JSON Canonicalization Scheme (JCS)
RFC 8785



Independent Submission                                       A. Rundgren
Request for Comments: 8785                                   Independent
Category: Informational                                        B. Jordan
ISSN: 2070-1721                                                 Broadcom
                                                              S. Erdtman
                                                              Spotify AB
                                                               June 2020

                   JSON Canonicalization Scheme (JCS)

Abstract

   Cryptographic operations like hashing and signing need the data to be
   expressed in an invariant format so that the operations are reliably
   repeatable.  One way to address this is to create a canonical
   representation of the data.  Canonicalization also permits data to be
   exchanged in its original form on the "wire" while cryptographic
   operations performed on the canonicalized counterpart of the data in
   the producer and consumer endpoints generate consistent results.

   This document describes the JSON Canonicalization Scheme (JCS).  This
   specification defines how to create a canonical representation of
   JSON data by building on the strict serialization methods for JSON
   primitives defined by ECMAScript, constraining JSON data to the
   Internet JSON (I-JSON) subset, and by using deterministic property
   sorting.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8785.

Copyright Notice

   Copyright (c) 2020 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
   (https://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.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Detailed Operation
     3.1.  Creation of Input Data
     3.2.  Generation of Canonical JSON Data
       3.2.1.  Whitespace
       3.2.2.  Serialization of Primitive Data Types
         3.2.2.1.  Serialization of Literals
         3.2.2.2.  Serialization of Strings
         3.2.2.3.  Serialization of Numbers
       3.2.3.  Sorting of Object Properties
       3.2.4.  UTF-8 Generation
   4.  IANA Considerations
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  ECMAScript Sample Canonicalizer
   Appendix B.  Number Serialization Samples
   Appendix C.  Canonicalized JSON as "Wire Format"
   Appendix D.  Dealing with Big Numbers
   Appendix E.  String Subtype Handling
     E.1.  Subtypes in Arrays
   Appendix F.  Implementation Guidelines
   Appendix G.  Open-Source Implementations
   Appendix H.  Other JSON Canonicalization Efforts
   Appendix I.  Development Portal
   Acknowledgements
   Authors' Addresses

1.  Introduction

   This document describes the JSON Canonicalization Scheme (JCS).  This
   specification defines how to create a canonical representation of
   JSON [RFC8259] data by building on the strict serialization methods
   for JSON primitives defined by ECMAScript [ECMA-262], constraining
   JSON data to the I-JSON [RFC7493] subset, and by using deterministic
   property sorting.  The output from JCS is a "hashable" representation
   of JSON data that can be used by cryptographic methods.  The
   subsequent paragraphs outline the primary design considerations.

   Cryptographic operations like hashing and signing need the data to be
   expressed in an invariant format so that the operations are reliably
   repeatable.  One way to accomplish this is to convert the data into a
   format that has a simple and fixed representation, like base64url
   [RFC4648].  This is how JSON Web Signature (JWS) [RFC7515] addressed
   this issue.  Another solution is to create a canonical version of the
   data, similar to what was done for the XML signature [XMLDSIG]
   standard.

   The primary advantage with a canonicalizing scheme is that data can
   be kept in its original form.  This is the core rationale behind JCS.
   Put another way, using canonicalization enables a JSON object to
   remain a JSON object even after being signed.  This can simplify
   system design, documentation, and logging.

   To avoid "reinventing the wheel", JCS relies on the serialization of
   JSON primitives (strings, numbers, and literals), as defined by
   ECMAScript (aka JavaScript) [ECMA-262] beginning with version 6.

   Seasoned XML developers may recall difficulties getting XML
   signatures to validate.  This was usually due to different
   interpretations of the quite intricate XML canonicalization rules as
   well as of the equally complex Web Services security standards.  The
   reasons why JCS should not suffer from similar issues are:

   *  JSON does not have a namespace concept and default values.

   *  Data is constrained to the I-JSON [RFC7493] subset.  This
      eliminates the need for specific parsers for dealing with
      canonicalization.

   *  JCS-compatible serialization of JSON primitives is currently
      supported by most web browsers as well as by Node.js [NODEJS].

   *  The full JCS specification is currently supported by multiple
      open-source implementations (see Appendix G).  See also Appendix F
      for implementation guidelines.

