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published: false title: “Docker Distribution JSON Canonicalization” description: “Explains registry JSON objects” keywords: [“registry, service, images, repository, json”]

Docker Distribution JSON Canonicalization

To provide consistent content hashing of JSON objects throughout Docker Distribution APIs, the specification defines a canonical JSON format. Adopting such a canonicalization also aids in caching JSON responses.

Note that protocols should not be designed to depend on identical JSON being generated across different versions or clients. The canonicalization rules are merely useful for caching and consistency.

Rules

Compliant JSON should conform to the following rules:

1. All generated JSON should comply with RFC 7159.
2. Resulting “JSON text” shall always be encoded in UTF-8.
3. Unless a canonical key order is defined for a particular schema, object keys shall always appear in lexically sorted order.
4. All whitespace between tokens should be removed.
5. No “trailing commas” are allowed in object or array definitions.
6. The angle brackets “<” and “>” are escaped to “\u003c” and “\u003e”. Ampersand “&” is escaped to “\u0026”.

Examples

The following is a simple example of a canonicalized JSON string:

{"asdf":1,"qwer":[],"zxcv":[{},true,1000000000,"tyui"]}

Reference

Other Canonicalizations

The OLPC project specifies Canonical JSON. While this is used in TUF, which may be used with other distribution-related protocols, this alternative format has been proposed in case the original source changes. Specifications complying with either this specification or an alternative should explicitly call out the canonicalization format. Except for key ordering, this specification is mostly compatible.

Go

In Go, the encoding/json library will emit canonical JSON by default. Simply using json.Marshal will suffice in most cases:

incoming := map[string]interface{}{
    "asdf": 1,
    "qwer": []interface{}{},
    "zxcv": []interface{}{
        map[string]interface{}{},
        true,
        int(1e9),
        "tyui",
    },
}

canonical, err := json.Marshal(incoming)
if err != nil {
  // ... handle error
}

To apply canonical JSON format spacing to an existing serialized JSON buffer, one can use json.Indent with the following arguments:

incoming := getBytes()
var canonical bytes.Buffer
if err := json.Indent(&canonical, incoming, "", ""); err != nil {
	// ... handle error
}