How come MD5 hash values are not reversible?

hash cryptography md5 cryptographic-hash-function
barfoon picture barfoon · Dec 1, 2008 · Viewed 34.8k times · Source

One concept I've always wondered about is the use of cryptographic hash functions and values. I understand that these functions can generate a hash value that is unique and virtually impossible to reverse, but here's what I've always wondered:

If on my server, in PHP I produce:

md5("stackoverflow.com") = "d0cc85b26f2ceb8714b978e07def4f6e"

When you run that same string through an MD5 function, you get the same result on your PHP installation. A process is being used to produce some value, from some starting value.

Doesn't this mean that there is some way to deconstruct what is happening and reverse the hash value?

What is it about these functions that makes the resulting strings impossible to retrace?

Answer

Serafina Brocious picture Serafina Brocious · Dec 1, 2008

The input material can be an infinite length, where the output is always 128 bits long. This means that an infinite number of input strings will generate the same output.

If you pick a random number and divide it by 2 but only write down the remainder, you'll get either a 0 or 1 -- even or odd, respectively. Is it possible to take that 0 or 1 and get the original number?

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