What it is
The Certification Authority Public Key Index identifies which scheme CA public key the terminal must use to verify the Issuer Public Key Certificate (tag 90) during offline data authentication. The index plus the RID selects a specific CA key from the terminal key store.
Where it appears
Read from the card record files; used at the start of offline data authentication.
Specification reference
Defined in: EMV 4.x Book 3 (Application Specification), Annex A — Data Element Dictionary
Example values
09CA public key index 09 for the application RID