Cryptographic Token Interface Standard

PKCS#11

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Vendor-defined callbacks

Library vendors can also define additional types of callbacks. Because of this extension capability, application-supplied notification callback routines should examine each callback they receive, and if they are unfamiliar with the type of that callback, they should immediately give control back to the library by returning with the value CKR_OK.

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RSA Security Inc. Public-Key Cryptography Standards - PKCS#11 - v220