28,913 reputation
453109
bio website bolet.org/~pornin
location Quebec City, Canada
age 38
visits member for 2 years, 9 months
seen 5 hours ago

Cryptographer, programmer in several languages (C, Java, several assemblies, Pascal, Forth...). I also have a life.


Nov
3
answered How does one measure the decrease in security, if any, of RC4 when mapping to a specific range of values?
Nov
2
comment Threshold Secret sharing - How to create a shared secret from pre existing secret parts?
That's the point. If you use Shamir's scheme, you get a whole new set of secret values to store (which I call $v_i$); but that storage can be a public shared disk (as opposed to, say, a smartcard per participant) because each participant already has a secret value $x_i$ and can use it as a symmetric key to encrypt his $v_i$.
Nov
1
reviewed Approve suggested edit on Threshold Secret sharing - How to create a shared secret from pre existing secret parts?
Nov
1
answered Threshold Secret sharing - How to create a shared secret from pre existing secret parts?
Oct
31
reviewed Approve suggested edit on Why would you expect to find a collision in a hash function after approximately $\sqrt{n}$ hashes?
Oct
31
answered Is Blowfish strong enough for VPN encryption?
Oct
31
revised Why can't one implement bcrypt in Cuda?
added 155 characters in body
Oct
30
awarded  Enlightened
Oct
30
awarded  Nice Answer
Oct
30
awarded  Nice Answer
Oct
28
reviewed Approve suggested edit on What is a hard-core predicate?
Oct
28
answered What is a hard-core predicate?
Oct
28
revised Why would you expect to find a collision in a hash function after approximately $\sqrt{n}$ hashes?
added 167 characters in body
Oct
27
answered Why would you expect to find a collision in a hash function after approximately $\sqrt{n}$ hashes?
Oct
27
reviewed Approve suggested edit on Should I use ECB or CBC encryption mode for my block cipher?
Oct
26
comment How can rainbow tables be used for a dictionary attack?
That's the point of my comments: to give information that anybody (including you) may use at their leisure (even for incorporating it into, for instance, an edit to this answer).
Oct
26
comment How can rainbow tables be used for a dictionary attack?
400 GB is cheap, I can have 5 times that much storage space for 100$. You may want to crank up the numbers a bit by considering 8-character passwords; they would look more impressive.
Oct
26
comment How can rainbow tables be used for a dictionary attack?
You could include the complexity: a rainbow table covers a set of N potential passwords. Let's call t the average chain length. Then, the building cost of the table is about 1.7*N (yes, it is 70% more expensive than a simple exhaustive search on the set). The storage cost is N/t elements of size at least log N (but not necessarily much bigger). Attacking a password has cost about t^2/2 hash computations, and t lookups (a "lookup" is when you are actually looking for data in the harddisk).
Oct
26
comment How can rainbow tables be used for a dictionary attack?
About salts: the effect of a salt can be seen as: we do not have one hash function, we have one per salt value. There is not a single hash function but a family of hash function, and the salt value tells you which family member is actually used. When you build a table, you do it for a given hash function; it cannot be used for any other.
Oct
26
comment How can rainbow tables be used for a dictionary attack?
The count of passwords is strange: it assumes that the same character may not appear twice in a given password. But what prevents that ? A more common count would be 62^6, close to 56 billions instead of your 44 billions. Storage cost is also overestimated: in a table of hashes for lookups, one would sort the pairs (password,hash) by hash value, keeping only as many bits of the hash value as necessary. The total cost would be closer to the cost of simply storing the passwords themselves, maybe with one or two extra bytes. About 400 GB, I would say.