Public Key Cryptosystem: A cryptographic system that uses pairs of keys – one which may be disseminated publicly to encrypt data (public key) and another which is known only to the owner (private key) for decryption. In a public key cryptosystem, anyone can encrypt a message using the receiver’s public key, but only the holder of the paired private key can decrypt it, thereby ensuring confidentiality. Additionally, these systems can be used for digital signatures, where a message is signed with a sender’s private key and can be verified by anyone having access to the sender’s public key, thus providing authentication and non-repudiation. Well-known examples of public key cryptosystems include RSA, DSA, and ECC (Elliptic Curve Cryptography). Public key cryptography is foundational to many aspects of digital security and is widely used in securing communications, data, and transactions over the Internet.