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Bramble QR Code Protocol (BQP) is a key agreement protocol that establishes a shared secret key between two peers. The peers must be near each other and equipped with screens and cameras. They must also have a short-range bidirectional transport over which they can communicate, such as a wireless LAN. The transport is not required to provide any security properties.
Each peer displays a QR code containing a commitment to an ephemeral public key and information about how to connect to the peer over various short-range transports. The peers scan each other's QR codes and use the transport information to establish an insecure connection. The peers then exchange public keys matching their commitments over the insecure connection.
Each peer derives a shared secret from its own private key and the received public key, then derives an ephemeral master key from the shared secret. The peers exchange confirmation codes to verify that they have received each other's public keys. The master key is returned to the calling application, which may use it to derive other keys for communicating securely over the transport connection, or for other purposes.
We assume the adversary can read, modify, delete and insert traffic on all transports at will. Anything displayed on a screen (specifically QR codes) is assumed to be seen by the adversary.
The initial exchange of QR codes is assumed to be secure against man-in-the-middle attacks because the users can see which screens they are scanning. This allows each user to be sure that the QR code they scanned was provided by the person with whom they intend to exchange keys.
Man-in-the-middle attacks against the subsequent exchange of public keys over the insecure connection can be detected by comparing the keys to the commitments contained in the QR codes.
- || denotes concatenation
- Double quotes denote an ASCII string
- len(x) denotes the length of x in bytes
- int\_n(x) denotes x represented as an unsigned, big-endian, n-bit integer
BQP uses three cryptographic primitives:
1. **A cryptographic hash function**, H(m)
2. **A key agreement function**, DH(pri, pub), where pri is one party's private key and pub is the other party's public key
3. **A message authentication code**, MAC(k, m), which must be a pseudo-random function
- HASH(x\_1, ..., x\_n) = H(int\_32(len(x\_1)) || x\_1 || ... || int\_32(len(x\_n)) || x\_n)
- KDF(k, x\_1, ..., x\_n) = MAC(k, int\_32(len(x\_1)) || x\_1 || ... || int\_32(len(x\_n)) || x\_n)
All hashes are HASH\_LEN bytes, all symmetric keys are KEY\_LEN bytes, and the output of MAC(k, m) is MAC\_LEN bytes. For simplicity we require that HASH\_LEN = KEY\_LEN = MAC\_LEN.
(*Note:* The current version of the protocol uses BLAKE2b as the hash function and keyed BLAKE2b as the message authentication code, both with an output length of 32 bytes. This gives HASH\_LEN = KEY\_LEN = MAC\_LEN = 32. The key agreement function is X25519.)
Each run of the protocol has four phases: preparation, connection establishment, key agreement, and master key derivation. The phases are described in the following sections.
## 2 Preparation Phase
### 2.1 Key Generation
Each peer starts by generating a fresh **ephemeral key pair** (pri, pub). The peer's **public key commitment** is the first COMMIT\_LEN bytes of HASH("org.briarproject.bramble.keyagreement/COMMIT", pub).
(*Note:* In the current version of the protocol, COMMIT\_LEN = 16. This is sufficient for 128-bit security because the adversary cannot make use of the birthday paradox: a successful man-in-the-middle attack requires a public key with a commitment that matches the victim's commitment, rather than two public keys with commitments that match each other.)
### 2.2 Scan Payloads
BQP uses **Bramble Data Format (BDF)** to represent structured data. BDF has six primitive types (null, boolean, integer, float, string, raw) and two container types (list, dictionary). BDF is specified in a separate document.
Each peer creates a **scan payload**, which starts with a byte containing the protocol version as an unsigned 8-bit integer. The current protocol version is 4. If a peer scans a QR code with a version lower than 4, or with version 89, it must abort the protocol. Peers may accept QR codes with higher versions if they know how to handle them, but version 89 is reserved.
The remainder of the scan payload is a BDF list with one or more elements. The first element is the public key commitment, represented as a BDF raw. The remaining elements are **transport descriptors**.
Each transport descriptor describes how to connect to the peer over a short-range transport. The descriptor is a BDF list with one or more elements. The first element is a **transport identifier**, represented as a BDF integer. The remaining elements are transport-dependent. Descriptors for the currently supported transports are defined below. Peers should ignore descriptors with unrecognised transport identifiers without aborting the protocol.
The peer encodes its scan payload as a QR code, which it displays.
Each peer scans the other peer's QR code and extracts the scan payload.
A peer may immediately connect to the other peer over any transports supported by both peers, simultaneously or in any order. This may result in one or more transport connections being established by either or both peers.
If no incoming or outgoing connection is established within CONNECTION\_TIMEOUT seconds of scanning the QR code, the peer aborts the protocol. (*Note:* The current version of BQP uses CONNECTION\_TIMEOUT = 60 seconds, which provides a reasonable tradeoff between reliability and responsiveness.)
