Key encapsulation mechanism

Not to be confused with Key wrap.

A key encapsulation mechanism, to confidentially transport a random secret key $k$ from a sender to a receiver, consists of three algorithms: Gen, Encap, and Decap. Circles shaded blue—the receiver's public key $pk$ and the encapsulation $c$—can be safely revealed to an adversary, while boxes shaded red—the receiver's private key $sk$ and the encapsulated secret key $k$—must be kept secret. The secret key $k$ is chosen at random inside the logic of Encap, and the sender has no control over it.

In cryptography, a key encapsulation mechanism (KEM) is a public-key cryptosystem that allows a sender to generate a short secret key and transmit it to a receiver confidentially, in spite of eavesdropping and intercepting adversaries. Modern standards for public-key encryption of arbitrary messages are usually based on KEMs.

A KEM allows a sender who knows a public key to simultaneously generate a short random secret key and an encapsulation or ciphertext of the secret key by the KEM's encapsulation algorithm. The receiver who knows the private key corresponding to the public key can recover the same random secret key from the encapsulation by the KEM's decapsulation algorithm.

The security goal of a KEM is to prevent anyone who does not know the private key from recovering any information about the encapsulated secret keys, even after eavesdropping or submitting other encapsulations to the receiver to study how the receiver reacts.