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# Thread: Discussion: Will an md5 Value ever repeat?

1. ## Discussion: Will an md5 Value ever repeat?

Ok, this is something that I have thought about before, and never really thought about any more until the other day.

If you use md5 to encrypt information, you get your md5 value returned... great... but are there any limits to the number of different inputs that can be encrypted with md5.

Basically, if enough combinations of md5 values were encrypted, would you eventually hit a repeat? (im talking alot).

• As input for this, I am going to say that "yes", it will repeat eventually... becuase with the limit of characters used for md5, there is more possible input combinations that could be used, and therefore, a repeat could be achieved...

Also, if anyone knows of any written resource regarding this, please do post it up.

• Also the MD5 function in the php by default (not sure if there's anothe option) is 32 characters long, whereas the original hash spits out 64.

The character range is a-f and 0-9.

To get more information on how the hash was designed, you can look it up on the MIT website under the guy who wrote it, I believe it was Ron Livest.

Consider the following, however. The MD5 creates a numeric representation of the contents of a message and displays it as a 16 character hexadecimal value. Theoretically, there are 2^128 possible MD5 hash values. Therefore, if you have a file system with 2^128+1 different files on it I can guarantee you that there are at least two different files that will generate duplicate hash values.

Consider the reality of 2^128 possibilities. There are actually 3.402 x 10^38 or 340 billion billion billion billion or a little more than 1/3 of a googol possibilities. When you consider that most people have never seen a million of anything the actual number becomes really difficult to conceptualize. (Leave it to my nine year old to teach me that a googol, which really exists, is actually 10^100.)

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