JSON Canonicalization Scheme (JCS)
RFC 8785
| Document | Type |
RFC - Informational
(June 2020; Errata)
Was draft-rundgren-json-canonicalization-scheme (individual)
|
|
|---|---|---|---|
| Authors | Anders Rundgren , Bret Jordan , Samuel Erdtman | ||
| Last updated | 2020-09-23 | ||
| Stream | Independent Submission | ||
| Formats | plain text html xml pdf htmlized with errata bibtex | ||
| IETF conflict review | conflict-review-rundgren-json-canonicalization-scheme | ||
| Stream | ISE state | Published RFC | |
| Consensus Boilerplate | Unknown | ||
| Document shepherd | Adrian Farrel | ||
| Shepherd write-up | Show (last changed 2019-11-27) | ||
| IESG | IESG state | RFC 8785 (Informational) | |
| Telechat date | |||
| Responsible AD | (None) | ||
| Send notices to | Adrian Farrel <rfc-ise@rfc-editor.org> | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | No IANA Actions | ||
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