[messaging] Can a pre-shared public key prevent MITM-attacks?

U.Mutlu for-gmane at mutluit.com
Thu Dec 3 18:03:27 PST 2015

On the following wiki page it's boldly claimed that "A pre-shared public key
also prevents man-in-the-middle attacks"
https://en.wikipedia.org/wiki/Diffie–Hellman_key_exchange#Public_key :
   "It is also possible to use Diffie–Hellman as part of a public key
infrastructure, allowing Bob to encrypt a message so that only Alice will be
able to decrypt it, with no prior communication between them other than Bob
having trusted knowledge of Alice's public key. Alice's public key is
(g^a mod p, g, p). To send her a message, Bob chooses a random b and then
sends Alice g^b mod p (un-encrypted) together with the message encrypted
with symmetric key (g^a)^b mod p. Only Alice can determine the symmetric key
and hence decrypt the message because only she has a (the private key).
A pre-shared public key also prevents man-in-the-middle attacks."

I have my doubts.
What do others think of 'MITM prevention by using public key encryption'?

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