWS Secured Input using RSASSA-PKCS1-V1_5-SIGN and the SHA-256 hash function with the desired private key. The output will be a byte array.
2. Base64url encode the resulting byte array.

The output is the Encoded JWS Signature for that JWS.

The RSA SHA-256 digital signature for a JWS is validated as follows:

1. Take the Encoded JWS Signature and base64url decode it into a byte array. If decoding fails, the JWS MUST be rejected.
2. Submit the bytes of the ASCII representation of the JWS Secured Input and the public key corresponding to the private key used by the signer to the RSASSA-PKCS1-V1_5-VERIFY algorithm using SHA-256 as the hash function.
3. If the validation fails, the JWS MUST be rejected.

Signing with the RSA SHA-384 and RSA SHA-512 algorithms is performed identically to the procedure for RSA SHA-256 - just using the corresponding hash algorithm with correspondingly larger result values: 384 bits for RSA SHA-384 and 512 bits for RSA SHA-512.

3.4. Digital Signature with ECDSA P-256 SHA-256, ECDSA P-384 SHA-384, or ECDSA P-521 SHA-512

The Elliptic Curve Digital Signature Algorithm (ECDSA) [DSS] provides for the use of Elliptic Curve cryptography, which is able to provide equivalent security to RSA cryptography but using shorter key sizes and with greater processing speed. This means that ECDSA digital signatures will be substantially smaller in terms of length than equivalently strong RSA digital signatures.

This specification defines the use of ECDSA with the P-256 curve and the SHA-256 cryptographic hash function, ECDSA with the P-384 curve and the SHA-384 hash function, and ECDSA with the P-521 curve and the SHA-512 hash function. The P-256, P-384, and P-521 curves are defined in [DSS]. The alg (algorithm) header parameter values ES256, ES384, and ES512 are used in the JWS Header to indicate that the Encoded JWS Signature contains a base64url encoded ECDSA P-256 SHA-256, ECDSA P-384 SHA-384, or ECDSA P-521 SHA-512 digital signature, respectively.

The ECDSA P-256 SHA-256 digital signature is generated as follows:

1. Generate a digital signature of the bytes of the ASCII representation of the JWS Secured Input using ECDSA P-256 SHA-256 with the desired private key. The output will be the pair (R, S), where R and S are 256 bit unsigned integers.
2. Turn R and S into byte arrays in big endian order, with each array being be 32 bytes long. The array representations MUST not be shortened to omit any leading zero bytes contained in the values.
3. Concatenate the two byte arrays in the order R and then S. (Note that many ECDSA implementations will directly produce this concatenation as their output.)
4. Base64url encode the resulting 64 byte array.

The output is the Encoded JWS Signature for the JWS.

The ECDSA P-256 SHA-256 digital signature for a JWS is validated as follows:

1. Take the Encoded JWS Signature and base64url decode it into a byte array. If decoding fails, the JWS MUST be rejected.
2. The output of the base64url decoding MUST be a 64 byte array. If decoding does not result in a 64 byte array, the JWS MUST be rejected.
3. Split the 64 byte array into two 32 byte arrays. The first array will be R and the second S (with both being in big endian byte order).
4. Submit the bytes of the ASCII representation of the JWS Secured Input R, S and the public key (x, y) to the ECDSA P-256 SHA-256 validator.
5. If the validation fails, the JWS MUST be rejected.

Note that ECDSA digital signature contains a value referred to as K, which is a random number generated for each digital signature instance. This means that two ECDSA digital signatures using exactly the same input parameters will output different signature values because their K values will be different. A consequence of this is that one cannot validate an ECDSA signature by recomputing the signature and comparing the results.

Signing with the ECDSA P-384 SHA-384 and ECDSA P-521 SHA-512 algorithms is performed identically to the procedure for ECDSA P-256 SHA-256 - just using the corresponding hash algorithm with correspondingly larger result values. For ECDSA P-384 SHA-384, R and S will be 384 bits each, resulting in a 96 byte array. For ECDSA P-521 SHA-512, R and S will be 521 bits each, resulting in a 132 byte array.

3.5. Using the Algorithm "none"

JWSs MAY also be created that do not provide integrity protection. Such a JWS is called a "Plaintext JWS". Plaintext JWSs MUST use the alg value none, and are formatted identically to other JWSs, but with an empty JWS Signature value.

