Is MD5 considered insecure?

Question

After all these articles circulating online about md5 exploits, I am considering switching to another hash algorithm. As far as I know it's always been the algorithm of choice among numerous DBAs. Is it that much of a benefit to use MD5 instead of (SHA1, SHA256, SHA384, SHA512), or is it pure performance issue?

What other hash do you recommend (taking into consideration data-bound applications as the platform)? I'm using salted hashes currently (MD5 salted hashes). Please consider both md5 file hashes and password hashes alike.

Answer 1

MD5 for passwords

Using salted md5 for passwords is a bad idea. Not because of MD5's cryptographic weaknesses, but because it's fast. This means that an attacker can try billions of candidate passwords per second on a single GPU.

What you should use are deliberately slow hash constructions, such as scrypt, bcrypt and PBKDF2. Simple salted SHA-2 is not good enough because, like most general purpose hashes, it's fast. Check out How to securely hash passwords? for details on what you should use.

MD5 for file integrity

Using MD5 for file integrity may or may not be a practical problem, depending on your exact usage scenario.

The attacks against MD5 are collision attacks, not pre-image attacks. This means an attacker can produce two files with the same hash, if he has control over both of them. But he can't match the hash of an existing file he didn't influence.

I don't know if the attacks applies to your application, but personally I'd start migrating even if you think it doesn't. It's far too easy to overlook something. Better safe than sorry.

The best solution in this context is SHA-2 (SHA-256) for now. Once SHA-3 gets standardized it will be a good choice too.

Answer 2

To complete @CodesInChaos' answer, MD5 is often used because of Tradition, not because of performance. People who deal with databases are not the same people as those who deal with security. They often see no problem in using weak algorithms (e.g. see the joke of an algorithm that MySQL was using for hashing passwords). They use MD5 because they used to use MD5 and are used to using MD5.

Performance is much more often discussed than measured; and yet, logically, there cannot be a performance issue if there is nothing to measure. Using one core of a basic CPU, you can hash more than 400 MBytes per second with MD5, closer to 300 MB/s with SHA-1, and 150 MB/s with SHA-256. On the other hand, a decent hard disk will yield data at an even lower rate (100 to 120 MB/s would be typical) so the hash function is hardly ever the bottleneck. Consequently, there is no performance issue relatively to hashing in databases.

The usual recommendations, for hash functions, are:

1. Don't do it. You should not use elementary cryptographic algorithms, but protocols which assemble several algorithms so that they collectively provide some security features (e.g. transfer of data with confidentiality and integrity).

2. Really, don't do it. For storing passwords (more accurately, password verification tokens), don't make a custom mix of a hash function and salts; use a construction which has been studied specifically for such a use. This normally means bcrypt or PBKDF2.

3. If a hash function is indeed what does the job, then use SHA-256. Consider using any other function only if some serious problem with SHA-256 (most probably its performance) has been duly detected and measured.

Answer 3

I'm using salted hashes currently (MD5 salted hashes).

If you are salting MD5 hashes, you definitely don't want to be using MD5. It sounds like you need to use PBKDF2 or bcrypt.

As far as I know it's always been the algorithm of choice among numerous DBAs.

That's not a compelling reason.

I have worked with a lot of DBAs that are at least 5 years behind in general technology (not using version control, unformatted perl scripts for everything, etc). They might have been particularly bad DBAs, but I think it comes with the extremely conservative mindset of not changing things.

Answer 4

Just to complement the answers already given (most of which are excellent) we now have a real world example of where a data breach (Ashley Madison) lead to the entire password table being leaked. They used bcrypt with a random salt to hash the passwords. A security researcher decided to take those hashes and brute force them. This was the result

As a result of all this, bcrypt is putting Herculean demands on anyone trying to crack the Ashley Madison dump for at least two reasons. First, 4,096 hashing iterations require huge amounts of computing power. In Pierce's case, bcrypt limited the speed of his four-GPU cracking rig to a paltry 156 guesses per second. Second, because bcrypt hashes are salted, his rig must guess the plaintext of each hash one at a time, rather than all in unison.

"Yes, that's right, 156 hashes per second," Pierce wrote. "To someone who's used to cracking MD5 passwords, this looks pretty disappointing, but it's bcrypt, so I'll take what I can get."

Pierce gave up once he passed the 4,000 mark. To run all six million hashes in Pierce's limited pool against the RockYou passwords would have required a whopping 19,493 years, he estimated. With a total 36 million hashed passwords in the Ashley Madison dump, it would have taken 116,958 years to complete the job.

At the end of the day, the only ones he was able to crack were ridiculously simple or common passwords (like "123456").

Answer 5

Yes MD5 is insecure and so is SHA-1, I recommend using SHA-256 if size of the digest is an issue. Remember, if you store it into a BINARY column, it will take less space that if stored into CHAR. Just make sure it is done properly. MD5 is a about 2.3x faster than SHA-256. More benchmarks are at http://www.cryptopp.com/benchmarks.html

Answer 6

To put it short it is pretty insecure now because of rainbow tables, Rainbow table is a list of MD5 hashes and their matching strings. So basically i would consider other alternatives such as SHA1

Answer 7

You are all talking about insecurity, but none of you has given a straight anwser to the question.

I have been working with MD5 and Sha512, both of them where easy to crack once I had put my little hacker expert on it, until it both hit us like a thunder! The most simple and effective way to stop "Mass Attacks" like the ones described above, is setting a limiter of login attemps, meaning :

function checkbrute($user_id, $mysqli) {
 // Get timestamp of current time
   $now = time();
   // All login attempts are counted from the past 2 hours. 
   $valid_attempts = $now - (2 * 60 * 60); 

   if ($stmt = $mysqli->prepare("SELECT time FROM login_attempts WHERE user_id = ? AND time > '$valid_attempts'")) { 
  $stmt->bind_param('i', $user_id); 
  // Execute the prepared query.
  $stmt->execute();
  $stmt->store_result();
  // If there has been more than 5 failed logins
  if($stmt->num_rows > 5) {
     return true;
  } else {
     return false;
  }
}
}

You get my point?
The maximum amount of combinations the hacker wants to try, he is limited to 5 fails per 2 hours. You can even block the user after X attempts.