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Re: encrypt password by md5 twice?


On Oct 10, 2007, at 6:41 PM, Zhang Weiwu wrote:

Dear everyone

I am planing to migrate an Intranet info system to authenticate with
OpenLDAP, so more of our business can be done with the same login. The
old system uses their own SQL table to store user information, no
problem, I can write a script to convert to LDIF format. But md5 was
used to encrypt user password, and the developer of that system knows
md5 is cracked, so he encrypted the md5 hash with md5 method again.

clear text password --> md5 hash --> md5 hash of the md5 hash

My question:

   1. Have you ever heard this solution to avoid md5 crack? Now as I
      cannot reach the original system author, I wonder how this idea
      come to be (e.g. why not using SHA).

Yes, but generally with more rounds than 2... and often with a salt. It's called key strengthening.
(See http://en.wikipedia.org/wiki/Key_strengthening).

2. Does it work? (is md5 hashed md5 hash much safer with no side- effect?)

md5(md5(p)) requires twice as many cycles as md5(p) and hence is a minor hinderance to those attempting dictionary attacks. The more rounds, the greater the hinderance. But without a salt, it's subject to pre-computed dictionary attacks. While it 2*md5 pre-computed dictionaries are likely rare, 1*md5 pre-computed dictionary aren't. And they could be used to save the attacker half the work in cracking a 2*md5 hash.

3. Now, how we can migrate this system to use openldap. 

Implement a custom hash comparator module.

I would recommend you design it to perform any number of rounds as determined from the value of userPassword. For instance, one could have a value such as {MD5:N}base64encodingOfHash. Then in migrating the data, I'd add some many more rounds, but not too many. One has to balance the server performance requirements with the attack mitigation requirements. Such an approach allows one to add more rounds later on, which is a plus.

For more general purpose, I'd suggest a {SSHA256:N} as a replacement for current set of seeded/unseeded SHA1 and MD5 hash schemes (or possibly s/256/512/). I note that {SMD5:N} or {SSHA1:N} might be useful in environments that have key length restrictions.

AFAIK
openldap have no direct support for such hash. There are a lot of
users of the system and there will be problems if migration is
done and everyone's password is reset..

Thanks in advance!

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