3.6. Additional Digital Signature/MAC Algorithms and Parameters

Additional algorithms MAY be used to protect JWSs with corresponding alg (algorithm) header parameter values being defined to refer to them. New alg header parameter values SHOULD either be registered in the IANA JSON Web Signature and Encryption Algorithms registry Section 6.1 or be a URI that contains a Collision Resistant Namespace. In particular, it is permissible to use the algorithm identifiers defined in XML DSIG [RFC3275], XML DSIG 2.0 [W3C.CR-xmldsig-core2-20120124], and related specifications as alg values.

As indicated by the common registry, JWSs and JWEs share a common alg value space. The values used by the two specifications MUST be distinct, as the alg value MAY be used to determine whether the object is a JWS or JWE.

Likewise, additional reserved header parameter names MAY be defined via the IANA JSON Web Signature and Encryption Header Parameters registry [JWS]. As indicated by the common registry, JWSs and JWEs share a common header parameter space; when a parameter is used by both specifications, its usage must be compatible between the specifications.

4. Cryptographic Algorithms for JWE

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

4.1. "alg" (Algorithm) Header Parameter Values for JWE

The table below is the set of alg (algorithm) header parameter values that are defined by this specification for use with JWE. These algorithms are used to encrypt the CMK, producing the JWE Encrypted Key, or to use key agreement to agree upon the CMK.

The use of "+" in the Implementation Requirements indicates that the requirement strength is likely to be increased in a future version of the specification.

4.2. "enc" (Encryption Method) Header Parameter Values for JWE

The table below is the set of enc (encryption method) header parameter values that are defined by this specification for use with JWE. These algorithms are used to encrypt the Plaintext, which produces the Ciphertext.

All the names are short because a core goal of JWE is for the representations to be compact. However, there is no a priori length restriction on alg values.

See Appendix Appendix B for a table cross-referencing the encryption alg (algorithm) and enc (encryption method) values used in this specification with the equivalent identifiers used by other standards and software packages.

4.3. "int" (Integrity Algorithm) Header Parameter Values for JWE

The table below is the set of int (integrity algorithm) header parameter values defined by this specification for use with JWE. Note that these are the HMAC SHA subset of the alg (algorithm) header parameter values defined for use with JWS Section 3.1.

4.4. "kdf" (Key Derivation Function) Header Parameter Values for JWE

The table below is the set of kdf (key derivation function) header parameter values defined by this specification for use with JWE.

4.5. Key Encryption with RSAES-PKCS1-V1_5

This section defines the specifics of encrypting a JWE CMK with RSAES-PKCS1-V1_5 [RFC3447]. The alg header parameter value RSA1_5 is used in this case.

A key of size 2048 bits or larger MUST be used with this algorithm.

4.6. Key Encryption with RSAES OAEP

This section defines the specifics of encrypting a JWE CMK with RSAES using Optimal Asymmetric Encryption Padding (OAEP) [RFC3447], with the default parameters specified by RFC 3447 in Section A.2.1. The alg header parameter value RSA-OAEP is used in this case.

A key of size 2048 bits or larger MUST be used with this algorithm.

4.7. Key Encryption with AES Key Wrap

This section defines the specifics of encrypting a JWE CMK with the Advanced Encryption Standard (AES) Key Wrap Algorithm [RFC3394] using 128 or 256 bit keys. The alg header parameter values A128KW or A256KW are used in this case.

4.8. Direct Encryption with a Shared Symmetric Key

This section defines the specifics of directly performing symmetric key encryption without performing a key wrapping step. In this case, the shared symmetric key is used directly as the Content Master Key (CMK) value for the enc algorithm. An empty byte array is used as the JWE Encrypted Key value. The alg header parameter value dir is used in this case.

4.9. Key Agreement with Elliptic Curve Diffie-Hellman Ephemeral Static (ECDH-ES)

This section defines the specifics of key agreement with Elliptic Curve Diffie-Hellman Ephemeral Static [RFC6090], and using the Concat KDF, as defined in Section 5.8.1 of [NIST.800-56A], where the Digest Method is SHA-256 and all OtherInfo parameters are the empty bit string. The key agreement result can be used in one of two ways: (1) directly as the Content Master Key (CMK) for the enc algorithm, or (2) as a symmetric key used to wrap the CMK with either the A128KW or A256KW algorithms. The alg header parameter values ECDH-ES, ECDH-ES+A128KW, and ECDH-ES+A256KW are respectively used in this case.

In the direct case, the output of the Concat KDF MUST be a key of the same length as that used by the enc algorithm; in this case, the empty byte array is used as the JWE Encrypted Key value. In the key wrap case, the output of the Concat KDF MUST be a key of the length needed for the specified key wrap algorithm, either 128 or 256 bits respectively.

A new epk (ephemeral public key) value MUST be generated for each key agreement transaction.