   JCS is compatible with some existing systems relying on JSON
   canonicalization such as JSON Web Key (JWK) Thumbprint [RFC7638] and
   Keybase [KEYBASE].

   For potential uses outside of cryptography, see [JSONCOMP].

   The intended audiences of this document are JSON tool vendors as well
   as designers of JSON-based cryptographic solutions.  The reader is
   assumed to be knowledgeable in ECMAScript, including the "JSON"
   object.

2.  Terminology

   Note that this document is not on the IETF standards track.  However,
   a conformant implementation is supposed to adhere to the specified
   behavior for security and interoperability reasons.  This text uses
   BCP 14 to describe that necessary behavior.

   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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Detailed Operation

   This section describes the details related to creating a canonical
   JSON representation and how they are addressed by JCS.

   Appendix F describes the RECOMMENDED way of adding JCS support to
   existing JSON tools.

3.1.  Creation of Input Data

   Data to be canonically serialized is usually created by:

   *  Parsing previously generated JSON data.

   *  Programmatically creating data.

   Irrespective of the method used, the data to be serialized MUST be
   adapted for I-JSON [RFC7493] formatting, which implies the following:

   *  JSON objects MUST NOT exhibit duplicate property names.

   *  JSON string data MUST be expressible as Unicode [UNICODE].

   *  JSON number data MUST be expressible as IEEE 754 [IEEE754] double-
      precision values.  For applications needing higher precision or
      longer integers than offered by IEEE 754 double precision, it is
      RECOMMENDED to represent such numbers as JSON strings; see
      Appendix D for details on how this can be performed in an
      interoperable and extensible way.

   An additional constraint is that parsed JSON string data MUST NOT be
   altered during subsequent serializations.  For more information, see
   Appendix E.

   Note: Although the Unicode standard offers the possibility of
   rearranging certain character sequences, referred to as "Unicode
   Normalization" [UCNORM], JCS-compliant string processing does not
   take this into consideration.  That is, all components involved in a
   scheme depending on JCS MUST preserve Unicode string data "as is".

3.2.  Generation of Canonical JSON Data

   The following subsections describe the steps required to create a
   canonical JSON representation of the data elaborated on in the
   previous section.

   Appendix A shows sample code for an ECMAScript-based canonicalizer,
   matching the JCS specification.

3.2.1.  Whitespace

   Whitespace between JSON tokens MUST NOT be emitted.

3.2.2.  Serialization of Primitive Data Types

   Assume the following JSON object is parsed:

     {
       "numbers": [333333333.33333329, 1E30, 4.50,
                   2e-3, 0.000000000000000000000000001],
       "string": "\u20ac$\u000F\u000aA'\u0042\u0022\u005c\\\"\/",
       "literals": [null, true, false]
     }

   If the parsed data is subsequently serialized using a serializer
   compliant with ECMAScript's "JSON.stringify()", the result would
   (with a line wrap added for display purposes only) be rather
   divergent with respect to the original data:

     {"numbers":[333333333.3333333,1e+30,4.5,0.002,1e-27],"string":
     "€$\u000f\nA'B\"\\\\\"/","literals":[null,true,false]}

   The reason for the difference between the parsed data and its
   serialized counterpart is due to a wide tolerance on input data (as
   defined by JSON [RFC8259]), while output data (as defined by
   ECMAScript) has a fixed representation.  As can be seen in the
   example, numbers are subject to rounding as well.

   The following subsections describe the serialization of primitive
   JSON data types according to JCS.  This part is identical to that of
   ECMAScript.  In the (unlikely) event that a future version of
   ECMAScript would invalidate any of the following serialization
   methods, it will be up to the developer community to either stick to
   this specification or create a new specification.

3.2.2.1.  Serialization of Literals

   In accordance with JSON [RFC8259], the literals "null", "true", and
   "false" MUST be serialized as null, true, and false, respectively.

3.2.2.2.  Serialization of Strings

   For JSON string data (which includes JSON object property names as
   well), each Unicode code point MUST be serialized as described below
   (see Section 24.3.2.2 of [ECMA-262]):

   *  If the Unicode value falls within the traditional ASCII control
      character range (U+0000 through U+001F), it MUST be serialized
      using lowercase hexadecimal Unicode notation (\uhhhh) unless it is
      in the set of predefined JSON control characters U+0008, U+0009,
      U+000A, U+000C, or U+000D, which MUST be serialized as \b, \t, \n,
      \f, and \r, respectively.

