Briar is protected against:

• Metadata surveillance. There are no central server and all traffic are either local or through TOR hidden services network.
• Content surveillance. All communication between devices is encrypted end-to-end, protecting the content from eavesdropping or tampering.
• Content filtering. Briar’s end-to-end encryption prevents keyword filtering, and because of its decentralized design there are no servers to block.
• Takedown orders. Every user who subscribes to a forum keeps a copy of its content, so there’s no single point where a post can be deleted.
• Denial of service attacks. Briar’s forums have no central server to attack, and every subscriber has access to the content even if they’re offline. Internet blackouts. Briar can operate over Bluetooth and Wi-Fi to keep information flowing during blackouts.

With a limited protection against:

• Spywares on the phone. The app prevents being drawn over by other apps, screen recording or screenshooting.
• Stealing and accessing the data. The app can be very easily signed out and data deleted. Meaning that if you are signed out, a stealer or hacker can’t decrypt your messages. And if everything is removed they can’t even know you ever used the app.

Here’s how Briar compares to other protocols in terms of protection, cryptographic abilities

Properties	Discord	Signal	Matrix/Element	Conversations (XMPP over TOR and with OMEMO)	Briar
Network type	Centralized	Centralized	Federated	Federated	Peer to Peer
E2E encrypted	❌	✅	✅	✅	✅
Peer-to-Peer	❌	❌	❌	❌	✅
Ability to work offline (bluetooth, LAN, etc)	❌	❌	❌	❌	✅
TOR Hidden Services	❌	❌	❌	✅	✅
Protection against theft (while signed out) or screen recording	❌	❌	❌	❌	✅
Protection against metadata surveillance	❌	❌	❌	✅	✅
Protection against content surveillance	❌	✅	✅	✅	✅
Protection against takedown and DDoS attacks	❌	❌	➕	➕	✅
Anonymity	❌	❌	❌	✅	✅
Unlinkability	❌	❌	❌	✅	✅
Unobservability	❌	❌	❌	✅	✅
Operating over several modes of transport	❌	❌	❌	❌	✅
Support for images	✅	✅	✅	✅	❌
Support for calls	✅	✅	✅	✅	❌
Support for chat history (receiving messages while offline)	✅	✅	✅	✅	❌

To be able to understand what follows we need to understand the difference betweeen symmetric and asymmetric cryptography and also understand what is called the Diffie-Hellman Key Exchange.

For the sake of time and your sanity (and mine) we’re gonna limit the difference between symmetric and asymmetric cryptography to this for the moment:

• Symmetric cryptography has one key that can encrypt and decrypt messages. It been used for hundreds of years and is very quick.
• Asymmetric cryptography has two keys, one for encrypting and one for decrypting. It’s much more recent but also much more resource-intensive and slow.

But we can get the best of both worlds by using the “Diffie-Hellman” algorithm. Which starts with asymmetric cryptography to exchange keys and generate a shared secret that can be used to continue in the less resource intensive symmetric cryptography.

The Diffie-Hellman algorithm consists of mixing the public key of the other person with your private key (the other person does the same). Which results at the end with the same shared secret.

TOR and hidden services is an important point in Briar when Briar operates a remote connection.

So let’s explain the basics of TOR Hidden Services. Let’s start with a server and a client, but just remember that in the case of Briar, both people are servers and clients at the same time. Those images have been found on the official Tor Project’s website and will explain it much better than I ever will.

When contacts are near-by (can use bluetooth and/or LAN) then they should use QR codes to connect, it’s the safest method to connect on Bramble. And it uses the BQP (Bramble Qr Code Protocol)

• First both generate a key pair
• Then both generate a QR code containing a public key commitement (the first 16 bytes of the hash of the public key), the bluetooth MAC address and LAN IP address.
• They both scan each other’s QR code and try connect either by LAN, bluetooth or both.
• Once connection established, they both send their public key and compare them to the committement made.
• Diffie-Hellman is done with the keys and is hashed in order to generate a shared secret.
• A confirmation is sent containing a hash of the shared secret, the 2 public keyrs and the 2 QR code payloads. Those are exchanged, compared and must be identical.
• A new key is derived from the shared secret and the old keypair is deleted.

Now there are 2 protocols for operating with briar:// links. The first one is called “Bramble Rendezvous Protocol” or BRP for short and is used to discover the peers in the network.

The second is called “Bramble Handshake Protocol” (or BHP) and is used to share keys and start chatting.

The goal of this protocol is to provide a way to flow data through the different transports (bluetooth, LAN, TOR, etc) with, among others,

• Flexibility. The protocol should be able to operate over a wide range of underlying transports with bandwidths varying from kilobits to gigabits per second, and with latencies varying from milliseconds to days.
• Concealability. The protocol shouldn’t reveal any information that could make the requests distinguishable from other protocols. If someone is listening to traffic, they shouldn’t be able to see that one is using Briar.
• Confidentiality. The adversary should not be able to learn what data is being carried by the protocol.
• Integrity. If the adversary truncates the data, the receiving peer should be able to detect that this has happened.
• Authenticity. The adversary should not be able to cause either peer to accept data from any third party as though it came from the other peer.
• Forward Secrecy. The adversary should not be able to learn what data was carried by the protocol if, at some later time, the adversary compromises one or both of the peer devices.

However, the protocol itself doesn’t grant anonymity, unlinkability or unobservability. If you want that you then should use TOR (see BRP and BHP above).

Those are my sources about Briar itself. Those go from the most basic to the most advanced.

• How it works - Briar Project is a very simple overview of Briar’s security feature and stuff.
• Diving deep into Briar - a closer look at its internals at XMPP Meetup Berlin is a video explaining how Briar works. You can find the slides of the presentation here.
• Briar Wiki explains how each component and protocol works in much greater details.

• Perfect Forward Secrecy in TLS Explained explains what is forward secrecy and how it works in practice in the example of TLS (forward secrecy is used to share keys in Briar)

Those are all the informations about TOR Hidden Services. They go from the basic components of it, to the full explanations.

• What are Distributed Hash Tables explains how DHT works (to understand hidden services)
• How TOR Works (Computerphile) explains how onion routing which is used for remote transport in Briar
• TOR Hidden Services (Computerphile) explains how hidden services work (requires understanding of onion routing and DHT understanding)
• How Does Tor Really Work? The Definitive Visual Guide (2020) Explains how tor and hidden services work in a non video format (requires understanding of DHT). Note: It seems part of this is inaccurate, but it does give a pretty good overview of TOR in general, beyond onion routing and hidden services.
• How do Onion Services Work is the official explanation from Tor Proect about hidden services (also called onion services) where all the images come from. It’s very easy to understand, but if you want a deeper understanding of it, I advise you to look at all the links above.