JSON Web Algorithms (JWA)
draft-ietf-jose-json-web-algorithms-01

Abstract

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

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. Cryptographic Algorithms for JWS
• 3.1. Creating a JWS with HMAC SHA-256, HMAC SHA-384, or HMAC SHA-512
• 3.2. Creating a JWS with RSA SHA-256, RSA SHA-384, or RSA SHA-512
• 3.3. Creating a JWS with ECDSA P-256 SHA-256, ECDSA P-384 SHA-384, or ECDSA P-521 SHA-512
• 3.4. Creating a Plaintext JWS
• 3.5. Additional Digital Signature/HMAC Algorithms
• 4. Cryptographic Algorithms for JWE
• 4.1. Encrypting a JWE with TBD
• 4.2. Additional Encryption Algorithms
• 5. IANA Considerations
• 6. Security Considerations
• 7. Open Issues and Things To Be Done (TBD)
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Appendix A. Digital Signature/HMAC 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] and JSON Web Encryption (JWE) [JWE] specifications. Enumerating the algorithms and identifiers for them in this specification, rather than in the JWS and JWE 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. This specification also describes the semantics and operations that are specific to these algorithms and algorithm families.

2. Terminology

This specification uses the terminology defined by the JSON Web Signature (JWS) [JWS] and JSON Web Encryption (JWE) [JWE] specifications.

3. Cryptographic Algorithms for JWS

JWS uses cryptographic algorithms to sign the contents of the JWS Header and the JWS Payload. The use of the following algorithms for producing JWSs is defined in this section.

The table below Table 1 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:

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

Of these algorithms, only HMAC SHA-256 and none MUST be implemented by conforming JWS implementations. It is RECOMMENDED that implementations also support the RSA SHA-256 and ECDSA P-256 SHA-256 algorithms. Support for other algorithms and key sizes is OPTIONAL.

3.1. Creating a JWS 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 cryptographic hash functions as defined in FIPS 180-3 [FIPS.180-3]. 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.

The HMAC SHA-256 MAC is generated as follows:

1. Apply the HMAC SHA-256 algorithm to the UTF-8 representation of the JWS Secured Input using the shared key to produce an HMAC value.
2. Base64url encode the resulting HMAC value.

The output is the Encoded JWS Signature for that JWS.

The HMAC SHA-256 MAC for a JWS is validated as follows:

1. Apply the HMAC SHA-256 algorithm to the UTF-8 representation of the JWS Secured Input of the JWS using the shared key.
2. Base64url encode the resulting HMAC value.
3. If the Encoded JWS Signature and the base64url encoded HMAC value exactly match, then one has confirmation that the shared key was used to generate the HMAC on the JWS and that the contents of the JWS have not be tampered with.
4. If the validation fails, the JWS MUST be rejected.

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.

Securing content with the HMAC SHA-384 and HMAC SHA-512 algorithms is performed identically to the procedure for HMAC SHA-256 - just with correspondingly longer minimum key sizes and result values.

3.2. Creating a JWS 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 RFC 3447 [RFC3447], Section 8.2 (commonly known as PKCS#1), using SHA-256, SHA-384, or SHA-512 as the hash function. The RSASSA-PKCS1-v1_5 algorithm is described in FIPS 186-3 [FIPS.186-3], Section 5.5, and the SHA-256, SHA-384, and SHA-512 cryptographic hash functions are defined in FIPS 180-3 [FIPS.180-3]. 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 2048-bit or longer key length MUST be used with this algorithm.

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

1. Generate a digital signature of the UTF-8 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 UTF-8 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 with correspondingly longer minimum key sizes and result values.

3.3. Creating a JWS with ECDSA P-256 SHA-256, ECDSA P-384 SHA-384, or ECDSA P-521 SHA-512

The Elliptic Curve Digital Signature Algorithm (ECDSA) is defined by FIPS 186-3 [FIPS.186-3]. ECDSA provides for the use of Elliptic Curve cryptography, which is able to provide equivalent security to RSA cryptography but using shorter key lengths 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 also defined in FIPS 186-3. 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 UTF-8 representation of the JWS Secured Input using ECDSA P-256 SHA-256 with the desired private key. The output will be the EC point (R, S), where R and S are unsigned integers.
2. Turn R and S into byte arrays in big endian order. Each array will be 32 bytes long.
3. Concatenate the two byte arrays in the order R and then S.
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.
3. Split the 64 byte array into two 32 byte arrays. The first array will be R and the second S. Remember that the byte arrays are in big endian byte order; please check the ECDSA validator in use to see what byte order it requires.
4. Submit the UTF-8 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.

The ECDSA validator will then determine if the digital signature is valid, given the inputs. 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. The consequence of this is that one must validate an ECDSA digital signature by submitting the previously specified inputs to an ECDSA validator.

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 with correspondingly longer minimum key sizes and result values.

3.4. Creating a Plaintext JWS

To support use cases where the content is secured by a means other than a digital signature or HMAC value, JWSs MAY also be created without them. These are called "Plaintext JWSs". Plaintext JWSs MUST use the alg value none, and are formatted identically to other JWSs, but with an empty JWS Signature value.

3.5. Additional Digital Signature/HMAC Algorithms

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 defined in the IANA JSON Web Signature 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 DSIG [RFC3275] and related specifications as alg values.

4. Cryptographic Algorithms for JWE

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 2 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 CEK, which produces the JWE Encrypted Key.

The table below Table 3 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.

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.

Of these algorithms, only RSA-PKCS1-1.5 with 2048 bit keys, AES-128-CBC, and AES-256-CBC MUST be implemented by conforming JWE 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.

4.1. Encrypting a JWE with TBD

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

4.2. Additional Encryption Algorithms

Additional algorithms MAY be used to protect JWEs with corresponding alg (algorithm) and enc (encryption method) 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.

5. IANA Considerations

This specification calls for:

• A new IANA registry entitled "JSON Web Signature Algorithms" for values of the JWS alg (algorithm) header parameter is defined in Section 3.5. Inclusion in the registry is RFC Required in the RFC 5226 [RFC5226] sense. The registry will just record the alg value and a pointer to the RFC that defines it. This specification defines inclusion of the algorithm values defined in Table 1.
• A new IANA registry entitled "JSON Web Encryption Algorithms" for values used with the JWE alg (algorithm) and enc (encryption method) header parameters is defined in Section 4.2. 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 2 and Table 3.

6. Security Considerations

TBD

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

The following items remain to be done in this draft:

• Specify minimum required key sizes for all algorithms.
• Specify which algorithms require Initialization Vectors (IVs) and minimum required lengths for those IVs.
• Since RFC 3447 Section 8 explicitly calls for people NOT to adopt RSASSA-PKCS1 for new applications and instead requests that people transition to RSASSA-PSS, we probably need some Security Considerations text explaining why RSASSA-PKCS1 is being used (it's what's commonly implemented) and what the potential consequences are.
• 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?
• Decide whether to move the JWK algorithm family definitions "EC" and "RSA" here. This would likely result in all the family-specific parameter definitions also moving here ("crv", "x", "y", "mod", "exp"), leaving very little normative text in the JWK spec itself. This seems like it would reduce spec readability and so was not done.
• 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.
• Write the Security Considerations section.

8. References

8.1. Normative References

8.2. Informative References

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

This appendix contains a table cross-referencing the digital signature and HMAC alg (algorithm) values used in this specification with the equivalent identifiers used by other standards and software packages. See XML DSIG [RFC3275] 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-core1-20110303], 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

-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".

Author's Address