   *  If the Unicode value is outside of the ASCII control character
      range, it MUST be serialized "as is" unless it is equivalent to
      U+005C (\) or U+0022 ("), which MUST be serialized as \\ and \",
      respectively.

   Finally, the resulting sequence of Unicode code points MUST be
   enclosed in double quotes (").

   Note: Since invalid Unicode data like "lone surrogates" (e.g.,
   U+DEAD) may lead to interoperability issues including broken
   signatures, occurrences of such data MUST cause a compliant JCS
   implementation to terminate with an appropriate error.

3.2.2.3.  Serialization of Numbers

   ECMAScript builds on the IEEE 754 [IEEE754] double-precision standard
   for representing JSON number data.  Such data MUST be serialized
   according to Section 7.1.12.1 of [ECMA-262], including the "Note 2"
   enhancement.

   Due to the relative complexity of this part, the algorithm itself is
   not included in this document.  For implementers of JCS-compliant
   number serialization, Google's implementation in V8 [V8] may serve as
   a reference.  Another compatible number serialization reference
   implementation is Ryu [RYU], which is used by the JCS open-source
   Java implementation mentioned in Appendix G.  Appendix B holds a set
   of IEEE 754 sample values and their corresponding JSON serialization.

   Note: Since Not a Number (NaN) and Infinity are not permitted in
   JSON, occurrences of NaN or Infinity MUST cause a compliant JCS
   implementation to terminate with an appropriate error.

3.2.3.  Sorting of Object Properties

   Although the previous step normalized the representation of primitive
   JSON data types, the result would not yet qualify as "canonical"
   since JSON object properties are not in lexicographic (alphabetical)
   order.

   Applied to the sample in Section 3.2.2, a properly canonicalized
   version should (with a line wrap added for display purposes only)
   read as:

     {"literals":[null,true,false],"numbers":[333333333.3333333,
     1e+30,4.5,0.002,1e-27],"string":"€$\u000f\nA'B\"\\\\\"/"}

   The rules for lexicographic sorting of JSON object properties
   according to JCS are as follows:

   *  JSON object properties MUST be sorted recursively, which means
      that JSON child Objects MUST have their properties sorted as well.

   *  JSON array data MUST also be scanned for the presence of JSON
      objects (if an object is found, then its properties MUST be
      sorted), but array element order MUST NOT be changed.

   When a JSON object is about to have its properties sorted, the
   following measures MUST be adhered to:

   *  The sorting process is applied to property name strings in their
      "raw" (unescaped) form.  That is, a newline character is treated
      as U+000A.

   *  Property name strings to be sorted are formatted as arrays of
      UTF-16 [UNICODE] code units.  The sorting is based on pure value
      comparisons, where code units are treated as unsigned integers,
      independent of locale settings.

   *  Property name strings either have different values at some index
      that is a valid index for both strings, or their lengths are
      different, or both.  If they have different values at one or more
      index positions, let k be the smallest such index; then, the
      string whose value at position k has the smaller value, as
      determined by using the "<" operator, lexicographically precedes
      the other string.  If there is no index position at which they
      differ, then the shorter string lexicographically precedes the
      longer string.

      In plain English, this means that property names are sorted in
      ascending order like the following:

              ""
              "a"
              "aa"
              "ab"

   The rationale for basing the sorting algorithm on UTF-16 code units
   is that it maps directly to the string type in ECMAScript (featured
   in web browsers and Node.js), Java, and .NET.  In addition, JSON only
   supports escape sequences expressed as UTF-16 code units, making
   knowledge and handling of such data a necessity anyway.  Systems
   using another internal representation of string data will need to
   convert JSON property name strings into arrays of UTF-16 code units
   before sorting.  The conversion from UTF-8 or UTF-32 to UTF-16 is
   defined by the Unicode [UNICODE] standard.

