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

U.Mutlu for-gmane at mutluit.com
Sun Dec 6 15:43:52 PST 2015

Justin King-Lacroix wrote on 12/06/2015 03:23 PM:
> There are basically three common ways to authenticate a DH key exchange:
>     1. If you and your partner *already have* a shared secret: you mix it
>     into into the key exchange somehow. This is the 'shared key' mechanism, and
>     is the basis of (eg) PAKE. The crux of this mechanism is that the key
>     exchange will fail unless both parties knew the shared secret.
>     2. If you and your partner have long-term public keys, and *each already
>     knows the public key of the other*: you use the corresponding private
>     keys to augment the key exchange. This one basically breaks down into two
>     categories, depending on what kinds of long-term public keys you have.
>        1. If you have signing keys, then you digitally sign the DH
>        transaction to authenticate it. You also need to hash the
> long-term public
>        keys into the shared secret, or you introduce an identity misbinding
>        vulnerability.
>        2. If you have DH-type keys, then you basically do the DH crypto
>        twice, using both sets of keys.
>     3. If you and your partner have long-term public keys, and you *don't
>     know* each other's public keys, then you need someone to vouch for you.
>     In most people's heads, that means "PKI", but SSH/OTR-style
>     check-the-fingerprint is potentially viable.
> There's also a whole raft of academic literature on more subtle ways you
> can authenticate your DH transaction using the properties of the
> environment, like the availability of secondary, very low-bandwidth
> channels (think Bluetooth pairing); I can direct you to one or two of those
> papers if you're interested. But those three are the 'classically strong'
> ones, and the ones that are the easiest to understand.
> Justin

Hello, thank you,
I'm interested in a practical and ultimate MITM-prevention method
to be used in computer communication using TCP.

All the papers and examples I read on MITM-prevention do expect
that one already has safely (pre-) exchanged the public keys
of a signing algo like RSA for signing+enc+dec.

And just that exactly is now the question I'm seeking an answer for,
ie. does there exist an algorithm for sending/exchanging the public keys
safely (that is guaranteed authentic) over the public internet,
w/o human interaction by the two parties?

Said differently:
Public keys are by definition of course public, but there is the "little"
problem of authentically transmitting it to the other party, for example
the public key of an ephemeral RSA method to be used as nonce in session
creation with PFS: here Alice needs to be sure that Bob receives her
new pubkey, and not that of someone else in the middle (MITM).
Any algorithm known for solving this elementary problem of securely
exchanging the public keys?


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