Chapter 7. Digital Identification II: Digital Certificates, CAs, and PKI

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In the previous chapter, we explored three techniques for establishing and authenticating a person's identity: the use of paper documents, biometrics, and digital signatures. We saw in that chapter that digital signatures had a significant security advantage over the first two systems for e-commerce: because the private key used to "sign" a digital signature is not used by the recipient to verify the signature, digital signatures are not easily subverted by replay attacks. Identity-proving signatures cannot be reused (if the nonces are created with care), but must be created new each time that a person's identity needs to be proven. But as we also saw, digital signatures had a problem as well; for you to prove your identity to someone using a digital signature, that person needs to have your public key already on file. That is, being able to create a digital signature doesn't actually authenticate your identity, it simply proves that you have possession of a private key.

The use of digital certificates and a public key infrastructure (PKI) are attempts to tie absolute identity to digital signatures. A digital certificate is a special kind of digital signature it is a digital signature that comes with an identity, which is designed to be interpreted by computers in an automated way. A public key infrastructure is a collection of technologies and policies for creating and using digital certificates. The effectiveness of these systems comes from a marriage of public key cryptography, carefully created and maintained policies, and the legal system.

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