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JCS

JSON Canonicalization

Cryptographic operations like hashing and signing depend on that the target data does not change during serialization, transport, or parsing. By applying the rules defined by JCS (JSON Canonicalization Scheme), data provided in the JSON [RFC8259] format can be exchanged "as is", while still being subject to secure cryptographic operations. JCS achieves this by building on the serialization formats for JSON primitives as defined by ECMAScript [ES], constraining JSON data to the I-JSON [RFC7493] subset, and through a platform independent property sorting scheme.

Public RFC: https://tools.ietf.org/html/rfc8785

The JSON Canonicalization Scheme concept in a nutshell:

  • Serialization of primitive JSON data types using methods compatible with ECMAScript's JSON.stringify()
  • Lexicographic sorting of JSON Object properties in a recursive process
  • JSON Array data is also subject to canonicalization, but element order remains untouched

Sample Input:

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

Expected Output:

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

Note: for platform interoperable canonicalization, the output must be converted to UTF-8 as well, 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

Implementations

Language URL
Rust https://crates.io/crates/serde_json_canonicalizer
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
Elixir https://github.com/pzingg/jcs
Ruby https://github.com/dryruby/json-canonicalization
PHP https://github.com/aywan/php-json-canonicalization

Combining JCS and JWS (RFC7515)

JWS-JCS

On-line Browser JCS Test

https://cyberphone.github.io/doc/security/browser-json-canonicalization.html

ECMAScript Proposal: JSON.canonify()

JSON.canonify()

Other Canonicalization Efforts

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

http://wiki.laptop.org/go/Canonical_JSON

https://gibson042.github.io/canonicaljson-spec/

https://gist.github.com/mikesamuel/20710f94a53e440691f04bf79bc3d756