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I want to upgrade the security of some existing databases of users' authentication tokens strictly for the purpose of making sure that if the database is stolen, attackers will not be able to guess any but the weakest passwords in a reasonable period of time. I know there's not much I can do for people who choose "password" for their password, but I think we can protect most people who use words from a bigger dictionary, e.g. "regenerating".

Some of the databases are from the 90's and the existing authentication tokens are stored as

 MD5(salt + password)

and some of the salts are pretty weak. Other databases use somewhat better hashes and salts, but still not strong enough to resist attacks from today's computers. However, we don't want to wait until people login to update their database entries. So my plan is to simply take the output of whatever hashing algorithm the password system is currently using and to feed it into bcrypt and store that result as the new authentication token, e.g.

 bcrypt(MD5(salt + password), new_secure_salt)

Does anyone see a weakness with this procedure? It seems to me that this will not be any worse than using bcrypt(password, new_secure_salt) but I know better than to rely on just myself for the evaluation of a cryptographic protocol.

I wouldn't recommend this procedure for new applications, but this method is the best I can come up with for immediately securing an entire database of insecurely stored passwords in a way that is transparent to users.

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Looks perfectly fine to me. – CodesInChaos Jun 17 '12 at 21:56
Do you mean? original_salt + bcrypt(MD5(original_salt + password), new_secure_salt) – Hendrik Brummermann Jun 17 '12 at 22:26
For the scheme to work at all, the original salt must be derivable from something stored. It is best if the original salt is a subfield of new_secure_salt: that insure MD5 collisions can't be turned into salt collisions; but even without that, yes bcrypt(MD5(salt + password), new_secure_salt) seems to be an improvement in every respect, as most of the entropy in password remains in MD5(salt + password). However I would recommend scrypt over bcrypt; see in particular table 1 of the paper defining scrypt. – fgrieu Jun 18 '12 at 7:11
@IlmariKaronen: done. The text expanded greatly :-) – fgrieu Jun 18 '12 at 15:53
up vote 13 down vote accepted

The overall idea is a sound migration strategy. The nice thing is that security for all users is upgraded in a one-shot operation, rather than at the hypothetic next login of each user. Of course, original salt and new_secure_salt shall be stored, or perhaps for some portion derivable exactly from material keyed-in by the user, e.g. user name lowercased.

Using bcrypt(MD5(salt + password), new_secure_salt) is a huge improvement over MD5(salt + password): the risk of password cracking for a given password entropy is greatly increased by the many iterations (and some use of memory) in bcrypt. Using MD5(salt + password) rather than password discard negligibly little password entropy for typical passwords (who chooses 20 random characters among 68 as password?)

However I would recommend scrypt over bcrypt. Scrypt uses the memory available during legitimate use of the password in order to further increase security; have a look at this table from the paper defining scrypt:

Estimated cost of hardware to crack a password in one year

The single weakness I can think of relates to MD5 collisions on salt. If it was possible for an adversary to choose salt and get the resulting authentication token with the same new_secure_salt a few times, it would become possible to recover the first few characters of the password, following the line of thought in Practical key-recovery attack against APOP, an MD5 based challenge-response authentication. That's likely mostly theoretical in the context, where salt turns out to be non-malleable. To rule out any possibility of that attack, it would be enough that salt be included in new_secure_salt.

The above theoretical attack exploits that it is feasible to find two 63-byte 127-byte salt0 and salt1 such that MD5(salt0 + 'e') = MD5(salt1 + 'e') with collision in the first two 64-byte MD5 blocks. Now assume we could get the authentication tokens for these salt0 and salt1; if the password starts with e, then these tokens match! And the converse is true with near certainty. This allows to construct a test for the first character of the password, and recover it in a moderate number of queries, then proceed with the second, and perhaps third character; after that, finding the collisions seem to become quite difficult.

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Thanks! I like scrypt, but it's too new for me to trust right now and with this scheme we can migrate to scrypt later. In all the systems I'm looking at, an adversary could at best predict, but not choose, the old salt and has no control over the new salt (securely random and unique per user), so the attack would not be practical. – Major Major Jun 18 '12 at 22:49
I trust the scrypt algorithm is free of any flaw, and appreciably safer than bcrypt: the security argument is simple, strong, and peer-reviewed. I have no informed opinion on scrypt implementations w.r.t. bcrypt. Also I find scrypt's definition a tad complex for my taste, and not entirely free of typo. In my work field (Smart Cards and HSMs), both bcrypt and scrypt suffer from using an exotic primitive (Blowfish for bcrypt, Salsa-20 for scrypt) with no hardware support, thus slow, thus much less secure than achievable; and also of using a hash in a mode requiring DPA/DFA resistance. – fgrieu Jun 19 '12 at 12:54
I submitted an edit to this answer but it was rejected. If you will make the changes I'll accept this answer. All I'm asking is that you re-order the statements so that they go from most practical to least. Start with that the proposal seems like in improvement, then recommend scrypt as a better solution, then discuss the theoretical attack. Please also note that using a random new_secure_salt that changes each time would protect against that attack, since that is the recommended usage of bcrypt. – Major Major Jun 22 '12 at 6:53
@fgrieu isn't being slow part of the point of bcrypt and scrypt? – ZoFreX Jul 16 '14 at 22:00
@fgrieu Could you explain that Table 1 in simple terms for me please? If I derive a work factor for bcrypt that say takes 1000ms on hardware x and derive another work factor for scrypt that takes also 1000ms on hardware x, then how is one hash more hardware expensive than the other? – cottsak Apr 8 at 8:49

One potential issue with this strategy revolves around compliance. You might be subject to various regulations (such as Government/DoD regulations) that prohibit unsafe hash functions. You might have a very unpleasant experience trying to explain to the audit team how using MD5 as part of user authentication does not mean "the terrorists win!"... In the best case you'll get a knowledgeable audit team that can separate checkbox security from actual security, but it will be very stressful for you (they don't really care about false positives as much as false negatives). In the worst case, they'll take away your authorization to operate - "just to be safe", of course.

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One potential consideration is that combinations of security principles may have unintended vulnerabilities. This is not to say they you cannot combine such approaches, but most combinations have been well studied and then recommended by trusted 3rd parties (academic or governmental or institutional).

While not in the same domain has hash functions, the Meet-in-the–middle attack is one example of unintended consequences when combining security primitives. For example, how much weaker is the hash function if the salt + password is known for any combination?

So what does one do with this information. You need to assess the approach your proposing which has a relatively unknown exposure profile to the one you do know, which you, rightly, identified has known issues. Are you willing to trade the operational simplicity for the uncertainty of a novel approach? As an implementer, you'll be able to assess what works the best for your situation or at least express the tradeoffs both approach bring.

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