JSON Web Signature (JWS)
draft-ietf-jose-json-web-signature-01

Abstract

JSON Web Signature (JWS) is a means of representing content secured with digital signatures or Hash-based Message Authentication Codes (HMACs) using JSON data structures. Cryptographic algorithms and identifiers used with this specification are enumerated in the separate JSON Web Algorithms (JWA) specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) 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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [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 13, 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 Signature (JWS) Overview
    3.1.  Example JWS
4.  JWS Header
    4.1.  Reserved Header Parameter Names
    4.2.  Public Header Parameter Names
    4.3.  Private Header Parameter Names
5.  Rules for Creating and Validating a JWS
6.  Securing JWSs with Cryptographic Algorithms
7.  IANA Considerations
8.  Security Considerations
    8.1.  Unicode Comparison Security Issues
9.  Open Issues and Things To Be Done (TBD)
10.  References
    10.1.  Normative References
    10.2.  Informative References
Appendix A.  JWS Examples
    A.1.  JWS using HMAC SHA-256
        A.1.1.  Encoding
        A.1.2.  Decoding
        A.1.3.  Validating
    A.2.  JWS using RSA SHA-256
        A.2.1.  Encoding
        A.2.2.  Decoding
        A.2.3.  Validating
    A.3.  JWS using ECDSA P-256 SHA-256
        A.3.1.  Encoding
        A.3.2.  Decoding
        A.3.3.  Validating
    A.4.  Example Plaintext JWS
Appendix B.  Notes on implementing base64url encoding without padding
Appendix C.  Acknowledgements
Appendix D.  Document History
§  Authors' Addresses

1.  Introduction

JSON Web Signature (JWS) is a compact format for representing content secured with digital signatures or Hash-based Message Authentication Codes (HMACs) intended for space constrained environments such as HTTP Authorization headers and URI query parameters. It represents this content using JSON [RFC4627] (Crockford, D., “The application/json Media Type for JavaScript Object Notation (JSON),” July 2006.) data structures. The JWS digital signature and HMAC mechanisms are independent of the type of content being secured, allowing arbitrary content to be secured. Cryptographic algorithms and identifiers used with this specification are enumerated in the separate JSON Web Algorithms (JWA) [JWA] (Jones, M., “JSON Web Algorithms (JWA),” March 2012.) specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) [JWE] (Jones, M., Rescorla, E., and J. Hildebrand, “JSON Web Encryption (JWE),” March 2012.) specification.

2.  Terminology

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 JSON object that describes the digital signature or HMAC applied to the JWS Header and the JWS Payload to create the JWS Signature value.

JWS Payload

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

JWS Signature

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

Encoded JWS Header

Base64url encoding of the bytes of the UTF-8 RFC 3629 (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) [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.

Header Parameter Names

The names of the members within the JSON object represented in a JWS Header.

Header Parameter Values

The values of the members within the JSON object represented in a JWS Header.

JWS Compact Serialization

A representation of the JWS as the concatenation of the Encoded JWS Header, the Encoded JWS Payload, and the Encoded JWS Signature in that order, with the three strings being separated by period ('.') characters.

Base64url Encoding

For the purposes of this specification, this term always refers to the URL- and filename-safe Base64 encoding described in RFC 4648 (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) [RFC4648], Section 5, with the (non URL-safe) '=' padding characters omitted, as permitted by Section 3.2. (See Appendix B (Notes on implementing base64url encoding without padding) for notes on implementing base64url encoding without padding.)

3.  JSON Web Signature (JWS) Overview

JWS represents digitally signed or HMACed content using JSON data structures and base64url encoding. The representation consists of three parts: the JWS Header, the JWS Payload, and the JWS Signature. In the Compact Serialization, the three parts are base64url-encoded for transmission, and represented as the concatenation of the encoded strings in that order, with the three strings being separated by period ('.') characters. (A JSON Serialization for this information is defined in the separate JSON Web Signature JSON Serialization (JWS-JS) [JWS‑JS] (Jones, M., Bradley, J., and N. Sakimura, “JSON Web Signature JSON Serialization (JWS-JS),” March 2012.) specification.)

