Serial numbers

The format of a Satyakod serial: what its 16 characters carry and how to read them.

Every code carries a 16-character serial such as 0021A0512PXK20J9, often printed in groups of four (0021-A051-2PXK-20J9). It is the same in every code family, and fits GS1 Application Identifier (21).

Characters

Serials use Crockford base32: the digits and the letters except I, L, O and U.

0123456789ABCDEFGHJKMNPQRSTVWXYZ

When reading a serial typed or spoken by a person, treat O as 0 and I and L as 1, and ignore hyphens and spaces. Case doesn’t matter.

What the 80 bits carry

Decoded, the 16 characters are an 80-bit number, most significant bit first:

Bits	Field	Size
79–56	Batch sequence: the number in the batch id (B-1045 → 1045)	24 bits
55–32	Item index in the batch, from 0	24 bits
31–0	Tag: the first 4 bytes of an HMAC-SHA256 signature	32 bits

The batch sequence and index are readable by anyone: your systems can tell which batch a serial belongs to and where it falls in it without a key. The tag can only be made with your workspace’s key, so it is what proves a serial genuine. Checking it needs either Satyakod (online) or the batch’s verification key (offline).

A guessed serial has a 1 in 4.3 billion chance of carrying a valid tag, and online checks are rate limited, so serials can’t be guessed in practice.

Limits

• Up to 16,777,216 codes per batch.
• Serials are never reused: each batch has its own sequence number.