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Public-key cryptography, also called asymmetric cryptography, uses a mathematically related pair of keys: a public key that can be shared and a private key that must be protected. Depending on the algorithm, the pair can support encryption, digital signatures, or key agreement—but not every public-key algorithm does all three.
How a public-key pair works
The two keys have different roles. Someone may share a public key openly, while keeping the corresponding private key secret. NIST’s glossary definition of public-key cryptography describes the approach as using separate keys for operations such as encryption and decryption or signing and verification. The exact operations depend on the cryptographic scheme.
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In practice, public-key methods can help people communicate securely without first sharing a secret key. They are also used in digital signatures and in protocols that establish shared secret material. These are related applications of asymmetric cryptography, not interchangeable names for the same operation.
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Encryption and decryption
For a confidentiality use, a sender can encrypt data or a key for a recipient using the recipient’s public key; the corresponding private key is used to decrypt it. This is a useful basic model, but actual algorithms and protocols have specific constraints. A public key should not be assumed to encrypt every kind of message directly.
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Digital signatures
A signer uses a private key to create a digital signature, and others can use the matching public key to verify it. A valid signature can provide evidence of authenticity and help detect changes to signed data. It does not make the message secret: anyone who can read the message may still be able to do so. NIST explains these protections in its SP 800-63-4 digital identity guidance.
Key agreement
In key agreement, parties use public-key techniques to establish shared secret material. That shared material can then be used in secure communications. Unlike the simplified encryption example, key agreement is not simply one person encrypting a message for another.
Public-key encryption, signatures, and key agreement compared
| Use | Primary purpose | Typical key roles |
|---|---|---|
| Encryption and decryption | Confidentiality | The recipient’s public key is used to encrypt data or a key; the matching private key is used to decrypt. |
| Digital signature | Authenticity and integrity | The signer’s private key creates the signature; the matching public key verifies it. |
| Key agreement | Establishing shared secret material | Parties use public-key techniques to compute or establish shared material. |
These roles are described in NIST’s public-key glossary entry. The table shows common patterns, not a guarantee that any one algorithm supports every use.
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No. A public key by itself does not establish the identity of the person or service that controls the matching private key. When identity matters, a public-key certificate can bind an identifier to a subscriber’s public key. Public-key infrastructure (PKI) refers to the policies, processes, and systems used to administer certificates and key pairs. A verifier must also rely on an appropriate trust process for that binding; simply finding or receiving a public key is not proof of its owner.
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