The JWS Header describes the signature or HMAC method and parameters employed. The JWS Payload is the message content to be secured. The JWS Signature ensures the integrity of both the JWS Header and the JWS Payload.

3.1.  Example JWS

The following example JWS Header declares that the encoded object is a JSON Web Token (JWT) [JWT] (Jones, M., Balfanz, D., Bradley, J., Goland, Y., Panzer, J., Sakimura, N., and P. Tarjan, “JSON Web Token (JWT),” March 2012.) and the JWS Header and the JWS Payload are secured using the HMAC SHA-256 algorithm:

{"typ":"JWT",
 "alg":"HS256"}

Base64url encoding the bytes of the UTF-8 representation of the JWS Header yields this Encoded JWS Header value:

eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9

The following is an example of a JSON object that can be used as a JWS Payload. (Note that the payload can be any content, and need not be a representation of a JSON object.)

{"iss":"joe",
 "exp":1300819380,
 "http://example.com/is_root":true}

Base64url encoding the bytes of the UTF-8 representation of the JSON object yields the following Encoded JWS Payload (with line breaks for display purposes only):

eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFt
cGxlLmNvbS9pc19yb290Ijp0cnVlfQ

Computing the HMAC of the UTF-8 representation of the JWS Secured Input (the concatenation of the Encoded JWS Header, a period ('.') character, and the Encoded JWS Payload) with the HMAC SHA-256 algorithm and base64url encoding the result, as per Appendix A.1 (JWS using HMAC SHA-256), yields this Encoded JWS Signature value:

dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

Concatenating these parts in the order Header.Payload.Signature with period characters between the parts yields this complete JWS representation (with line breaks for display purposes only):

eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9
.
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFt
cGxlLmNvbS9pc19yb290Ijp0cnVlfQ
.
dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

This computation is illustrated in more detail in Appendix A.1 (JWS using HMAC SHA-256).

4.  JWS Header

The members of the JSON object represented by the JWS Header describe the digital signature or HMAC applied to the Encoded JWS Header and the Encoded JWS Payload and optionally additional properties of the JWS. The Header Parameter Names within this object MUST be unique. Implementations MUST understand the entire contents of the header; otherwise, the JWS MUST be rejected.

The JWS Header MUST contain an alg (algorithm) parameter, the value of which is a string that unambiguously identifies the algorithm used to secure the JWS Header and the JWS Payload to produce the JWS Signature.

There are three classes of Header Parameter Names: Reserved Header Parameter Names, Public Header Parameter Names, and Private Header Parameter Names.

4.1.  Reserved Header Parameter Names

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

Additional reserved header parameter names MAY be defined via the IANA JSON Web Signature Header Parameters registry, as per Section 7 (IANA Considerations). 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 JWSs. However, in order to prevent collisions, any new header parameter name or algorithm value SHOULD either be defined in the IANA JSON Web Signature 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 JWS.

4.3.  Private Header Parameter Names

A producer and consumer of a JWS may agree to any header parameter name that is not a Reserved Name Section 4.1 (Reserved Header Parameter Names) or a Public Name Section 4.2 (Public Header Parameter Names). 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 JWSs.

5.  Rules for Creating and Validating a JWS

To create a JWS, one MUST perform these steps. The order of the steps is not significant in cases where there are no dependencies between the inputs and outputs of the steps.

