You can only rehash if you have the plaintext password

Drupal 7 used such a technique for upgrades from Drupal 6, migrating from MD5 to a salted sha512 hash, but it's not an uncommon technique.

The old passwords are stored as MD5 hashes in the databases. The MD5 hash is processed through the same techniques as new passwords: a salt and the new sha512 hash. Provide a way to identify whether the origin was a password, or an MD5 hash.

Either way, you end up with a hash. You can identify whether the origin was a password, or an MD5 hash, but you can neither determine the origin MD5 hash, nor the origin password, as the new hash is secure. So even if the original MD5 hash was insecure, the new hash is secure.

When someone attempts to login, you still need to determine which password-validation to use: hash = sha512(salt + password), or hash = sha512(salt + MD5(password)), but the security level is the same.

Passing the password through MD5 reduces the complexity to 128 bits, you can't get that back.

So the security level is not the same, though it may be resistant to some attacks on MD5.

And it's probably not important for most people, since there are less than 2^56 eight character ASCII passwords.

  > "Passing the password through MD5 reduces the complexity to 128 bits, you can't get that back."

Attacks against MD5 are based around knowing the original MD5 hash. In this instance, the original MD5 hash is unknown, so there is no mathematical shortcut to finding a collision.

The attacker needs a password with a specific hash, and the best reported attack for that is around 2^128.

Personally, I'm willing to chance that my password will be discovered via a brute-force attack within the next 0.65 billion billion years [1]

[1] http://bitcoin.stackexchange.com/questions/2847/how-long-wou...

A new preimage attack could be discovered - or might already have been, secretly.

No, this is not the problem with MD5. You are not going to find two user-memorizeable-and-typeable passwords with an MD5 collision.

If you are bringing a password with more than 128 bits of complexity to the party, any password storage scheme better than plaintext will have your password safe.

Collisions are a problem for digital signatures, not for passwords.

But some people do want and use more than 2^128 bit passwords, for whatever reason, and an MD5 intermediate stage limits that.

Somewhere in the organization, a product team is going to throw a fit about usability and churn over the decision to reset user passwords en masse, or to force users to change them when they first log in. This isn't a slight against product managers, but one of the clearest indications of a company's overall security culture "health" is how the security, engineering and product teams choose to compromise and "pick their battles." Risk accepting vulnerabilities has a legitimate place when you have to balance product development and usability, but so does pushing back on egregious issues.

I don't have privileged insight into Yahoo's organization, but in this case it's pretty clear the security team should have either been more diligent in conveying the ramifications or less kneecapped by the surrounding org units, depending on the circumstance. More importantly, Yahoo should have "migrated" their passwords in the manner a parallel comment explains in this thread. This is what Facebook and other companies did after maturing their security programs (see "Facebook Onion" on how Facebook transitioned away from MD5).

Also good to note - there is evidence Yahoo's security culture improved over the years. The decision to go with MD5 almost certainly happened in the 90s, and when Tumblr suffered a breach all users were forced to reset their passwords. The capability and awareness was clearly there.

The password "foo" may encrypt to the hash "12345". If an attacker were to discover that the hash is "12345", they would look for a password that hashes to "12345", which could, hypothetically, be the password "bar". They don't know the original password "foo", they've simply discovered an alternative, which happens to match the algorithm enough to unlock access.

In general, rainbow tables are used for identifying and attacking common passwords, but that doesn't mean that the algorithm is insecure.

Insecure algorithms can be attacked through collisions, which don't necessarily give you the original password, they just provide an alternative password which is accepted by the algorithm. The distinction matters when it comes to password reuse, because if Site A uses MD5, but Site B uses sha512, finding a collision that grants access on Site A doesn't necessarily give you a password that will grant access on Site B.