Public-key cryptography uses a related public and private key pair to support operations such as encryption or signatures without sharing the private key.

The public key can be distributed, while the private key must remain protected. Which operations a pair supports depends on the algorithm and its intended use. A key created for signing should not be treated as a universal key for every cryptographic purpose.

How public-key cryptography works

For a digital signature, a signer uses the private key to produce a value that can be checked with the public key. Verification provides evidence that the signed data has not changed and that the signature was produced using the corresponding private key, assuming the algorithm and keys remain secure.

For public-key encryption, the recipient’s public key supports protecting information that the matching private key can recover. Key-agreement schemes work differently: participants use their key material to establish a shared secret without sending that secret directly.

Practical protocols often combine asymmetric operations with symmetric encryption. The asymmetric part can establish trust and key material, while a symmetric algorithm efficiently protects the larger data exchange.

A passkey example

When a customer registers a passkey, the service receives a public key associated with that credential. During authentication, the authenticator uses the private key to sign a challenge in the protocol’s required context. The service verifies the response with the registered public key.

The service does not need a reusable customer password for that exchange. A biometric check or device PIN may authorize use of the credential locally; that local check is distinct from the cryptographic proof sent to the service.

This arrangement does not mean every private key is permanently restricted to one physical device. Some passkeys can be securely synchronized through a credential provider. The storage and recovery model needs to be understood separately from the basic public/private key relationship.

Trust in the key still matters

A valid signature proves little about a claimed organization if the verifier obtained the public key from an untrusted source. The application needs a trustworthy association between the key and the expected identity.

Public key infrastructure can provide that association through certificates and certificate authorities. Other systems establish trust by directly registering a public key, as in a passkey registration flow. Public-key cryptography is the underlying technique; PKI is one way to manage trust around it.

Losing a private key can make a credential unusable, while theft may let an attacker act as its holder. Key protection and replacement procedures therefore belong in the identity lifecycle from the start.

Frequently asked questions

Does the public key need to be secret?

No. It can be shared. Its authenticity matters, because a substituted key can cause a verifier to trust the wrong party.

Can every key pair both encrypt and sign?

No. Algorithms and key-usage policies differ. Use keys only for the operations and protocols for which they were created.

Is public-key cryptography the same as PKI?

No. Public-key cryptography describes cryptographic techniques using key pairs. PKI provides infrastructure and policy for managing certificates and trust.

Related: Passkeys · WebAuthn · FIDO2 · Machine Identity · Verifiable Credentials · Passwordless Authentication