JOSE Working Group M. Jones
Internet-Draft Microsoft
Updates: 7519 (if approved) December 13, 2015
Intended status: Standards Track
Expires: June 15, 2016

JWS Unencoded Payload Option


JSON Web Signature (JWS) represents the payload of a JWS as a base64url encoded value and uses this value in the JWS Signature computation. While this enables arbitrary payloads to be integrity protected, some have described use cases in which the base64url encoding is unnecessary and/or an impediment to adoption, especially when the payload is large and/or detached. This specification defines a means of accommodating these use cases by defining an option to change the JWS Signing Input computation to not base64url-encode the payload. This option is intended to broaden the set of use cases for which the use of JWS is a good fit.

This specification updates RFC 7519 by prohibiting the use of the unencoded payload option in JSON Web Tokens (JWTs).

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Table of Contents

1. Introduction

The "JSON Web Signature (JWS)" [JWS] specification defines the JWS Signing Input as the input to the digital signature or MAC computation, with the value ASCII(BASE64URL(UTF8(JWS Protected Header)) || '.' || BASE64URL(JWS Payload)). While this works well in practice for many use cases, including those accommodating arbitrary payload values, other use cases have been described in which base64url-encoding the payload is unnecessary and/or an impediment to adoption, particularly when the payload is large and/or detached.

This specification introduces a new JWS Header Parameter value that generalizes the JWS Signing Input computation in a manner that makes base64url-encoding the payload selectable and optional. The primary set of use cases where this enhancement may be helpful are those in which the payload may be very large and where means are already in place to enable the payload to be communicated between the parties without modifications. Appendix F of [JWS] describes how to represent JWSs with detached content, which would typically be used for these use cases.

The advantages of not having to base64url-encode a large payload are that allocation of the additional storage to hold the base64url-encoded form is avoided and the base64url-encoding computation never has to be performed. In summary, this option can help avoid unnecessary copying and transformations of the potentially large payload, resulting in sometimes significant space and time improvements for deployments.

1.1. Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "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]. The interpretation should only be applied when the terms appear in all capital letters.

BASE64URL(OCTETS) denotes the base64url encoding of OCTETS, per Section 2 of [JWS].

UTF8(STRING) denotes the octets of the UTF-8 [RFC3629] representation of STRING, where STRING is a sequence of zero or more Unicode [UNICODE] characters.

ASCII(STRING) denotes the octets of the ASCII [RFC20] representation of STRING, where STRING is a sequence of zero or more ASCII characters.

The concatenation of two values A and B is denoted as A || B.

2. Terminology

This specification uses the same terminology as the "JSON Web Signature (JWS)" [JWS] and "JSON Web Algorithms (JWA)" [JWA] specifications.

3. The "b64" Header Parameter

This Header Parameter modifies the JWS Payload representation and the JWS Signing Input computation in the following way:


The b64 (base64url-encode payload) Header Parameter determines whether the payload is represented in the JWS and the JWS Signing Input as ASCII(BASE64URL(JWS Payload)) or as the JWS Payload value itself with no encoding performed. When the b64 value is false, the payload is represented simply as the JWS Payload value; otherwise, it is represented as ASCII(BASE64URL(JWS Payload)). The b64 value is a JSON boolean, with a default value of true. When used, this Header Parameter MUST be integrity protected; therefore, it MUST occur only within the JWS Protected Header. Use of this Header Parameter is OPTIONAL. If the JWS has multiple signatures and/or MACs, the b64 Header Parameter value MUST be the same for all of them. Note that unless the payload is detached, many payload values would cause errors parsing the resulting JWSs, as described in Section 5.

The following table shows the JWS Signing Input computation, depending upon the value of this parameter:

"b64" JWS Signing Input Formula
true ASCII(BASE64URL(UTF8(JWS Protected Header)) || '.' || BASE64URL(JWS Payload))
false ASCII(BASE64URL(UTF8(JWS Protected Header)) || '.') || JWS Payload

4. Examples

This section gives examples of JWSs showing the difference that using the b64 Header Parameter makes. The examples all use the JWS Payload value [36, 46, 48, 50]. This octet sequence represents the ASCII characters $.02; its base64url-encoded representation is JC4wMg.

