License key format

SMS-A1B2-C3D4-E5F6-G7H8

Prefix

2-4 uppercase letters. The default generated prefix is SMS.

Body

Four blocks of four characters.

Alphabet

A-Z and 2-9. The generator avoids visually confusing 0/O and 1/I.

Checksum

The final block is derived from SHA-256 of the prefix plus the first three blocks.

Regex

^([A-Z]{2,4})-([A-Z2-9]{4})-([A-Z2-9]{4})-([A-Z2-9]{4})-([A-Z2-9]{4})$

• Split the key into prefix and four 4-character blocks.
• Reject if the regex does not match.
• Compute SHA-256 over prefix-block1-block2-block3.
• Map the first four hash bytes into the key alphabet.
• Accept only when that generated checksum equals block 4.