Re: Cryptographic Functions

[Steve Friedl <steve () unixwiz net>]

On Wed, Aug 19, 2009 at 08:31:33AM +0200, M.D.Mufambisi wrote:
> Ok. Thanks. I have an SHA-1 hash of a file and the digest is
> DA39A3EE5E6B4B0D3255BFEF95601890AFD80709. Is this160 bit? How does the
> output map to 160 bits?

That hash is 40 characters, and since each hex digit is 4 bits,
40 x 4 = 160 bits.

This might help:

        An Illustrated Guide to Cryptographic Hashes 
        http://unixwiz.net/techtips/iguide-crypto-hashes.html

Steve


> On 8/18/09, Shailesh Rangari <shailesh.sf () gmail com> wrote:
> > Hi Munyaradzi,
> >
> > On Tue, Aug 18, 2009 at 2:02 PM, Jeffrey Walton <noloader () gmail com> wrote:
> >
> > > Hi Munyaradzi,
> > >
> > > > When a passphrase is used a key in symetric
> > > > cryptography, how does the pass phrase map to
> > > > the key in an algorithm like AES
> > >
> > > The passphrase should be derived using a KDF. KDFs includes salts and
> > > iteration counts. Quite a few bodies offer guidance on KDFs - NIST,
> > > RFC, IETF, and ANSI to name a few.
> > >
> > > > how many letters correspond to 1 bit?
> > > Don't know what you are asking here. The KDF should provide sufficent
> > > 'mixing' such that no information can be gained from 1 bit of output
> > > (either 1 or 0 is equally probable). Otherwise, its not a very good
> > > KDF.
> >
> >
> > I second that.
> > Also, assuming that a strong Hash Function is being used, then it is
> > difficult to ascertain how many letter(s) would correspond to 1 bit - for
> > one of the essential properties of a Hash Function is that it takes in an
> > 'Arbitrary' length of input(key, passphrase, message, etc) and converts it
> > into a 'Unique', 'Fixed' length output (hash). A Key Len of 128, 256, 512
> > Bit if hashed with SHA-1, then the output would necessarily be 160 Bits
> > only.
> >
> > > Jeff
> > >
> > > On 8/18/09, M.D.Mufambisi <mufambisi () gmail com> wrote:
> > > > Hello people.
> > > >
> > > > 1. When a passphrase is used a key in symetric cryptography, how does
> > > > the pass phrase map to the key in an algorithm like AES? ie....how
> > > > many letters correspond to 1 bit? etc?
> > > >
> > > >
> > > > Regards
> > > >
> > > > Munyaradzi Mufambisi
>
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-- 
Stephen J Friedl  | Security Consultant |  UNIX Wizard  | 714 694-0494
steve () unixwiz net | Orange County, CA   | Microsoft MVP |  unixwiz.net

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Prove to peers and potential employers without a doubt that you can actually do a proper penetration test. IACRB CPT 
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