Can “Something You Know” be Saved?

“Something you know,” in the form of passwords, has been the cornerstone of authentication for some time; however the inability to survive replay attack threatens this state of affairs. While “something you know” may always be used in addition to “something you have” we examine whether it can be salvaged as the solo factor for authentication. A recent surge of interest in Challenge Response authentication schemes raises the question whether a secret shared between the user and the server can allow secure access even in the presence of spyware. Our conclusion is negative. Assuming only a limit on the amount that a user can remember and calculate we find that any scheme likely to be usable is too easily brute forced if the attacker observes several logins. This is true irrespective of the details of the scheme. The vital parameter is the number of bits of the secret involved in each bit of the response. When this number is too low the scheme is easily brute-forced, but making it high makes the scheme unworkable for the user. Our conclusion is that single factor “something you know” schemes have a fundamental weakness unless the number of logins the attacker observes can be restricted.