Hybrid cryptosystem: Difference between revisions
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'''Hybrid cryptosystems''' combine [[public key]] (asymmetric) [[cryptography]] with secret key (symmetric) techniques such as [[block cipher]]s, [[stream cipher]]s and [[cryptographic hash]]es. Because asymmetric techniques are typically slower than symmetric techniques by orders of magnitude, it is more efficient to only [[Digital signature|sign]] the hash of a message than the message itself. Similarly, public-key encryption is only used to encrypt a symmetric key, which is then used to encrypt the message. Thus, the [[public key]] techniques provide [[information security#source authentication|source authentication]] and [[key management]] services while the faster symmetric techniques do the high-volume data processing. | '''Hybrid cryptosystems''' combine [[public key]] (asymmetric) [[cryptography]] with [[Symmetric key cryptography|secret key]] (symmetric) techniques such as [[block cipher]]s, [[stream cipher]]s and [[cryptographic hash]]es. Because asymmetric techniques are typically slower than symmetric techniques by orders of magnitude, it is more efficient to only [[Digital signature|sign]] the hash of a message than the message itself. Similarly, public-key encryption is only used to encrypt a symmetric key, which is then used to encrypt the message. Thus, the [[public key]] techniques provide [[information security#source authentication|source authentication]] and [[key management]] services while the faster symmetric techniques do the high-volume data processing. | ||
For the Internet, there are a number of security systems that are hybrid cryptosystems — [[PGP]] for email, [[TLS]] for the web, [[SSH]] for remote login, [[IPsec]] as a general protection mechanism, and [[DNS security]]. All require a source of cryptographic quality [[random number]]s. | For the Internet, there are a number of security systems that are hybrid cryptosystems — [[PGP]] for email, [[TLS]] for the web, [[SSH]] for remote login, [[IPsec]] as a general protection mechanism, and [[DNS security]]. All require a source of cryptographic quality [[random number]]s. | ||
Revision as of 21:48, 1 March 2010
Hybrid cryptosystems combine public key (asymmetric) cryptography with secret key (symmetric) techniques such as block ciphers, stream ciphers and cryptographic hashes. Because asymmetric techniques are typically slower than symmetric techniques by orders of magnitude, it is more efficient to only sign the hash of a message than the message itself. Similarly, public-key encryption is only used to encrypt a symmetric key, which is then used to encrypt the message. Thus, the public key techniques provide source authentication and key management services while the faster symmetric techniques do the high-volume data processing.
For the Internet, there are a number of security systems that are hybrid cryptosystems — PGP for email, TLS for the web, SSH for remote login, IPsec as a general protection mechanism, and DNS security. All require a source of cryptographic quality random numbers.
Take PGP as an example. If Alice wants to securely send a message to Bob, she cannot just use a symmetric method without first securely delivering the key to him. Securely delivering a key is not a trivial problem; anything sent over the net might be monitored, phones might be tapped and so on. Bob might be some distance away and sending an officer with a briefcase handcuffed to his wrist is often impractical.
If she can get Bob's public key and verify that it is valid, she can securely send a message using a public key system. However, such systems are generally much slower than symmetric methods; using them for large messages is expensive. In PGP, therefore, the public key technique is used only to provide authentication and to securely transport the symmetric keys for a block cipher.
The steps for Alice are:
- generate a random key (call it K) for the symmetric cipher, using a good random number generator
- encrypt the actual message with a block cipher, using K as the key
- encrypt K with the public key system
- package the encrypted message, the encrypted K, and some authentication data into a PGP message
- send it off to Bob
Bob checks the authentication data, does a public key decryption to get K, then uses K to decrypt the actual message.
RFC 3766 has guidance on choosing appropriate key sizes for the public key algorithms in hybrid systems, to match the strength of other algorithms.