Objecthash in Elm

Implementation of Ben Laurie's objecthash in Elm.

Usage

There are a few interfaces available. The main module offers two functions objecthash and fromJson. The former expects an Objecthash.Value and the latter a JSON string.

import Objecthash exposing (fromJson, objecthash)
import Objecthash.Value exposing (Value(..))

fromJson """{"k1":"v1","k2":"v2","k3":"v3"}"""
-- Ok "ddd65f1f7568269a30df7cafc26044537dc2f02a1a0d830da61762fc3e687057"

objecthash <| VSet [VInteger 1, VInteger 2]
-- "ee104c03e5465735a9fb3fa5d0f19199297a135486fa76930c69cec825f8dac8"

The Objecthash.Json module exposes a set of functions to decode JSON built on top of the Json.Decode from core. For example, the Objecthash.fromJson mentioned above is implemented as:

import Objecthash.Json exposing (decode)

fromJson : String -> Result String String
fromJson input =
    input
        |> decode
        |> Result.map objecthash

The function Json.decode is a combined decoder that assumes all arrays in the JSON blob are lists and all numbers floats. You can use your own decoder with the Json.decodeWith function.

Note that Json.decode aligns with other objecthash “Common JSON” implementations. This is the reason for having numbers as floats.

Finally, the Objecthash.Hash module exposes a set of functions to operate on Elm values and return the hashed bytes.

import Dict
import Objecthash.Hash exposing (dict, unicode, toHex)

toHex <| dict (Dict.fromList [ ( "foo", unicode "bar" ) ])
-- "7ef5237c3027d6c58100afadf37796b3d351025cf28038280147d42fdc53b960"

Redaction

This implementation offers a similar mechanism for redaction as the one offered by Ben Laurie's Python implementation.

In short, objecthash lets you redact parts of a data structure and keep the final hash intact. To do that, is represents redacted parts as tagged strings of the form **REDACTED***{hexadecimal representation of the hashed value}.

For example, given the JSON blob:

fromJson """{"foo": {"x": 1}, "bar": {"x": 2}}"""
Ok "56b425f5e640238f9481dbf227d3b3aa023905b91e9941e6c987e56bd37ec6a3"

And the fact that {"x": 1} has the hash 480499ec4efe0e177793c217c8227d4096d2352beee2d6816ba8f4e8a421a138:

fromJson """{"x": 1}"""
Ok "480499ec4efe0e177793c217c8227d4096d2352beee2d6816ba8f4e8a421a138"

You can redact the original value for the foo key as follows:

fromJson """{"foo": "**REDACTED**480499ec4efe0e177793c217c8227d4096d2352beee2d6816ba8f4e8a421a138", "bar": {"x
": 2}}"""
Ok "56b425f5e640238f9481dbf227d3b3aa023905b91e9941e6c987e56bd37ec6a3"

And still get the same result.

Contribute

To run a clone of this project locally, you need Yarn (and nodejs):

yarn install

To run the test suite:

yarn test

Note that the elm tooling is installed local to the project so as long as you use commands via yarn you should be fine.

Notice

This library inlines elm-crypto to address the need for composing a sha256 hash from a list of words rather than from a string. This is the corresponding license:

Copyright (c) 2017 Kevin Tonon kevin@betweenconcepts.com

Licensed under the MIT license <LICENSE or http://opensource.org/licenses/MIT>, at your option. This file may not be copied, modified, or distributed except according to those terms.

License

Copyright (c) 2018-2019 elm-objecthash contributors licensed under BSD-3-clause.