   The following JSON test data can be used for verifying the
   correctness of the sorting scheme in a JCS implementation:

     {
       "\u20ac": "Euro Sign",
       "\r": "Carriage Return",
       "\ufb33": "Hebrew Letter Dalet With Dagesh",
       "1": "One",
       "\ud83d\ude00": "Emoji: Grinning Face",
       "\u0080": "Control",
       "\u00f6": "Latin Small Letter O With Diaeresis"
     }

   Expected argument order after sorting property strings:

     "Carriage Return"
     "One"
     "Control"
     "Latin Small Letter O With Diaeresis"
     "Euro Sign"
     "Emoji: Grinning Face"
     "Hebrew Letter Dalet With Dagesh"

   Note: For the purpose of obtaining a deterministic property order,
   sorting of data encoded in UTF-8 or UTF-32 would also work, but the
   outcome for JSON data like above would differ and thus be
   incompatible with this specification.  However, in practice, property
   names are rarely defined outside of 7-bit ASCII, making it possible
   to sort string data in UTF-8 or UTF-32 format without conversion to
   UTF-16 and still be compatible with JCS.  Whether or not this is a
   viable option depends on the environment JCS is used in.

3.2.4.  UTF-8 Generation

   Finally, in order to create a platform-independent representation,
   the result of the preceding step MUST be encoded in UTF-8.

   Applied to the sample in Section 3.2.3, this should yield the
   following bytes, here shown in hexadecimal notation:

     7b 22 6c 69 74 65 72 61 6c 73 22 3a 5b 6e 75 6c 6c 2c 74 72
     75 65 2c 66 61 6c 73 65 5d 2c 22 6e 75 6d 62 65 72 73 22 3a
     5b 33 33 33 33 33 33 33 33 33 2e 33 33 33 33 33 33 33 2c 31
     65 2b 33 30 2c 34 2e 35 2c 30 2e 30 30 32 2c 31 65 2d 32 37
     5d 2c 22 73 74 72 69 6e 67 22 3a 22 e2 82 ac 24 5c 75 30 30
     30 66 5c 6e 41 27 42 5c 22 5c 5c 5c 5c 5c 22 2f 22 7d

   This data is intended to be usable as input to cryptographic methods.

4.  IANA Considerations

   This document has no IANA actions.

5.  Security Considerations

   It is crucial to perform sanity checks on input data to avoid
   overflowing buffers and similar things that could affect the
   integrity of the system.

   When JCS is applied to signature schemes like the one described in
   Appendix F, applications MUST perform the following operations before
   acting upon received data:

   1.  Parse the JSON data and verify that it adheres to I-JSON.

   2.  Verify the data for correctness according to the conventions
       defined by the ecosystem where it is to be used.  This also
       includes locating the property holding the signature data.

   3.  Verify the signature.

   If any of these steps fail, the operation in progress MUST be
   aborted.

6.  References

6.1.  Normative References

   [ECMA-262] ECMA International, "ECMAScript 2019 Language
              Specification", Standard ECMA-262 10th Edition, June 2019,
              <https://www.ecma-international.org/ecma-262/10.0/
              index.html>.

   [IEEE754]  IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE
              754-2019, DOI 10.1109/IEEESTD.2019.8766229,
              <https://ieeexplore.ieee.org/document/8766229>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,
              <https://www.rfc-editor.org/info/rfc7493>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [UCNORM]   The Unicode Consortium, "Unicode Normalization Forms",
              <https://www.unicode.org/reports/tr15/>.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard",
              <https://www.unicode.org/versions/latest/>.

6.2.  Informative References

   [JSONCOMP] Rundgren, A., ""Comparable" JSON (JSONCOMP)", Work in
              Progress, Internet-Draft, draft-rundgren-comparable-json-
              04, 13 February 2019, <https://tools.ietf.org/html/draft-
              rundgren-comparable-json-04>.

   [KEYBASE]  Keybase, "Canonical Packings for JSON and Msgpack",
              <https://keybase.io/docs/api/1.0/canonical_packings>.

   [NODEJS]   OpenJS Foundation, "Node.js", <https://nodejs.org>.

   [OPENAPI]  OpenAPI Initiative, "The OpenAPI Specification: a broadly
              adopted industry standard for describing modern APIs",
              <https://www.openapis.org/>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7638]  Jones, M. and N. Sakimura, "JSON Web Key (JWK)
              Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
              2015, <https://www.rfc-editor.org/info/rfc7638>.

   [RYU]      "Ryu floating point number serializing algorithm", commit
              27d3c55, May 2020, <https://github.com/ulfjack/ryu>.

   [V8]       Google LLC, "What is V8?", <https://v8.dev/>.

