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FIDO authentication is a standards-based way to sign in by proving possession of a cryptographic credential. Instead of sending a password to the service, an authenticator uses a private key to answer a service-specific challenge; the service checks the response with the matching public key. FIDO stands for Fast Identity Online. A passkey is one kind of FIDO credential, while a hardware security key is one optional type of authenticator.
What FIDO authentication means
FIDO is an umbrella for authentication standards developed by the FIDO Alliance. Its approach uses public-key cryptography: a credential is created for a particular account and service, and the service verifies sign-in using the public key registered for that credential. The user proves possession of the corresponding private key without transmitting it to the service. FIDO Alliance’s specifications overview describes the standards as providing phishing-resistant authentication with cryptographic key pairs called passkeys.
FIDO authentication does not always mean a password-free sign-in. Depending on the standard and how a service implements it, FIDO can provide a second factor, a passwordless sign-in, or a multi-factor experience. For example, FIDO U2F supports second-factor authentication; FIDO2 supports several sign-in patterns.
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Registration
When you register a FIDO credential with a website or app, an authenticator creates or accesses a public-and-private key pair for that service. The service—called the relying party—stores the public key. The private key remains with the authenticator or the passkey provider, rather than being sent to the service.
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- POWERFUL SECURITY KEY: The YubiKey 5C NFC is the most versatile physical passkey, protecting your digital life from phishing attacks. It ensures only you can access your accounts
- WORKS WITH 1000+ ACCOUNTS: Compatible with popular accounts like Google, Microsoft, and Apple. A single YubiKey 5C NFC secures 100+ of your favorite accounts, including email, password managers, and more
- FAST & CONVENIENT LOGIN: Plug in your YubiKey 5C NFC via USB and tap it, or tap it against your phone (NFC), to authenticate. No batteries, no internet connection, and no extra fees required
- MOST SECURE PASSKEY: Supports FIDO2/WebAuthn, FIDO U2F, Yubico OTP, OATH-TOTP/HOTP, Smart card (PIV), and OpenPGP. That means it’s versatile, working almost anywhere you need it
- PRIMARY & SPARE KEYS: Just like having a spare house key, we recommend buying two YubiKeys - one for daily use and one as a spare. That way you’ll never get locked out of your accounts
Authentication
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The relying party sends a fresh challenge to the client.
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You verify locally, for example with a biometric, PIN, or pattern, if the authenticator requires user verification.
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Yubico - Security Key C NFC - Basic Compatibility - Multi-Factor authentication (MFA) Security Key and passkey, Connect via USB-C or NFC, FIDO Certified- POWERFUL SECURITY KEY: The Security Key C NFC is the essential physical passkey for protecting your digital life from phishing attacks. It ensures only you can access your accounts.
- WORKS WITH 1000+ ACCOUNTS: Compatible with Google, Microsoft, and Apple. A single Security Key C NFC secures 100 of your favorite accounts, including email, password managers, and more.
- FAST & CONVENIENT LOGIN: Plug in your Security Key C NFC via USB-C and tap it, or tap it against your phone (NFC) to authenticate. No batteries, no internet connection, and no extra fees required.
- TRUSTED PASSKEY TECHNOLOGY: Uses the latest passkey standards (FIDO2/WebAuthn & FIDO U2F) but does not support One-Time Passwords. For complex needs, check out the YubiKey 5 Series.
- BUILT TO LAST: Made from tough, waterproof, and crush-resistant materials. Manufactured in Sweden and programmed in the USA with the highest security standards.
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The authenticator uses the credential’s private key to sign a response to the challenge.
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The relying party checks the response using the public key registered for that credential. If it verifies, the service accepts the authentication.
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Yubico - YubiKey 5 NFC - Multi-Factor authentication (MFA) Security Key and passkey, Connect via USB-A or NFC, FIDO Certified - Protect Your Online Accounts- POWERFUL SECURITY KEY: The YubiKey 5 NFC is the most versatile physical passkey, protecting your digital life from phishing attacks. It ensures only you can access your accounts
- WORKS WITH 1000+ ACCOUNTS: Compatible with popular accounts like Google, Microsoft, and Apple. A single YubiKey 5 NFC secures 100+ of your favorite accounts, including email, password managers, and more
- FAST & CONVENIENT LOGIN: Plug in your YubiKey 5 NFC via USB and tap it, or tap it against your phone (NFC), to authenticate. No batteries, no internet connection, and no extra fees required
- MOST SECURE PASSKEY: Supports FIDO2/WebAuthn, FIDO U2F, Yubico OTP, OATH-TOTP/HOTP, Smart card (PIV), and OpenPGP. That means it’s versatile, working almost anywhere you need it
- PRIMARY & SPARE KEYS: Just like having a spare house key, we recommend buying two YubiKeys - one for daily use and one as a spare. That way you’ll never get locked out of your accounts
The FIDO Alliance certification overview describes this public-key and challenge-response model. The biometric or PIN is a local way to unlock or authorize use of the credential; it is not itself the public-key credential.