The following table shows a set of Header Parameter values without using a false b64 Header Parameter value and a set using it, with the resulting JWS Signing Input values represented as ASCII characters:

JWS Protected Header JWS Signing Input Value
{"alg":"HS256"} eyJhbGciOiJIUzI1NiJ9.JC4wMg
{"alg":"HS256","b64":false} eyJhbGciOiJIUzI1NiIsImI2NCI6ZmFsc2V9.$.02

These examples use the HMAC key from Appendix A.1 of [JWS], which is represented below as a JWK [JWK] (with line breaks within values for display purposes only):


The rest of this section shows complete representations for the two JWSs above.

4.1. Example with Header Parameters {"alg":"HS256"}

The complete JWS representation for this example using the JWS Compact Serialization and a non-detached payload (with line breaks for display purposes only) is:


Note that this JWS uses only features defined by [JWS] and does not use the new b64 Header Parameter. It is the "control", so that differences when it is used can be easily seen.

The equivalent representation for this example using the flattened JWS JSON Serialization is:


4.2. Example with Header Parameters {"alg":"HS256","b64":false}

The complete JWS representation for this example using the JWS Compact Serialization and a detached payload (with line breaks for display purposes only) is:


Note that the payload $.02 cannot be represented in this JWS in its unencoded form because it contains a period ('.') character, which would cause parsing problems. This JWS is therefore shown with a detached payload.

The complete JWS representation for this example using the flattened JWS JSON Serialization and a non-detached payload is:


If using a detached payload with the JWS JSON Serialization, the payload element would be omitted.

5. Unencoded Payload Content Restrictions

When the b64 value is false, different restrictions on the payload contents apply, depending upon the circumstances, as described in this section. The restrictions prevent the use of payload values that would cause errors parsing the resulting JWSs.

Note that because the character sets that can be used for unencoded non-detached payloads differ between the two serializations, some JWSs using a b64 value of false cannot be syntactically converted between the JWS JSON Serialization and the JWS Compact Serialization. See Section 7 for security considerations on using unencoded payloads.

5.1. Unencoded Detached Payload

Appendix F of [JWS] describes how to represent JWSs with detached content. A detached payload can contain any octet sequence representable by the application. The payload value will not cause problems parsing the JWS, since it is not represented as part of the JWS. If an application uses a content encoding when representing the payload, then it MUST specify whether the signature or MAC is performed over the content-encoded representation or over the unencoded content.

5.2. Unencoded JWS Compact Serialization Payload

When using the JWS Compact Serialization, unencoded non-detached payloads using period ('.') characters would cause parsing errors; such payloads MUST NOT be used with the JWS Compact Serialization. Similarly, if a JWS using the JWS Compact Serialization and a non-detached payload is to be transmitted in a context that requires URL safe characters, then the application MUST ensure that the payload contains only the URL-safe characters 'a'-'z', 'A'-'Z', '0'-'9', dash ('-'), underscore ('_'), and tilde ('~'). The payload value is the ASCII representation of the characters in the payload string. The ASCII space character and all printable ASCII characters other than period ('.') (those characters in the ranges %x20-2D and %x2F-7E) MAY be included in a non-detached payload using the JWS Compact Serialization, provided that the application can transmit the resulting JWS without modification.

No meaning or special semantics are attached to any characters in the payload. For instance, the percent ('%') character represents itself, and is not used by JWS objects for percent-encoding [RFC3986]. Applications, of course, are free to utilize content encoding rules of their choosing, provided that the encoded representations utilize only allowed payload characters.

5.3. Unencoded JWS JSON Serialization Payload

When using the JWS JSON Serialization, unencoded non-detached payloads must consist of the octets of the UTF-8 encoding of a sequence of Unicode code points that are representable in a JSON string. The payload value is determined after performing any JSON string escape processing, per Section 8.3 of RFC 7159 [RFC7159], and then UTF-8-encoding the resulting Unicode code points. This means, for instance, that these payloads represented as JSON strings are equivalent ("$.02", "\u0024.02"). Unassigned Unicode code point values MUST NOT be used to represent the payload.

