[cryptography]
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Public key authentication
Public key authentication refers to the use of public key cryptography in verifying a person´s identity. The idea is simple: since no one else has access an individual´s secret key, a message that can be decrypted using an individual´s public key must (at least originally) come from that individual. Another fact that can be used is that only the holder of the secret key can decrypt a message encrypted with the corresponding public key.The problem is in key management (getting an individual´s public key).
In public key authentication concept the authenticator MUST know the correct
public key of the individual he wishes to authenticate. If he takes someone
else´s public key, this other person can then masquerade as the said individual.
The keys could of course be stored centrally on a server run by some authority.
This idea, however, is brings another problem of authenticating this server.
Therefore, so called certificates or
digital ID´s are used. Essentially they connect the holders name to his
public key.
![[public key authentication is simple]](Image1.gif)
Figure: An example of public key authentication
In the above figure, a malicious person cannot get the session key because he doesn't have Bob's secret key. Also, he cannot provide Alice with his own public key, because owing to the fact that he doesn't have the certificate authority's private key, he cannot forge Bob's certificate.
The simple and interesting principles of public key authentication can be read here.
Plusses and minuses of public key authentication
The main advantage is that a secure connection can be established even when the parties have no prior relationship. The client doesn't need his own certificate either if only the server is authenticated. An accessible trusted third party is not needed. These properties make concept ideal for web communication where there is a large user base which is not known in advance.Note however, that usually a certificate revocation list is used. The revocation server then must be a highly available third party. The heavy calculation needed in public key encryption causes some delay when a connection is opened.