How FIDO2, WebAuthn, and CTAP fit together
FIDO2 is a pair of complementary standards: WebAuthn and CTAP. They handle different parts of the interaction, so they are related but not interchangeable terms.
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- POWERFUL SECURITY KEY: The Security Key NFC is the essential physical passkey for protecting your digital life from phishing attacks. It ensures only you can access your accounts.
- WORKS WITH 1000+ ACCOUNTS: Compatible with Google, Microsoft, and Apple. A single Security Key NFC secures 100 of your favorite accounts, including email, password managers, and more.
- FAST & CONVENIENT LOGIN: Plug in your Security Key NFC via USB-A and tap it, or tap it against your phone (NFC) to authenticate. No batteries, no internet connection, and no extra fees required.
- TRUSTED PASSKEY TECHNOLOGY: Uses the latest passkey standards (FIDO2/WebAuthn & FIDO U2F) but does not support One-Time Passwords. For complex needs, check out the YubiKey 5 Series.
- BUILT TO LAST: Made from tough, waterproof, and crush-resistant materials. Manufactured in Sweden and programmed in the USA with the highest security standards.
| Term | What it does |
|---|---|
| FIDO | The Fast Identity Online standards and ecosystem for authentication. |
| FIDO2 | The combination of WebAuthn and CTAP. |
| WebAuthn | A W3C web API that lets a website create and use public-key credentials. |
| CTAP | The protocol that lets a client platform or browser communicate with an external authenticator. |
In a typical FIDO2 sign-in, the website uses WebAuthn to request authentication, while the client platform or browser uses CTAP to communicate with an external authenticator when needed. CTAP supports external authenticators connected through USB, NFC, or Bluetooth Low Energy (BLE). Built-in authenticators can also be used. See the FIDO User Authentication Specifications and the CTAP 2.2 Proposed Standard.
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Passkeys, authenticators, and security keys
Passkey
A passkey is the user-facing name for a FIDO credential, not another name for the whole FIDO system. Depending on the implementation, a passkey may be synced across devices or bound to a single device. In either case, you typically unlock or authorize its use locally with a method such as a biometric, PIN, or pattern. FIDO Alliance’s U.S. government guidance, revised March 14, 2025, distinguishes synced and device-bound passkeys.
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- FIDO2 & Passkey Ready: Business-ready and FIDO2 L1 certified. This key is supported by major management suites and is ideal for both individual and enterprise deployment. Works seamlessly with Gmail, Facebook, GitHub, Dropbox, Coinbase, and more.
- Universal Connectivity (USB-A ): Features a built-in USB-A connector—simply unfold the key and plug it into your compatible PC or laptop for seamless authentication on the go.
- Dedicated Manager App: Use the Thetis Manager App for the initial hardware PIN setup. Setting the PIN on the device first ensures a smooth registration process. Once the PIN is configured, you can begin registering the key across your favorite FIDO2-compatible online services.
- Ultra-Durable & Portable: Featuring a rotating metal cover, this key is water, crush, and tamper-resistant. It fits easily on a keychain and requires no batteries or network connectivity.
- Check FIDO2 compatibility before purchase - Known limitations: ID Austria is not supported (requires FIDO2 Level 2). Windows Hello login only works with Windows Enterprise editions that support Entra ID, and NFC is NOT supported.
Authenticator
An authenticator is the component that holds or accesses credential material and performs the cryptographic operation. It may be built into a platform, such as a phone or computer, or be an external device. The service that registers the credential and verifies the response is the relying party.
Hardware security key
A hardware security key is an optional external authenticator—not a requirement for FIDO authentication. If you are choosing one, check that the service supports security keys, that your operating system and browser are compatible, and that the key has a transport your device can use, such as USB, NFC, or BLE. Compatibility depends on the service, platform, and key; a FIDO2 label alone does not guarantee that a key works with every account.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What FIDO’s security and privacy design does—and does not—mean
FIDO credentials are service-specific. The FIDO Alliance says each passkey is unique and bound to the online service domain, a design intended to help prevent phishing. Its stated privacy properties also include that the protocols do not provide information for tracking a user across services, and that biometric data, when used, stays on the user’s device. These describe FIDO’s design; they are not a blanket guarantee about every implementation, the security of a device, or the rest of an account’s protection. Details can depend on the authenticator, passkey provider, service, and deployment.
How older FIDO specifications relate
FIDO includes specifications beyond FIDO2. U2F is referred to as CTAP1 in the FIDO2 context and supports second-factor use; CTAP2 adds capabilities used by passwordless flows. UAF supports a separate passwordless experience. Therefore, “FIDO,” “FIDO2,” “WebAuthn,” “passkey,” and “security key” refer to different layers or options, not synonyms.
For developers choosing an implementation, check the FIDO Alliance’s authentication specifications catalog and applicable certification requirements. The Alliance’s server certification page lists active server requirements for WebAuthn Level 3 and CTAP2.3 dated February 26, 2026. A Proposed Standard or Working Draft is not the same as a finalized standard, so verify the status of the specific specification before building against it.
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