In cryptography, a hybrid cryptosystem is one which combines the convenience of a public-key cryptosystem with the efficiency of a symmetric-key cryptosystem. Public-key cryptosystems are convenient in that they do not require the sender and receiver to share a common secret in order to communicate securely. However, they often rely on complicated mathematical computations and are thus generally much more inefficient than comparable symmetric-key cryptosystems. In many applications, the high cost of encrypting long messages in a public-key cryptosystem can be prohibitive. This is addressed by hybrid systems by using a combination of both.
A hybrid cryptosystem can be constructed using any two separate cryptosystems:
a key encapsulation mechanism, which is a public-key cryptosystem
a data encapsulation scheme, which is a symmetric-key cryptosystem
The hybrid cryptosystem is itself a public-key system, whose public and private keys are the same as in the key encapsulation scheme.
Note that for very long messages the bulk of the work in encryption/decryption is done by the more efficient symmetric-key scheme, while the inefficient public-key scheme is used only to encrypt/decrypt a short key value.
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