4.10. Plaintext Encryption with AES CBC Mode

This section defines the specifics of encrypting the JWE Plaintext with Advanced Encryption Standard (AES) in Cipher Block Chaining (CBC) mode with PKCS #5 padding [AES] [NIST.800-38A] using 128 or 256 bit keys. The enc header parameter values A128CBC or A256CBC are used in this case.

Use of an initialization vector of size 128 bits is REQUIRED with this algorithm.

4.11. Plaintext Encryption with AES GCM

This section defines the specifics of encrypting the JWE Plaintext with Advanced Encryption Standard (AES) in Galois/Counter Mode (GCM) [AES] [NIST.800-38D] using 128 or 256 bit keys. The enc header parameter values A128GCM or A256GCM are used in this case.

Use of an initialization vector of size 96 bits is REQUIRED with this algorithm.

The "additional authenticated data" parameter is used to secure the header and key values, as specified for AEAD algorithms in Section 5 of [JWE].

The requested size of the "authentication tag" output MUST be 128 bits, regardless of the key size.

As GCM is an AEAD algorithm, the JWE Integrity Value is set to be the "authentication tag" value produced by the encryption.

4.12. Integrity Calculation with HMAC SHA-256, HMAC SHA-384, or HMAC SHA-512

This section defines the specifics of computing a JWE Integrity Value with HMAC SHA-256, HMAC SHA-384, or HMAC SHA-512 [SHS]. Other than as stated below, these computations are performed identically to those specified in Section 3.2.

A key of the same size as the hash output (for instance, 256 bits for HS256) MUST be used with this algorithm.

Per Section 9 of [JWE], the JWS Secured Input value used contains the header, encrypted key, and ciphertext.

4.13. Key Derivation with Concat KDF and SHA-256, SHA-384, or SHA-512

The key derivation process derives CEK and CIK values from the CMK. It uses as a primitive a Key Derivation Function (KDF) which notionally takes three arguments: [NIST.800-56A], where the Digest Method is one of SHA-256, SHA-384, or SHA-512, 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 desired length of the use key

This section defines the specifics of using the Concat KDF, as defined in Section 5.8.1 of

The kdf (key derivation function) header parameter values CS256, CS384, and CS512 are respectively used in the JWE Header to indicate the use of the Concat KDF as above with the respective digest methods. If the kdf header parameter is omitted when an AEAD enc algorithm is not used, this is equivalent to specifying use of the CS256 key derivation function.

To compute the CEK from the CMK, the ASCII label "Encryption" ([69, 110, 99, 114, 121, 112, 116, 105, 111, 110]) is used. Use the key size for the enc algorithm as the CEK desired key length.

To compute the CIK from the CMK, the ASCII label "Integrity" ([73, 110, 116, 101, 103, 114, 105, 116, 121]) is used. Use the minimum key size for the int algorithm (for instance, 256 bits for HS256) as the CIK desired key length.

4.14. Additional Encryption Algorithms and Parameters

Additional algorithms MAY be used to protect JWEs with corresponding alg (algorithm), enc (encryption method), and int (integrity algorithm) header parameter values being defined to refer to them. New alg, enc, and int header parameter values SHOULD either be registered in the IANA JSON Web Signature and Encryption Algorithms registry Section 6.1 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-core1-20120313], and related specifications as alg, enc, and int values.

As indicated by the common registry, JWSs and JWEs share a common alg value space. The values used by the two specifications MUST be distinct, as the alg value MAY be used to determine whether the object is a JWS or JWE.

Likewise, additional reserved header parameter names MAY be defined via the IANA JSON Web Signature and Encryption Header Parameters registry [JWS]. As indicated by the common registry, JWSs and JWEs share a common header parameter space; when a parameter is used by both specifications, its usage must be compatible between the specifications.

5. Cryptographic Algorithms for JWK

A JSON Web Key (JWK) [JWK] is a JavaScript Object Notation (JSON) [RFC4627] data structure that represents a public key. A JSON Web Key Set (JWK Set) is a JSON data structure for representing a set of JWKs. This section specifies a set of algorithm families to be used for those public keys and the algorithm family specific parameters for representing those keys.

5.1. "alg" (Algorithm Family) Parameter Values for JWK

The table below is the set of alg (algorithm family) parameter values that are defined by this specification for use in JWKs.

All the names are short because a core goal of JWK is for the representations to be compact. However, there is no a priori length restriction on alg values.

The use of "+" in the Implementation Requirements indicates that the requirement strength is likely to be increased in a future version of the specification.

