Nicholas Morrison - Networking Specialist

S2E7 - The Onion Routing project

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Workshop Goals

  • understand and discuss TOR and why it’s called Onion Routing
  • understand and discuss its uses and its limitations

Symmetric encryption

Asymmetric encryption / public key encryption

  • Wikipedia: https://en.wikipedia.org/wiki/Public-key_cryptography
  • A key pair consists of a Public Key and a Private Key
  • great for allowing anyone to send you encrypted data, without you needing to share your key with them
  • or vice versa
  • Public Key
    • encrypt data
    • verify a signature
  • Private Key
    • decrypt data
    • sign data
  • If I have your Public Key, I can send you data that only you can decrypt with your Private Key.
  • I can sign a blob of data with my Private Key, which you can then verify using my Public Key.
  • Public Keys can be shared freely, advertised, uploaded to directory servers
  • Private Keys must be kept secret

Using them both together

  • Use asymmetric encryption to establish an encrypted communications channel
  • Use this channel to share an ephemeral symmetric key
  • Use symmetric encryption to communicate securely using the ephemeral key
  • Ephemeral key can be rotated every x seconds or every x bytes (forward secrecy)

Onion Routing

  • Wikipedia: https://en.wikipedia.org/wiki/Onion_routing
  • Forward your data along a chain of nodes
  • Only the exit node knows that it is an exit node
  • Other nodes don’t know if they are receiving packets from the originator or from another node
  • No node can see your original packet
    • Except for the “exit node”
    • So it’s very very important to always use your normal security practices! (HTTPS, SSH, SSL)

What it can do

  • hide your IP address from your ISP
  • hide your IP address from the server you are connecting to
  • disguise your activity
  • let you connect to .onion sites (the dark web)

What it cannot do

  • provide total anonymity

The Onion Routing project

Steps: setup

  • You = OP = Onion Proxy
  • OP finds a list of nodes from a TOR directory server (https://metrics.torproject.org/rs.html#search/flag:authority)
  • OP selects some (3 by default) relay nodes, with an exit node at the end (nodes A (guard), B (relay) and C (exit))
  • OP gets public keys of all nodes in the chain
  • OP uses these keys and receives (3) ephermeral keys for symmetric encryption with all nodes in the chain

Steps: send a packet

  • Encrypt your packet with the key of node C
  • Encrypt again with the key of node B
  • Encrypt again with the key of node A
  • Send your packet to Node A
  • Node A decrypts, and now has a packet encrypted for node B
  • Node A sends this packet to node B
  • Node B decrypts, and now has a packet encrypted for node C
  • Node B sends this packet to node C
  • Node C decrypts, and now has a packet to send to the internet
  • Node C sends this packet to the server on the internet

Steps: receive a reply

  • The internet server sees a connection with a source IP of the TOR exit relay
  • It sends reply packets back to this IP (node C)
  • Node C encrypts the packet with the ephemeral symmetric key
  • Node C sends the packet to node B
  • Node B encrypts the packet with its ephemeral symmetric key
  • Node B sends the packet to node A
  • Node A encrypts the packet with its ephemeral symmetric key
  • Node A sends the packet to you
  • You decrypt the packet three times, with those three ephemeral keys
  • Now you have the original packet!

Being a relay

  • You can choose to become a TOR Relay node yourself
    • Your computer will forward traffic to and from other TOR nodes
  • You can also become a TOR Exit Relay
    • Your computer will forward traffic to the internet from TOR users
  • If you just run the TOR Browser you’re not a relay

Attacks

  • Attackers want to deanonymise you
  • Timing attacks (control all the nodes)
  • User errors (oops I logged in with my username and password)
  • Side channel attacks (javascript bugs, malicious ads, etc)