FCP Data Structures and Algorithms 2

Question 1

Which data structure does the FIDO2 authenticator use to store the counter value that prevents replay attacks?

Accepted Answer

A monotonically increasing integer stored in non-volatile memory

Answer

FIDO2 authenticators maintain a monotonically increasing signature counter in persistent (non-volatile) memory to detect cloned authenticators.

Question 2

In FIDO's CTAP2 protocol, what encoding format is used to serialize data structures for transmission between the authenticator and the platform?

Accepted Answer

CBOR (Concise Binary Object Representation)

Answer

CTAP2 uses CBOR for compact binary serialization of authenticator request and response data structures.

Question 3

What algorithmic technique does FIDO2's authenticatorGetInfo command use to enumerate supported algorithms in a defined priority order?

Accepted Answer

An ordered array (list) where earlier entries have higher priority

Answer

CTAP2 specifies that the algorithms array in authenticatorGetInfo is ordered so that entries earlier in the list are preferred by the authenticator.

Question 4

Which tree-based data structure is commonly used by platform authenticators to efficiently look up stored credentials by Relying Party ID?

Accepted Answer

B-tree or B+ tree index

Answer

Platform credential stores typically use B-tree or B+ tree indices to provide O(log n) lookup of credentials by RP ID.

Question 5

In the FIDO2 WebAuthn spec, the clientDataJSON object is serialized and then processed by which algorithm before being included in the signed authenticatorData?

Accepted Answer

SHA-256 hashing

Answer

The client hashes the clientDataJSON using SHA-256, and that hash is concatenated with authenticatorData before the authenticator signs it.

(FCP) FIDO Certified Professional Practice Test

(FCP) FIDO Certified Professional Practice Test

FCP Data Structures and Algorithms 2