ZyanWu

What's a MD5 hash???

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“Hell begins on the day God grants us a clear vision
of all that we might have achieved,
of all the gifts which we have wasted,
of all that we might have done which
- we did not do -”

bakuryu

MD5

MD5 (Message-Digest algorithm 5) is a widely used cryptographic hash function with a 128-bit hash value. An MD5 hash is typically a 32-character hexadecimal number.

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ZyanWu

And how do i use it to verify files?

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“Hell begins on the day God grants us a clear vision
of all that we might have achieved,
of all the gifts which we have wasted,
of all that we might have done which
- we did not do -”

Strider

Every file will have a unique MD5 hash. Change in even a single byte of the file will change the MD5 value. So it can be used for verifying file integrity.

Download a freeware MD5 calculator here:
MD5 Calculator

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ZyanWu

So... why can't we compress huge files into one string and use it instead of the big one?

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“Hell begins on the day God grants us a clear vision
of all that we might have achieved,
of all the gifts which we have wasted,
of all that we might have done which
- we did not do -”

bakuryu

have u read the algorithm ?? it's not such a easy task

Strider

It's not about compressing. It's a unique checksum of all the bytes of the files calculating through a complex MD5 hashing algorithm. That value can assure you that the file is original or malformed.

eg. For the file MD5.exe the unique hash value is 7480640e9e85b3b9de7afe4f0c400d5e.

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Night_virus

thnx all re....
i didnt know tht...

anniepoo

"So... why can't we compress huge files into one string and use it instead of the big one?"

MD5 is an example of a hash code. Suppose you're going to keep track of student records for a few hundred students in a school using file folders and paper (gasp!)

To make it quicker to find records, you set up a file cabinet with those divider things and file each students folder under their last name in the appropriate
cubbyhole. If you have 200 students you have an *average* of 8 folders per cubbyhole.

But when you try this you discover that something's wrong. There are lots of P's, R's, and V's, but not many 'nya's' , 'zh's - so you're still looking through lots of folders to find students named Varma.

So instead you make some complicated crazy function. Exactly what it is isn't important, only that, when you stick a name in it gives a letter back that is randomly distributed in the alphabet. And every time you put in the same name, you get the same letter out. Call that function F

Now, put the folder for Varma in whatever letter F("Varma") is

We call F a hash function.

Now, after we put all the folders in their new locations, we have an equal distribution of items in the cubbyholes.

That's one of many cool uses of a hash function. Another one is figuring out whether two things are equal. Suppose Strider decides that no two people can use the same avatar on Techtalkz. That'd be really computationally intensive to check by brute force.
Here's a fast way to do it.
Construct a hash function that maps an image into a number. Lets say it maps into 10 bits. Then we have an array bucket[1024]. We look through all the images. Compute the hash code and look in the bucket[hachscode(image)]. Now, it might be that two different images have the same hashcode. But it's pretty easy to check the few images in that bucket. If non are identical, we add this one to the bucket, and go to the next image.
Java, for example, keeps a hash code for each object, and when you do
foo.equals(bar), it checks whether the hash codes are equal (few caveats here, this isn't an essay on Java hashcodes)

So, the MD5 digest is a hash code whose input is a whole file and whose output is some hex number.
Suppose D1pSh1T the cracker wants to act cool by causing you grief. He gets on dad's computer and writes a program that erases your hard drive or something. He also writes some stupid little utility that shows JPG's or something, and uploads this to tucows. Tucows decides it's not a virus and puts it on their site, with a pointer to D1pSh1T's web page.
Now D1pSh1T puts the destructive Trojan where the innocent utility was. You download it, run it, and your day is ruined.
How to protect yourself?
Tucow, when they decide 'Super JPG viewer' isn't a Trojan Horse, computes the MD5 hash, and publishes this along with the link to where you can download it. Lets say it's 2342AE239800AC1244FF1136CA1288FF

ZyanWu

Yea... i've got it... it's just like the compiler sums up 2 and 3... gets 5... but the decompiler doesn't know how to put the bytes back: 0 and 5 / 1 and 4 / 2 and 3 etc... i getit!
Also... this means that not every file has it's unique hash... 2^32640 files can have the same MD5 hash... but a different content... doesn't this mean that with a little bit of work a MD5 calculator can be tricked?