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Concept
Hybrid cryptosystem
Summary
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.
All practical implementations of public key cryptography today employ the use of a hybrid system. Examples include the TLS protocol and the SSH protocol, that use a public-key mechanism for key exchange (such as Diffie-Hellman) and a symmetric-key mechanism for data encapsulation (such as AES). The OpenPGP file format and the PKCS#7 file format are other examples.
Hybrid Public Key Encryption (HPKE, published as RFC 9180) is a modern standard for generic hybrid encryption. HPKE is used within multiple IETF protocols, including MLS and TLS Encrypted Hello.
Envelope encryption is an example of a usage of hybrid cryptosystems in cloud computing. In a cloud context, hybrid cryptosystems also enable centralized key management.
To encrypt a message addressed to Alice in a hybrid cryptosystem, Bob does the following:
Obtains Alice's public key.
Official source
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