210 reputation
17
bio website
location Tel Aviv, Israel
age 36
visits member for 1 year, 8 months
seen Oct 15 '13 at 15:02

Coding in Tel Aviv


Aug
30
comment How can I validate a hashed password if all I have is another hash?
SRP has minimal back-and-forth: Just two messages to derive a shared key. There are two more messages if you want authentication.
Aug
18
comment How do I store encrypted files on a web server and decrypt them locally?
Wow, I was surprised! PBKDF2 is very expensive to widen! jsperf.com/1-pbkdf2-versus-many-decrypts I'll drop the key authentication and just use the keys for decryption.
Aug
18
comment How do I store encrypted files on a web server and decrypt them locally?
Regarding B, it depends which is more expensive to the user: Decrypting with a wrong key once per image (fast but many times) or doing the extra PBKDF2 (slow but just once).
Aug
18
comment How do I calculate the maximum plain text length allowable for a certain cipher text length?
Can VARCHAR(512) actually store 512 or does it need some sort of end-of-string marker?
Aug
17
comment How do I store encrypted files on a web server and decrypt them locally?
How come I need all the addition or modulo arithmetic? Couldn't I just use PBKDF2 to generate 512 bits, half which I would store in the index file for authentication, and the other half which I would use to decrypt the image's key? Per image I would store the first half of the 512 bits and the encryption of K_f. The advantage would be that I would only have to authenticate each password once, not once per file. The disadvantage: it would leak info about which files are encrypted with the same key. Other disadvantages?
Aug
17
comment Can I use PBKDF2 for authentication and decryption?
The point about halving the attacker's workload is convincing!
Aug
16
comment How do I store encrypted files on a web server and decrypt them locally?
I like the idea of being able to change the password without modifying all the images, though. That's pretty cool!
Aug
16
comment How do I store encrypted files on a web server and decrypt them locally?
Thanks for the detailed reply. I'm not having the client upload images nor encrypt them. Assume that those are done securely and uploaded securely to a server. I also want the access to the images to require just a plain browser and knowledge of the password so I have to live with the javascript hole. I don't even have user names: Anyone can download any file, but only people that know a password should be able to view and it should happen without fuss.
Aug
16
comment PBKDF2 and salt
Would it be inappropriate to use PBKDF2 for both authentication and password stretching (assuming different salts)?