5.2. JWK Parameters for Elliptic Curve Keys

JWKs can represent Elliptic Curve [DSS] keys. In this case, the alg member value MUST be EC. Furthermore, these additional members MUST be present:

5.2.1. "crv" (Curve) Parameter

The crv (curve) member identifies the cryptographic curve used with the key. Curve values from [DSS] used by this specification are: crv values MAY be used, provided they are understood by implementations using that Elliptic Curve key. The crv value is a case sensitive string.

• P-256
• P-384
• P-521

Additional

5.2.2. "x" (X Coordinate) Parameter

The x (x coordinate) member contains the x coordinate for the elliptic curve point. It is represented as the base64url encoding of the coordinate's big endian representation as a byte array. The array representation MUST not be shortened to omit any leading zero bytes contained in the value. For instance, when representing 521 bit integers, the byte array to be base64url encoded MUST contain 66 bytes, including any leading zero bytes.

5.2.3. "y" (Y Coordinate) Parameter

The y (y coordinate) member contains the y coordinate for the elliptic curve point. It is represented as the base64url encoding of the coordinate's big endian representation as a byte array. The array representation MUST not be shortened to omit any leading zero bytes contained in the value. For instance, when representing 521 bit integers, the byte array to be base64url encoded MUST contain 66 bytes, including any leading zero bytes.

5.3. JWK Parameters for RSA Keys

JWKs can represent RSA [RFC3447] keys. In this case, the alg member value MUST be RSA. Furthermore, these additional members MUST be present:

5.3.1. "mod" (Modulus) Parameter

The mod (modulus) member contains the modulus value for the RSA public key. It is represented as the base64url encoding of the value's unsigned big endian representation as a byte array. The array representation MUST not be shortened to omit any leading zero bytes. For instance, when representing 2048 bit integers, the byte array to be base64url encoded MUST contain 256 bytes, including any leading zero bytes.

5.3.2. "exp" (Exponent) Parameter

The exp (exponent) member contains the exponent value for the RSA public key. It is represented as the base64url encoding of the value's unsigned big endian representation as a byte array. The array representation MUST utilize the minimum number of bytes to represent the value. For instance, when representing the value 65537, the byte array to be base64url encoded MUST consist of the three bytes [1, 0, 1].

5.4. Additional Key Algorithm Families and Parameters

Public keys using additional algorithm families MAY be represented using JWK data structures with corresponding alg (algorithm family) parameter values being defined to refer to them. New alg parameter values SHOULD either be registered in the IANA JSON Web Key Algorithm Families registry Section 6.2 or be a URI that contains a Collision Resistant Namespace.

Likewise, parameters for representing keys for additional algorithm families or additional key properties SHOULD either be registered in the IANA JSON Web Key Parameters registry [JWK] or be a URI that contains a Collision Resistant Namespace.

6. IANA Considerations

The following registration procedure is used for all the registries established by this specification.

Values are registered with a Specification Required [RFC5226] after a two week review period on the [TBD]@ietf.org mailing list, on the advice of one or more Designated Experts. However, to allow for the allocation of values prior to publication, the Designated Expert(s) may approve registration once they are satisfied that such a specification will be published.

Registration requests must be sent to the [TBD]@ietf.org mailing list for review and comment, with an appropriate subject (e.g., "Request for access token type: example"). [[ Note to RFC-EDITOR: The name of the mailing list should be determined in consultation with the IESG and IANA. Suggested name: jose-reg-review. ]]

Within the review period, the Designated Expert(s) will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful.

IANA must only accept registry updates from the Designated Expert(s), and should direct all requests for registration to the review mailing list.

6.1. JSON Web Signature and Encryption Algorithms Registry

This specification establishes the IANA JSON Web Signature and Encryption Algorithms registry for values of the JWS and JWE alg (algorithm), enc (encryption method), and int (integrity algorithm) header parameters. The registry records the algorithm name, the algorithm usage locations from the set alg, enc, and int, implementation requirements, and a reference to the specification that defines it. The same algorithm name may be registered multiple times, provided that the sets of usage locations are disjoint. The implementation requirements of an algorithm may be changed over time by the Designated Experts(s) as the cryptographic landscape evolves, for instance, to change the status of an algorithm to DEPRECATED, or to change the status of an algorithm from OPTIONAL to RECOMMENDED or REQUIRED.

6.1.1. Registration Template

Algorithm Name:

The name requested (e.g., "example"). This name is case sensitive. Names that match other registered names in a case insensitive manner SHOULD NOT be accepted.

Algorithm Usage Location(s):

The algorithm usage, which must be one or more of the values alg, enc, int, or kdf.

