How to check the CRC of an EMV payment QR code

Copy the original test payload

Use a payload you are authorized to inspect. In EMV QR Reader, use the raw-content copy option on the inspection screen. Work from a local copy so you can preserve the original order, spaces and capitalization.

The reader shows CRC-related information and parser results. Use the copied bytes for a separate calculation even if you see a CRC badge. The CRC field belongs at the end of an EMVCo merchant-presented payload.

Include the CRC header in the input

Look at the final field. In 6304FD79, 63 identifies the CRC, 04 says the value is four characters long, and FD79 is the expected result. Include 6304 in the input to the calculation.

Find the field at the end of the payload by following its structure. Searching for the first occurrence of 6304 can pick up those characters inside another value. Keep the exact text from the start through the final CRC length.

Reproduce a checksum with a synthetic example

Use 0002015907TEST QR6304FD79 as a checksum-only example. It has a format indicator, a fictional name and a CRC field. The string is a partial fixture for arithmetic only. It lacks the account fields needed for a payment payload.

In Python, import binascii and call binascii.crc_hqx(b"0002015907TEST QR6304", 0xFFFF). Format the result with format(result, "04X"). This exact ASCII input gives FD79. The space in TEST QR is part of the input.

Change only the R in TEST QR to S and recompute. The result becomes 5728. That gives you a deliberate mismatch to compare with the unchanged encoded FD79.

Investigate a mismatch before changing the payload

Check whether you included 6304, accidentally appended a line break, or copied the checksum value into the calculation. Compare the working text with the original copy. Record the input boundary and both results so another developer can reproduce the check.

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