The following is a quoted description of the protocol as described by Nash Kaminski. See https://kaminski.io Full credit to Mr. Kaminski for his work in explaining the details of creating a CSRMesh packet.

2.1Network Key🔗 i

"The 128 bit network key used in CSRMesh networks is derived by concatenating the ASCII representation of the PIN with a null byte and the string ’MCP’, computing the SHA256 hash of the string, reversing the order of the bytes in the resulting hash, and taking the first 16 bytes as the key."

2.2Authenticated Packets🔗 i

"Packets sent to CSRMesh devices require authentication as well as encryption. All multibyte types are represented in little endian format. To form a valid packet, the sequence/nonce value, constant 0x0080, and 10 null bytes are concatenated together to form a 128 bit initialization vector (IV). This IV, as well as the network key derived earlier is then used to initialize AES-128 in OFB mode. The arbitrary length data payload is then encrypted using this AES-OFB instance to form the encrypted payload. Next, a message authentication code is computed using HMAC-SHA256, using the network key as the secret, of the following data: 8 null bytes, sequence number, constant 0x80 and encrypted payload. The order of the bytes in the resulting hash are then reversed and the hash truncated to 8 bytes. The final output packet can then be built by contatenating the sequence/nonce value, constant 0x80, encrypted payload, truncated HMAC, and the constant 0xff."

3API🔗 i

(require csrmesh ) package: csrmesh

3.1Types🔗 i

value
Pin :bytes? =(pinbytes? )

A (define-new-subtypePin (pinBytes)) that represents a 4-digit ascii pin.

value
NetKey :bytes? =(netkeybytes? )

A (define-new-subtypeNetKey (netkeyBytes)) that represents a CSRMesh Netkey.

value
Encrypted-Payload:bytes? =(encrypted-payloadbytes? )

A (define-new-subtypeEncrypted-Payload(encrypted-payloadBytes)) that represents an encrypted CSRMesh payload.

value
Payload :bytes? =(payloadbytes? )

A (define-new-subtypePayload (payloadBytes)) that represents a payload of bytes TO BE rendered into the payload of a CSRMech packet.

value
Packet :bytes? =(packetbytes? )

A (define-new-subtypePacket (packetBytes)) that represents a ready to send CSRMesh packet of some payload/message.

value
SeqNo:Word32=(seqnoWord32)

A (define-new-subtypeSeqno(seqnoWord32)) that represents a packet sequence number used in the creation of a CSRMesh packet.

The sequence number of a packet is not only essential to the packet encryption but is also used by the mesh network in packet transmission. The mesh "remembers" recently transmitted packet sequence numbers and will drop packets with the same sequence number. An application using this library should ensure non-duplication of packet sequence numbers by either a global incrementing counter, a random generation of the next sequence number or similar scheme.

3.2CSRMesh Packets🔗 i

procedure
( string->Pin pin)→Pin
pin:String

Constructs a Pin from an ascii string. Typically this is a 4 digit ascii value, e.g. "1234".

procedure
( network-key pin)→NetworkKey
pin:Pin

Cryptographically calculates the CSRMesh NetworkKey from a given Pin . This is deterministic as the same Pin always creates the same NetworkKey.

procedure
( build-packet-with-key keysnopayload)→Packet
key:NetKey
sno:SeqNo
payload:Payload

Creates a ready to transmit CSRMesh Packet for the give payload, network key and sequence number.

procedure
( build-packet-with-pin pinsnopayload)→Packet
pin:Pin
sno:SeqNo
payload:Payload

Creates a ready to transmit CSRMesh Packet for the give payload, pin and sequence number.

Re-calculates the NetworkKey from the givein Pin and then calls build-packet-with-key .

As a NetworkKey is uniquely cryptographically generated from a Pin, an application may generate the Network Key once for a Pin via network-key and then using build-packet-with-key directly.

3.3Example Sketch🔗 i

(define pin:Pin (string->Pin "1234"))
(define net-key:NetworkKey(network-key pin))
(define my-payload:Payload (payload(bytes 123)))
(define next-seq-no:SeqNo(seqno(random 1000000)))
(define csr-pkt:Packet (build-packet-with-key net-keynext-seq-nomy-payload))

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