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From: Tony Johansson on 22 Apr 2010 12:35
Hi!

I'm reading in a book from microsoft press (exam 70-536) and there is
something that I don't fully understand what they mean.
I hope somebody could explain what this text mean. The part that I don't
understand is the last part of the text below and start from where it says
Therefore, reverse mapping databases have been developed. These . . .

I know what keyed hash is but I find the text very far from an explanation
of what keyed hash is.

"Even with the flaws in the MD5 hashing algorithm, both the MD5 and SHA1
hashing algorithms are considered secure for most uses, especially if a salt
value is added to the hash. However, most hashing algorithms are very well
known. Therefore, reverse mapping databases have been developed. These
databases allow a malicious user to attempt to reverse hash a hashed value
to extract the input value. As a result, a new family of hashing classes
evolved. This added an additional iteration to the transformation process,
which makes it more complex to extract an input value. This new family of
hashing algorithms is called the HMAC algorithms."

//Tony


From: rossum on 22 Apr 2010 18:31
On Thu, 22 Apr 2010 18:35:39 +0200, "Tony Johansson"
<johansson.andersson(a)telia.com> wrote:

>Hi!
>
>I'm reading in a book from microsoft press (exam 70-536) and there is
>something that I don't fully understand what they mean.
>I hope somebody could explain what this text mean. The part that I don't
>understand is the last part of the text below and start from where it says
>Therefore, reverse mapping databases have been developed. These . . .
>
>I know what keyed hash is but I find the text very far from an explanation
>of what keyed hash is.
>
>"Even with the flaws in the MD5 hashing algorithm, both the MD5 and SHA1
>hashing algorithms are considered secure for most uses, especially if a salt
>value is added to the hash. However, most hashing algorithms are very well
>known.
MD5 and SHA1 are cryptographic hashes. If you do not intend to use a
cryptographic hash in your application then you do not need this
stuff.

>Therefore, reverse mapping databases have been developed. These
>databases allow a malicious user to attempt to reverse hash a hashed value
>to extract the input value.
Look up "Rainbow Tables" in a cryptographic context. They are
precalculated by an attacker and can be used to attempt to de-hash a
password. Once calculated they greatly reduce the time needed to
attempt to reverse a hash.

>As a result, a new family of hashing classes
>evolved. This added an additional iteration to the transformation process,
>which makes it more complex to extract an input value. This new family of
>hashing algorithms is called the HMAC algorithms."
HMAC is resistant to many attack techniques that a simple hash is not
resistant to.

In general Wikipedia is a good source of basic cryptographic
information with good links to more detailed papers and definitions.

rossum

>
>//Tony
>

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