SSH Keystroke Timing Attacks

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SSH Keystroke Timing Attacks
Mike Hogye Thad Hughes Josh Sarfaty Joe Wolf
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SSH
The Secure SHell protocol was created by Tatu
Ylönen and others to provide encrypted data
transfers between remote machines
MmmmSSH
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SSH Weaknesses

• SSH can leak information about passwords
• Approximate length of password can be inferred by
  examining number of packets.
• Keystroke Timing Analysis can reduce the search
  space for brute force attacks.

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Password Keystroke Timing

• Users type passwords often
• Password keystrokes develop consistent rhythm due
  to optimized hand motion
• This rhythm can be used to determine
  characteristics about the password

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SSH Immediate Mode

• Each keystroke is sent IMMEDIATELY from client to
  server, one character per packet
• Allows interactive user experience

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Passwords SSH

• SSH login does NOT used immediate mode
• Password (and username) packets are padded to
  fixed lengths
• No problems, right?

WRONG !!
Maybe?
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su

• UNIX Switch User command (used to get root
  access)
• Executed in IMMEDIATE mode

SSH1 su command
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Nested SSH

• Start new SSH session from within a running SSH
  session
• Username and password sent to server B in
  immediate mode

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So What?

• Password lengths can be determined
• Reveals timing information of password keystrokes
• Academically speaking, this is a lot of
  information

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Is This Practical?

• How to detect an su command?
• How to detect a nested SSH session?
• Network latency

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Detecting the su
I am a su

• Look for the su signature
• Not as easy as it sounds

SSH2 su command
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SSH! (nested)

• Theoretically similar to detecting su
• In practice, much harder to detect
• No definite packet signature for calling ssh

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How late(ncy) is your network

• Random network delay influences observed packet
  times
• Songs paper considered latency statistics
• Determined that latency is not an issue
• Used eight year old statistics
• Songs estimated network latency 10 ms
• Modern latency easily reaches 170 ms

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Internet Latency
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Conclusions

• Song Timing analysis can reduce brute-force
  password search by a factor of 50
• In practice, this is unlikely
• Use SSH2
• PuTTY defaults to SSH1

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