9.6. Summary
Cryptographic infrastructure provides the mechanisms needed to use cryptography. The infrastructure sees to the distribution of keys and the security of the procedures and mechanisms implementing cryptographic algorithms and protocols.
Key exchange and authentication protocols, although distinct in principle, are often combined because the first step in most communications is to prove identity. Exchanging a session key in the process saves another exchange. Both public key and classical cryptosystems can provide authentication and key exchange, provided that the appropriate infrastructure is present.
A key element of such an infrastructure is a mechanism for binding cryptographic keys to identity. This mechanism leads to the distinction between session keys (generated once per session, and associated with that session) and interchange keys (generated once per principal, and associated with that principal). It also leads to certification, in which a representation of identity, along with other information such as expiration time, is cryptographically signed and distributed as a unit. The name of the signer (issuer) is included so that the certificate can be verified.
The mechanism used to sign certificates and other documents is a digital signature. A disinterested third party, called a judge, must be able to confirm or disprove that the (alleged) sender computed the digital signature of the (alleged) signed message.
Session keys require pseudorandom number generation. Of the many algorithms in use, the best are mixing algorithms in which every bit of the output depends on every bit of the input, and no bit can be predicted even if all previous bits are known.
The management of keys involves storing them and revoking them, both of which involve system issues as well as cryptographic ones. Another aspect is the idea of key recovery.
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