Phone Numbers and Emails
When you want to communicate with people you know on the Internet, Keybase is great because you can address them by their social identities (Twitter, Reddit, Mastodon, etc.).
But identity proofs aren’t that useful for many people you know in real life. They might not be on the same social media you’re on, or you might have just met this person at a coffee shop.
In Keybase 4.3.0, users can now add and verify their phone numbers and emails, as well as send messages to phone numbers and emails. Like with social identities, you can send messages even if the recipient isn’t on Keybase yet.
First, ensure you’ve updated to the latest Keybase version.
To add a phone number or email address, go to the Settings tab (or on mobile, hamburger menu→Your account).
To start a conversation with a phone number or email, go to the Chat tab, click the new conversation button at the top left (Ctrl-N or ⌘N on Mac).
Type in a phone number or email address. If a user on Keybase has verified it and made it searchable, they’ll appear; otherwise, you’ll still be able to start a conversation, and the recipient will get the messages when they join Keybase, verify their contact, and make it searchable.
On mobile, you can import your address book to quickly find which of your contacts are already on Keybase. You can toggle this setting in hamburger menu→Phone contacts.
Unlike identity proofs, which are public and checkable by all Keybase devices, phone number and email verifications are only verified by the Keybase server. That means that when you start a conversation with a phone number or email, you trust the Keybase server to give you the right user the first time—this is called trust on first use (TOFU).
After the initial conversation is started, the Keybase server can’t play any more tricks, since both users’ devices now verify each other. Then when users chat, their devices encrypt messages only for the devices they've already verified.
On apps like Signal, Wire, and Riot, you can optionally verify a key fingerprint or safety number to be sure you’re talking to the right person.
Because of the global Merkle tree, to achieve the same level of security on Keybase when starting conversations with phone numbers and emails, you only need to exchange Keybase usernames outside of Keybase (in person, over a phone call, etc.).
Some users may not want the Keybase servers to know their phone numbers or email addresses. Users will never be required to add a phone number on Keybase, and although email addresses are currently required to signup for Keybase, in a future Keybase version, we will remove this requirement as well.
Some users may want to have phones and emails registered for notifications from Keybase, but not want other users to find them by those phone numbers or email addresses. This can be accomplished by deselecting “Allow friends to find you by this number/email address” option when adding them. You can toggle this option in the Settings tab by making it “unsearchable.”
All users who signed up before the rollout of this feature have their emails set as unsearchable by default.
Some users may not want the Keybase servers to know their address book. Contact syncing is an optional feature intended to help you find friends who may already be on Keybase. When contact syncing, devices send the server the phone numbers and emails of the contacts in their address book, but not names, labels or other metadata. The server will reply with any Keybase users with accounts which match those phone numbers and emails, and then immediately discards the data: user address books are not stored permanently on the server.
We do not currently prove this via a system like private set intersection or Signal’s private contact discovery via SGX. In the future, we hope to increase the privacy of contact syncing so users do not have to trust the Keybase server to discard address book data.