   [XMLDSIG]  W3C, "XML Signature Syntax and Processing Version 1.1",
              W3C Recommendation, April 2013,
              <https://www.w3.org/TR/xmldsig-core1/>.

Appendix A.  ECMAScript Sample Canonicalizer

   Below is an example of a JCS canonicalizer for usage with ECMAScript-
   based systems:

     ////////////////////////////////////////////////////////////
     // Since the primary purpose of this code is highlighting //
     // the core of the JCS algorithm, error handling and      //
     // UTF-8 generation were not implemented.                 //
     ////////////////////////////////////////////////////////////
     var canonicalize = function(object) {

         var buffer = '';
         serialize(object);
         return buffer;

         function serialize(object) {
             if (object === null || typeof object !== 'object' ||
                 object.toJSON != null) {
                 /////////////////////////////////////////////////
                 // Primitive type or toJSON, use "JSON"        //
                 /////////////////////////////////////////////////
                 buffer += JSON.stringify(object);

             } else if (Array.isArray(object)) {
                 /////////////////////////////////////////////////
                 // Array - Maintain element order              //
                 /////////////////////////////////////////////////
                 buffer += '[';
                 let next = false;
                 object.forEach((element) => {
                     if (next) {
                         buffer += ',';
                     }
                     next = true;
                     /////////////////////////////////////////
                     // Array element - Recursive expansion //
                     /////////////////////////////////////////
                     serialize(element);
                 });
                 buffer += ']';

             } else {
                 /////////////////////////////////////////////////
                 // Object - Sort properties before serializing //
                 /////////////////////////////////////////////////
                 buffer += '{';
                 let next = false;
                 Object.keys(object).sort().forEach((property) => {
                     if (next) {
                         buffer += ',';
                     }
                     next = true;
                     /////////////////////////////////////////////
                     // Property names are strings, use "JSON"  //
                     /////////////////////////////////////////////
                     buffer += JSON.stringify(property);
                     buffer += ':';
                     //////////////////////////////////////////
                     // Property value - Recursive expansion //
                     //////////////////////////////////////////
                     serialize(object[property]);
                 });
                 buffer += '}';
             }
         }
     };

Appendix B.  Number Serialization Samples

   The following table holds a set of ECMAScript-compatible number
   serialization samples, including some edge cases.  The column "IEEE
   754" refers to the internal ECMAScript representation of the "Number"
   data type, which is based on the IEEE 754 [IEEE754] standard using
   64-bit (double-precision) values, here expressed in hexadecimal.

   +==================+===========================+====================+
   |     IEEE 754     |    JSON Representation    |      Comment       |
   +==================+===========================+====================+
   | 0000000000000000 | 0                         | Zero               |
   +------------------+---------------------------+--------------------+
   | 8000000000000000 | 0                         | Minus zero         |
   +------------------+---------------------------+--------------------+
   | 0000000000000001 | 5e-324                    | Min pos number     |
   +------------------+---------------------------+--------------------+
   | 8000000000000001 | -5e-324                   | Min neg number     |
   +------------------+---------------------------+--------------------+
   | 7fefffffffffffff | 1.7976931348623157e+308   | Max pos number     |
   +------------------+---------------------------+--------------------+
   | ffefffffffffffff | -1.7976931348623157e+308  | Max neg number     |
   +------------------+---------------------------+--------------------+
   | 4340000000000000 | 9007199254740992          | Max pos int    (1) |
   +------------------+---------------------------+--------------------+
   | c340000000000000 | -9007199254740992         | Max neg int    (1) |
   +------------------+---------------------------+--------------------+
   | 4430000000000000 | 295147905179352830000     | ~2**68         (2) |
   +------------------+---------------------------+--------------------+
   | 7fffffffffffffff |                           | NaN            (3) |
   +------------------+---------------------------+--------------------+
   | 7ff0000000000000 |                           | Infinity       (3) |
   +------------------+---------------------------+--------------------+
   | 44b52d02c7e14af5 | 9.999999999999997e+22     |                    |
   +------------------+---------------------------+--------------------+
   | 44b52d02c7e14af6 | 1e+23                     |                    |
   +------------------+---------------------------+--------------------+
   | 44b52d02c7e14af7 | 1.0000000000000001e+23    |                    |
   +------------------+---------------------------+--------------------+
   | 444b1ae4d6e2ef4e | 999999999999999700000     |                    |
   +------------------+---------------------------+--------------------+
   | 444b1ae4d6e2ef4f | 999999999999999900000     |                    |
   +------------------+---------------------------+--------------------+
   | 444b1ae4d6e2ef50 | 1e+21                     |                    |
   +------------------+---------------------------+--------------------+
   | 3eb0c6f7a0b5ed8c | 9.999999999999997e-7      |                    |
   +------------------+---------------------------+--------------------+
   | 3eb0c6f7a0b5ed8d | 0.000001                  |                    |
   +------------------+---------------------------+--------------------+
   | 41b3de4355555553 | 333333333.3333332         |                    |
   +------------------+---------------------------+--------------------+
   | 41b3de4355555554 | 333333333.33333325        |                    |
   +------------------+---------------------------+--------------------+
   | 41b3de4355555555 | 333333333.3333333         |                    |
   +------------------+---------------------------+--------------------+
   | 41b3de4355555556 | 333333333.3333334         |                    |
   +------------------+---------------------------+--------------------+
   | 41b3de4355555557 | 333333333.33333343        |                    |
   +------------------+---------------------------+--------------------+
   | becbf647612f3696 | -0.0000033333333333333333 |                    |
   +------------------+---------------------------+--------------------+
   | 43143ff3c1cb0959 | 1424953923781206.2        | Round to even  (4) |
   +------------------+---------------------------+--------------------+

