JA4's Split Format Saved Our Metrics Cardinality

Originally published at harshit.cloud on 2025-10-14.

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TIL: JA4's Split Format Saved Our Metrics Cardinality

We had a service that rotated TLS ciphers on every connection. Our client classification metrics exploded to 50k unique fingerprints. Prometheus cardinality alert fired.

the problem

JA3 gives you one hash. When ciphers rotate, you get a new hash:

Request 1: 5c4fba4a0f93c6f2a3e52e9c8d4a7b21
Request 2: 3d8a9f2c4e6b1a7c5f9e3d8b2a6c4e1f
Request 3: 7f2e4a8c3b9d1f6e5a2c8d4b7e9f3a1c

Each one looks like a different client in your metrics. Cardinality explosion.

JA4's solution

JA4 splits the fingerprint into three parts:

t13d190900_5d65cb28da5c_02713d6af862
│          │              │
a          b              c

a = protocol + TLS version + counts + ALPN
b = cipher hash (changes on rotation)
c = extension hash

When the service rotates ciphers, only b changes. a and c stay constant.

the fix

Instead of grouping by full fingerprint, group by ac:

def normalize_fingerprint(fp):
    parts = fp.split('_')
    return f"{parts[0]}_{parts[2]}"  # ignore cipher part

# Before: 50k unique fingerprints
# After: 47 unique fingerprints

Cardinality back to normal. Metrics useful again.

when this matters

If you track client types in metrics and see unexplained cardinality spikes, check if they're rotating ciphers. The a_b_c format lets you ignore the changing parts.

Saved us from having to increase our Prometheus retention limits.