OSNews

siimo,

"SHA1 is a one way hash like MD5. So can't get passwords from it, its pretty much useless to the hacker.

All the google results are pointing to the same hacked paste dump. How do you figure that they are not salted from that?"



It may not be possible to reverse the SHA1 hash *directly*, but it is possible to enumerate password combinations to build a reverse index.

Attackers have databases of enumerated passwords (many gigabytes to terrabytes in size) which are organized to allow one to effectively look up the original text for any SHA1 hash which was enumerated during the generation of the database.

I found an interface to one such database here:
http://www.hashcrack.com/

aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d -> "hello"

Note that you have to hash values without a CR:
> echo -n "hello" | sha1sum

If you have an SHA1 hashed password, it is very likely you can obtain the original password.


edbd1887e772e13c251f688a5f10c1ffbb67960d -> "secretpassword"


The difference with "hash salting" is that each uniquely salted password would need it's own index of cleartext->hash values. So at the very least an attacker would have to generate a new hash database from scratch for each unique salt instead of using a pre-existing database.

However even salting alone is insecure since building the reverse index is feasible using free software. A single desktop processor can try over 10M hashes/sec. That's nearly a trillion per day. A cluster (or running on amazon S3) will crack a salted password in very little time.

The solution is to make the the hash function much slower by applying it recursively. Therefor it takes far more resources to generate the reverse index. However it cannot be too slow otherwise it will effect the legitimate use of the hash.

Edited 2011-07-05 05:01 UTC