The long tail of MD5

Yeah a couple years ago I was trying to determine whether to support Content-MD5 in some HTTP software I was writing I could find nobody actually using it. There was an attempt a decade ago to make Firefox validate with it, but that died: https://bugzilla.mozilla.org/show_bug.cgi?id=232030

Also the author is wrong that there isn't a proposed replacement -- it's the "Digest:" header (RFC 3230) although I don't think anybody uses that either.

donavanm• 12 years ago

Id hazard the most prolific use case of md5 as http checksum is AWS S3. You can, IIRC, send content-md5 on PUT and POST and theyll reject with a 400 if the body hash doesnt match. Conversely the default etag is the body md5.

JoachimS2• 12 years ago

Good article that really shows why design decisions can have impact 20, 30 years or more.

This is why I get fairly upset when people design something new I n 2014 that uses md5. Yes even if your application does not use md5 for anything security related, the mere fact that you use a bad, slow algorithm should be considered wrong. And adding new dependencies will make it harder for us to migrate away and extend that tail even further.

If you need a good hash function that is fast, use SipHas. If you need a really secure one, use SHA-512, SHA-3, BLAKE or just SHA-256.

pbsd3• 12 years ago

Do NOT use SipHash as a cryptographic hash function. It is designed to be a PRF, and its output length is way too small to make it collision-resistant when used as a hash. SHA-3, SHA-512/256 or BLAKE(2) (in increasing order of performance) are suitable cryptographic hash functions.

lisper3• 12 years ago

I think you meant: it [SipHash] is NOT designed to be a PRF.

Andys• 12 years ago

Slow? I thought md5 was faster than the SHA-* family

WhitneyLand• 12 years ago

Yes, way faster. It would be great if it weren't for those pesky collisions.

jonstewart• 12 years ago

MD5 also remains quite popular for file hashing/hash set analysis. I.e., is this file a member of a known set? For example, NIST's NSRL still includes MD5 hashes.

* This is not an endorsement of MD5 for such use cases.

smackfu• 12 years ago

Do the security problems with MD5 make it bad as a hashing algorithm?

tkmcc• 12 years ago

Not necessarily, but if that's your use case (and the hashing speed isn't extremely critical), why not use a better algorithm with a larger output?

derobert• 12 years ago

Depends. Can an attacker introduce files? If so, he can probably introduce collisions. In most uses of a hash, that's an issue—a performance (DoS) issue if nothing else.

If the application was just looking for a longer CRC/Adler32/etc. and not depending on the hash to be strong against attack, then MD5 wasn't an appropriate choice in the first place, as its much slower than need be. But the security problems are irrelevant.

ChuckMcM• 12 years ago

Generally the tuple (<length>,<hash>) is unique since the attacks on MD5 all involve changing the number of octets in the hashed source. That said, it isn't secure if you can change the length independently of changing the hash. Hence the challenge of using it cryptographically. If you're confident you know the correct 'length' value (acts as a sort of openly shared secret in this case) then you trust the hash.

ketralnis• 12 years ago

Not unless hash collisions are also a security issue (see http://bugs.python.org/issue13703). But it's much slower than a traditional hashtable hashing function and not much slower than a sha family hash.

Maybe that's the sweet spot for your project, but I think most non-security uses of MD5 are because it already exists in most languages and is pretty universal

dredmorbius• 12 years ago

Do you know the contexts your hashing requirement is likely to be used in, and/or may be adapted to?

The worst and most persistent security problems emerge when someone defends half-assery as acceptable because, take your pick, it's a quick hack / it's a personal project / it's a small project / it's not used for critical infrastructure.

Until eventually is one or more of the above are violated.

The primary advantage of MD5 is that the hashes are (slightly) shorter than those of other checksum methods. This makes it slightly more convenient to manually compare or transmit hashes.

My problem is that I happen to have used md5sum so often and for so long that it's wired into my own wetware and muscle-memory, and I'm not sure which of the alternative SHA sums I should use, and which of those are widely available. I honestly don't know the answer to that off the top of my head, and "what to use instead of md5sum" as a DDG or Google search doesn't turn up a clearly useful answer. "sha1 sha256 sha384 sha512 which to use" does better.