      Table 1: ECMAScript-Compatible JSON Number Serialization Samples

   Notes:

   (1)  For maximum compliance with the ECMAScript "JSON" object, values
        that are to be interpreted as true integers SHOULD be in the
        range -9007199254740991 to 9007199254740991.  However, how
        numbers are used in applications does not affect the JCS
        algorithm.

   (2)  Although a set of specific integers like 2**68 could be regarded
        as having extended precision, the JCS/ECMAScript number
        serialization algorithm does not take this into consideration.

   (3)  Values out of range are not permitted in JSON.  See
        Section 3.2.2.3.

   (4)  This number is exactly 1424953923781206.25 but will, after the
        "Note 2" rule mentioned in Section 3.2.2.3, be truncated and
        rounded to the closest even value.

   For a more exhaustive validation of a JCS number serializer, you may
   test against a file (currently) available in the development portal
   (see Appendix I) containing a large set of sample values.  Another
   option is running V8 [V8] as a live reference together with a program
   generating a substantial amount of random IEEE 754 values.

Appendix C.  Canonicalized JSON as "Wire Format"

   Since the result from the canonicalization process (see
   Section 3.2.4) is fully valid JSON, it can also be used as "Wire
   Format".  However, this is just an option since cryptographic schemes
   based on JCS, in most cases, would not depend on that externally
   supplied JSON data already being canonicalized.

   In fact, the ECMAScript standard way of serializing objects using
   "JSON.stringify()" produces a more "logical" format, where properties
   are kept in the order they were created or received.  The example
   below shows an address record that could benefit from ECMAScript
   standard serialization:

     {
       "name": "John Doe",
       "address": "2000 Sunset Boulevard",
       "city": "Los Angeles",
       "zip": "90001",
       "state": "CA"
     }

   Using canonicalization, the properties above would be output in the
   order "address", "city", "name", "state", and "zip", which adds
   fuzziness to the data from a human (developer or technical support)
   perspective.  Canonicalization also converts JSON data into a single
   line of text, which may be less than ideal for debugging and logging.

Appendix D.  Dealing with Big Numbers

   There are several issues associated with the JSON number type, here
   illustrated by the following sample object:

     {
       "giantNumber": 1.4e+9999,
       "payMeThis": 26000.33,
       "int64Max": 9223372036854775807
     }

   Although the sample above conforms to JSON [RFC8259], applications
   would normally use different native data types for storing
   "giantNumber" and "int64Max".  In addition, monetary data like
   "payMeThis" would presumably not rely on floating-point data types
   due to rounding issues with respect to decimal arithmetic.

   The established way of handling this kind of "overloading" of the
   JSON number type (at least in an extensible manner) is through
   mapping mechanisms, instructing parsers what to do with different
   properties based on their name.  However, this greatly limits the
   value of using the JSON number type outside of its original, somewhat
   constrained JavaScript context.  The ECMAScript "JSON" object does
   not support mappings to the JSON number type either.