In the remainder of the protocol, the peers are assigned roles according to the lexicographic order of their scan payloads, compared as byte strings. The peer with the earlier payload plays the role of **Alice**, while the peer with the later payload plays the role of **Bob**. Each peer can determine its role after scanning the other peer's QR code. The roles are identical except for some key derivation constants. It does not matter which peer plays which role, as long as one is Alice and the other Bob.
If multiple transport connections are established, Alice decides which connection to use and closes any other connections. Bob waits for Alice to start communicating over one of the connections and then closes any other connections.
## 4 Key Agreement Phase
### 4.1 Records
The peers exchange a series of **records** over the connection chosen by Alice. Each record has the following format:
- record\_header = int\_8(protocol\_version) || int\_8(record\_type) || int\_16(len(payload))
The current version of the protocol is 4, which has three record types:
**0: KEY** - The payload consists of the sender's ephemeral public key.
**1: CONFIRM** - The payload consists of a confirmation code.
**2: ABORT** - The payload is empty.
If a peer aborts the protocol it must send an ABORT record unless it has already sent one. If a peer receives an ABORT record it must abort the protocol.
If a peer receives a record with a protocol version lower than 4, or with version 89, it must abort the protocol. Peers may accept records with higher versions if they know how to handle them, but version 89 is reserved.
Peers should ignore any records with unrecognised record types without aborting the protocol. If a peer receives a record with a recognised protocol version and record type at an unexpected stage in the protocol, it must abort the protocol.
Alice begins by sending a KEY record containing her ephemeral public key, pub\_a. Bob compares the first COMMIT\_LEN bytes of HASH("org.briarproject.bramble.keyagreement/COMMIT", pub\_a) to the commitment in Alice's scan payload. If the key does not match the commitment, Bob aborts the protocol.
Bob sends a KEY record containing his ephemeral public key, pub\_b. Alice compares the first COMMIT\_LEN bytes of HASH("org.briarproject.bramble.keyagreement/COMMIT", pub\_b) to the commitment in Bob's scan payload code. If the key does not match the commitment, Alice aborts the protocol.
(*Note:* If a peer calculates an X25519 raw shared secret that is all zeroes, the peer must abort the protocol.)
If the adversary has not modified the KEY records, both peers will calculate the same shared secret. The peers then derive a "cooked" shared secret that incorporates the protocol version and both peers' public keys:
- cooked\_secret = HASH("org.briarproject.bramble.keyagreement/SHARED\_SECRET", raw\_secret, int\_8(protocol\_version), pub\_a, pub\_b)
Each peer knows it has received the correct public key because it has compared the received public key to the commitment in the other peer's scan payload. However, each peer also needs to know that the other peer has received the correct public key. To confirm this, both peers derive a **confirmation key** from the cooked shared secret and use it to calculate two message authentication codes over the scan payloads, q\_a and q\_b, and the public keys, pub\_a and pub\_b:
- confirmation\_key = KDF(cooked\_secret, "org.briarproject.bramble.keyagreement/CONFIRMATION\_KEY")
- confirm\_a = KDF(confirmation\_key, "org.briarproject.bramble.keyagreement/CONFIRMATION\_MAC", q\_a, pub\_a, q\_b, pub\_b)
- confirm\_b = KDF(confirmation\_key, "org.briarproject.bramble.keyagreement/CONFIRMATION\_MAC", q\_b, pub\_b, q\_a, pub\_a)
Alice sends a CONFIRM record containing confirm\_a. Bob compares the received confirm\_a to the confirm\_a he calculated. If the values do not match, Bob aborts the protocol.
Bob sends a CONFIRM record containing confirm\_b. Alice compares the received confirm\_b to the confirm\_b she calculated. If the values do not match, Alice aborts the protocol.
Finally, each peer derives the ephemeral master key from the cooked shared secret:
- ephemeral\_master\_key = KDF(cooked\_secret, "org.briarproject.bramble.keyagreement/MASTER\_SECRET")
The peers must then delete the raw and cooked shared secrets, allowing the calling application to use the master key for forward secret communication if required.
The master key is retured to the calling application, together with the open transport connection and a flag indicating whether the peer played the role of Alice or Bob. The application may use the master key and flag to derive keys for communicating securely over the transport connection, or for any other purpose.
## 6 Transport Descriptors
The current version of the protocol supports the following transports:
### 6.1 Bluetooth
The peer registers a Bluetooth service to accept RFCOMM connections. The service's name is "RFCOMM". The UUID is generated by converting the first 16 bytes of the peer's public key commitment into a UUID, as specified in section 4.4 of RFC 4122. The peer unregisters the service when the protocol terminates.
The transport identifier is 0. After the identifier the descriptor may contain the peer's Bluetooth address, represented as a BDF raw. If the peer supports Bluetooth but does not know its own address, it should omit the address from the descriptor and make itself discoverable so the other peer can discover its address.
The peer connects to a local area network and opens a port to accept TCP connections. The peer closes the port when the protocol terminates.
The transport identifier is 1. After the identifier the descriptor contains the peer's IP address, represented as a BDF raw, and the port number as a BDF integer. The address must have link-local or site-local scope.