jcs — JSON Canonicalization (RFC 8785)

Cryptographic hashing and signing need a stable byte representation of JSON. The same logical value can be serialized many ways — key order, spacing, number formatting — making naive JSON unsuitable as a signing payload.

JCS (RFC 8785) solves this by defining a canonical form:

• Serializes primitives like ECMAScript JSON.stringify
• Restricts to the I-JSON subset (RFC 7493)
• Sorts object keys by UTF-16 code unit order, recursively
• Preserves array element order

This library is a v1.0 stable implementation in Go. The public API (Transform, NumberToJSON) will not have breaking changes within the v1 major version.

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Install

Requires Go 1.24+.

go get github.com/deszhou/jcs

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Usage

Transform — canonicalize a JSON document

import "github.com/deszhou/jcs"

canonical, err := jcs.Transform(jsonBytes)
if err != nil {
    log.Fatal(err)
}
// canonical is a UTF-8 JCS byte slice, safe to hash or sign directly

Transform accepts any valid JSON root value: object, array, or scalar (true / null / number / string). Leading and trailing whitespace around the root value is allowed.

Typical use — sign a canonical payload:

canonical, err := jcs.Transform(jsonBytes)
if err != nil {
    return err
}
digest := sha256.Sum256(canonical)
signature := sign(privateKey, digest[:])

NumberToJSON — ES6-style float64 formatting

formatted := jcs.NumberToJSON(1e30)   // "1e+30"
formatted  = jcs.NumberToJSON(4.5)    // "4.5"
formatted  = jcs.NumberToJSON(0.002)  // "0.002"

Use this when you need to format a standalone float64 in JCS-compatible form — for example, when constructing a canonical payload manually rather than round-tripping through json.Marshal.

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Errors

Transform returns an error for any input that violates RFC 8785 or I-JSON constraints:

Condition	Example
Invalid JSON	{key: value}
Duplicate object keys	{"a":1,"a":2}
Number out of safe-integer range	integers beyond ±2⁵³
Lone UTF-16 surrogate in a string	"\uD800" (unpaired)

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Example

Input:

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

Canonical output (single line, keys sorted, numbers normalized):

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

Key differences from the input: "literals" sorts before "numbers" and "string"; numbers are normalized to ES6 form; control characters use the shortest valid escape sequence.

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Performance

Benchmarks run on Apple M4 (darwin/arm64, Go 1.24). Each result is the average of 5 runs:

go test -bench=. -benchmem -count=5

Time per operation — lower is better

Scenario	cyberphone	gowebpki	this library	vs cyberphone	vs gowebpki
Structures (nested object)	1878 ns	1708 ns	1428 ns	−24%	−16%
Arrays (mixed array)	595 ns	483 ns	471 ns	−21%	−2%
Unicode (non-ASCII values)	401 ns	374 ns	245 ns	−39%	−34%
Weird (special chars in keys)	2569 ns	2359 ns	1887 ns	−27%	−20%

Allocations per operation — lower is better

Scenario	cyberphone	gowebpki	this library	reduction
Structures	95	95	74	−22%
Arrays	27	27	23	−15%
Unicode	16	16	12	−25%
Weird	95	95	67	−29%

To reproduce, run go test -bench=. -benchmem -count=5 in the root of this repo.

What drives the gains

Optimization	Effect
256-byte escape lookup table	O(1) escape decisions, eliminates per-byte branching
Direct \u00XX byte writes	Removes one fmt.Sprintf call per control character
slices.SortFunc + slices.Compare	Replaces container/list insertion sort for object keys
ASCII-only fast path in sort-key builder	Avoids []rune allocation for pure-ASCII keys
strings.Builder.Grow pre-sized to len(input)+2	Cuts reallocations during string serialization

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Contributing

Bug reports, test cases, and pull requests are welcome.

1. Open an issue to discuss what you'd like to change before a large PR
2. Run go test ./... and go vet ./... before submitting
3. For benchmark changes, include before/after numbers in the PR description

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Attribution

Derived from cyberphone/json-canonicalization (Anders Rundgren), the original multi-language JCS reference implementation. Licensed Apache-2.0 — see LICENSE.

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See also

• RFC 8785 — the specification
• RFC 7493 — I-JSON subset
• JWS-JCS — combining JCS with JWS (RFC 7515)
• Browser JCS demo