Unformatted text preview: on 24.9). A complicated noncryptographic issue surrounds this type of system. What is the meaning of certification? Or, to put it another way, who is trusted to issue certificates to whom? Anyone may sign anyone else’s certificate, but there needs to be some way to filter out questionable certificates: for example, certificates for employees of one company signed by the CA of another company. Normally, a certification chain transfers trust: A single trusted entity certifies trusted agents, trusted agents certify company CAs, and company CAs certify their employees. Here are some more things to think about: — What level of trust in someone’s identity is implied by his certificate? — What are the relationships between a person and the CA that certified his public key, and how can those relationships be implied by the certificate? — Who can be trusted to be the “single trusted entity” at the top of the certification chain? — How long can a certification chain be? Ideally, Bob would follow some kind of authentication procedure before the CA signs his certificate. Additionally, some kind of timestamp or an indication of the certificate’s validity period is important to guard against compromised keys . Timestamping is not enough. Keys may be invalidated before they have expired, either through compromise or for administrative reasons. Hence, it is important the CA keep a list of invalid certificates, and for users to regularly check that list. This key revocation problem is still a difficult one to solve. And one public-key/private-key pair is not enough. Certainly any good implementation of public-key cryptography needs separate keys for encryption and digital signatures. This separation allows for different security levels, expiration times, backup procedures, and so on. Someone might sign messages with a 2048-bit key stored on a smart card and good for twenty years, while they might use a 768-bit key stored in the computer and good for six months for encryption. And a single pair of encryption and signature keys isn’t enough, either. A private key authenticates...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips