public key certificate [closed]

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what role does public key certificate play in public key cryptosystem?

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A public key refers to a cryptographic system where there are two parts: the public key (which is given out freely) and the private key. Anyone can use the public key to encrypt a piece of information. However, to decrypt this information you need to use the private key. This is a powerful technique to ensure that information can only be read by the intended recipient (who would be the private key holder).

The "certificate" is a means of relating the public key with a specific identify. So for example, a company "Widgets Incorporated" provides the certificate with a public key, and an external certificate authority confirms that this certificate really does belong to "Widgets Incorporated" (and not, for example, "Credit Card Scams-R-Us"). The external authority signs the certificate with a digital signature to confirm that it is genuinely from the identified company, and that it has not been tampered with.

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You might be interested in reading http://www.schneier.com/book-applied.html or a similar book.

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There's no such thing as "public key certificate". X.509 certificates, used in Public Key Infrastructure, contain a public key of a keypair and certain other information. The certificate is signed (using PKI means and public key algorithm).

Overall the question is too broad and good reading is recommended. For example, we have an explanation article on our site.