Answer Posted / ramkumar
Suppose Alice wants to find Bob's public key. There are
several possible ways of doing this. She could call him up
and ask him to send his public key via e-mail. She could
request it via e-mail, exchange it in person, as well as
many other ways. Since the public key is public knowledge,
there is no need to encrypt it while transferring it,
though one should verify the authenticity of a public key.
A mischievous third party could intercept the transmission,
replace Bob's key with his or her own and thereby be able
intercept and decrypt messages that are sent from Alice to
Bob and encrypted using the ``fake'' public key. For this
reason one should personally verify the key (for example,
this can be done by computing a hash of the key and
verifying it with Bob over the phone) or rely on certifying
authorities (see Question 4.1.3.12 for more information on
certifying authorities). Certifying authorities may provide
directory services; if Bob works for company Z, Alice could
look in the directory kept by Z's certifying authority.
Today, full-fledged directories are emerging, serving as on-
line white or yellow pages. Along with ITU-T X.509
standards (see Question 5.3.2), most directories contain
certificates as well as public keys; the presence of
certificates lower the directories' security needs.
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