   Due to the above, numbers that do not have a natural place in the
   current JSON ecosystem MUST be wrapped using the JSON string type.
   This is close to a de facto standard for open systems.  This is also
   applicable for other data types that do not have direct support in
   JSON, like "DateTime" objects as described in Appendix E.

   Aided by a system using the JSON string type, be it programmatic like

     var obj = JSON.parse('{"giantNumber": "1.4e+9999"}');
     var biggie = new BigNumber(obj.giantNumber);

   or declarative schemes like OpenAPI [OPENAPI], JCS imposes no limits
   on applications, including when using ECMAScript.

Appendix E.  String Subtype Handling

   Due to the limited set of data types featured in JSON, the JSON
   string type is commonly used for holding subtypes.  This can,
   depending on JSON parsing method, lead to interoperability problems,
   which MUST be dealt with by JCS-compliant applications targeting a
   wider audience.

   Assume you want to parse a JSON object where the schema designer
   assigned the property "big" for holding a "BigInt" subtype and "time"
   for holding a "DateTime" subtype, while "val" is supposed to be a
   JSON number compliant with JCS.  The following example shows such an
   object:

     {
       "time": "2019-01-28T07:45:10Z",
       "big": "055",
       "val": 3.5
     }

   Parsing of this object can be accomplished by the following
   ECMAScript statement:

     var object = JSON.parse(JSON_object_featured_as_a_string);

   After parsing, the actual data can be extracted, which for subtypes,
   also involves a conversion step using the result of the parsing
   process (an ECMAScript object) as input:

     ... = new Date(object.time); // Date object
     ... = BigInt(object.big);    // Big integer
     ... = object.val;            // JSON/JS number

   Note that the "BigInt" data type is currently only natively supported
   by V8 [V8].

   Canonicalization of "object" using the sample code in Appendix A
   would return the following string:

     {"big":"055","time":"2019-01-28T07:45:10Z","val":3.5}

   Although this is (with respect to JCS) technically correct, there is
   another way of parsing JSON data, which also can be used with
   ECMAScript as shown below:

     // "BigInt" requires the following code to become JSON serializable
     BigInt.prototype.toJSON = function() {
         return this.toString();
     };

     // JSON parsing using a "stream"-based method
     var object = JSON.parse(JSON_object_featured_as_a_string,
         (k,v) => k == 'time' ? new Date(v) : k == 'big' ? BigInt(v) : v
     );

   If you now apply the canonicalizer in Appendix A to "object", the
   following string would be generated:

     {"big":"55","time":"2019-01-28T07:45:10.000Z","val":3.5}

   In this case, the string arguments for "big" and "time" have changed
   with respect to the original, presumably making an application
   depending on JCS fail.

   The reason for the deviation is that in stream- and schema-based JSON
   parsers, the original string argument is typically replaced on the
   fly by the native subtype that, when serialized, may exhibit a
   different and platform-dependent pattern.

   That is, stream- and schema-based parsing MUST treat subtypes as
   "pure" (immutable) JSON string types and perform the actual
   conversion to the designated native type in a subsequent step.  In
   modern programming platforms like Go, Java, and C#, this can be
   achieved with moderate efforts by combining annotations, getters, and
   setters.  Below is an example in C#/Json.NET showing a part of a
   class that is serializable as a JSON object:

     // The "pure" string solution uses a local
     // string variable for JSON serialization while
     // exposing another type to the application
     [JsonProperty("amount")]
     private string _amount;

     [JsonIgnore]
     public decimal Amount {
         get { return decimal.Parse(_amount); }
         set { _amount = value.ToString(); }
     }

   In an application, "Amount" can be accessed as any other property
   while it is actually represented by a quoted string in JSON contexts.

   Note: The example above also addresses the constraints on numeric
   data implied by I-JSON (the C# "decimal" data type has quite
   different characteristics compared to IEEE 754 double precision).

E.1.  Subtypes in Arrays

   Since the JSON array construct permits mixing arbitrary JSON data
   types, custom parsing and serialization code may be required to cope
   with subtypes anyway.

