MD5 Tutorial

Discussion in 'Engineering Concepts' started by pradeep, May 24, 2005.

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What is MD5 or MD5sum?

Its a formula - a way to take a message of an arbitrary length, and create a 128-bit "fingerprint" or "message digest" of the message. MD5 is a way to verify data integrity. On these forums, it comes up fairly often in discussions about storing user passwords and other sensitive data.

Is MD5 encryption?

No. It is simply a one-way fingerprint of the message. It doesn't include the original message, and you can't (generally) use the fingerprint (the md5sum) to 'figure out' the original message.

Okay, so you take a message - like a password - and generate an MD5sum from it.. Can't you brute-force that?

Like any password system, you could attempt to brute force the answer. However, MD5sum's are in a 128-bit space, meaning that to brute force it would take 2^128 attempts - thats over 3 with 38 zeroes after it.

Neat! Thats a lot. Are there any flaws in the algorithm that could speed it up?

A birthday attack is based on the theory that there *might* be *one* md5sum that matches multiple inputs. In theory, it is possible that a "birthday" attack could be possible - two md5sum hashes could be the same. But even then, the total number of brute forces is at 2^64 attempts - still a heck of a lot.

Okay. But couldn't (insert super-sneaky government agency here) build an md5 dictionary, and know what the password was with the md5?

Yes. Its entirely possible. However - it would take some work to do so. For example, just for a dictionary consisting of Alphabet letters (upper and lower), and numbers, there would be 46,656,000,000 entries - all at 32 characters each. Thats over 1 terabyte of data to store and search! It could be done - absolutely. But is it likely?

So its hard to brute force, what about dictionary attacks?

Dictionary attacks are a way of attacking poor passwords - most people use words in their passwords. If you can guess the word - for example, "love", then you can cut down the number of tries it would take. Of course if you guess right, then your # of attacks = 1. However, in general, using common computers as of the writing of this (2005), you can generally get roughly 5 million attacks per second, or fast enough to guess all 8-character Alphanumericals within 497 days.

Thats pretty strong - but is there anything stronger?

A similar method is SHA1 - a more secure 160-bit hashing algorithm. That makes it *much* more secure against brute-force, birthday attacks, and other forms of assault. There are yet more hashing algorithms that are even stronger - but MD5 and SHA1 are both natively supported in the latest PHP, and should be sufficient for most projects.

Allright - I'm sold. Tell me how to use it to store passwords and check them

There are three things we are protecting against - the stored passwords, the transmission of the passwords, and the replay of the password. Each is very different. Lets start with the stored password. We need to take a password, and store it in a variable. Then we need to check that variable against what the user entered:
PHP:
``` \$secret_password = md5("password");   if (md5(\$_POST['password']) == \$secret_password) {     echo "Correct password";   } else {       echo "Incorrect password"; }  ```
Simple enough. However, the password is being sent cleartext in \$_POST['password']. Which brings us to another thing to protect against - the cleartext transmission. Thankfully, there is an opensource (GPL'd) javascript MD5 implementation available online which can be found here. If you use that javascript library to md5 the password before sending it, the server code would look like this instead:
PHP:
``` \$secret_password = md5("password");   if (\$_POST['password'] == \$secret_password) {     echo "Correct password";   } else {       echo "Incorrect password"; }  ```
Again, fairly simple. However, I mentioned the other problem - replay attacks. If someone could manage to 'sniff' the connection, and capture the md5sum, they could simply use that to login!

The solution to that can be very complex and involved - the same site for the javascript md5 function goes into great detail discussing how to implement a truly secure solution. It's called a "CHAP" login system, and here is a link to his page on it - including complete working PHP and javascript code to implement it.

MD5 is a very useful means to protect user's passwords online - if used correctly. Its not encryption, but it does help prevent whole databases of passwords being compromised.

2. AhmedHanNew Member

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Thanks for the splendid info.

Do you know if there is any Win32 API or some stuff like that that generates MD5 code. I mean, I am looking for a function which is corresponding to the MD5() function of PHP, for using in C++. A DLL would be nice.

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In dot net you can use the Cryptography interface to do it for you but in Win32 if you use the dotnet compiler the SDK can help you out but probably need the 3rd party DLL or Some SDK

Here is the MSDN link for the same MD5

4. BecksboxNew Member

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Hi

Is it somehow possible to reverse engineer md5 hashes ?
I mean, i just found http://md5.rednoize.com and i wondered how this site works.

5. AhmedHanNew Member

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Its not possible to reverse engineer MD5 hashes, but what you can do is, make a database of of MD5 hashes and their corresponding words,and when you want to lookup what the hash orginally was generated from, just lookup the hash with the database to get the corresponding word.
But its not feasible to create a database of all possible strings.

7. ReekenXNew Member

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Thanks. But I md5 is not safe in my opinion. It is possible that two or more words hashed will have the same value of md5?

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The probability of a collision is negligible.

9. kush_2207New Member

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Rainbow attacks have become the major threat for md5 and is put to use to crack md5 hashes....md5 would be gone in few years...SHA-1 and SHA-256 (its successor) would be put into practice more often....

10. AhmedHanNew Member

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What is a rainbow attack?

11. kush_2207New Member

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Rainbow tables store all the predetermined hashed along with the corresponding text.....They signify the trade-off between memory and performance.....
Rainbow tables are of 50-100 GiB (Depending on the amount of information it stores) and can help you find the plaintext of the hash within seconds....
Link -> http://www.antsight.com/zsl/rainbowcrack/ This will give you a better idea....You can google it and find alot.

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Yeah! That's right!

13. AhmedHanNew Member

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I knew that method, but I didn't know its name was "rainbow attack".
You say 50-100 GB but I heard that the tables had sizes which are in units of TB, and only a few supercomputers in the world have it.

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No, servers these days support TB of storage. All major RDBMS's support data files in TB!

15. codestormNew Member

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I think MD5 has been succeeded by SHA1, what say?

16. blacklight332New Member

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A lot of people use PHP for md5(), and when they use a C++ implementation it doesn't always return the same hash.

This implementation's hashes match that of php:

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You mean the link in the signature?

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