cryptography - What are the important points about cryptographic hash functions? -

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I was reading on MD5 hash values ​​and the accepted answer confuses me one of the main properties, as I understand I have a cryptographic hash function, it is unable to find two different messages (input) with the same hash value.

Yet the consensus answer question is why MD5 hash values ​​are not reversible? is because an infinite number of input strings will produce the same output. This seems completely contradictory to me.

In addition to this, I suffer from some degree to the fact that the algorithms are public, yet the hash values ​​are still immutable. This is because there is always a loss of data in a hash function, so there is no way to tell which data was removed?

What happens when the size of the input data is smaller than the specified output data size (such as the password "ABC" hashing?)

edit:

Ok, I have to see that this is straightforward:

  1. It is really hard to guess input from hash < Strong> Because the input strings have an infinite amount that will generate the same output (irreversible asset).
  2. However, too many input strings generating the same output are also really hard (collision-resistant properties) find .

You may be confused, because is of Confusing answers One requirement for the cryptographic hash function is that it should be preimage resistant. That is, if you know the MD5 (X) but the message is not x, then it is difficult to get any X (either equal to x or x), such as MD5 (x ') = MD5 (x).

Being a primes-resistant, there is a different asset than being reversible. A function is reversible if y = f (x) is equal to an x ​​that fits (whether it is easy or not). For example define F (x) = x mod 10. Then there is no reversal. With F (x) = 7, you can not determine whether x 17, 27 or something else is there. But F. Premiege is not resistant because it is easy to get value x 'like F (x) = 7. X '= 17, 27, 12341237 etc. All work.

When you do crypto, you usually need functions that require preimage resistant (and other properties like collision resistance), and not something that is not reversible.


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