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canonical_json

Rust library for serializing and deserializing Canonical JSON.

What is Canonical JSON?

Canonical JSON is a variant of JSON in which values each have a single, unambiguous serialized form. It provides meaningful and repeatable hashes of encoded data. Canonical JSON can be parsed by regular JSON parsers provided that no control characters appear in strings.

Compared to JSON:

  • Whitespace between tokens is disallowed. Leading and trailing whitespace is likewise disallowed.
  • Floating point numbers, exponents and "minus zero" are all disallowed.
  • Object keys must appear in lexiographical order and must not be repeated.
  • Strings are uninterpreted bytes, with the only escaped byte values being backslash and quote. Escaping is mandatory for those two characters.
  • String contents are not guaranteed be parsable as UTF-8. Be aware that encoded data may contain embedded control characters and nulls.

Full grammar in spec.txt.

Note: This library deviates from the spec by additionally requiring that strings are, in fact, valid UTF-8. This is a convenience tradeoff so that users get to handle strings directly rather than converting back-and-forth between raw bytes.

Example

This JSON value:

{
  "foo": "bar",
  "abc": 9e3,
  "snowman": "\u2603",
  "zoo":
    [
      "zorilla",
      "anteater"
    ]
}

becomes this in Canonical JSON:

{"abc":9000,"foo":"bar","snowman":"☃","zoo":["zorilla","anteater"]}

Usage

This crate is currently only available on nightly Rust.

Add this to your Cargo.toml:

[dependencies]
canonical_json = "0.8"
serde_derive = "0.8" # Optional, for deriving Serialize and Deserialize
serde = "0.8"        # Optional, for deriving Serialize and Deserialize
serde_json = "0.8"   # Optional, for converting to/from regular JSON

and this to your crate root:

extern crate canonical_json;

Type-based serialization and deserialization

Structs and enums can be serialized and deserialized to/from Canonical JSON without writing boilerplate code. To do this, it must implement the Serialize and Deserialize traits. Serde provides provides an annotation to automatically derive these traits.

To derive Serialize and Deserialize, add this to your crate root:

#![feature(proc_macro)]

#[macro_use]
extern crate serde_derive;

then annotate your data structure like this:

#[derive(Serialize, Deserialize)]
struct Point {
    x: i64,
    y: i64,
}

Note: Struct fields must be defined in lexiographical order when deriving Serialize.

To customize how a data structure is serialized, for example by renaming fields, see the Serde documentation on attributes.

Examples of use

Serializing and deserializing a struct

#![feature(proc_macro)]

#[macro_use]
extern crate serde_derive;
extern crate canonical_json;

#[derive(Debug, Serialize, Deserialize)]
struct Point {
    x: i64,
    y: i64,
}

fn main() {
    let point = Point { x: 1, y: 2 };

    let point_string: String = canonical_json::to_string(&point).unwrap();
    println!("{}", point_string);
    // {"x":1,"y":2}

    let point: Point = canonical_json::from_str(&point_string).unwrap();
    println!("{:?}", point);
    // Point { x: 1, y: 2 }
}

Parsing a str into a generic Canonical JSON Value

extern crate canonical_json;

use canonical_json::Value;

fn main() {
    let value: Value = canonical_json::from_str(r#"{"bar":"baz","foo":13}"#).unwrap();
    println!("value: {:?}", value);
    // value: {"bar":"baz","foo":13}
    println!("object? {}", value.is_object());
    // object? true

    let obj = value.as_object().unwrap();
    let foo = obj.get("foo").unwrap();
    println!("array? {:?}", foo.as_array());
    // array? None
    println!("u64? {:?}", foo.as_u64());
    // u64? Some(13u64)

    for (key, value) in obj.iter() {
        println!("{}: {}", key, match *value {
            Value::U64(v) => format!("{} (u64)", v),
            Value::String(ref v) => format!("{} (string)", v),
            _ => format!("other")
        });
    }
    // bar: baz (string)
    // foo: 13 (u64)
}

Calculating a checksum of a regular JSON document

#![feature(try_from)]

extern crate canonical_json;
extern crate serde_json;
extern crate ring;

use std::convert::TryFrom;

use canonical_json as cjson;
use serde_json as json;
use ring::digest;

fn main() {
    // Whitespace and the order of keys can be changed here
    // while the checksum below will stay the same
    let json_str: &'static str = r#"
        {
            "when you press": {
                "a": "parachute goes up",
                "b": "parachute turns green"
            }
        }
    "#;

    let value: json::Value = json::from_str(json_str).unwrap();
    let canonical_value: cjson::Value = cjson::Value::try_from(value).unwrap();
    let canonical_json_str: String = cjson::to_string(&canonical_value).unwrap();
    let checksum = digest::digest(&digest::SHA256, canonical_json_str.as_bytes());
    println!("{}", hex_from_bytes(checksum.as_ref()));
    // 8b3199db606876d3ac0d9e678090c87e96ba4ba2c241e27e3e44e2bb102ce1
}

fn hex_from_bytes(bytes: &[u8]) -> String {
    use std::fmt::Write;

    let mut hex = String::new();
    for &byte in bytes {
        write!(&mut hex, "{:x}", byte).unwrap();
    }
    hex
}

Acknowledgements

Thanks to Erick Tryzelaar, David Tolnay and other contributors to the excellent serde_json library. This library is a derivative of serde_json and shares much of its code.