143 reputation
5
bio website echogtgt.wordpress.com
location Rome, Italy
age 48
visits member for 2 years, 1 month
seen Mar 30 '13 at 13:57
Old school programmer, I still prefer C over other languages.

Jun
2
asked Using compression to test encryption
Apr
8
awarded  Scholar
Apr
8
accepted Twofish key length
Apr
6
asked Twofish key length
Mar
22
awarded  Supporter
Mar
22
comment Checking a key before actually using it
@Grant: Imagine that the nature of the data is such that, once decoded, the user couldn't tell if it's correct or not before trying to use it (and failing in the attempt). I prefer to avoid the user having to fail to understand that the key was incorrect, I'd like to be able to warn him beforehand.
Mar
21
awarded  Editor
Mar
21
comment Checking a key before actually using it
@Henrick: I've added some more info to better explain what is the scenario. I'm not clear what the user can do to guess the key, apart from trying all the 2^128 keys.
Mar
21
revised Checking a key before actually using it
added 1313 characters in body
Mar
21
comment Checking a key before actually using it
@HenrickHellström: Yes we may assume that the user is an adversary. It's not a license scheme, just a block of data that needs to be decrypted at a given time. If they modify the cipher text, what would this imply?
Mar
21
comment Checking a key before actually using it
The key has to be provided once by the user and is used only to crypt the secret file (and the check file). 128 bits can be expressed with a string of 26 characters like ABBCD-EFGHK-12345-LMNOP-XYZ12-3 which is not too bad to me. I can't imagine other scenarios except the one you mention: the adversary has no access to the key but (might have) read access to the crypted file. What else could go wrong?
Mar
21
awarded  Student
Mar
21
asked Checking a key before actually using it
Mar
21
awarded  Autobiographer