Cryptography Related

Dear All,

I have certain questions related to certain Cryptographic algorithms. Would appreciate if anybody is able to help me out with this.

A> Why can digital signatures be never modified?

B> The second question is regarding public / private keys. Assuming I am an end user, how do I get authenticated everytime with a public key of some other customer or user. I am seen many a times that in certain examples, I am asked for a certificate to be signed only once when I visit a page. The next time I revist I am not asked for any sign ups. Whereas in certain examples I have to accept the certificate everytime I visit the page. How does the authentication happen in such cases if I do not have an Internet connect and am just an intranet user.

C> Last is what are various ways of storing the keys on different platforms ?

I would be very happy if anybody can help me out with these queries.

Many Thanks,
Mihir