Appendix F.  Implementation Guidelines

   The optimal solution is integrating support for JCS directly in JSON
   serializers (parsers need no changes).  That is, canonicalization
   would just be an additional "mode" for a JSON serializer.  However,
   this is currently not the case.  Fortunately, JCS support can be
   introduced through externally supplied canonicalizer software acting
   as a post processor to existing JSON serializers.  This arrangement
   also relieves the JCS implementer from having to deal with how
   underlying data is to be represented in JSON.

   The post processor concept enables signature creation schemes like
   the following:

   1.  Create the data to be signed.

   2.  Serialize the data using existing JSON tools.

   3.  Let the external canonicalizer process the serialized data and
       return canonicalized result data.

   4.  Sign the canonicalized data.

   5.  Add the resulting signature value to the original JSON data
       through a designated signature property.

   6.  Serialize the completed (now signed) JSON object using existing
       JSON tools.

   A compatible signature verification scheme would then be as follows:

   1.  Parse the signed JSON data using existing JSON tools.

   2.  Read and save the signature value from the designated signature
       property.

   3.  Remove the signature property from the parsed JSON object.

   4.  Serialize the remaining JSON data using existing JSON tools.

   5.  Let the external canonicalizer process the serialized data and
       return canonicalized result data.

   6.  Verify that the canonicalized data matches the saved signature
       value using the algorithm and key used for creating the
       signature.

   A canonicalizer like above is effectively only a "filter",
   potentially usable with a multitude of quite different cryptographic
   schemes.

   Using a JSON serializer with integrated JCS support, the
   serialization performed before the canonicalization step could be
   eliminated for both processes.

Appendix G.  Open-Source Implementations

   The following open-source implementations have been verified to be
   compatible with JCS:

   *  JavaScript: <https://www.npmjs.com/package/canonicalize>

   *  Java: <https://github.com/erdtman/java-json-canonicalization>

   *  Go: <https://github.com/cyberphone/json-
      canonicalization/tree/master/go>

   *  .NET/C#: <https://github.com/cyberphone/json-
      canonicalization/tree/master/dotnet>

   *  Python: <https://github.com/cyberphone/json-
      canonicalization/tree/master/python3>

Appendix H.  Other JSON Canonicalization Efforts

   There are (and have been) other efforts creating "Canonical JSON".
   Below is a list of URLs to some of them:

   *  <https://tools.ietf.org/html/draft-staykov-hu-json-canonical-form-
      00>

   *  <https://gibson042.github.io/canonicaljson-spec/>

   *  <http://wiki.laptop.org/go/Canonical_JSON>

   The listed efforts all build on text-level JSON-to-JSON
   transformations.  The primary feature of text-level canonicalization
   is that it can be made neutral to the flavor of JSON used.  However,
   such schemes also imply major changes to the JSON parsing process,
   which is a likely hurdle for adoption.  Albeit at the expense of
   certain JSON and application constraints, JCS was designed to be
   compatible with existing JSON tools.

Appendix I.  Development Portal

   The JCS specification is currently developed at:
   <https://github.com/cyberphone/ietf-json-canon>.

   JCS source code and extensive test data is available at:
   <https://github.com/cyberphone/json-canonicalization>.

Acknowledgements

   Building on ECMAScript number serialization was originally proposed
   by James Manger.  This ultimately led to the adoption of the entire
   ECMAScript serialization scheme for JSON primitives.

   Other people who have contributed with valuable input to this
   specification include Scott Ananian, Tim Bray, Ben Campbell, Adrian
   Farell, Richard Gibson, Bron Gondwana, John-Mark Gurney, Mike Jones,
   John Levine, Mark Miller, Matthew Miller, Mark Nottingham, Mike
   Samuel, Jim Schaad, Robert Tupelo-Schneck, and Michal Wadas.

   For carrying out real-world concept verification, the software and
   support for number serialization provided by Ulf Adams, Tanner
   Gooding, and Remy Oudompheng was very helpful.

Authors' Addresses

   Anders Rundgren
   Independent
   Montpellier
   France

   Email: anders.rundgren.net@gmail.com
   URI:   https://www.linkedin.com/in/andersrundgren/

   Bret Jordan
   Broadcom
   1320 Ridder Park Drive
   San Jose, CA 95131
   United States of America

   Email: bret.jordan@broadcom.com

   Samuel Erdtman
   Spotify AB
   Birger Jarlsgatan 61, 4tr
   SE-113 56 Stockholm
   Sweden

   Email: erdtman@spotify.com