Unpredictable Number

What it is

The Unpredictable Number (UN) is a 4-byte random value the terminal generates for each transaction and feeds into the cryptogram. It guarantees freshness: because the card signs over the UN, a captured ARQCApplication Request CryptogramThe cryptogram a card produces when it wants the transaction sent online to the issuer for approval.View in glossary → cannot be replayed for a different transaction.

Where it appears

Generated by the terminal; supplied to the card via CDOL1 for GENERATE ACApplication CryptogramA cryptogram the card generates to approve or decline a transaction: TC (approve offline), ARQC (go online), or AAC (decline)..

Specification reference

Defined in: EMV 4.x Book 3 (Application Specification), Annex A — Data Element Dictionary

Example values

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