In cryptography, a hybrid cryptosystem is one which combines the convenience of a public-key cryptosystem with the efficiency of a symmetric-key cryptosystem.[1] 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.[2] 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.[3]
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.[4]
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.[3]
All practical implementations of public key cryptography today employ the use of a hybrid system. Examples include the TLS protocol [5] and the SSH protocol,[6] 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[7] file format and the PKCS#7[8] 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.[9][10]
^Shoukat, Ijaz Ali (2013). "A Generic Hybrid Encryption System (HES)".
^Paar, Christof; Pelzl, Jan; Preneel, Bart (2010). "Chapter 6: Introduction to Public-Key Cryptography". Understanding Cryptography: A Textbook for Students and Practitioners(PDF). Springer. ISBN 978-3-642-04100-6.
^ abDeng, Juan; Brooks, Richard (2012). "Chapter 26 - Cyber-Physical Security of Automotive Information Technology". Handbook on Securing Cyber-Physical Critical Infrastructure. Elsevier. pp. 655–676. ISBN 978-0-12-415815-3.
^Cite error: The named reference cramer-shoup was invoked but never defined (see the help page).
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