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Normally you hear that using an SSH Key is far more secure than using a regular password. However I've been wondering if this statement is really true.

Let's say we have the following variables for this example:

  • The SSH Server is publicly accessible
  • The SSH Server has fail2ban and will perma-ban any ip that tries to ssh to root or fails more than 3 times.
  • Users have Full Disk Encryption on all devices using SSH
  • The SSH Server requires two factor authentication from a authy on an android device with FDE
  • Users are using complex 48 character passwords for FDE (ex: YUba3KR06/62.Dzq!955513s0k2m123;sF32a60G6n2PYu0Z8_vv8u) with the guarantee of not writing them down

Which would of the following would truly be more secure?

  1. Users use memorized passwords (not written down) for SSH. All passwords would be complex with 16+ characters (ex: S_s7vEn93z0!.Lid)
  2. Users use Passowrdless SSH Keys
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There are 2 awnser. They differ on the attack vector under consideration .

  • Password based login. Is more secure when the machine that would have held the ssh-key is not protected against access to the key. (Use password protected keys always to defeat this weakness and agents for convenience).

  • keys are more secure when transmission matters. The ssh-keys influence the way the initial key exchange is in such a way that it is already partly encrypted with the private key. Similar to how client side certificates work.

The difference is minor and the specific threats you want more protection against determines what choice is better here.

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