ways to ensure that GPG public key belongs to right person in business to business communication

[Martin T]

Hi,

thanks for reply! Unfortunately, Alice and Bob cannot meet in person
because of geographical distance. If they could, then this would
definitely be the best way to exchange public keys. I further
simplified my initial idea:

Alice from company A asks Bob from company B to send her Bobs public
key using an e-mail. Both Alice and Bob know each other e-mail
addresses because they have been in contact before during a project
which involves both company A and company B. Now when Alice receives
Bobs public key, she will send hers in return to same e-mail address
which she received the Bobs public key. Then she looks up the phone
number of the customer support department of company B from company B
official website and calls there and asks for Bob. Once she gets Bob
on the phone, she asks Bob to tell the fingerprint of his public key
and then Alice tells her public key fingerprint to Bob and asks Bob to
confirm that it matches.

I guess this provides reasonable security?


thanks,
Martin


On Wed, Oct 26, 2016 at 11:51 PM, Daniel Kahn Gillmor
<dkg at fifthhorseman.net> wrote:
> Hi Martin--
>
> On Wed 2016-10-26 16:21:48 -0400, Martin T wrote:
>
>> let's say that Alice from company A and Bob from company B need to
>> exchange some private data with each other. Alice and Bob need to
>> encrypt data just that one time, they do not belong to web-of-trust,
>> but both company A and company B websites are trusted by certification
>> authority, secure and available only over TLS. This gives a first
>> option where both Alice and Bob ask their IT departments to publish
>> their public keys on the company website so Alice can get Bobs public
>> key over TLS from company B website and the other way around. Or when
>> for example website of company B is not trusted by CA, then Alice can
>> pick up the phone, call the customer-support of the company B and ask
>> for Bob and then ask Bob to send her an e-mail with a public key and
>> verify the fingerprint of the public key over a phone? Are there
>> better(easier to use or more secure) ways to ensure that GPG public
>> key belongs to right person in business to business communication?
>
> It depends on how much involvement you want the IT department to have.
>
> There are a few more options:
>
>  * if Alice and Bob can meet in person, they can give each other
>    business cards with their fingerprints on them.  If this is how Alice
>    finds Bob's e-mail address in the first place, this is a natural
>    place to exchange cryptographic details as well.
>
>  * the two companies could use WKD (web key directory), which is in its
>    infancy, but is at least supported by GnuPG 2.1.x.
>
>  * Alice and Bob could submit their keys to a third-party notary like
>    Symantec's PGP Global Directory (if such a thing still exists)
>
>  * Alice and Bob could publish their public keys in the public
>    keyservers (e.g. gpg --send-key $FINGERPRINT) when they create their
>    keys.  Then they could look each other up in the public keyservers;
>    if Alice finds only one public key associated with Bob's e-mail
>    address, she might just decide to assume it's the right one.
>
> These all have slightly different security properties and failure modes,
> which might have different value to Alice and Bob, depending on their
> threat model and any other economic or logistical pressure they're
> under.
>
>       --dkg