[Cryptography Team] Hash function properties

[scal rahotguy]

On 5/3/06, Ron Teitelbaum <Ron at usmedrec.com> wrote:
>
>  Hi Scal,
>
>
>
> Can you give us some background of what you are doing?  Is this a school
> assignment?
>
>
>

yes. it is.

  If it is then maybe you could give us an idea of what you do understand
> and what you do not, so that we can help either with a discussion or
> suggestions for other reference material.
>
>
>
yes. certainly for example in (b). I do not think I understood the question.
Eve knows the hash function and I also assumed she also knows the private
key of  Bob. How can anybody knows once she sends a message with Bob's
signature? How can hash funtion's property help here? Can somebody throw
light here?

thanks
Akhil


  Ron Teitelbaum
>
>
>   ------------------------------
>
> *From:* cryptography-bounces at lists.squeakfoundation.org [mailto:
> cryptography-bounces at lists.squeakfoundation.org] *On Behalf Of *scal
> rahotguy
> *Sent:* Tuesday, May 02, 2006 11:17 PM
> *To:* Cryptography at lists.squeakfoundation.org
> *Subject:* [Cryptography Team] Hash function properties
>
>
>
>
>
> For each of the following applications of hash functions, explain which
> of hash function properties (preimage resistant, second preimage resistant
> and collision resistant) are needed and which are not.
>
>
> (a) Cryptographic signatures are produced by computing a hash of a
> message, then applying a signature function to the hash of the message.
> Suppose Eve has a list of messages m1,...mn, and their signatures computed
> using Bob's signing key, but does not have Bob's signing key. Assuming that
> the signature function is not susceptible to attack, it should not be
> possible for Eve to present Bob's signature on any message other than
> m1,...mn.
>
> (b) Suppose that Eve works for a Certificate Authority. She does not have
> access to the special harware that computes digital signatures, but she
> knows the hash function. In addition, Eve can get messages signed, but every
> message that is signed automatically goes into a log file that Eve cannot
> change. Eve should not be able to produce a certificate signed by the
> Certificate Authority that does not appear in the log file.
>
> Anybody can explain these to me?
>
> thanks
> Akhil
>
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