And locally I've got utilities for SHA 1, 224, 256, and 512 installed, that I can tell.

Looks as if SHA-2 and SHA-3, which include keylengths of 224-512, are considered secure:

http://en.wikipedia.org/wiki/Secure_Hash_Algorithm

Then there are some openssl utilities. But let's not go there.

MagicWishMonkey2• 12 years ago

Are collisions really that much of a concern (other than for security reasons)? IIRC they are still incredibly rare even with older hashing algorithms like MD5.

nly• 12 years ago

Collisions render hash functions useless as a component of most (but not all) digital signature algorithms

ekimekim• 12 years ago

Yes, if you don't care about collisions* or being able to reverse the hash (a pre-image attack) then there's nothing wrong with MD5 per se. However, MD5 was designed as a secure algorithm, and makes tradeoffs, mainly in speed, to that effect. If you don't need security, you should use something designed for that, like CRC.

* note that even if it isn't immediately obvious, in most situations you really don't want collisions to be an easy thing to cause. For example, many algorithms and structures involving hashes (even a simple in-memory hash map) suffer massively degraded performance when there's a large number of collisions. If your user can craft an input that causes operations to take orders of magnitude longer, that's a denial of service attack.

stubborndude• 12 years ago

Was going to make this point. But lets instead talk about the endorsement aspect, because lots of people in the forensic community take the "its popular so i use them, but i neither endorse nor oopose" position.

There is certainly risk of hash collisions in files, but most attacks require you to generate two different files with the same hash? Do people think a pre-image attack is feasible? If feasible, at what scale? What type of actor can give me a file that has different content, but the same hash as winword.exe. If they can, what is the likelihood that said file will also be executable or contain a linkable library

*similarly, and hypocritically, not an endorsement of md5

IvyMike2• 12 years ago

Want to kill MD5?

Provide public-domain easy-to-compile/use versions for all languages, and furthermore, get their google page ranks high.

Do not underestimate laziness. If Joe Random can find a suitable MD5 algorithm in 10 seconds but it takes 30 seconds to find a suitable SHA algorithm, guess which one gets used?

awalton• 12 years ago

Honestly, this is not so much a problem. Cryptographers like to write simple C public domain implementations of their algorithms and then people go to work for months and years squeezing performance out of them, cryptanalyzing them, recoding them in various languages and releasing their own implementations under new licenses. You can see this happening with pretty much every newly designed crypto component from hashes to ciphers to entire crypto systems.

The problem is exactly the one elucidated in the blog post above: the long tail of baked in brokenness. These systems were never designed to be extensible, they are cooked into code that hasn't been serviced in years, perhaps even decades in some cases. They're bolted into specifications in ways that either obsolete the technology completely, or make it so incredibly complicated to update the technology that doing so outweighs the apparent cost. And that's without considering problems like deployment and phase-out.

These types of problems make it very likely we will be stuck with the stupidity of DES and MD5 in strange places until it becomes a fire drill and then all the sudden people will be baking in SHA-1/SHA-2 or BLAKE2 and we'll be going through these very same motions again in 5-15 years, wondering why we didn't learn from the mistakes we made last time.

nathan7• 12 years ago

These days, I find using MD5 absolutely inexcusable. SHA-1 is available pretty much everywhere, SHA-2 algorithms are quite readily available too. BLAKE2 (https://blake2.net) is slightly faster than MD5, and significantly more secure. It's based on the SHA-3 finalist BLAKE. Additionally, there are versions of the algorithm that parallelise using SIMD, and there are both 32-bit and 64-bit optimised versions too. It also gives you the possibility of customising your hash, using it as an HMAC at no extra cost, salting it, or adding a personalisation key to effectively have different hash functions for different purposes. As a nice extra, it also uses a third less RAM than SHA-2 or SHA-3. If output length is a concern, truncating the output (with a corresponding increase in the likeliness of collisions) is perfectly fine with any good hash function.

w8rbt• 12 years ago

HMAC MD5 is a perfectly reasonable choice for some situations.

astrodust5• 12 years ago

A situation like not caring.

The long tail of MD5

Related Stories