Security In Andrew

Summary

• State of Computing circa 1988
• Andrew Architecture
• Encryption Primer
• Challenges for Andrew File System
  • Solution
• Login Process
• RPC Session Establishment
  • Login is a Special Case of Bind
• Putting it All Together

State of Computing circa 1988

Workstations conneccted to central server over lan

• local disks served as caches

Andrew Architecture

Client workstation

• Unix
• Called virtues

Servers are clusted in secure environment

• Called vice
• Secure

Clients need to access servers over insecure links

Process called Venus used for auth and client caching

Encryption Primer

Private Key System

• Symmetric keys used in login
  • like password

Public Key System

• Asymmetric keys
  • Public key published
    • Used to encrypt
  • Private key hidden
    • Used to decrypt

Challenges for Andrew File System

• Authenticate users
• Authenticate server
• Prevent replay attacks
• Isolate users

Private key crypto system

Identity of sender in cleartext

Overuse of both username and password is a security hole

• Over-exposing key makes it easier to break key

Solution

• Username and Password only for login
• Use ephemeral id and keys for subsequent venus-vice communication

3 classes of client-server interaction

• login
• RPC Session Establishment
• FS Access during session

Login Process

2 Data strucutures:

• cleartoken
  • extract handshake key client (HKC)
• secrettoken
  • cleartoken encrypted with key known only to vice
  • unique for this login session
    • used as ephemeral client_id

Venus uses secretoken as the client id and the RPC messages are encoded with HKC

RPC Session Establishment

Incrementing random number by 1 proves that sender is genuine

num is the starting sequence number for RPC

Login is a Special Case of Bind

• Username is used for clientIdent
• Password is used as HKC
• Get back two tokens, encrypted with password
  • secrettoken
  • cleartoken

Putting it All Together

Username and password are only used once per login session

HKC used only for a new RPC session

SK used for all RPC calls to a filesystem