Passwords are dead – long live passkeys

What are passkeys?

Passkeys are essentially a replacement for traditional passwords as they employ modern cryptographic public/private keys instead of a shared ‘word as a secret’. Instead of exchanging a ‘shared secret’ over the network, a challenge is signed with a private key and sent to the service requesting authentication (also known as the relying party). The relying party verifies the signature using the public key obtained during registration. The private key must be kept secret, it never leaves the device (device-bound passkey) or the vendor-specific ecosystem (synced passkey). This authentication paradigm significantly changes the security posture of the credentials used, as they never leave the defined perimeter.

The Passkeys user experience is designed to be consistent across user devices. Figure 1 shows how a passkey is created by simply providing the username or email. Figure 2 shows the logon interaction pattern – a simple fingerprint or face verification, or a device PIN that users are familiar with and encounter frequently. Before an authenticator signs a given challenge, the relaying party is confirmed to match the domain for which the key was created, giving passkeys so called 'verifier impersonation resistance' and making it phishing-resistant. In addition, passkeys give organisations the peace of mind that their users' credentials will remain secure even if their database is compromised because all a relying party needs to store is a user's public certificate. Passkeys cannot be forgotten or mistyped leading to a 95% reduction in password reset tickets, eliminating expensive and potentially insecure password recovery tasks.

Because passkeys are discoverable by browsers, new and unique ways of interacting with users emerge. Soon, users may only be asked to select which authenticator (platform, password manager, USB device, etc.) they wish to authenticate to, using their stored passkey. But for this to happen, organisations need to adapt their authentication process.

What are synced passkeys?

In May 2022, Apple, Google, and Microsoft announced plans to support synced passkeys. As mentioned above, unlike device-bound passkeys, synced passkeys can be synchronised between devices within the same vendor ecosystem, such as iCloud for Apple devices or within a cloud-based password manager. While these credentials have a different need for security protection than device-bound passkeys, they share the same security and usability features and they offer significant benefits to both end users and organisations, balancing convenience with security.

The figure summarises the differences between synced and device-bound passkeys:

But wait, there is more …

The development does not stop at synced passkeys. CTAP2 supported connections to external authenticators via USB, NFC, or Bluetooth Low Energy (BLE). In March 2023, an extension to the protocol was proposed to enable Cross-Device Authentication Flows. This proposal is already supported by Apple and Android, allowing users to securely log in to their accounts on a shared computer (e.g. library) without risking exposure of private key material. 

Figure 4 shows the cross-device flow: (1) the client requires authentication (2) presents a QR code, which can then (3) be scanned by an authenticator. Next, the authenticator (4) interacts with the server, requiring authentication over the Internet and (5) signs the presented challenge to (6) prove possession of a valid passkey. The device presenting the QR code continuously checks whether an authenticator has signed the challenge (6). If so, a valid session is received.

The cross-device authentication flow allows authenticators with cameras to be used to authenticate users. BLE is used to confirm physical proximity, ensuring that the authentication process remains secure and seamless.