Apple iOS Security Guide

As the result of the Link and Provision process, two things occur:
• The device begins to download the Passbook pass file representing the
credit or debit card
• The device begins to bind the card to the Secure Element
The pass file contains URLs to download card art, metadata about the card such as
contact information, the related bank app, and supported features. It also contains the
pass state, which includes information such as whether the personalizing of the Secure
Element has completed, whether the card is currently suspended by the card issuer or
whether additional verification is required before the card will be able to make
payments with Apple Pay.
Adding credit or debit cards from an iTunes Store account to Apple Pay
For a credit or debit card on file with iTunes, the user may be required to re-enter their
Apple ID password. The card number is retrieved from iTunes and the Check Card
process is initiated. If the card is eligible for Apple Pay, the device will download and
display terms and conditions, then send along the terms ID and the CVV to the Link and
Provision process. Additional verification may occur for iTunes account cards on file.
Additional verification
A bank can decide whether a credit or debit card requires additional verification.
Depending on what is offered by the card issuer, the user may be able to choose
between different options for additional verification, such as a text message, email,
customer service call, or a method in an approved third-party app to complete the
verification. For text messages or email, the user selects from contact information the
bank has on file. A code will be sent, which the user will need to enter into Passbook.
For customer service or verification using an app, the bank performs their own
communication process.
Payment authorization
The Secure Element will only allow a payment to be made after it receives authorization
from the Secure Enclave, confirming the user has authenticated with Touch ID or the
device passcode. Touch ID is the default method if available but the passcode can be
used at any time instead of Touch ID. A passcode is automatically offered after three
unsuccessful attempts to match a fingerprint and after five unsuccessful attempts, the
passcode is required. A passcode is also required when Touch ID is not configured or not
enabled for Apple Pay.
Communication between the Secure Enclave and the Secure Element takes place over a
serial interface, with the Secure Element connected to the NFC controller, which in turn
is connected to the application processor. Even though not directly connected, the
Secure Enclave and Secure Element can communicate securely using a shared pairing
key that is provisioned during the manufacturing process. The pairing key is generated
inside the Secure Enclave from its UID key and the Secure Element’s unique identifier.
The pairing key is then securely transferred from the Secure Enclave to a hardware
security module (HSM) in the factory, which has the key material required to then inject
the pairing key into the Secure Element. The encryption and authentication of the
communication is based on AES, with cryptographic nonces used by both sides to
protect against replay attacks.
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