SSH, VPNs, and IPSec

SSH

• provides an encrypted tunnel between two hosts

  • typically, we think of a tunnel as a way to connect two machines over a network that doesn’t support the service we want
  • in this case, ssh provides an encrypted connection for remote access, over an unencrypted network

• we used to have cleartext utilities for working with remote hosts, and now there are encrypted equivalents, see Table 10.2, page 293

• see Figure 10.5, page 293 for an illustration of how SSH provides a tunnel between a client ssh and a server sshd

  • the applications on the remote host do not need to be “ssh-aware”

• protocols

  • transport protocol: authentication, encryption, integrity, negotiates parameters and keys
  • user authentication protocol: handles user authentication, runs on top of the transport protocol
  • connection protocol: enables multiple logical connections over a single physical connection, also runs over the transprt protocol

• user authentication

  • password
  • Kerberos ticket
  • client public key

• public key authentication

  • client sends public key and signature over data, including session ID
  • public key must be pre-registered as an authorized key
  • signature must be valid
  • session data must match
  • you should setup public key authentication when you use ssh and you should put a password on your private key

• SSH server authentication

  • outside of busineses, we often use TOFU (trust on first use) for a server key
    • can verify a hash of the fingerprint if you can get a valid hash over an independent channel — not done in practice
  • in enterprises, use a CA model, with a CA managed by the enterprise

• SSH port forwarding

  • see Figure 10.6, page 295

• How scp works

  • see Figure 10.7, page 296

• old model of trusted logins illustrates the dangers of transited trust

VPNs

• Virtual Private Network

• want to extend a private network (home or enterprise) to other locations

• see Figure 10.8, page 298

• can use hosts or gateways as endpoints, see FIgure 10.9, page 299

• encrypted tunnels make it difficult for an enterprise to filter malicious content, catch data exfiltration, block unwanted protocols or sites

IPsec

• provide security at the network layer, see Figure 10.10, page 300

• used for VPNs

• Internet Key Exchange (IKE)

  • automated key exchange using Diffie-Hellman, plus parameter negotiation
  • security association (SA) - shared state of algorithms, sequence numbers, cryptographic keys

• Authentication Header (AH)

  • used for authentication only (no encryption)
  • see Figure 3.11, page 301
    • remember, the IP header contains a Protocol field, this field is set to IPsec, so the receiving router knows that it will next see an AH
    • the Next Header field tells what protocol comes next in the payload, e.g. TCP
    • the SPI is an index to the SA that was created

• Encapsulating Security Payload (ESP)

  • used for confidentiality in addition to authentication
  • see Figure 3.12, page 301
  • Next Header indicates what type of protocol was encrypted (e.g. TCP or IP)

• small differences in transport mode vs tunnel mode, see Figure 3.13, page 302

  • note that in tunnel mode, the IPsec payload includes an IP header — this is to send the packet to the host after exiting the tunnel, which terminates at a router