Implementation Requirements:

The algorithm implementation requirements, which must be one the words REQUIRED, RECOMMENDED, OPTIONAL, or DEPRECATED. Optionally, the word may be followed by a "+" or "-". The use of "+" indicates that the requirement strength is likely to be increased in a future version of the specification. The use of "-" indicates that the requirement strength is likely to be decreased in a future version of the specification.

Change Controller:

For standards-track RFCs, state "IETF". For others, give the name of the responsible party. Other details (e.g., postal address, e-mail address, home page URI) may also be included.

Specification Document(s):

Reference to the document that specifies the parameter, preferably including a URI that can be used to retrieve a copy of the document. An indication of the relevant sections may also be included, but is not required.

6.1.2. Initial Registry Contents

• Algorithm Name: HS256
• Algorithm Usage Location(s): alg, int
• Implementation Requirements: REQUIRED
• Change Controller: IETF
• Specification Document(s): Section 3.1 and Section 4.3 of [[ this document ]]

• Algorithm Name: HS384
• Algorithm Usage Location(s): alg, int
• Implementation Requirements: OPTIONAL
• Change Controller: IETF
• Specification Document(s): Section 3.1 and Section 4.3 of [[ this document ]]

• Algorithm Name: HS512
• Algorithm Usage Location(s): alg, int
• Implementation Requirements: OPTIONAL
• Change Controller: IETF
• Specification Document(s): Section 3.1 and Section 4.3 of [[ this document ]]

• Algorithm Name: RS256
• Algorithm Usage Location(s): alg
• Implementation Requirements: RECOMMENDED
• Change Controller: IETF
• Specification Document(s): Section 3.1 of [[ this document ]]

• Algorithm Name: RS384
• Algorithm Usage Location(s): alg
• Implementation Requirements: OPTIONAL
• Change Controller: IETF
• Specification Document(s): Section 3.1 of [[ this document ]]

• Algorithm Name: RS512
• Algorithm Usage Location(s): alg
• Implementation Requirements: OPTIONAL
• Change Controller: IETF
• Specification Document(s): Section 3.1 of [[ this document ]]

• Algorithm Name: ES256
• Algorithm Usage Location(s): alg
• Implementation Requirements: RECOMMENDED+
• Change Controller: IETF
• Specification Document(s): Section 3.1 of [[ this document ]]

• Algorithm Name: ES384
• Algorithm Usage Location(s): alg
• Implementation Requirements: OPTIONAL
• Change Controller: IETF
• Specification Document(s): Section 3.1 of [[ this document ]]

• Algorithm Name: ES512
• Algorithm Usage Location(s): alg
• Implementation Requirements: OPTIONAL
• Change Controller: IETF
• Specification Document(s): Section 3.1 of [[ this document ]]

• Algorithm Name: none
• Algorithm Usage Location(s): alg
• Implementation Requirements: REQUIRED
• Change Controller: IETF
• Specification Document(s): Section 3.1 of [[ this document ]]

• Algorithm Name: RSA1_5
• Algorithm Usage Location(s): alg
• Implementation Requirements: REQUIRED
• Change Controller: IETF
• Specification Document(s): Section 4.1 of [[ this document ]]

• Algorithm Name: RSA-OAEP
• Algorithm Usage Location(s): alg
• Implementation Requirements: OPTIONAL
• Change Controller: IETF
• Specification Document(s): Section 4.1 of [[ this document ]]

• Algorithm Name: A128KW
• Algorithm Usage Location(s): alg
• Implementation Requirements: RECOMMENDED
• Change Controller: IETF
• Specification Document(s): Section 4.1 of [[ this document ]]

• Algorithm Name: A256KW
• Algorithm Usage Location(s): alg
• Implementation Requirements: RECOMMENDED
• Change Controller: IETF
• Specification Document(s): Section 4.1 of [[ this document ]]

• Algorithm Name: dir
• Algorithm Usage Location(s): alg
• Implementation Requirements: RECOMMENDED
• Change Controller: IETF
• Specification Document(s): Section 4.1 of [[ this document ]]

• Algorithm Name: ECDH-ES
• Algorithm Usage Location(s): alg
• Implementation Requirements: RECOMMENDED+
• Change Controller: IETF
• Specification Document(s): Section 4.1 of [[ this document ]]

• Algorithm Name: ECDH-ES+A128KW
• Algorithm Usage Location(s): alg
• Implementation Requirements: RECOMMENDED
• Change Controller: IETF
• Specification Document(s): Section 4.1 of [[ this document ]]

• Algorithm Name: ECDH-ES+A256KW
• Algorithm Usage Location(s): alg
• Implementation Requirements: RECOMMENDED
• Change Controller: IETF
• Specification Document(s): Section 4.1 of [[ this document ]]