1. Create the content to be used as the JWS Payload.
2. Base64url encode the bytes of the JWS Payload. This encoding becomes the Encoded JWS Payload.
3. Create a JWS Header containing the desired set of header parameters. Note that white space is explicitly allowed in the representation and no canonicalization need be performed before encoding.
4. Base64url encode the bytes of the UTF-8 representation of the JWS Header to create the Encoded JWS Header.
5. Compute the JWS Signature in the manner defined for the particular algorithm being used. The JWS Secured Input is always the concatenation of the Encoded JWS Header, a period ('.') character, and the Encoded JWS Payload. (Note that if the JWS represents a JWT, this corresponds to the portion of the JWT representation preceding the second period character.) The alg (algorithm) header parameter MUST be present in the JSON Header, with the algorithm value accurately representing the algorithm used to construct the JWS Signature.
6. Base64url encode the representation of the JWS Signature to create the Encoded JWS Signature.
7. The three encoded parts, taken together, are the result. The Compact Serialization of this result is the concatenation of the Encoded JWS Header, the Encoded JWS Payload, and the Encoded JWS Signature in that order, with the three strings being separated by period ('.') characters.

When validating a JWS, the following steps MUST be taken. 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 the listed steps fails, then the JWS MUST be rejected.

1. Parse the three parts of the input (which are separated by period characters when using the JWS Compact Serialization) into the Encoded JWS Header, the Encoded JWS Payload, and the Encoded JWS Signature.
2. The Encoded JWS Header MUST be successfully base64url decoded following the restriction given in this specification that no padding characters have been used.
3. The JWS Header MUST be completely valid JSON syntax conforming to RFC 4627 (Crockford, D., “The application/json Media Type for JavaScript Object Notation (JSON),” July 2006.) [RFC4627].
4. The JWS Header MUST be validated to only include parameters and values whose syntax and semantics are both understood and supported.
5. The Encoded JWS Payload MUST be successfully base64url decoded following the restriction given in this specification that no padding characters have been used.
6. The Encoded JWS Signature MUST be successfully base64url decoded following the restriction given in this specification that no padding characters have been used.
7. The JWS Signature MUST be successfully validated against the JWS Secured Input (the concatenation of the Encoded JWS Header, a period ('.') character, and the Encoded JWS Payload) in the manner defined for the algorithm being used, which MUST be accurately represented by the value of the alg (algorithm) header parameter, which MUST be present.

Processing a JWS inevitably requires comparing known strings to values in the header. For example, in checking what the algorithm is, the Unicode string encoding alg will be checked against the member names in the JWS Header to see if there is a matching header parameter name. A similar process occurs when determining if the value of the alg header parameter represents a supported algorithm.

Comparisons between JSON strings and other Unicode strings MUST be performed as specified below:

1. Remove any JSON applied escaping to produce an array of Unicode code points.
2. Unicode Normalization (Davis, M., Whistler, K., and M. Dürst, “Unicode Normalization Forms,” 09 2009.) [USA15] MUST NOT be applied at any point to either the JSON string or to the string it is to be compared against.
3. Comparisons between the two strings MUST be performed as a Unicode code point to code point equality comparison.

6.  Securing JWSs with Cryptographic Algorithms

JWS uses cryptographic algorithms to digitally sign or HMAC the contents of the JWS Header and the JWS Payload. The JSON Web Algorithms (JWA) [JWA] (Jones, M., “JSON Web Algorithms (JWA),” March 2012.) specification enumerates a set of cryptographic algorithms and identifiers to be used with this specification. Specifically, Section 3, Table 1 enumerates a set of alg (algorithm) 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 digital signing can be identified using the Header Parameter methods described in Section 4.1 (Reserved Header Parameter Names) or can be distributed using methods that are outside the scope of this specification.

7.  IANA Considerations

This specification calls for:

• A new IANA registry entitled "JSON Web Signature Header Parameters" for reserved header parameter names is defined in Section 4.1 (Reserved Header Parameter Names). Inclusion in the registry is RFC Required in the RFC 5226 (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.) [RFC5226] sense for reserved JWS 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 (Reserved Header Parameter Definitions).

8.  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: Write security considerations about the implications of using a SHA-1 hash (for compatibility reasons) for the x5t (x.509 certificate thumbprint).

