[cryptography] Password non-similarity?

Adam Back adam at cypherspace.org
Mon Jan 2 04:41:06 EST 2012

On 2 January 2012 03:01, ianG <iang at iang.org> wrote:
>>> When I was a rough raw teenager doing this, I needed around 2 weeks to
>>> pick up 5 letters from someone typing like he was electrified.  The other 3
>>> were crunched in 4 hours on a vax780.
>> how many samples? (distinct shoulder surf events)
> About 1 a day, say 10, without making it obvious.

The trick to counter-acting shoulder surfing is to touch type and hold the
shoulder suffers gaze so you know they are not looking at your
key-presses. Computer teacher in high school used to do that I

Seperately and relatedly I was thinking of having a go at designing a human
computable challenge response for occasional when you know or believe your
typing is being observed.  eg Human remembers single digit numeric
coefficients to a 8 mod 10 simultaneous equations (16 digits):

r1 = a.x1+b.x2 mod 10
r2 = c.x3+d.x4 mod 10
r8 = o.x15+p.x16 mod 10

computer generates x1 - x16 at random between -9 and +9.  Now a shoulder
surfer sees less than 8 challenges responded to and they have only 1
equation for each pair of unknowns.  The challenges are one use.

The response (what is typed to login) are r1.. r8 an 8 digit number.

That was just the rough idea, no calculations done yet, maybe one can reduce
the number of terms and safely allow more than one use with a bit of

I was thinking it might be interesting for encrytped file systems
also. Normally you login with your passphrase when you are confident
you are not being shoulder surfed, or no public video surveillance in place (eg
airport).  But this way you have a second login mechanism with limited
number of logins that are safe to use.  The challenges and the disk key
encrypted with salted, iterated hash of the challenge response can be stored
separately, one per login, and over-written after use, preventing hostile
reuse.  After login they can be replaced with a new one.


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