FIDO2 and WebAuthn

FIDO2 is an authentication standard developed by the FIDO Alliance that enables passwordless authentication using public key cryptography.

FIDO2 authentication provides:

• No passwords - Public key cryptography replaces shared secrets

• Phishing resistant - Origin binding prevents credential theft

• Privacy preserving - No tracking between services

• Strong authentication - Hardware-backed security

It consists of two components:

1. WebAuthn: W3C web standard for browser-based authentication

2. CTAP2: Client to Authenticator Protocol for communication with hardware authenticators

The Arculus FIDO2 Server implements the server-side components of these standards.

How FIDO2 Works

Key Concepts

Relying Party (RP)

The relying party is your application or service that wants to authenticate users. The RP is identified by:

• RP ID: The domain name of your service (e.g., example.com)

• RP Name: Human-readable name displayed to users

The RP ID must match the origin of the authentication request for security.

Authenticator

An authenticator is a hardware or software component that:

• Generates and stores cryptographic key pairs

• Performs user verification (PIN, biometric)

• Signs authentication challenges

Types of Authenticators:

Type	Description	Examples
Platform	Built into device	Touch ID, Face ID, Windows Hello
Cross-Platform	External, portable	Arculus Card, YubiKey, security keys

Credential

A credential is a public key pair registered with a relying party:

• Private Key: Stored securely on the authenticator (never leaves device)

• Public Key: Stored on the server for verification

• Credential ID: Unique identifier for the credential

Authenticator Data

The authenticator data is a binary structure returned during registration and authentication.

Structure

Flags

Flag	Bit	Name	Description
UP	0x01	User Present	User was present during operation
UV	0x04	User Verified	User was verified (PIN/biometric)
BE	0x08	Backup Eligible	Credential can be backed up
BS	0x10	Backup State	Credential is currently backed up
AT	0x40	Attested	Contains attested credential data
ED	0x80	Extension Data	Contains extension data

Counter

The signature counter is a 32-bit unsigned integer that:

• Increments with each authentication

• Helps detect cloned authenticators

• Should always increase; a decrease indicates potential cloning

Attested Credential Data

During registration, the authenticator returns attested credential data containing:

Structure

AAGUID

The Authenticator Attestation GUID (AAGUID) is a 128-bit identifier that:

• Identifies the authenticator model

• Used for metadata lookup (MDS)

• All zeros indicates no identification

Arculus Card AAGUID: Registered with FIDO Alliance MDS

Attestation

Attestation provides cryptographic proof about the authenticator during registration.

Attestation Types

Type	Description	Use Case
none	No attestation, privacy-preserving	General consumer use
direct	Full authenticator attestation	Enterprise, high-security
indirect	Anonymized attestation	Balance of privacy and trust

Attestation Formats

Format	Description
packed	Standard WebAuthn attestation format
tpm	TPM-based attestation
android-key	Android Keystore attestation
android-safetynet	Android SafetyNet attestation
fido-u2f	Legacy FIDO U2F format
none	No attestation statement

User Verification

User verification ensures the person using the authenticator is authorized.

Levels

Level	Description	When to Use
discouraged	Skip verification	Low-security, convenience-focused
preferred	Verify if possible	Balanced security (default)
required	Must verify	High-security applications

Verification Methods

• PIN: 4-8 digit code entered on device

• Biometric: Fingerprint, face recognition

• Pattern: Screen unlock pattern

Discoverable Credentials (Resident Keys)

Discoverable credentials are stored on the authenticator and can be used without providing a credential ID.

Benefits

• Enables truly passwordless authentication

• No need to remember usernames

• User selects from available credentials

Considerations

• Limited storage on some authenticators

• May require authenticator management

COSE Algorithms

Credentials use COSE (CBOR Object Signing and Encryption) algorithms.

Supported Algorithms

Name	COSE ID	Description
ES256	-7	ECDSA with P-256 and SHA-256
RS256	-257	RSASSA-PKCS1-v1_5 with SHA-256
EdDSA	-8	Edwards-curve Digital Signature Algorithm

The Arculus FIDO2 Server supports ES256 and RS256 by default.

Extensions

WebAuthn extensions provide additional functionality.

Common Extensions

Extension	Description
credProtect	Credential protection level
appid	Legacy U2F application ID
largeBlob	Store data on authenticator
minPinLength	Minimum PIN length requirement

Security Properties

What FIDO2 Protects Against

Attack	Protection
Phishing	Origin binding prevents cross-site attacks
Man-in-the-Middle	Challenge-response prevents replay
Password Reuse	No shared secrets
Credential Theft	Private keys never leave authenticator
Server Breach	Only public keys stored on server

What Remains Your Responsibility

• Secure session management after authentication

• Proper TLS configuration

• Account recovery mechanisms

• Authenticator loss procedures

WebAuthn API Overview

Registration (navigator.credentials.create)

const credential = await navigator.credentials.create({
  publicKey: {
    challenge: /* from server */,
    rp: { id: "example.com", name: "Example" },
    user: { id: /* bytes */, name: "[email protected]", displayName: "User" },
    pubKeyCredParams: [{ type: "public-key", alg: -7 }],
    authenticatorSelection: {
      authenticatorAttachment: "cross-platform",
      userVerification: "preferred"
    },
    attestation: "none"
  }
});

Authentication (navigator.credentials.get)

const assertion = await navigator.credentials.get({
  publicKey: {
    challenge: /* from server */,
    rpId: "example.com",
    allowCredentials: [{ type: "public-key", id: /* credential id */ }],
    userVerification: "preferred"
  }
});

References

• FIDO2 Alliance Overview

• FIDO Alliance Specifications

• FIDO Metadata Service

• FIDO CTAP2 Protocol Reference

• W3C WebAuthn Specification

• IANA COSE Algorithm Registry