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

8.1.  Unicode Comparison Security Issues

Header parameter names in JWSs are Unicode strings. For security reasons, the representations of these names must be compared verbatim after performing any escape processing (as per RFC 4627 (Crockford, D., “The application/json Media Type for JavaScript Object Notation (JSON),” July 2006.) [RFC4627], Section 2.5).

This means, for instance, that these JSON strings must compare as being equal ("sig", "\u0073ig"), whereas these must all compare as being not equal to the first set or to each other ("SIG", "Sig", "si\u0047").

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, JWS 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.

9.  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.
• Describe the relationship between the JWS, JWE, and JWT header parameters. In particular, point out that the set of alg values defined by each must be compatible and non-overlapping.
• Combine the JWS and JWE alg parameter registries and possibly also the header parameter registries.
• Clarify the intended use of the typ Header Parameter across the JWS, JWE, and JWT specifications. Decide whether a registry of typ values is appropriate.
• Add normative text that requires rejecting headers in which member names occur multiple times, as apparently this is legal JSON.
• Clarify the semantics of the kid (key ID) header parameter. Open issues include: What happens if a kid header is received with an unrecognized value? Is that an error? Should it be treated as if it's empty? What happens if the header has a recognized value but the value doesn't match the key associated with that value, but it does match another key that is associated with the issuer? Is that an error?
• Consider whether a key type parameter should also be introduced.
• It would be good to have a confirmation method element so it could be used with holder-of-key.
• EDITORIAL: Think about how to best describe the concept currently described as "the bytes of the UTF-8 representation of". Possible terms to use instead of "bytes of" include "byte sequence", "octet series", and "octet sequence". Also consider whether we want to add an overall clarifying statement somewhere in each spec something like "every place we say 'the UTF-8 representation of X', we mean 'the bytes of the UTF-8 representation of X'". That would potentially allow us to omit the "the bytes of" part everywhere else.
• Write a note in the Security Considerations section about how x5t (x.509 certificate thumbprint) should be deprecated because of known problems with SHA-1.
• Add Security Considerations text on timing attacks.
• Finish the Security Considerations section.
• EDITORIAL: Add an example in which the payload is not a base64url encoded JSON object.

10.  References

10.1. Normative References

10.2. Informative References

Appendix A.  JWS Examples

This section provides several examples of JWSs. While these examples all represent JSON Web Tokens (JWTs) [JWT] (Jones, M., Balfanz, D., Bradley, J., Goland, Y., Panzer, J., Sakimura, N., and P. Tarjan, “JSON Web Token (JWT),” March 2012.), the payload can be any base64url encoded content.

A.1.  JWS using HMAC SHA-256

A.1.1.  Encoding

The following example JWS Header declares that the data structure is a JSON Web Token (JWT) [JWT] (Jones, M., Balfanz, D., Bradley, J., Goland, Y., Panzer, J., Sakimura, N., and P. Tarjan, “JSON Web Token (JWT),” March 2012.) and the JWS Secured Input is secured using the HMAC SHA-256 algorithm.

{"typ":"JWT",
 "alg":"HS256"}

The following byte array contains the UTF-8 characters for the JWS Header:

[123, 34, 116, 121, 112, 34, 58, 34, 74, 87, 84, 34, 44, 13, 10, 32, 34, 97, 108, 103, 34, 58, 34, 72, 83, 50, 53, 54, 34, 125]

Base64url encoding this UTF-8 representation yields this Encoded JWS Header value:

eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9

The JWS Payload used in this example follows. (Note that the payload can be any base64url encoded content, and need not be a base64url encoded JSON object.)

