Hybrid cryptosystem

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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.

PGP

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.

Tunnelling systems

There are a number of network protocols which create secure encrypted tunnels between two endpoints, IPsec, SSL and SSH. Details vary, but in principle they all work the same way:

  • provide an authentication mechanism, usually some public key system but in some cases just a shared secret
  • use a Diffie-Hellman exchange to generate some shared keying material. D-H is insecure without authentication, so apply your authentication here.
  • use a symmetric cipher — block cipher or stream cipher — and the shared keying material to encrypt data
  • (in most cases) use a hashed message authentication code and some additional shared keying material to authenticate messages

This requires that both players have a good random number generator for the Diffie-Hellman protocol. Some variants also require a secure method of expanding a relatively small shared secret into a larger set of shared keys.

Key sizes

RFC 3766 has guidance on choosing appropriate key sizes for the public key algorithms in hybrid systems, to match the strength of other algorithms.