Generate md5 hash of JSON and compare in Python and JavaScript

Question

I have a use case where i have to generate md5 hash of a JSON object and compare the hashes in the server and the browser.

The browser client generates hash and then asks the server for the hash of the same resource[ which happens to be a JSON object], and compares both the hashes to decide what to do next.

For server i am using Python and browser client is in Javascript.

For me the hashes generated in both cases do not match. Here's my code:

Python:

>>> import hashlib
>>> import json

>>> a = {"candidate" : 5, "data": 1}
>>> a = json.dumps(a, sort_keys = True).encode("utf-8")
>>> hashlib.md5(a).hexdigest()
>>> 12db79ee4a76db2f4fc48624140adc7e

JS: I am using md5 for hashing in browser

> var hash = require("md5")
> var data = {"candidate":5, "data":1}
> data = JSON.stringify(data)
> md5(data)
> 92e99f0a99ad2a3b5e02f717a2fb83c2

What is it that i am doing wrong?

Answer 1

You're assuming that both languages generate JSON that looks identical.

>>> json.dumps({"candidate" : 5, "data": 1}, sort_keys=True)
'{"candidate": 5, "data": 1}'

js> JSON.stringify({"candidate" : 5, "data": 1})
"{\"candidate\":5,\"data\":1}"

Fortunately, they can.

>>> a = json.dumps({"candidate" : 5, "data": 1}, sort_keys=True, indent=2)
'{\n  "candidate": 5,\n  "data": 1\n}'

js> var a = JSON.stringify({"candidate" : 5, "data": 1}, null, 2)
"{\n  \"candidate\": 5,\n  \"data\": 1\n}"

And now the hashes would be same as well.

Python:

>>> hashlib.md5(a.encode("utf-8")).hexdigest()
>>> d77982d217ec5a9bcbad5be9bee93027

JS:

>>> md5(a)
>>> d77982d217ec5a9bcbad5be9bee93027