Alice wants to communicate with Bob, using public-key cryptography. She establis

Question

Alice wants to communicate with Bob, using public-key cryptography. She establishes

a connection to someone she hopes is Bob. She asks him for his public key and

he sends it to her in plaintext along with an X.509 certificate signed by the root CA.

Alice already has the public key of the root CA. What steps does Alice carry out to

verify that she is talking to Bob? Assume that Bob does not care who he is talking to

(e.g., Bob is some kind of public service).

Please type answer do not hand write. Thank you.

Explanation / Answer

Let us assume that your friend Alice sent you her public key but Bob substituted it with his one. Then Bob sends you message as if he were Alice. In that case, you cannot find out that the message was really written by Bob as you think his public key to be Alice's one. It is necessary to have the possibility to find out that the sender is the one he gives himself out to be.

Certificates contain information about the person or organization to which it belongs, public key, information about certificate issuer, time limits when this certificate can be used and operations for which it can be used. The private key is not a part of the certificate. Unlike certificates, the private key is not public and it is an element you must store at the most secure place.

There are several different certificate formats used. One of the most used is X.509. This type allows the use of the certificates for various purposes due to its wide distribution and numerous extensions. Such certificates can be used for encrypting, signing messages, signature verification, as addition or replacement password system of authentication while resources access, furthermore such certificates are used in the SSL protocol.

Each certificate contains some mandatory and some optional fields. To understand functions of some of these fields you need to acquaint yourself with Certificate Authority concept and certificate issuing technology.

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