{"iss":"joe",
 "exp":1300819380,
 "http://example.com/is_root":true}

The following byte array contains the UTF-8 characters for the JWS Payload:

[123, 34, 105, 115, 115, 34, 58, 34, 106, 111, 101, 34, 44, 13, 10, 32, 34, 101, 120, 112, 34, 58, 49, 51, 48, 48, 56, 49, 57, 51, 56, 48, 44, 13, 10, 32, 34, 104, 116, 116, 112, 58, 47, 47, 101, 120, 97, 109, 112, 108, 101, 46, 99, 111, 109, 47, 105, 115, 95, 114, 111, 111, 116, 34, 58, 116, 114, 117, 101, 125]

Base64url encoding the above yields the Encoded JWS Payload value (with line breaks for display purposes only):

eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFt
cGxlLmNvbS9pc19yb290Ijp0cnVlfQ

Concatenating the Encoded JWS Header, a period character, and the Encoded JWS Payload yields this JWS Secured Input value (with line breaks for display purposes only):

eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9
.
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFt
cGxlLmNvbS9pc19yb290Ijp0cnVlfQ

The UTF-8 representation of the JWS Secured Input is the following byte array:

[101, 121, 74, 48, 101, 88, 65, 105, 79, 105, 74, 75, 86, 49, 81, 105, 76, 65, 48, 75, 73, 67, 74, 104, 98, 71, 99, 105, 79, 105, 74, 73, 85, 122, 73, 49, 78, 105, 74, 57, 46, 101, 121, 74, 112, 99, 51, 77, 105, 79, 105, 74, 113, 98, 50, 85, 105, 76, 65, 48, 75, 73, 67, 74, 108, 101, 72, 65, 105, 79, 106, 69, 122, 77, 68, 65, 52, 77, 84, 107, 122, 79, 68, 65, 115, 68, 81, 111, 103, 73, 109, 104, 48, 100, 72, 65, 54, 76, 121, 57, 108, 101, 71, 70, 116, 99, 71, 120, 108, 76, 109, 78, 118, 98, 83, 57, 112, 99, 49, 57, 121, 98, 50, 57, 48, 73, 106, 112, 48, 99, 110, 86, 108, 102, 81]

HMACs are generated using keys. This example uses the key represented by the following byte array:

[3, 35, 53, 75, 43, 15, 165, 188, 131, 126, 6, 101, 119, 123, 166, 143, 90, 179, 40, 230, 240, 84, 201, 40, 169, 15, 132, 178, 210, 80, 46, 191, 211, 251, 90, 146, 210, 6, 71, 239, 150, 138, 180, 195, 119, 98, 61, 34, 61, 46, 33, 114, 5, 46, 79, 8, 192, 205, 154, 245, 103, 208, 128, 163]

Running the HMAC SHA-256 algorithm on the UTF-8 representation of the JWS Secured Input with this key yields the following byte array:

[116, 24, 223, 180, 151, 153, 224, 37, 79, 250, 96, 125, 216, 173, 187, 186, 22, 212, 37, 77, 105, 214, 191, 240, 91, 88, 5, 88, 83, 132, 141, 121]

Base64url encoding the above HMAC output yields the Encoded JWS Signature value:

dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

A.1.2.  Decoding

Decoding the JWS first requires removing the base64url encoding from the Encoded JWS Header, the Encoded JWS Payload, and the Encoded JWS Signature. We base64url decode the inputs and turn them into the corresponding byte arrays. We translate the header input byte array containing UTF-8 encoded characters into the JWS Header string.

A.1.3.  Validating

Next we validate the decoded results. Since the alg parameter in the header is "HS256", we validate the HMAC SHA-256 value contained in the JWS Signature. If any of the validation steps fail, the JWS MUST be rejected.

First, we validate that the JWS Header string is legal JSON.

To validate the HMAC value, we repeat the previous process of using the correct key and the UTF-8 representation of the JWS Secured Input as input to a SHA-256 HMAC function and then taking the output and determining if it matches the JWS Signature. If it matches exactly, the HMAC has been validated.

