Asymmetric encryption

Asymmetric encryption uses a mathematically linked key pair: what one key encrypts, only the other can decrypt. The public key is distributed freely; the private key never leaves its owner. To send something confidential you encrypt with the recipient’s public key, and only their private key can open it. The same key-pair mathematics, used in the opposite direction, underpins digital signatures. RSA, elliptic curve cryptography, and Diffie-Hellman key agreement are the exam’s named algorithms.

The caveat is cost: asymmetric operations are orders of magnitude slower than symmetric encryption, so nobody encrypts bulk data with RSA. Real systems are hybrid: asymmetric encryption establishes identity and moves a symmetric session key, then the symmetric cipher does the heavy lifting. Key management also inverts: n users need only n key pairs (2n keys), but trusting that a public key really belongs to its claimed owner requires Public Key Infrastructure.

Exam relevance: no prior shared secret is the trigger; if two strangers must communicate securely over an open network, the answer is asymmetric (or hybrid). Watch the key-direction traps: confidentiality means encrypting with the recipient’s public key, signing means the sender’s private key. The confusable is symmetric encryption: shared key, fast, bulk data, but helpless at first contact, which is exactly the gap asymmetric cryptography fills.