MD5 To Be Considered Harmful Someday

Effugas writes "I've completed an applied security analysis (pdf) of MD5 given Xiaoyun Wang et al's collision attack (covered here and here). From an applied perspective, the attack itself is pretty limited -- essentially, we can create 'doppelganger' blocks (my term) anywhere inside a file that may be swapped out, one for another, without altering the final MD5 hash. This lets us create any number of binary-inequal files with the same md5sum. But MD5 uses an appendable cascade construction -- in other words, if you happen to find yourself with two files that MD5 to the same hash, an arbitrary payload can be applied to both files and they'll still have the same hash. Wang released the two files needed (but not the collision finder itself). A tool, Stripwire, demonstrates the use of colliding datasets to create two executable packages with wildly different behavior but the same MD5 hash. The faults discovered are problematic but not yet fatal; developers (particularly of P2P software) who claim they'd like advance notice that their systems will fail should take note."

Comments

md5 vs sha1 vs ?

[alexandre]

http://en.wikipedia.org/wiki/Md5

here is a very good link about the algo... :)

The "Detailed Summary"

[Effugas]

[This is the author]

I've been doing some analysis on MD5 collision announced by Wang et al. Short version: Yes, Virginia, there is no such thing as a safe hash collision -- at least in a function that's specified to be cryptographically secure. The full details may be acquired at the following link:

http://www.doxpara.com/md5_someday.pdf

A tool, Stripwire, has been assembled to demonstrate some of the attacks described in the paper. It may be acquired at the following address:

http://www.doxpara.com/stripwire-1.1.tar.gz

Incidentally, the expectations management is by no means accidental -- the paper's titled "MD5 To Be Considered Harmful Someday" for a reason. Some people have said there's no applied implications to Joux and Wang's research. They're wrong; arbitrary payloads can be successfully integrated into a hash collision. But the attacks are not wildly practical, and in most cases exposure remains thankfully limited, for now. But the risks are real enough that responsible engineers should take note: This is not merely an academic threat, systems designed with MD5 now need to take far more care than they would if they were employing an unbroken hashing algorithm, and the problems are only going to get worse.

Some highlights from the paper:

* The attack itself is pretty limited -- essentially, we can create "doppelganger" blocks (my term) anywhere inside a file that may be swapped out, one for another, without altering the final MD5 hash. This lets us create any number of binary-inequal files with the same md5sum.

* MD5 uses an appendable cascade construction -- in other words, if you happen to find yourself with two files that MD5 to the same hash, an arbitrary payload can be applied to both files and they'll still have the same hash. This leads to...

* Attacks are possible using only the proof of concept test vectors released by Wang -- the actual attack is not necessary.

* Stripwire emits two binary packages. They both contain an arbitrary payload, but the payload is encrypted with AES. Only one of the packages ("Fire") is decryptable and thus dangerous; the other ("Ice") shields its data behind AES. Both files share the same MD5 hash.

* Digital Signature systems are vulnerable, as they almost always sign a hashed representation of data rather than the data itself.

* This is an excellent vector for malicious developers to get unsafe code past a group of auditors, perhaps to acquire a required third party signature. Alternatively, build tools themselves could be compromised to embed safe versions of dangerous payloads in each build. At some later point, the embedded payload could be safely "activated", without the MD5 changing. This has implications for Tripwire, DRM, and several package management architectures.

* HMAC's invulnerability has been slightly overstated. It's definitely possible, given the key, to create two datasets with the same HMAC. Attacker possession of the key violates MAC presumptions, so the impact of this is particularly questionable.

* Very interesting possibilities open up once the full attack is made available -- among other things, we can create self-decrypting executables (fire.exe and ice.exe) that exhibit differential behavior based on their internal colliding payloads. They'll still have the same MD5 hash.

* Several doppelgangers may (relatively quickly, as per Joux) be computed within a single multicollision-friendly block. As such, the particular selection of doppelganger sets within a file can itself be made to represent data. It's relatively straightforward to embed a 128 bit signature inside an arbitrary file, in such a way that no matter the value of the signature, a constant MD5 hash is maintained. This is curiously steganographic.

* Many popular P2P networks (and innumerable distributed content databases) use MD5 hashes as both a reliable search handle and a mechanism to ensure file integrity. This makes them blind to any sign

Re:In english

[Anonymous Brave Guy]

Short version: A common technology for verifying that a file you've downloaded is legitimate and untampered-with, known as MD5, isn't as secure as people thought.

Slightly longer version: MD5 is a way of generating a checksum -- a single, comparable value -- from a file. Ideally it is supposed to give you different numbers for different files, so if a web site advertises the checksum a file should have, you can compare that with one generated from the file you actually got to see whether the file you've downloaded has been modified, potentially maliciously.

The research shows that it is possible for someone to construct a drop-in replacement for the file you thought you had that generates the same MD5 checksum as the original, so anyone attempting to validate the file this way would think they had the real thing. If it turns out that you can construct a damaging replacement for a common file -- perhaps an installer for a popular application like Firefox or OpenOffice that's usually downloaded from a public server -- then this could open a loophole for viruses, worms, etc. that would slip through the security net often used by cautious people when downloading such programs.

You're wrong.

[Piquan]

He can create a file that MD5sum's to the same result as a legitimate file, but does not have full control over the content or size of the result (making this a mostly useless avenue of exploitation except for people who want to spread trash on P2P networks -- I.E. it shouldn't particularly bother anyone except people who already don't care about security).

Suppose you're storing passwords as encrypted hashes, so that intercepting the hashes doesn't tell you what the password is. But if you can generate a password to match that MD5...

You know that GPG keys are identified and signed by their MD5 hashes? Suppose that I can generate a GPG key that would be identified as yours, and distributed it.

Or he can create two files that MD5sum to the same result. But he has to have control over both files, which offers effectively no advantage to someone who is trying to spread malware or tamper with existing archives that have been MD5summed.

There's a coin-flipping protocol that goes as follows. Suppose that Alice and Bob want to flip a coin (over the Internet), but they don't trust each other.

1. Alice generates a file with random data.
2. Alice sends Bob the MD5 hash of the file.
3. Bob picks a bit in the file, and whether he thinks it's a 0 or a 1.
4. Bob wins if and only if he picked right.
5. To verify, Alice sends Bob the file she generated at the beginning.

Now, suppose that Alice generated multiple files in step 1. When Bob makes his guess, she tries to pick a file that will make her win. If she generated only two files, completely randomly, this would let Alice win 75% of the time.

These are just the first ideas I thought of. If I were looking for other problems, I'd think about undeniable signatures, keysigning (which as GPG and X.509 SSL are heavily based on) and other specialized signature systems. In particular, I expect that the first type of crack could cause issues with SSH keys, both user keys (used for authentication) and host keys (to prevent man-in-the-middle attacks).

Digital signatures are used for much more than just testing for file tampering.

Re:Two files with the same md5 hash?

[Phleg]

See you in the next life.