specifications.md

Bridge Specification

Table of Contents

• Content Format
• Payload Format
  • Auth Request Payload
  • Auth Code Payload
  • Auth Code and Payload
  • Payload Only

Content Format

The Bridge supports the following content formats:

1. Email: to:cc:bcc:subject:body
   • Example: Email Bridge

Payload Format

Payload Type	Switch	Description
Auth Request Payload	0	Contains a client public key
Auth Code Payload	1	Contains an authentication code
Auth Code and Payload	2	Contains an authentication code and encrypted content
Payload Only	3	Contains encrypted content only

Auth Request Payload

• Switch: 0
• Format:
  • 1 byte: Content Switch
  • 4 bytes: Public Key Length (integer)
  • Variable: Client Public Key

Visual Representation:

+------------------+----------------------------+-------------------+
| Content Switch   | Length of Client Public Key| Client Public Key |
| (1 byte)         | (4 bytes, integer)         | (variable size)   |
+------------------+----------------------------+-------------------+

content_switch = bytes([0])  # b'\x00'
client_public_key = b"client_pub_key"

payload = (
    content_switch +
    struct.pack("<i", len(client_public_key)) +
    client_public_key
)
encoded = base64.b64encode(payload).decode("utf-8")
print(encoded)

Auth Code Payload

• Switch: 1
• Format:
  • 1 byte: Content Switch
  • 1 byte: Auth Code Length (integer)
  • Variable: Auth Code

Visual Representation:

+------------------+---------------------------+-----------------+
| Content Switch   | Length of Auth Code       | Auth Code       |
| (1 byte)         | (1 byte, integer)         | (variable size) |
+------------------+---------------------------+-----------------+

content_switch = bytes([1])  # b'\x01'
auth_code = b"123456"

payload = (
    content_switch +
    bytes([len(auth_code)]) +
    auth_code
)
encoded = base64.b64encode(payload).decode("utf-8")
print(encoded)

Auth Code and Payload

• Switch: 2
• Format:
  • 1 byte: Content Switch
  • 1 byte: Auth Code Length (integer)
  • 4 bytes: Ciphertext Length (integer)
  • 1 byte: Bridge Letter
  • Variable: Auth Code
  • Variable: Ciphertext

Note

For detailed instructions on using the Double Ratchet algorithm to create ciphertext, refer to the smswithoutborders_lib_sig documentation.

Visual Representation:

+------------------+---------------------------+----------------------------+-------------------+------------------+----------------------------+
| Content Switch   | Length of Auth Code       | Length of Ciphertext       | Bridge Letter     | Auth Code        | Ciphertext                 |
| (1 byte)         | (1 byte, integer)         | (4 bytes, integer)         | (1 byte)          | (variable size)  | (variable size)            |
+------------------+---------------------------+----------------------------+-------------------+------------------+----------------------------+

content_switch = bytes([2])  # b'\x02'
auth_code = b"123456"
bridge_letter = b"e"
ciphertext = b"Hello world!"

payload = (
    content_switch +
    bytes([len(auth_code)]) +
    struct.pack("<i", len(ciphertext)) +
    bridge_letter +
    auth_code +
    ciphertext
)
encoded = base64.b64encode(payload).decode("utf-8")
print(encoded)

Payload Only

• Switch: 3
• Format:
  • 1 byte: Content Switch
  • 4 bytes: Ciphertext Length (integer)
  • 1 byte: Bridge Letter
  • Variable: Ciphertext

Note

For detailed instructions on using the Double Ratchet algorithm to create ciphertext, refer to the smswithoutborders_lib_sig documentation.

Visual Representation:

+------------------+-----------------------------+------------------+----------------------------+
| Content Switch   | Length of Ciphertext        | Bridge Letter    | Ciphertext                 |
| (1 byte)         | (4 bytes, integer)          | (1 byte)         | (variable size)            |
+------------------+-----------------------------+------------------+----------------------------+

content_switch = bytes([3])  # b'\x03'
bridge_letter = b"e"
ciphertext = b"Hello world!"

payload = (
    content_switch +
    struct.pack("<i", len(ciphertext)) +
    bridge_letter +
    ciphertext
)
encoded = base64.b64encode(payload).decode("utf-8")
print(encoded)