Provably fair verifier

The method used

1. Message = client_seed + ":" + nonce (for example lab-reader-01:0).
2. HMAC-SHA256 with the server seed as the key, output as hex.
3. Take the first 8 hex characters (32 bits) as an integer n.
4. Roll = floor(n × 10000 / 2³²) / 100, giving 0.00 to 99.99.

This is one common convention, not a universal standard. Sites differ in message format, how many bytes they read and how they map them to an outcome. If a site's own documentation describes a different method, its results won't match this tool — and a site that won't document its method at all is a red flag.

Reproduce it in Python

import hmac, hashlib
server_seed = "9f2c4e1a7b3d8e6f0a5c2b9d4e7f1a3c"
client_seed, nonce = "lab-reader-01", 0
print(hashlib.sha256(server_seed.encode()).hexdigest())
h = hmac.new(server_seed.encode(), f"{client_seed}:{nonce}".encode(), hashlib.sha256).hexdigest()
n = int(h[:8], 16)
print(h, n, (n * 10000 >> 32) / 100)
# a5e39de9361cb47dd6ccdf84bb0cbeff1e402575f217b91bb2e560cfe1ac582d
# c6b1e6abfb75912f63fb7d8e96d037910dd26cd81e57d5e70c7fc855c3fdb59e 3333547691 77.61

The default values are example seeds made up for this page. For the full walk-through, read how to verify a roll step by step.