A.2.  JWS using RSA SHA-256

A.2.1.  Encoding

The JWS Header in this example is different from the previous example in two ways: First, because a different algorithm is being used, the alg value is different. Second, for illustration purposes only, the optional "typ" parameter is not used. (This difference is not related to the algorithm employed.) The JWS Header used is:

{"alg":"RS256"}

The following byte array contains the UTF-8 characters for the JWS Header:

[123, 34, 97, 108, 103, 34, 58, 34, 82, 83, 50, 53, 54, 34, 125]

Base64url encoding this UTF-8 representation yields this Encoded JWS Header value:

eyJhbGciOiJSUzI1NiJ9

The JWS Payload used in this example, which follows, is the same as in the previous example. Since the Encoded JWS Payload will therefore be the same, its computation is not repeated here.

{"iss":"joe",
 "exp":1300819380,
 "http://example.com/is_root":true}

Concatenating the Encoded JWS Header, a period character, and the Encoded JWS Payload yields this JWS Secured Input value (with line breaks for display purposes only):

eyJhbGciOiJSUzI1NiJ9
.
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFt
cGxlLmNvbS9pc19yb290Ijp0cnVlfQ

The UTF-8 representation of the JWS Secured Input is the following byte array:

[101, 121, 74, 104, 98, 71, 99, 105, 79, 105, 74, 83, 85, 122, 73, 49, 78, 105, 74, 57, 46, 101, 121, 74, 112, 99, 51, 77, 105, 79, 105, 74, 113, 98, 50, 85, 105, 76, 65, 48, 75, 73, 67, 74, 108, 101, 72, 65, 105, 79, 106, 69, 122, 77, 68, 65, 52, 77, 84, 107, 122, 79, 68, 65, 115, 68, 81, 111, 103, 73, 109, 104, 48, 100, 72, 65, 54, 76, 121, 57, 108, 101, 71, 70, 116, 99, 71, 120, 108, 76, 109, 78, 118, 98, 83, 57, 112, 99, 49, 57, 121, 98, 50, 57, 48, 73, 106, 112, 48, 99, 110, 86, 108, 102, 81]

The RSA key consists of a public part (n, e), and a private exponent d. The values of the RSA key used in this example, presented as the byte arrays representing big endian integers are:

The RSA private key (n, d) is then passed to the RSA signing function, which also takes the hash type, SHA-256, and the UTF-8 representation of the JWS Secured Input as inputs. The result of the digital signature is a byte array S, which represents a big endian integer. In this example, S is:

Base64url encoding the digital signature produces this value for the Encoded JWS Signature (with line breaks for display purposes only):

cC4hiUPoj9Eetdgtv3hF80EGrhuB__dzERat0XF9g2VtQgr9PJbu3XOiZj5RZmh7
AAuHIm4Bh-0Qc_lF5YKt_O8W2Fp5jujGbds9uJdbF9CUAr7t1dnZcAcQjbKBYNX4
BAynRFdiuB--f_nZLgrnbyTyWzO75vRK5h6xBArLIARNPvkSjtQBMHlb1L07Qe7K
0GarZRmB_eSN9383LcOLn6_dO--xi12jzDwusC-eOkHWEsqtFZESc6BfI7noOPqv
hJ1phCnvWh6IeYI2w9QOYEUipUTI8np6LbgGY9Fs98rqVt5AXLIhWkWywlVmtVrB
p0igcN_IoypGlUPQGe77Rw

A.2.2.  Decoding

Decoding the JWS from this example requires processing the Encoded JWS Header and Encoded JWS Payload exactly as done in the first example.

A.2.3.  Validating

Since the alg parameter in the header is "RS256", we validate the RSA SHA-256 digital signature contained in the JWS Signature. If any of the validation steps fail, the JWS MUST be rejected.

First, we validate that the JWS Header string is legal JSON.

Validating the JWS Signature is a little different from the previous example. First, we base64url decode the Encoded JWS Signature to produce a digital signature S to check. We then pass (n, e), S and the UTF-8 representation of the JWS Secured Input to an RSA signature verifier that has been configured to use the SHA-256 hash function.

