A social network built on public-key encryption.


  1. Make it easy and fun to have private conversations between individuals and groups

  2. Make it impossible for anyone other than the intended user (system administrators, server operators, etc.) to view private information, or be coerced into sharing private information with others

  3. Make it harder for accounts to be stolen, hacked or for users to be impersonated


Public key encryption a great way to keep information private, almost the best (the best being one-time pad) .  It’s a little bit of work, but for one-on-one communication there are tools that make it almost as easy as using no encryption at all.  However, due to the nature of individual-specific keys it’s not great for sharing information privately with more than one person.

I’ve thought a lot about how to leverage the power of public-key encryption in an environment where people share information with not only one another, but also with groups of trusted friends.  I wanted to preserve the privacy of encrypting information client-side (before it goes out over the network) and I wanted to avoid any possibility of unencrypted data being stored on a somewhere other than on the computer of the users it is intended for.  I essentially wanted the same level of privacy you get from sending messages using OpenPGP, but also the utility of a basic forum or social network.  What I came up with is something I call “Crypton”*.

For the user, the experience is similar to familiar systems like Twitter or Facebook with the exception that a user needs to have a GPG keypair setup in advance.  The public key from this pair is then used to authenticate to Crypton.

When an account is setup, a new user supplies their public key to the system, which in turn sends them a message including a link to log-in to the site. Once logged-in, the user can send direct messages to other users, or post messages to a feed which is visible to everyone on the user’s “trusted friends” list.  The user can view messages sent directly to them, and can browse a feed containing messages posted by their trusted friends.  All of these messages are encrypted before they leave the user’s computer, and are only decrypted when they are displayed on the user’s computer.  Any data stored elsewhere is stored in encrypted form only .

How does this work?

When a user sends a direct message to another user, it works just like sending an OpenPGP email: the message is encrypted using the public key of the destination user.  However when messages are posted to the feed, we have to do things a bit differently.

When a user posts a message to their feed, the message needs to be readable by everyone in their “trusted friends” list.  The only way this is possible is if the message is encrypted using each friend’s public key; so this is what is done.

When a user posts the message to their feed, it is encrypted multiple times, once for each member of their friends list, using the friends public key.  The system then displays these messages to the viewer of the feed so that only the messages encrypted with their key are displayed, which gives the appearance of a shared feed.


One of the first issues that comes to my mind with this approach is the impact of changes to the trusted friends list.  In the system described, a friend added to the list cannot view messages posted to the feed in the past, as no copy of the message was encrypted for them.  This may be desirable, but it’s different from how existing systems work so it’s worth noting.  There may be work-arounds for this, but it would require some careful thought to do so without creating situations that violate the premise of the platform.

The more troublesome issue occurs when someone is removed from the trusted friends list.  Since messages are encrypted with the users public key, technically friends removed from the list can still decrypt messages sent and posted before they were removed.  Again this may or may not be desirable, but it may differ from what’s expected.  This could be managed by the system via deleting the copies of posts and messages encrypted for the removed friend, but this opens up the issue described above should the user be added-back to the trusted friends list.

Another potential challenge is sharing, because once a message has been decrypted by a trusted friend, it’s contents could be re-posted by that friend and made accessible to users outside of the original author’s trusted friends group.  There is probably no mitigation for this and hence the term “trusted friends” is used throughout this document, because users on this list should be limited to people trusted not to do things like that.

Other complexities like this potentially arise when considering how comments or responses to posts might work, or how to integrate features beyond basic messaging and posting in a way that maintains the principle privacy of the system.

Finally, there are implementation details that have not been discussed that would allow the system to work efficiently.  Given the amount of duplicate data, there may be ways to avoid the exponential resource consumption that could be associated with a system like this if this was overlooked.  I have in mind several strategies to mitigate these concerns but I won’t go into them here.

Going Further

The system described provides basic functionality for a social network system with strong privacy, but it doesn’t have a lot of features.  This was done deliberately to focus on what makes Crypton different and keep the explanation as simple as possible.

That said, it’s easy to imagine additional features that would make Crypton more useful.  The first one that comes to my mind are “scopes” that allow the user to group their trusted friends so that messages can target subsets of the friends list.  It’s easy to imagine how you might want to share some information with family and other information with co-workers, but not both, etc.

Something else that isn’t discussed is discovery, or how one builds a list of trusted friends.  Given the nature of GPG and GPG users, it seems like the starting point would be the ability to allow a user to share their GPG keyring with the system so anyone the user already communicates with via OpenPGP messages could be added to their trusted friends on Crypton.  Given the encrypted nature of the system, traditional means of searching content to find friends is less of an option, but there may be ways to accomplish that for users who opt-in to participating in an index, etc.



1140 Words

2016-08-19 00:00 +0000