6. Intended Use by Applications

It is intended that application profiles specify up front whether b64 with a false value is to be used by the application in each application context or not, with it then being consistently applied in each application context. For instance, an application that uses detached payloads might specify that b64 with a false value always be used. It is NOT RECOMMENDED that this parameter value be dynamically varied with different payloads in the same application context.

For interoperability reasons, JSON Web Tokens (JWTs) [JWT] MUST NOT use b64 with a false value.

7. Security Considerations

[JWS] base64url-encodes the JWS Payload to restrict the set of characters used to represent it so that the representation does not contain characters used for delimiters in JWS representations. Those delimiters are the period ('.') character for the JWS Compact Serialization and the double-quote ('"') character for the JWS JSON Serialization. When the b64 (base64url-encode payload) value is false, these properties are lost. It then becomes the responsibility of the application to ensure that payloads only contain characters that will not cause parsing problems for the serialization used, as described in Section 5. The application also incurs the responsibility to ensure that the payload will not be modified during transmission.

Note that if a JWS is created with a b64 value of false and is received by an implementation not supporting the b64 Header Parameter, then the signature or MAC may still verify but the recipient will believe that the intended JWS Payload value is the base64url decoding of the payload value received, rather than the payload value received itself. For example, if the payload value received is NDA1, an implementation not supporting this extension will think that the intended payload is the base64url decoding of this value, which is 405.

There are several ways for applications using this extension to address this situation:

  1. They can require that only JWS implementations that support this extension be used. Then the potential confusion between encoded and unencoded payload values cannot occur. This is necessary for JWSs using this extension to be successfully used by any application.
  2. They can require the use of the crit Header Parameter with b64 in the set of values. Then any JWS using this extension will be rejected by implementations not supporting this extension.
  3. They can always send unencoded payloads that contain characters outside the set used for base64url-encoding (such as including a '$' character), if this is reasonable for the application to do. Then JWS implementations not understanding this extension will reject the JWS because the attempt to base64url-decode the payload value will fail.

Note that methods 2 and 3 are sufficient to cause JWSs using this extension to be rejected by implementations not supporting this extension but they are not sufficient to enable JWSs using this extension to be successfully used by applications. Thus, method 1 - requiring support for this extension - is the preferred approach and the only means for this extension to be practically useful to applications. Method 2 - requiring the use of crit - while theoretically useful to ensure that confusion between encoded and unencoded payloads cannot occur, is not particularly useful in practice, since method 1 is still required for the extension to be usable. When method 1 is employed, method 2 doesn't add any value and since it increases the size of the JWS, its use is not required by this specification.

8. IANA Considerations

8.1. JWS and JWE Header Parameter Registration

This specification registers the b64 Header Parameter defined in Section 3 in the IANA "JSON Web Signature and Encryption Header Parameters" registry [IANA.JOSE] established by [JWS].

8.1.1. Registry Contents

9. References

9.1. Normative References

[IANA.JOSE] IANA, "JSON Object Signing and Encryption (JOSE)"
[JWA] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, DOI 10.17487/RFC7518, May 2015.
[JWS] Jones, M., Bradley, J. and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2015.
[JWT] Jones, M., Bradley, J. and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015.
[RFC20] Cerf, V., "ASCII format for Network Interchange", STD 80, RFC 20, October 1969.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014.
[UNICODE] The Unicode Consortium, "The Unicode Standard"

9.2. Informative References

[JWK] Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/RFC7517, May 2015.
[RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005.

Appendix A. Acknowledgements

Anders Rundgren, Richard Barnes, Phillip Hallam-Baker, Jim Schaad, Matt Miller, Martin Thomson, and others have all made the case for being able to use a representation of the payload that is not base64url-encoded in contexts in which it safe to do so.

Thanks to Sergey Beryozkin, Benjamin Kaduk, James Manger, Kathleen Moriarty, Axel Nennker, Anders Rundgren, Nat Sakimura, Jim Schaad, Robert Sparks, and Matias Woloski for their reviews of the specification and thanks to Vladimir Dzhuvinov for verifying the examples.

Appendix B. Document History

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









Author's Address

Michael B. Jones Microsoft EMail: URI: