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

Daniel Kahn Gillmor dkg at fifthhorseman.net
Wed Oct 26 22:51:40 CEST 2016

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

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