• Algorithm Name: A128CBC
• Algorithm Usage Location(s): enc
• Implementation Requirements: REQUIRED
• Change Controller: IETF
• Specification Document(s): Section 4.2 of [[ this document ]]

• Algorithm Name: A256CBC
• Algorithm Usage Location(s): enc
• Implementation Requirements: REQUIRED
• Change Controller: IETF
• Specification Document(s): Section 4.2 of [[ this document ]]

• Algorithm Name: A128GCM
• Algorithm Usage Location(s): enc
• Implementation Requirements: RECOMMENDED
• Change Controller: IETF
• Specification Document(s): Section 4.2 of [[ this document ]]

• Algorithm Name: A256GCM
• Algorithm Usage Location(s): enc
• Implementation Requirements: RECOMMENDED
• Change Controller: IETF
• Specification Document(s): Section 4.2 of [[ this document ]]

• Algorithm Name: CS256
• Algorithm Usage Location(s): kdf
• Implementation Requirements: REQUIRED
• Change Controller: IETF
• Specification Document(s): Section 4.4 of [[ this document ]]

• Algorithm Name: CS384
• Algorithm Usage Location(s): kdf
• Implementation Requirements: OPTIONAL
• Change Controller: IETF
• Specification Document(s): Section 4.4 of [[ this document ]]

• Algorithm Name: CS512
• Algorithm Usage Location(s): kdf
• Implementation Requirements: OPTIONAL
• Change Controller: IETF
• Specification Document(s): Section 4.4 of [[ this document ]]

6.2. JSON Web Key Algorithm Families Registry

This specification establishes the IANA JSON Web Key Algorithm Families registry for values of the JWK alg (algorithm family) parameter. The registry records the alg value and a reference to the specification that defines it. This specification registers the values defined in Section 5.1.

6.2.1. Registration Template

"alg" Parameter Value:

The name requested (e.g., "example"). This name is case sensitive. Names that match other registered names in a case insensitive manner SHOULD NOT be accepted.

Change Controller:

For standards-track RFCs, state "IETF". For others, give the name of the responsible party. Other details (e.g., postal address, e-mail address, home page URI) may also be included.

Implementation Requirements:

The algorithm implementation requirements, which must be one the words REQUIRED, RECOMMENDED, OPTIONAL, or DEPRECATED. Optionally, the word may be followed by a "+" or "-". The use of "+" indicates that the requirement strength is likely to be increased in a future version of the specification. The use of "-" indicates that the requirement strength is likely to be decreased in a future version of the specification.

Specification Document(s):

Reference to the document that specifies the parameter, preferably including a URI that can be used to retrieve a copy of the document. An indication of the relevant sections may also be included, but is not required.

6.2.2. Initial Registry Contents

• "alg" Parameter Value: EC
• Implementation Requirements: RECOMMENDED+
• Change Controller: IETF
• Specification Document(s): Section 5.1 of [[ this document ]]

• "alg" Parameter Value: RSA
• Implementation Requirements: REQUIRED
• Change Controller: IETF
• Specification Document(s): Section 5.1 of [[ this document ]]

6.3. JSON Web Key Parameters Registration

This specification registers the parameter names defined in Section 5.2 and Section 5.3 in the IANA JSON Web Key Parameters registry [JWK].

6.3.1. Registry Contents

• Parameter Name: crv
• Change Controller: IETF
• Specification Document(s): Section 5.2.1 of [[ this document ]]

• Parameter Name: x
• Change Controller: IETF
• Specification Document(s): Section 5.2.2 of [[ this document ]]

• Parameter Name: y
• Change Controller: IETF
• Specification Document(s): Section 5.2.3 of [[ this document ]]

• Parameter Name: mod
• Change Controller: IETF
• Specification Document(s): Section 5.3.1 of [[ this document ]]

• Parameter Name: exp
• Change Controller: IETF
• Specification Document(s): Section 5.3.2 of [[ this document ]]

7. Security Considerations

All of the security issues faced by any cryptographic application must be faced by a JWS/JWE/JWK agent. Among these issues are protecting the user's private key, preventing various attacks, and helping the user avoid mistakes such as inadvertently encrypting a message for the wrong recipient. The entire list of security considerations is beyond the scope of this document, but some significant concerns are listed here.

The security considerations in [AES], [DSS], [JWE], [JWK], [JWS], [NIST.800-38A], [NIST.800-38D], [NIST.800-56A], [RFC2104], [RFC3394], [RFC3447], [RFC5116], [RFC6090], and [SHS] apply to this specification.

