Public and Private Keys: Digital Signing Roles

What are public and private signing keys?

Public and private keys are mathematically related components of an asymmetric cryptographic system. In a digital-signature scheme, the private key signs and the corresponding public key verifies. NIST’s public-key definitions describe this signing relationship.

Why are the roles different?

The private key must be controlled so that only appropriately authorized signing operations occur. The public key can be distributed to support verification, but the recipient still needs a trustworthy connection between that key and the claimed signer.

A signing certificate can provide a public-key binding under an issuer’s policy. Sharing a public key alone does not establish the person’s identity, the agreed document or their authority to represent a company.

Is signing encryption with a private key?

That is not a reliable general description. Digital-signature algorithms perform signing and verification operations; they should not be explained as universally encrypting with the private key and decrypting with the public key. Signing also does not, by itself, hide the document’s contents.

NIST’s private-key definitions distinguish key uses across cryptographic schemes. The algorithm and intended operation matter.

Key custody and verification

Signing keys may be protected by software, a device such as an HSM, or a remote cryptographic service. Where the key is held and who can authorize its use are separate questions. A reviewer normally needs the appropriate public credential and signature evidence, not the signer’s private key.