Is it safe to reuse the hash object created by the crypto module in NodeJS?

Question

To hash a payload with nodejs, I can use the crypto module and do:

const crypto = require('crypto');
const payload = "abc123";
const hashObj = crypto.createHash('sha256');
const res = hashObj.update(payload).digest('hex');

Now say I want to hash a second payload, could I safely reuse the hashObj I just instantiated and call update and digest on it with the new payload? Or do I have to call createHash for every payload?

Edit: To clarify, by reuse I mean extending the above code to the following for example:

const crypto = require('crypto');
const payload1 = "abc123";
const payload2 = "def456";
const hashObj = crypto.createHash('sha256');

const res1 = hashObj.update(payload1).digest('hex');
const res2 = hashObj.update(payload2).digest('hex');

note that `.digest()` finalizes the hasher, so you techncially can't even run this code, it'll error out (thankfully =) — Mike 'Pomax' Kamermans, May 29 '22 at 18:04

score 1 · Accepted Answer · answered May 29 '22 at 17:59

The hasher does not reset, calling update() only adds more data to the hasher. So this:

const res1 = hashObj.update(payload1).digest('hex');
const res2 = hashObj.update(payload2).digest('hex');

would assign the hash for payload1 to res1, and the digest for payload1 + payload2 to res2, except for the part where calling .digest() finalizes the hasher:

The Hash object can not be used again after hash.digest() method has been called. Multiple calls will cause an error to be thrown.

So for any discrete piece of data, you need a separate hasher, and for any data stream you can update() as much as you need to, and then you call .digest() once you have everything packed up.

Is it safe to reuse the hash object created by the crypto module in NodeJS?

1 Answers1