Ttyjukl

I read an article about password schemes that makes two seemingly conflicting claims:

MD5 is broken; it’s too slow to use as a general purpose hash; etc

The problem is that MD5 is fast

I know that MD5 should not be used for password hashing, and that it also should not be used for integrity checking of documents. There are way too many sources citing MD5 preimaging attacks and MD5s low computation time.

However, I was under the impression that MD5 still can be used as a non-cryptgraphic hash function:

1. Identifying malicious files, such as when Linux Mint's download servers were compromised and an ISO file was replaced by a malicious one; in this case you want to be sure that your file doesn't match; collision attacks aren't a vector here.


2. Finding duplicate files. By MD5-summing all files in a directory structure it's easy to find identical hashes. The seemingly identical files can then be compared in full to check if they are really identical. Using SHA512 would make the process slower, and since we compare files in full anyway there is no risk in a potential false positive from MD5. (In a way, this would be creating a rainbow table where all the files are the dictionary)

There are checksums of course, but from my experience, the likelihood of finding two different files with the same MD5 hash is very low as long as we can rule out foul play.

When the password scheme article states that "MD5 is fast", it clearly refers to the problem that hashing MD5 is too cheap when it comes to hashing a large amount of passwords to find the reverse of a hash. But what does it mean when it says that "[MD5 is] too slow to use as a general purpose hash"? Are there faster standardized hashes to compare files, that still have a reasonably low chance of collision?

But what does it mean when it says that "[MD5 is] too slow to use as a general purpose hash"? Are there faster standardized hashes to compare files, that still have a reasonably low chance of collision?

BLAKE2 is faster than MD5 and currently known to provide 64-bit collision resistence when truncated to the same size as MD5 (compare ~30 of that of MD5).

There's not a compelling reason to use MD5; however, there are some embedded systems with a MD5 core that was used as a stream verifier. In those systems, MD5 is still used. They are moving to BLAKE2 because it's smaller in silicon, and it has the benefit of being faster than MD5 in general.

The reason that MD5 started fall out of favor with hardware people was that the word reordering of the MD5 message expansions seems to be simple, but actually
they require a lot of circuits for demultiplexing and interconnect, and the hardware efficiencies are greatly degraded compared to BLAKE. In contrast, the message expansion blocks for BLAKE algorithms can be efficiently implemented as simple feedback shift registers.

The BLAKE team did a nice job of making it work well on silicon and in instructions.

A case where the use of the MD5-hash would still make sense (and low risk of deleting duplicated files):

If you want to find duplicate files you can just use CRC32.

As soon as two files return the same CRC32-hash you recompute the files with MD5 hash. If the MD5 hash is again identical for both files then you know that the files are duplicates.

In a case of high risk by deleting files:

You want the process to be fast: Instead use a hash function that's not vulnerable for a second hash of the files, i.e. SHA2 or SHA3. It's extremely unlikely that these hashes would return an identical hash.

Speed is no concern: Compare the files byte per byte.

MD5 is used throughout the world both at home and in the enterprise. It's the file change mechanism within *nix's rsync if you opt for something other than changed timestamp detection. It's used for backup, archiving and file transfer between in-house systems. Even between enterprises over VPNs.

Your comment that it "should not be used for integrity checking of documents" is interesting, as that's kinda what is done when transferring files (aka documents). A hacked file/document is philosophically a changed file/document. Yet if the adversary takes advantage of MD5's low collision resistance, file/document propagation through systems can be stopped. Carefully made changes can therefore go unnoticed by rsync, and attacks can occur. I expect that this is somewhat of a niche attack but illustrates the concept vis-à-vis MD5 being at the heart of many computer systems.