A.3.  JWS using ECDSA P-256 SHA-256

A.3.1.  Encoding

The JWS Header for this example differs from the previous example because a different algorithm is being used. The JWS Header used is:

{"alg":"ES256"}

The following byte array contains the UTF-8 characters for the JWS Header:

[123, 34, 97, 108, 103, 34, 58, 34, 69, 83, 50, 53, 54, 34, 125]

Base64url encoding this UTF-8 representation yields this Encoded JWS Header value:

eyJhbGciOiJFUzI1NiJ9

The JWS Payload used in this example, which follows, is the same as in the previous examples. Since the Encoded JWS Payload will therefore be the same, its computation is not repeated here.

{"iss":"joe",
 "exp":1300819380,
 "http://example.com/is_root":true}

Concatenating the Encoded JWS Header, a period character, and the Encoded JWS Payload yields this JWS Secured Input value (with line breaks for display purposes only):

eyJhbGciOiJFUzI1NiJ9
.
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFt
cGxlLmNvbS9pc19yb290Ijp0cnVlfQ

The UTF-8 representation of the JWS Secured Input is the following byte array:

[101, 121, 74, 104, 98, 71, 99, 105, 79, 105, 74, 70, 85, 122, 73, 49, 78, 105, 74, 57, 46, 101, 121, 74, 112, 99, 51, 77, 105, 79, 105, 74, 113, 98, 50, 85, 105, 76, 65, 48, 75, 73, 67, 74, 108, 101, 72, 65, 105, 79, 106, 69, 122, 77, 68, 65, 52, 77, 84, 107, 122, 79, 68, 65, 115, 68, 81, 111, 103, 73, 109, 104, 48, 100, 72, 65, 54, 76, 121, 57, 108, 101, 71, 70, 116, 99, 71, 120, 108, 76, 109, 78, 118, 98, 83, 57, 112, 99, 49, 57, 121, 98, 50, 57, 48, 73, 106, 112, 48, 99, 110, 86, 108, 102, 81]

The ECDSA key consists of a public part, the EC point (x, y), and a private part d. The values of the ECDSA key used in this example, presented as the byte arrays representing big endian integers are:

The ECDSA private part d is then passed to an ECDSA signing function, which also takes the curve type, P-256, the hash type, SHA-256, and the UTF-8 representation of the JWS Secured Input as inputs. The result of the digital signature is the EC point (R, S), where R and S are unsigned integers. In this example, the R and S values, given as byte arrays representing big endian integers are:

Concatenating the S array to the end of the R array and base64url encoding the result produces this value for the Encoded JWS Signature (with line breaks for display purposes only):

DtEhU3ljbEg8L38VWAfUAqOyKAM6-Xx-F4GawxaepmXFCgfTjDxw5djxLa8ISlSA
pmWQxfKTUJqPP3-Kg6NU1Q

A.3.2.  Decoding

Decoding the JWS from this example requires processing the Encoded JWS Header and Encoded JWS Payload exactly as done in the first example.

A.3.3.  Validating

Since the alg parameter in the header is "ES256", we validate the ECDSA P-256 SHA-256 digital signature contained in the JWS Signature. If any of the validation steps fail, the JWS MUST be rejected.

First, we validate that the JWS Header string is legal JSON.

Validating the JWS Signature is a little different from the first example. First, we base64url decode the Encoded JWS Signature as in the previous examples but we then need to split the 64 member byte array that must result into two 32 byte arrays, the first R and the second S. We then pass (x, y), (R, S) and the UTF-8 representation of the JWS Secured Input to an ECDSA signature verifier that has been configured to use the P-256 curve with the SHA-256 hash function.

As explained in Section 3.3 of the JSON Web Algorithms (JWA) [JWA] (Jones, M., “JSON Web Algorithms (JWA),” March 2012.) specification, the use of the k value in ECDSA means that we cannot validate the correctness of the digital signature in the same way we validated the correctness of the HMAC. Instead, implementations MUST use an ECDSA validator to validate the digital signature.