Eventually the algorithms and/or key sizes currently described in this specification will no longer be considered sufficiently secure and will be removed. Therefore, implementers and deployments must be prepared for this eventuality.

Algorithms of matching strength should be used together whenever possible. For instance, when AES Key Wrap is used with a given key size, using the same key size for AES CBC or GCM is recommended. Likewise, when AES CBC is used with a 128 bit key, using HMAC SHA-256 as the integrity algorithm is recommended, whereas when AES CBC is used with a 256 bit key, using HMAC SHA-512 as the integrity algorithm is recommended.

While Section 8 of RFC 3447 [RFC3447] explicitly calls for people not to adopt RSASSA-PKCS1 for new applications and instead requests that people transition to RSASSA-PSS, this specification does include RSASSA-PKCS1, for interoperability reasons, because it commonly implemented.

Keys used with RSAES-PKCS1-v1_5 must follow the constraints in Section 7.2 of RFC 3447 [RFC3447]. In particular, keys with a low public key exponent value must not be used.

Plaintext JWSs (JWSs that use the alg value none) provide no integrity protection. Thus, they must only be used in contexts where the payload is secured by means other than a digital signature or MAC value, or need not be secured.

Receiving agents that validate signatures and sending agents that encrypt messages need to be cautious of cryptographic processing usage when validating signatures and encrypting messages using keys larger than those mandated in this specification. An attacker could send certificates with keys that would result in excessive cryptographic processing, for example, keys larger than those mandated in this specification, which could swamp the processing element. Agents that use such keys without first validating the certificate to a trust anchor are advised to have some sort of cryptographic resource management system to prevent such attacks.

8. Open Issues

[[ to be removed by the RFC editor before publication as an RFC ]]

The following items remain to be considered or done in this draft:

• Should we use the alg value as the AlgorithmID input to the Concat KDF when doing key agreement? Or is an AlgorithmID value unnecessary in the way that we are using Concat for key agreement?
• Similarly, should we use a combination of the enc and int values as the AlgorithmID input to the Concat KDF when doing key derivation? Or is an AlgorithmID value unnecessary in the way that we are using Concat for key derivation?
• Do we want to add the output key length to the Concat KDF input parameters?
• Do we need non-empty PartyUInfo and PartyVInfo values when using the Concat KDF for key agreement? Or given that we already require the use of a random unique Ephemeral Public Key (EPK), is this superfluous, as duplicate keys will not be generated unless a duplicate EPK is used? If we do decide we need PartyUInfo and PartyVInfo values, how can we dynamically generate them from information already carried in the header, rather than requiring that they be explicitly passed as header parameters?
• Similarly, do we need non-empty PartyUInfo and PartyVInfo values when using the Concat KDF for key derivation? Or given that we already require the use of a random unique Content Master Key (CMK), is this superfluous, as duplicate keys will not be generated unless a duplicate CMK is used? If we do decide we need PartyUInfo and PartyVInfo values, how can we dynamically generate them from information already carried in the header, rather than requiring that they be explicitly passed as header parameters?
• We use the same secret multiple times with the Concat KDF with different parameters to generate multiple derived keys. Do we want to continue this practice or instead use the KDF once to generate a big enough byte array for all derived keys, and use different parts of the byte array for each key?
• Do we want to add AES ECB as a (non-authenticated) key wrap algorithm? Is there any problem with doing key wrap without an integrity check, given that a separate integrity check already covers the wrapped key?
• Do we want the ability to perform symmetric encryption directly with a shared key, without using a separate CMK? This would save space and time in the single recipient case. It would also be parallel to this option for key agreement, which doesn't use a separate CMK.
• Do we want the ability to perform symmetric encryption directly with an agreed upon key, without using a separate CMK? The arguments for this parallel those for direct encryption using a shared key.

9. References

9.1. Normative References

9.2. Informative References

Appendix A. Digital Signature/MAC Algorithm Identifier Cross-Reference

This appendix contains a table cross-referencing the digital signature and MAC alg (algorithm) values used in this specification with the equivalent identifiers used by other standards and software packages. See XML DSIG [RFC3275], XML DSIG 2.0 [W3C.CR-xmldsig-core2-20120124], and Java Cryptography Architecture [JCA] for more information about the names defined by those documents.

Appendix B. Encryption Algorithm Identifier Cross-Reference

This appendix contains a table cross-referencing the alg (algorithm) and enc (encryption method) 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-core1-20120313], and Java Cryptography Architecture [JCA] for more information about the names defined by those documents.

