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Secure Shell – SSH

Secure Shell – SSH. Tam Ngo Steve Licking cs265. Overview. Introduction Brief History and Background of SSH Differences between SSH-1 and SSH-2 Brief Overview of how SSH works Attack on SSH Key-Stroke Timing Attack Conclusion. History and Background. Password-sniffing attack

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Secure Shell – SSH

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  1. Secure Shell – SSH Tam Ngo Steve Licking cs265

  2. Overview • Introduction • Brief History and Background of SSH • Differences between SSH-1 and SSH-2 • Brief Overview of how SSH works • Attack on SSH • Key-Stroke Timing Attack • Conclusion

  3. History and Background • Password-sniffing attack • SSH-1 was developed, Finland, 1995 • SSH Communications Security Ltd. • Replacement for telnet and r-commands • Version 2, SSH-2 released in 1998

  4. All in one protocol CRC-32 integrity check One session per connection No password change No public-key certificate authentication Separate protocols Strong integrity check Multiple sessions per connection Password change provide public-key certificate authentication SSH-1 vs. SSH-2

  5. How SSH Works • (1) Client contacts server • (2) If SSH protocol versions do not agree, no connection • (3) Server identifies itself. Server sends host key, server key, check bytes, list of methods. Client looks in its DB for hosts. • (4) Client sends a secret key, encrypted using server’s public key • Both begins encryption. Server authentication is completed • Client authentication on the server side. Example, password and public-key authentication

  6. SSH-2 Protocol

  7. What SSH does: Packets are padded up to the first 8 byte multiple Input is sent as each key-down is read Not all input is echoed by the server What it means: Data size can be estimated Keystroke timing is feasible Password sessions are identifiable SSH2’s “Secure” Channel

  8. Identifying Password Transfers • Doesn’t SSH transfer passwords all at once? Yes, but… • Only when logging into the server • Not when running any applications (e.g. su) • Not when chaining logins

  9. Is this Useful? • Everything is encrypted, more information is required than just a password • What good is a password if you don’t know the host/user/application it is for • Attackers can sniff traffic to determine the host it is destined for • With access to the ps command attackers can narrow it down to a user running a specific application

  10. Keystroke Timing Various key pairs have different delays

  11. Keystroke Timing

  12. Keystroke Pair Probabilities

  13. Hidden Markov Model • State machine • The current state cannot be observed, only the output • Transition to next state depends only on current state • The likely state path can be deduced from observed output • Let each state be a key pair and the output be the delay between the two key presses

  14. Does It Work • The HMM can be solved using known algorithms to find a likely solution • The large amount of guesswork involved means the most likely solution isn’t always the correct one • Instead look at the n most likely solutions

  15. Does It Work • Given a subset of all possible 8 character random passwords • This method can reduce work by a factor of 50 • Translates to roughly 1 bit per character entered

  16. Does It Work • Can timing information be collected? • Yes • Are the timing metrics useful if the user creating them isn’t pre-tested? • Yes • Is it feasible to use a HMM to crack passwords? • Depends on who you ask

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