CODEWITHSUNDEEP
node.jshashcryptographysha256
Kevin Burke
Kevin Burke

Reputation: 64844

Generate a consistent sha256 hash from an object in Node

I have an object that I'd like to hash with sha256 in Node. The contents of the object are simple Javascript types. For example's sake, let's say:

var payload = {
    "id": "3bab3f00-7d55-11e7-9b0a-4c32759242a5",
    "foo": "a message",
    "version": 7,
};

I create a hash like this:

const crypto = require('crypto');
var hash = crypto.createHash('sha256');
hash.update( ... ).digest('hex');

The question is, what to pass to update? The documentation for crypto says you can pass a <string> | <Buffer> | <TypedArray> | <DataView>, which seems to suggest an object is not a good thing to pass.

I can't use toString() because that prints "[object Object]". I could use JSON.stringify, however I have read elsewhere that the output from stringify is not guaranteed to be deterministic for the same input.

Are there any other options? I do not want to download a package from NPM.

Upvotes: 4

Views: 3757

Answers (1)

Maarten Bodewes
Maarten Bodewes

Reputation: 93968

The right terms are "canonical" and the action is called "canonicalization" (I'm assuming EN-US here), you can find a stringify that produces canonical output here.

Beware that you must make sure that the output also has the right character set (UTF-8 should be preferred) and line endings. Spurious data should not be present, e.g. a byte order mark or NUL termination string is enough to void the hash value.

After that you can pass it as string I suppose.

You can of course use any canonical encoding. Note that XML has defined XML-digsig, which contains canonicalization during signature generation and signing, which means that the verification will even succeed if the XML code is altered (without altering the structure or contents of course, but whitespace / indentation will not matter).

I'd still recommend regression testing between implementations and even version updates of the libraries.

Upvotes: 4

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