Why is Md5 insecure?

Answer 1

Technically

The algorithm has a greater chance of hash collisions than other, similar hashing algorithms. This means it is less computationally expensive to crack than similar ones (such as SHA-1) and is thus less secure.

For the Layman, briefly:

MD5 is insecure because computer attackers have figured out a way to trick the MD5 security system into verifying that no tampering has occurred, when in fact the attacker has injected malicious code. It's like purchasing a lock for your bike that has a 5 buttons. Pressing one button will open the lock. The criminal can just try every possible combination in a short time, and thus the lock is considered insecure.

Why is MD5 Insecure? Technical answer:

Bob owns some web space on blue host, and he uploads an executable there that he intends other people to download and run so they can install his video game: angry_birds_installer.exe.

Bob uploads angry_birds_installer.exe to blue host and has allowed other people to download it through the browser and install the game so they can install/play it. Bob generates profit by people buying the unlocked version of the game, therefore Bob wants to make absolutely sure that the attacker doesn't use the installer to install malware on the customers who install the game.

Description of the security problem:

Bob's executable is super-insecure now because suppose an attacker breaks in to his blue host account and somehow gets the ability to make a change to angry_birds_installer.exe. The attacker could drop in the new executable, and nobody is the wiser. Now, users who install angry_birds_installer.exe also install Conficker, a malicious bot-net on the customer's computer.

Why would an attacker go to all this trouble?

Answer: profit. The attacker wants to get in between Bob and his customers so he can install Conficker. With that, he can do the following things:

• Do Distributed Denial of Service attacks to take down the internet in other countries by taking out the name servers and DNS servers.
• Launching trillions of spam emails.
• Acquire credit card numbers and incur charges via identity theft.
• Draining bank accounts of people who don't check their balance often.
• Many other brilliant illegal for-profit ideas to freeze your soul.

Bob has a lot to be worried about on behalf of his customers who don't know the difference between browser security, antivirus software and a hole in the ground.

Bob increases security with MD5 Sums

Bob has a plan, he will protect his executable by taking an MD5 check sum on angry_birds_installer.exe and store the result somewhere else (say on his go daddy web space) that has a different security system. The security conscious users who want to run angry_birds_installer.exe will also run an MD5 check sum against the executable they downloaded, and make sure that the check sums exactly match. If they do match, we can be relatively certain that the attacker has not modified the original angry_birds_installer.exe.

Security problem solved?

No. Mal-ware creators are clever, they were able to figure out that MD5 is insecure. They were able to find a way to break the algorithm for MD5 in order to run trillions of MD5 sums every second and find a "hash collision" between the original angry_birds_installer.exe and the modified: angry_birds_with_malware.exe. The attacker only has to compromise the blue host security and can drop-in the new executable that generates the SAME MD5 sum as the original, and the users who do check sums declare that the executable is safe, when it contains malware to install Conficker.

MD5 sum is insecure because Bob thinks he is protecting the users from evil mal ware creators, but in reality, the mal ware creators have figured out a way to completely bypass the security model. MD5 Sums do not deliver any significant security enhancements as it was originally envisioned to be.