Check content hash between server and client

What's a hash?

Given two pieces of data, a cryptographic hash function will return two different (fixed-length) values.

The hash function should never return the same result for two different inputs, a.k.a «collision».

In this article, we'll use SHA-256 (Secure Hash Algorithm, with a 256 bits output). Given any content, the function will return 256 bits (or 32 bytes). Each byte can be represented in hexadecimal (2 characters, from 00 to FF), and thus becomes a 64 characters string (a.k.a. «hex digest»).

I really enjoyed watching How secure is 256 bit security? by 3Blue1Brown. Note that in 2017, Google presented a practical technique to break SHA1 (used in Git for example).

On the server

In Python (as with most languages) it is straightforward:

import hashlib

content = "Get up, stand up, don't give up the fight"  # or ``file.read()``

hasher = hashlib.sha256()
hasher.update(content)
hash = hasher.hexdigest()

# "04cb9657d1a1a34ccd4f30252a061c36e45b2a5afff86e4c91fa778fa70400eb"

Ideally you would deliver this string to the client somewhere in your application data, or in the HTTP response headers etc.

On the client

In JavaScript, we can leverage the Crypto API to compute the SHA-256 of some content.

First, obtain the bytes array of the content.

For a String:

const text = "Get up, stand up, don't give up the fight";
const encoder = new TextEncoder();
const bytes = encoder.encode(text);

Or for a URL:

const resp = await fetch(url);
const buffer = await resp.arrayBuffer();
const bytes = new Uint8Array(buffer);

And then compute the hash:

const hashBuffer = await crypto.subtle.digest("SHA-256", bytes);
const hashBytes = new Uint8Array(hashBuffer);

// hex digest of bytes
const hash = Array.from(hashBytes)
  .map(b => b.toString(16).padStart(2, "0"))
  .join("");

if (hash != serverHash) {
  throw new Error("Bad content");
}

Going further

This hash verification is pretty solid to make sure that your data was downloaded and fetched successfully. However, it does not guarantee authenticity, since anybody can compute the SHA-256 function result without having any specific private key.

In order to prevent man-in-the-middle attacks, where someone could alter the content and deliver the modified hash values to the client, you should use signatures. In this model, the server computes a hash using a private key, and the client verifies the hash using a public key.

Usually, we use Elliptic Curve DSA for that.