SSH Keystroke Timing Attacks Mike Hogye Thad Hughes
SSH Keystroke Timing
Attacks
Mike Hogye
Thad Hughes
Josh Sarfaty
Joe Wolf
SSH
The Secure SH ell protocol was created by Tatu Ylönen and others to provide encrypted data transfers between remote machines
Mmmm…
SSH
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.
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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Time Between Adjacent Keystrokes vs. Key Typed
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Key Typed
[ : 4 Enter
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Trial 8
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Time Between Adjacent Keystrokes vs. Key Typed
With Network Latency
] i p J
Key Typed
[ : 4 Enter
Trial 1
Trial 2
Trial 3
Trial 4
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Trial 6
Trial 7
Trial 8
SSH Immediate Mode
• Each keystroke is sent IMMEDIATELY from client to server, one character per packet
• Allows interactive user experience
Passwords & SSH
•SSH login does NOT used immediate mode
•Password (and username) packets are padded to fixed lengths
•No problems, right?
WRONG !!
Maybe?
su
•UNIX “Switch User” command (used to get root access)
•Executed in IMMEDIATE mode
SSH1 su command
Nested SSH
•Start new SSH session from within a running SSH session
•Username and password sent to server B in immediate mode
So What?
• Password lengths can be determined
• Reveals timing information of password keystrokes
• Academically speaking, this is a lot of information
Is This Practical?
• How to detect an “su” command?
• How to detect a nested SSH session?
• Network latency
“ I am a su ”
Detecting the “su”
• Look for the ‘su’ signature
• Not as easy as it sounds
Server ack ack ack ack ack ack ack ack ack
40 40 40 40 40 40 48 48 40 40 40 40 40 40 56 64 56 ack ack ack ack
Client
“s” “u”
SSH2 su command
Return “a” “b” “c” “d” Return Server Response
SSH! (nested)
• Theoretically similar to detecting ‘su’
• In practice, much harder to detect
• No definite packet signature for calling
‘ssh’
How late(ncy) is your network
• Random network delay influences observed packet times
• Song’s paper considered latency statistics
– Determined that latency is not an issue
– Used eight year old statistics
– Song’s estimated network latency: 10 ms
• Modern latency easily reaches 170 ms
Internet Latency
Conclusions
• Song: Timing analysis can reduce bruteforce password search by a factor of 50
• In practice, this is unlikely
• Use SSH2
– PuTTY defaults to SSH1