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

Justin King-Lacroix justin.king-lacroix at cs.ox.ac.uk
Mon Dec 7 05:52:53 PST 2015

No. Even in principle, this is essentially impossible -- two parties with
no relationship basically can't communicate securely.
There are lots of approaches to the problem, but they all involve breaking
the 'no relationship' constraint. PKI -- and thus iMessage, WhatsApp --
does it by introducing a well-known trusted third-party. PGP / Web of Trust
does it by relying on social graphs. OTR and SSH leave it up to you: they
show you the key fingerprint, and it's up to you to work out whether it's
the right one.
But in general, the problem you're describing has no solution.

(Once you've exchanged keys, of course, there are a multitude of way to
create a secure channel on that basis. But you need to exchange keys
somehow first.)


On 6 December 2015 at 23:43, U.Mutlu <for-gmane at mutluit.com> wrote:

> 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?
> --
> U.Mutlu
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