A.4.  Example Plaintext JWS

The following example JWS Header declares that the encoded object is a Plaintext JWS:

{"alg":"none"}

Base64url encoding the bytes of the UTF-8 representation of the JWS Header yields this Encoded JWS Header:

eyJhbGciOiJub25lIn0

The JWS Payload used in this example, which follows, is the same as in the previous examples. Since the Encoded JWS Payload will therefore be the same, its computation is not repeated here.

{"iss":"joe",
 "exp":1300819380,
 "http://example.com/is_root":true}

The Encoded JWS Signature is the empty string.

Concatenating these parts in the order Header.Payload.Signature with period characters between the parts yields this complete JWS (with line breaks for display purposes only):

eyJhbGciOiJub25lIn0
.
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFt
cGxlLmNvbS9pc19yb290Ijp0cnVlfQ
.

Appendix B.  Notes on implementing base64url encoding without padding

This appendix describes how to implement base64url encoding and decoding functions without padding based upon standard base64 encoding and decoding functions that do use padding.

To be concrete, example C# code implementing these functions is shown below. Similar code could be used in other languages.

static string base64urlencode(byte [] arg)
{
  string s = Convert.ToBase64String(arg); // Standard base64 encoder
  s = s.Split('=')[0]; // Remove any trailing '='s
  s = s.Replace('+', '-'); // 62nd char of encoding
  s = s.Replace('/', '_'); // 63rd char of encoding
  return s;
}

static byte [] base64urldecode(string arg)
{
  string s = arg;
  s = s.Replace('-', '+'); // 62nd char of encoding
  s = s.Replace('_', '/'); // 63rd char of encoding
  switch (s.Length % 4) // Pad with trailing '='s
  {
    case 0: break; // No pad chars in this case
    case 2: s += "=="; break; // Two pad chars
    case 3: s += "="; break; // One pad char
    default: throw new System.Exception(
      "Illegal base64url string!");
  }
  return Convert.FromBase64String(s); // Standard base64 decoder
}

As per the example code above, the number of '=' padding characters that needs to be added to the end of a base64url encoded string without padding to turn it into one with padding is a deterministic function of the length of the encoded string. Specifically, if the length mod 4 is 0, no padding is added; if the length mod 4 is 2, two '=' padding characters are added; if the length mod 4 is 3, one '=' padding character is added; if the length mod 4 is 1, the input is malformed.

An example correspondence between unencoded and encoded values follows. The byte sequence below encodes into the string below, which when decoded, reproduces the byte sequence.

3 236 255 224 193

A-z_4ME

Appendix C.  Acknowledgements

Solutions for signing JSON content were previously explored by Magic Signatures (Panzer (editor), J., Laurie, B., and D. Balfanz, “Magic Signatures,” January 2011.) [MagicSignatures], JSON Simple Sign (Bradley, J. and N. Sakimura (editor), “JSON Simple Sign,” September 2010.) [JSS], and Canvas Applications (Facebook, “Canvas Applications,” 2010.) [CanvasApp], all of which influenced this draft. Dirk Balfanz, Yaron Y. Goland, John Panzer, and Paul Tarjan all made significant contributions to the design of this specification.

Appendix D.  Document History

-01

• Moved definition of Plaintext JWSs (using "alg":"none") here from the JWT specification since this functionality is likely to be useful in more contexts that just for JWTs.
• 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 JWS Compact Serialization. Referenced the new JWS-JS spec, which defines the JWS JSON Serialization.
• Added text "New header parameters should be introduced sparingly since an implementation that does not understand a parameter MUST reject the JWS".
• Clarified that the order of the creation and validation steps is not significant in cases where there are no dependencies between the inputs and outputs of the steps.
• Changed "no canonicalization is performed" to "no canonicalization need be performed".
• 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 with no normative changes.
• Changed terminology to no longer call both digital signatures and HMACs "signatures".

Authors' Addresses