FCP Public-Key Cryptography in FIDO 4

Question 1

What happens to the FIDO credential key pair when a user performs a factory reset of their authenticator?

Accepted Answer

All stored private keys are permanently destroyed and credentials become unusable

Answer

The key pairs are backed up to the FIDO Alliance cloud and can be restored

Answer

The private keys are migrated to the new authenticator using a QR code transfer protocol

Answer

The relying party is notified and automatically issues replacement credentials

Question 2

Which field in the FIDO2 PublicKeyCredentialCreationOptions specifies which cryptographic algorithms the relying party will accept?

Accepted Answer

pubKeyCredParams

Answer

excludeCredentials

Answer

authenticatorSelection

Answer

attestation

Question 3

How does the FIDO2 challenge prevent replay attacks during authentication?

Accepted Answer

The server generates a fresh random challenge each session; the authenticator signs it, and the server verifies it was used only once

Answer

The challenge is encrypted with the user's password so it cannot be reused

Answer

Challenges are timestamped with an atomic clock and expire after 30 seconds

Answer

The authenticator increments a counter that the challenge must match exactly

Question 4

What is the significance of the signature counter in FIDO U2F and FIDO2 authenticators?

Accepted Answer

It allows relying parties to detect cloned authenticators by identifying non-sequential counter values

Answer

It tracks the number of failed authentication attempts to trigger lockout

Answer

It measures the cryptographic strength of each signature produced

Answer

It counts how many credentials are stored on the authenticator

Question 5

In the context of FIDO attestation, what is 'self attestation' (also called surrogate attestation)?

Accepted Answer

The authenticator uses its credential key pair itself as the attestation key, providing no independent proof of device model

Answer

The FIDO Alliance server attests to the authenticator on behalf of the manufacturer

Answer

The user self-certifies their identity using a government-issued credential

Answer

The relying party generates the attestation certificate for each new credential

Question 6

Why does FIDO2 specify that the clientDataHash (hash of clientDataJSON) — not the raw clientDataJSON — is sent to the authenticator for signing?

Accepted Answer

To reduce the payload size transmitted over the CTAP2 transport layer

Answer

To prevent the authenticator from learning the origin URL and session details, preserving privacy

Answer

To allow the authenticator to validate the JSON structure independently

Answer

Because authenticators lack the processing power to handle variable-length JSON

Question 7

Which FIDO2 attestation format uses an X.509 certificate chain signed by the authenticator manufacturer to prove device provenance?

Accepted Answer

FIDO U2F attestation format

Answer

Packed attestation with self attestation

Answer

None — FIDO2 never uses X.509 certificates

Answer

Android Key attestation