How to do Onion Routing?

Onion routing is achieved by wrapping data in multiple layers of encryption and routing it through a series of network nodes, each of which decrypts one layer, revealing the next destination. This hides the data's origin and content from intermediate nodes and observers.

Here's a breakdown of the process:

• Encryption Layers (The "Onion"):

  • The sender (entry node) creates the message.
  • The sender encrypts the message with the public key of the last node in the path (exit node).
  • Then, they encrypt the result with the public key of the second-to-last node, and so on, working backwards up the path.
  • Each layer of encryption acts as a "layer of the onion." Each layer contains instructions on where to send the data next and the key necessary for the next node to decrypt its portion.

• Path Selection: The sender (entry node) chooses a random path (a "circuit") of nodes through the network. These nodes are often called "routers" or "onion routers." The path typically includes:

  • Entry Node (Guard Node): The first node in the circuit. It knows the sender's IP address.
  • Middle Nodes (Relay Nodes): Intermediate nodes that relay the data. They only know the previous and next nodes in the circuit.
  • Exit Node: The final node in the circuit. It decrypts the final layer of encryption and delivers the message to its final destination.

• Data Transmission:

  1. The sender sends the fully encrypted "onion" to the entry node.
  2. The entry node decrypts the outermost layer of encryption, revealing the next node in the path and instructions for forwarding the data.
  3. The entry node forwards the partially decrypted message to the next node.
  4. Each subsequent node decrypts one layer of encryption, reveals the next node, and forwards the message.
  5. The exit node decrypts the final layer of encryption and sends the original message to the intended recipient.

• Decryption at Each Node: Each node only knows the identity of the node that sent it the data and the node to which it should forward the data. They do not know the ultimate origin or destination of the message. Each node only has the key to decrypt one layer of encryption.

• Reply Traffic (Optional): To send a reply, a similar "onion" can be created by the recipient, encrypting the data layer by layer, going backwards through the same or a different circuit. This provides anonymity for the recipient as well.

Illustrative Example (Simplified):

Let's say Alice wants to send a message to Bob using onion routing with three nodes: Entry Node (E), Middle Node (M), and Exit Node (X).

1. Alice encrypts the message with X's public key: Encrypted_X(Message)
2. Alice encrypts the result with M's public key: Encrypted_M(Encrypted_X(Message))
3. Alice encrypts the result with E's public key: Encrypted_E(Encrypted_M(Encrypted_X(Message)))
4. Alice sends the fully encrypted message to E.
5. E decrypts the outer layer using its private key: Encrypted_M(Encrypted_X(Message)) and forwards it to M.
6. M decrypts its layer: Encrypted_X(Message) and forwards it to X.
7. X decrypts the final layer: Message and sends it to Bob.

Key Advantages of Onion Routing:

• Anonymity: Hides the sender's and receiver's IP addresses.
• Security: Encryption protects the message content from eavesdropping.
• Resilience: Difficult to trace the communication path.

Common Implementations:

• Tor (The Onion Router): The most well-known implementation of onion routing, used for anonymous web browsing and other applications.

In summary, onion routing provides a layered approach to encrypting and routing data, making it difficult to trace the communication back to its origin or decipher its content. Each node in the path only knows its immediate predecessor and successor, enhancing anonymity and security.