I too was skeptical, but the author actually reversed the email addresses for half of the entries in the leak in 3 minutes.

The second to last paragraph in the article says: “With my Nvidia RTX 3080 it only took 3 minutes and 17 seconds to process. Hashcat was able to recover 51.81% (972,933) of the hashes.”

MD5 was intended to be a message digest hash, so its primary function was to provide non-collidable checksums. (Edit: which it does poorly in an adversarial environment so is now retired for security uses.)

Unlike hashing algorithms designed to be hard to reverse, MD5 has a very fast algorithm, so rainbow tables are fast and easy to create.

A public list of emails only speeds up the process and is not required as you could just build a completely random rainbow table. Prioritizing common email patterns in the table also is just another speed boost.

This is probably going back 20+ years, but given old tools like john the ripper, newer tools like ighashgpu, or just writing your own iterative tool to go through [a-z][A-Z][0-9]@<populardomain>, and the advent of high speed CPUs and GPUs, you'd be able to generate rainbow tables of millions of md5 hashes in a few hours/days/weeks (depending on length and complexity of course).

So yes, literally every security expert out there knows md5 hashes are weak, hence the advent of salted hashes, and nowadays sha and variants, as well as well-known hash types for passwords like bcrypt/argon2.

In 2011 I attempted to reverse email MD5s published by stackoverflow. I recovered ~28% of 95k unique email hashes using a slow CPU based hasher, by trying simple patterns based on common names and providers. I did not use any already known email addresses for this.

https://meta.stackexchange.com/a/84734/152255

I think a bit later somebody else did a similar attack on a different dataset, and recovered about 50% using a GPU based hasher (a GPU can burn through billions of MD5 hashes per second).

https://arstechnica.com/information-technology/2013/12/crypt...

Not many people have unique addresses.

ETA: the author isn't claiming to have found a preimage attack, but brute force is a legitimate tactic when the search space is constrained, and I'd argue it counts for claiming that those particular hashes were reversed.

You can do many billion MD5 hashes a second and there are a few common suffixes @gmail.com @outlook.com etc.

The search space is mostly lowercase and maybe . or +

So it literally isn’t a reverse lookup but it’s still pretty broken.