What attribute characterizes a public key in PKI?

The characteristic that defines a public key in Public Key Infrastructure (PKI) is that it is disclosed to the public for encryption purposes. This public availability is essential for its function in secure communications. When a public key is shared widely, it allows anyone to encrypt messages meant for the owner of that key. This is a fundamental aspect of asymmetric encryption, which uses a pair of keys: a public key for encryption and a private key for decryption.

The public key facilitates secure communication by allowing individuals to send encrypted messages without needing to share a secret with the sender. It also plays a role in digital signatures, enabling verification of the signature by anyone who has access to the public key. This transparency and openness of the public key are what allows a secure framework for communications and transactions in PKI.