A public key can be shared; its matching private key must stay secret. They are related parts of an asymmetric cryptographic key pair, but they have different roles: depending on the algorithm, the private key may sign or decrypt, while the public key may verify a signature or encrypt. Some algorithms use the pair for key agreement instead.
What is the difference between a public key and a private key?
The public key is associated with an owner and may be distributed. The private key is uniquely associated with that owner and is not made public. NIST explains that deriving the private key from its public counterpart is computationally infeasible.
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These terms describe disclosure and cryptographic roles, not interchangeable credentials. A public key alone does not prove who owns it: in an application, it must be reliably associated with the person or system it is claimed to belong to.
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What does each key do?
| Use | Public key | Private key |
|---|---|---|
| Disclosure | May be made public. | Kept secret; not made public. |
| Digital signatures | Verifies a signature made with the matching private key. | Generates the signature. |
| Public-key encryption | Depending on the scheme, encrypts data or keys. | Depending on the scheme, decrypts data or keys encrypted with the matching public key. |
| Key agreement | Can contribute to a shared-secret calculation. | Can contribute to a shared-secret calculation. |
These are possible roles across public-key cryptography, not a promise that every algorithm supports all of them. For example, NIST says ECDSA keys are used for ECDSA signature generation and verification, not for encryption.
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How do public and private keys work with signatures?
The signer uses the private key to create a digital signature. Someone checking the signature uses the corresponding public key. If verification succeeds, it establishes that the signature is valid for that key and message; the public key itself does not establish that the key belongs to a claimed person or organization.
A public key may remain useful for verifying signatures even after the associated private key’s cryptoperiod has ended. NIST’s guidance on ECDSA distinguishes the signing key’s period of use from continued verification of signatures already made.
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Does the public key encrypt or decrypt?
In public-key encryption schemes, the public key encrypts data or keys, and the matching private key decrypts them. This is an algorithm-dependent use. Public-key cryptography can also be used to establish or agree on a shared secret rather than directly encrypting bulk data.
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- Share the public key only in a way that lets others confirm it is associated with the intended owner.
- Protect the private key from disclosure; anyone who obtains it may be able to perform operations the key permits, such as signing or decryption.
- Check what the specific algorithm and key are intended to do. Signing, encryption, and key agreement are not universal functions of every key pair.
NIST’s definitions are in its public-key glossary and private-key glossary. For the ECDSA-specific use and verification-lifetime detail, see FIPS 186-5.
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