[Bitcoin-development] two comments on brain-wallet security (and BIP 38)

[Adam Back <adam@cypherspace.org>]

So I had a go at deciphering BIP 038 in summary what I think its doing is
(ommitting lot and sequence and deterinistic IVs for simplicity):

user:

x1 = Scrypt( salt=random, pass )
P = x1*G

send (salt, P) to coin manufacturer -> 

				manufacturer:

				x2 = random 24bytes
				Q = x2*P
				k = Scrypt( salt2=H(Q)||salt, pass=P )
				e = AES_k( x2 )

				manufacturer puts es inside coin.

		<- send coin, (salt, e, Q) to user

				then optionally creates conf code:

				B = x2*G
				c = AES_k( B )

		<- send conf code c to user

verify code c:

(by recreating P, then k from Q & P, decrypt c to get B, check Q = x1*B)

x1 = Scrypt( salt, pass )
P = x1*G
k = Scrypt( salt2=H(Q)||salt, pass=P )

Which seems reasonable enough, however its unfortunate that you have to
repeat the Scrypt work at setup.

One thing that occurs to me eg as mentioned by Rivest et al in their
time-lock puzzle paper is that it is easy to create work, if you are ok with
parallelizable symmetric constructions (like scrypt(i) or PBKDF2(i) with i
iterations) without *doing* the work during setup.

It seems to me therefore that the above protocol could avoid the javascript
overhead issue that forces users to choose a weak iteration level if they
want to create the wallet in that way.

eg create a 32-bit random salt, replace scrypt(i=16384, salt, pass) with
scrypt(i=1,salt, pass) to be brute forced based on deleted salt.  Immediate
2^32 = 4billion iteration salt without any significant setup cost.  (Or if
you want to limit the parallelism say scrypt(i=65536, salt, pass) with a
deleted 16-bit salt.  That should be parallelizable up to 65536 GPU cores
(32x 7970 chips).

Symmetric time-lock puzzles can achieve decrypt asymmetry without repeating
the work at setup...

(Rivest et al goes on to avoid using that symmetric construct with an RSA
related mechanism, because they are trying to lock information for an
approximate future date, rather than protected by a specific amount of
grinding work.)


I proposed a different blind (securely server-offloadable) deterministic
proof of work relating to (asymmetric RSA-style) time-lock puzzles.  The
difference from time-lock is it is made blind (so the work can be securely
offloaded without the server learning your password or resulting key) and
can be easily made parallelizable also which is desirable for server
offload.

https://bitcointalk.org/index.php?topic=311000.new#new

I think that could take brain-wallets to a new level of security, if you
protect the amount by an amount of computation proportional to the value, eg
0.1% or 0.01% redemption cost paid to blind proof of work miners.

Adam