Protecting private key on USB flash drive: how to? // secret-sharing

[vedaal at hush.com]

Roscoe eocsor at gmail.com
wrote Mon May 12 09:02:32 CEST 2008 :

>> For my curiosity, has anyone used threshold (split-key)
>> crypto for key protection? 

> http://point-at-infinity.org/ssss/ works good for passwords to 
keys :)

no,
the Shamir split-key/secret sharing,
works for shares of 'keys',
not for passwords

here is a quote from the site:

=====[begin quote]=====
Note that Shamir's scheme is provable secure, that means: in a 
(t,n) scheme one can prove that it makes no difference whether an 
attacker has t-1 valid shares at his disposal or none at all; as 
long as he has less than t shares, there is no better option than 
guessing to find out the secret.
=====[end quote]=====

key structures are much more complex than passphrases

example:
assume a passphrase of 16 characters that is shared among two people
each having 8 characters protected by Shamir's secret sharing

even though each person cannot 'decrypt' the other person's 'share',
(that part is true),
each one can start from scratch and do a brute force attack on the 
other 8 characters when combined with the 8 characters already 
known, and recover the passphrase

when Shamir uses the technique to share 'Keys'
the 'key', which is far more complex than a simple password string,
cannot be reconstructed from a brute force attack, even when t-1 
shares are known

'split-keys' have been used by pgp since 6.x,
(usually for 'corporate signing'
when a certain majority t/n is needed for approval of a measure,
although it could work as well for decrypting too)


vedaal 

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