Appendix C. Acknowledgements

Solutions for signing and encrypting JSON content were previously explored by Magic Signatures [MagicSignatures], JSON Simple Sign [JSS], Canvas Applications [CanvasApp], JSON Simple Encryption [JSE], and JavaScript Message Security Format [I-D.rescorla-jsms], all of which influenced this draft. Dirk Balfanz, John Bradley, Yaron Y. Goland, John Panzer, Nat Sakimura, and Paul Tarjan all made significant contributions to the design of this specification and its related specifications.

Appendix D. Document History

[[ to be removed by the RFC editor before publication as an RFC ]]

-05

• Support both direct encryption using a shared or agreed upon symmetric key, and the use of a shared or agreed upon symmetric key to key wrap the CMK. Specifically, added the alg values dir, ECDH-ES+A128KW, and ECDH-ES+A256KW to finish filling in this set of capabilities.
• Updated open issues.

-04

• Added text requiring that any leading zero bytes be retained in base64url encoded key value representations for fixed-length values.
• Added this language to Registration Templates: "This name is case sensitive. Names that match other registered names in a case insensitive manner SHOULD NOT be accepted."
• Described additional open issues.
• Applied editorial suggestions.

-03

• Always use a 128 bit "authentication tag" size for AES GCM, regardless of the key size.
• Specified that use of a 128 bit IV is REQUIRED with AES CBC. It was previously RECOMMENDED.
• Removed key size language for ECDSA algorithms, since the key size is implied by the algorithm being used.
• Stated that the int key size must be the same as the hash output size (and not larger, as was previously allowed) so that its size is defined for key generation purposes.
• Added the kdf (key derivation function) header parameter to provide crypto agility for key derivation. The default KDF remains the Concat KDF with the SHA-256 digest function.
• Clarified that the mod and exp values are unsigned.
• Added Implementation Requirements columns to algorithm tables and Implementation Requirements entries to algorithm registries.
• Changed AES Key Wrap to RECOMMENDED.
• Moved registries JSON Web Signature and Encryption Header Parameters and JSON Web Signature and Encryption Type Values to the JWS specification.
• Moved JSON Web Key Parameters registry to the JWK specification.
• Changed registration requirements from RFC Required to Specification Required with Expert Review.
• Added Registration Template sections for defined registries.
• Added Registry Contents sections to populate registry values.
• No longer say "the UTF-8 representation of the JWS Secured Input (which is the same as the ASCII representation)". Just call it "the ASCII representation of the JWS Secured Input".
• Added "Collision Resistant Namespace" to the terminology section.
• Numerous editorial improvements.

-02

• For AES GCM, use the "additional authenticated data" parameter to provide integrity for the header, encrypted key, and ciphertext and use the resulting "authentication tag" value as the JWE Integrity Value.
• Defined minimum required key sizes for algorithms without specified key sizes.
• Defined KDF output key sizes.
• Specified the use of PKCS #5 padding with AES-CBC.
• Generalized text to allow key agreement to be employed as an alternative to key wrapping or key encryption.
• Clarified that ECDH-ES is a key agreement algorithm.
• Required implementation of AES-128-KW and AES-256-KW.
• Removed the use of A128GCM and A256GCM for key wrapping.
• Removed A512KW since it turns out that it's not a standard algorithm.
• Clarified the relationship between typ header parameter values and MIME types.
• Generalized language to refer to Message Authentication Codes (MACs) rather than Hash-based Message Authentication Codes (HMACs) unless in a context specific to HMAC algorithms.
• Established registries: JSON Web Signature and Encryption Header Parameters, JSON Web Signature and Encryption Algorithms, JSON Web Signature and Encryption "typ" Values, JSON Web Key Parameters, and JSON Web Key Algorithm Families.
• Moved algorithm-specific definitions from JWK to JWA.
• Reformatted to give each member definition its own section heading.

-01

• Moved definition of "alg":"none" for JWSs here from the JWT specification since this functionality is likely to be useful in more contexts that just for JWTs.
• Added Advanced Encryption Standard (AES) Key Wrap Algorithm using 512 bit keys (A512KW).
• Added text "Alternatively, the Encoded JWS Signature MAY be base64url decoded to produce the JWS Signature and this value can be compared with the computed HMAC value, as this comparison produces the same result as comparing the encoded values".
• Corrected the Magic Signatures reference.
• Made other editorial improvements suggested by JOSE working group participants.

-00

• Created the initial IETF draft based upon draft-jones-json-web-signature-04 and draft-jones-json-web-encryption-02 with no normative changes.
• Changed terminology to no longer call both digital signatures and HMACs "signatures".

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