Translation. Original: faktury/numer-ksef.md

KSeF Number – Structure and Validation

The KSeF number is a unique invoice identifier assigned by the system. It always has a length of 35 characters and is globally unique – it uniquely identifies each invoice in KSeF.

General number structure

9999999999-RRRRMMDD-FFFFFFFFFFFF-FF

Where:

• 9999999999 – seller's NIP (10 digits),
• RRRRMMDD – invoice acceptance date (year, month, day) for further processing,
• FFFFFFFFFFFF – technical part consisting of 12 characters in hexadecimal notation, only [0–9 A–F], uppercase letters,
• FF – CRC-8 checksum - 2 characters in hexadecimal notation, only [0–9 A–F], uppercase letters.

Example

5265877635-20250826-0100001AF629-AF

• 5265877635 - seller's NIP,
• 20250826 - invoice acceptance date for further processing,
• 0100001AF629 - technical part,
• AF - CRC-8 checksum.

KSeF number validation

The validation process includes:

1. Checking if the number has exactly 35 characters.
2. Separating the data part (32 characters) and the checksum (2 characters).
3. Calculating the checksum from the data part using the CRC-8 algorithm.
4. Comparing the calculated checksum with the value contained in the number.

CRC-8 Algorithm

The CRC-8 algorithm with the following parameters is used to calculate the checksum:

• Polynomial: 0x07
• Initial value: 0x00
• Result format: 2-character hexadecimal notation (HEX, uppercase letters)

Example: if the calculated checksum is 0x46, "46" will be added to the KSeF number.

Example in C#:

using KSeF.Client.Core;

bool isValid = KsefNumberValidator.IsValid(ksefNumber, out string message);

Example in Java:

OnlineSessionIntegrationTest.java

KSeFNumberValidator.ValidationResult result = KSeFNumberValidator.isValid(ksefNumber);