TLS fingerprints (JA3 and JA4)

A fingerprint here is like describing someone by the exact order they list their skills. Write the list down, squeeze it into a short code, and you can compare codes instead of long lists.

JA3: join everything in order, then hash

JA3, published in 2017, writes five fields as decimal numbers, joins each list with dashes, joins the five fields with commas, and takes the MD5 hash. GREASE values are left out.

version,ciphers,extensions,groups,point_formats
771,4866-4865-4867-49196,0-10-11-13-16,29-23-25-24,0     (shortened)
md5 -> a 32-character hex code

Because it keeps the order, two browsers offering the same features in a different order get different JA3 codes. That was a strength, until browsers changed.

The 2023 change: shuffled extensions

From Chrome 110 (early 2023), Chrome began putting its TLS extensions in a random order on every connection (checked October 2026). The aim was to stop servers depending on one fixed order, which makes the protocol harder to change. A side effect: Chrome's JA3 code became different on almost every connection, so JA3 could no longer recognise it as one thing.

JA4: sort first, then hash

JA4, released in 2023, fixes that by sorting cipher suites and extensions before hashing, so shuffling makes no difference. It also starts with a short readable part. A JA4 code has this shape (the two hashes here are made up): t13d1516h2_1a2b3c4d5e6f_0f9e8d7c6b5a:

• t for TCP, 13 for TLS 1.3, d because a domain name was sent (i if not), 15 cipher suites, 16 extensions, h2 from the first ALPN value's first and last characters.
• Then the first 12 hex characters of a SHA-256 of the sorted cipher suites.
• Then 12 characters from the sorted extensions (without the server name and ALPN, which depend on the site, not the browser).

Our lab builds a simplified JA4-style code with the same three parts. Real JA4 also includes signature algorithms in the last part; we leave those out to keep it short. A licensing note for builders: JA4 (the TLS client fingerprint) is under the BSD licence, while most of the rest of the JA4+ family uses the FoxIO licence (checked October 2026).

Python you need

import hashlib
hashlib.md5("abc".encode()).hexdigest()          # 32 hex characters
hashlib.sha256("abc".encode()).hexdigest()[:12]  # first 12
"-".join(str(n) for n in [771, 4865])            # '771-4865'
"%04x" % 4865                                     # '1301' (4-digit hex)
"%02d" % 7                                        # '07'

The practice world gives you client_hello(browser, connection, seed) for three fictional browsers: Brook (adds GREASE and shuffles its extensions), Cobalt (adds GREASE, fixed order) and Fennel (neither). Their numbers are real registry codes in combinations no real browser uses, so nothing here imitates a real browser. GREASE is the set of GREASE values.

// JavaScript (Node.js)
const crypto = require("crypto");
const clean = xs => xs.filter(x => !GREASE.has(x)).join("-");
const s = [h.version, clean(h.ciphers), clean(h.extensions),
           clean(h.groups), clean(h.point_formats)].join(",");
const ja3 = crypto.createHash("md5").update(s).digest("hex");

// Go
sum := md5.Sum([]byte(s))
ja3 := hex.EncodeToString(sum[:])
// sorting for JA4: sort.Slice(ciphers, func(i, j int) bool { return ciphers[i] < ciphers[j] })

// Rust (sha2 crate)
use sha2::{Digest, Sha256};
let mut c = ciphers.clone();
c.sort();
let text = c.iter().map(|n| format!("{:04x}", n)).collect::<Vec<_>>().join(",");
let part_b = &format!("{:x}", Sha256::digest(text.as_bytes()))[..12];

The defence

You cannot stop sending a ClientHello. What protects ordinary people is being common: an up-to-date, popular browser shares its fingerprint with millions of others, so it says little about you. An unusual setup, such as an old or heavily modified browser, stands out. Lesson 106 returns to this.