Passkey

A passkey is a FIDO (Fast Identity Online) credential that enables passwordless authentication for websites and applications. It consists of a public key stored by the service (relying party) and a corresponding private key securely stored on a user's device, such as a smartphone, computer, or hardware security key. This cryptographic key pair is used to authenticate a user without transmitting a shared secret, making it inherently resistant to phishing, credential stuffing, and server breaches.

The user experience for a passkey is simple: instead of typing a password, they authenticate using a device-native biometric (like a fingerprint or face scan) or a device PIN. This action cryptographically signs a challenge from the service, proving possession of the private key. Passkeys are often synced across a user's devices via a cloud platform (e.g., iCloud Keychain, Google Password Manager) for convenience, or they can be device-bound (resident) for higher security use cases. This model is standardized by the FIDO Alliance and the World Wide Web Consortium (W3C) as part of the WebAuthn specification.

From a security architecture perspective, passkeys eliminate common attack vectors. Since no secret is shared with or stored on the server, a database breach exposes only public keys, which are useless for authentication. The requirement for a local user gesture (biometric/PIN) prevents unauthorized use of stolen credentials. For developers, implementing passkeys typically involves integrating the WebAuthn API, which handles the complex cryptographic operations, allowing the service to request and verify public-key credentials instead of passwords.

A passkey is a cryptographic credential, based on the WebAuthn (Web Authentication) standard, that replaces passwords with public-key cryptography. When a user creates a passkey for a website or application, their device (e.g., a smartphone, laptop, or security key) generates a unique, mathematically linked public-private key pair. The public key is registered with the online service, while the private key remains securely stored, never leaving the user's device. This fundamental architecture eliminates shared secrets, making passkeys inherently resistant to phishing, data breaches, and credential stuffing attacks.

The authentication flow is user-centric and biometric-driven. To sign in, the user visits the service's login page, which sends a cryptographic challenge. The user's device (the authenticator) prompts for a local biometric verification—such as a fingerprint or face scan—or a device PIN. Upon successful verification, the device uses the stored private key to sign the challenge, creating an assertion. This signed assertion is sent back to the service, which verifies it using the registered public key. This process ensures that only the legitimate user, in possession of the specific device and able to pass the local unlock, can authenticate.

Passkeys are designed for seamless cross-device and platform use through syncing and discoverable credentials. Major platforms like Apple, Google, and Microsoft allow passkeys to sync securely across a user's devices via their cloud accounts (e.g., iCloud Keychain, Google Password Manager). Furthermore, a user can sign in on a new device by scanning a QR code with their phone, which acts as a cross-device authenticator. The private key is never exposed during this process; the phone performs the cryptographic signing and relays the result. This creates a smooth, secure user experience without memorizing passwords.

Passkey integration with ERC-4337 fundamentally redefines user authentication for blockchain accounts by enabling smart contract wallets to use FIDO2/WebAuthn credentials as their signing mechanism. Instead of a private key stored in software, the user's signing authority is derived from a cryptographic key pair secured by a hardware authenticator—such as a biometric sensor (fingerprint, face ID) or a physical security key—effectively making the user's own device the signer for their smart account. This integration is achieved by implementing a custom verification logic module within the account's smart contract, which validates signatures conforming to the WebAuthn standard.

The technical flow involves the UserOperation object, the core transaction packet in ERC-4337. When a user initiates a transaction, their client (a wallet app or browser) creates a WebAuthn assertion—a cryptographic signature from the secure hardware. This assertion is bundled into the UserOperation and sent to a Bundler. The Bundler forwards it to the user's smart contract wallet, which executes its custom verification function. This function checks the assertion's authenticity against the public credential ID stored on-chain, verifying the signature was generated by the legitimate hardware authenticator, thus authorizing the transaction.

This architecture provides significant security and user experience benefits. It eliminates seed phrase vulnerabilities like phishing and accidental exposure, as the cryptographic secret never leaves the secure hardware enclave. It enables familiar, passwordless login flows using biometrics. Furthermore, because the signer is abstracted, users can leverage cross-device synchronization (e.g., iCloud Keychain, Google Password Manager) for recovery without compromising security, a stark contrast to the fragility of mnemonic backups. The signer becomes a replaceable component of the account abstraction stack.

For developers, integration requires implementing an ERC-4337-compliant smart contract with a validateUserOp function that can process WebAuthn assertions. This often involves using libraries that handle the elliptic curve cryptography (specively the secp256r1 curve common in WebAuthn, as opposed to Ethereum's native secp256k1) and the complex signature format. The contract must securely store at least the credential ID and the public key corresponding to the user's passkey. Wallets like Stackup's Passkey Provider and ZeroDev's Kernel with passkey modules serve as practical, open-source references for this implementation.

Looking forward, passkey integration is a cornerstone for mass adoption, lowering the technical barrier to entry by mirroring web2 authentication patterns. It enables transaction signing on any device where the user is biometrically authenticated, not just a single device holding a private key. Combined with other account abstraction features like gas sponsorship, batched transactions, and session keys, passkeys create a seamless, secure, and powerful user-centric wallet experience, moving blockchain interaction beyond the era of seed phrase management.