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| Authored by: Winter on Wednesday, May 29 2013 @ 05:32 AM EDT |
"they are difficult to convert back as the hash functions are of a
many-to-one type of function - many originals map into the same hash-value: this
is what makes them [effectively] one-way."
As I understood, an ideal message digest functions convert a block the length of
the output into a new bit pattern one-on-one. That is, running an N bit hash in
reverse would return an N bit block of bits. For N bit messages, this might be
reversible in theory. That is why cryptographic hash algorithm tend to use
functions, e.g., a modulus over prime factors, that when run in reverse have
been chalked down as "difficult" by mathematicians, i.e., "NP
hard". Which is another way to say the function cannot be run in reverse.
http://diovo.com/2011/01/why-it-is-hard-to-break-hash-functions/
Indeed, when a message is longer than the N bits of the output hash, you lose
information. For a x*N bit message, there are 2^[(x-1)*N] (2 to the power of)
originals for each digest (hash). If N=128 (16 byte) and x=4 (64 bytes), you end
up with 2^384 ~ 4 × 10^115 possible messages per hash.
However, this is the ideal. Practice is different. There seem to be tricks to
coach MD5 to get fitting inputs on easier terms. MD5 is called broken for that
reason.
But in a password cracker, you do not need to find the real original sequence.
Any sequence that generates the hash will do. It is a pity most password forms
simply do no accept full binary sequences.
In the end, it is easier to simply try out all reasonable inputs. That is what
password crackers do.
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Some say the sun rises in the east, some say it rises in the west; the truth
lies probably somewhere in between.[ Reply to This | Parent | # ]
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| Authored by: Anonymous on Wednesday, May 29 2013 @ 05:39 AM EDT |
But is the conversion "mathmatical"?
My impression is that the conversion is discovered by guessing -- creating
passwords, putting them through a hash function, then comparing the results.
This isn't a mathmatical (reverse hash) function.[ Reply to This | Parent | # ]
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| Authored by: Anonymous on Wednesday, May 29 2013 @ 05:53 AM EDT |
| that would explain the relative ease of finding a long password. [ Reply to This | Parent | # ]
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| Authored by: Christian on Wednesday, May 29 2013 @ 04:32 PM EDT |
| "They are difficult to convert back as the hash functions are of a
many-to-one type of function - many originals map into the same hash-value: this
is what makes them [effectively] one-way.
Not exactly. Being
"many-to-one" is what makes it a hash, rather than an encryption. "One way"
means there is no known way to easily recover the original text (or a possible
original text). If you have the key, decrypting a 3DES encrypted text takes
about the same amount of time as the original encryption. In contrast, there is
AFAIK no way faster than brute force to find a text that has a particular SHA256
hash. [ Reply to This | Parent | # ]
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