AID

AID

An Application Identifier (AID) is the unique address used to select a specific application on a multi-application smart card. Defined by ISO 7816 Part 5, the AID is a 5-to-16 byte binary string composed of two parts: a mandatory RID (Registered Application Provider Identifier) and an optional PIX (Proprietary Application Identifier Extension).

AID Structure

For example, the Visa credit/debit AID is A000000003 1010, where A000000003 is Visa's registered RID and 1010 is the PIX for Visa credit. Mastercard's RID is A000000004, and its credit application PIX is 1010.

How AID Selection Works

To interact with an application, the reader sends a SELECT APDU (INS=A4) with the AID in the data field. The card's operating system locates the matching application and makes it the active context. All subsequent commands execute within that application's security domain until another SELECT is issued.

GlobalPlatform-managed cards maintain a registry of installed applications. The Issuer Security Domain (ISD) acts as the default selected application and controls applet installation and deletion. Each installed JavaCard applet is registered with its own AID.

Partial AID Selection

ISO 7816-4 supports partial AID matching — the reader can send only the RID (or a prefix) and the card returns the first matching application. This is useful for discovering which applications from a given provider are present, and is how EMV terminals perform Proximity Payment System Environment (PPSE) selection on contactless cards.

AID Registration

Organizations register RIDs through their national ISO member body (e.g., ANSI in the US, BSI in the UK). The registration fee is a one-time charge. Payment networks, government agencies, and telecom operators typically hold their own RIDs. Smaller application developers often operate under a parent organization's RID and use the PIX portion to differentiate their applications.