RE: Basic Questions about PKI

["David Gillett" <gillettdavid () fhda edu>]

  Yes, yes, and yes.

  For authentication, it's sufficient to sign the message
digest with your private key, but you could about as easily
encrypt the whole message with your private key -- the 
difference being that people without encryption software
couldn't read it in the latter case.  Think of it as an
alternative way to SIGN the message.

  In symmetric crypotosystems, the ENCRYPT and DECRYPT
algorithms are functional inverses when used with the same
key.  In asymmetric systems like PKI, there is a single
algorithm for both operations, and pairs of keys are used 
which are one another's arithmetical inverses under that
algorithm.  It's not that there's an ENCRYPT and a decrypt,
as that applying either key gets you a ciphertext to which
application of the other key will recover the plaintext.

  Trivial example:

A. Symmetric implementation
  The key is "2", the ENCRYPTION algorithm is "add" and
the DECRYPTION algorithm is "subtract".

B. Asymmetric implementation
  The ALGORITHM is "add" and the keys are "+2" and "-2".

  NOTE:  The problem with "add" as an algorithm for such
an asymmetric implementation is that the derivation of the
second key from the first key is *trivial*.
  The challenge of modern cryptography is to find algorithms 
where the algorithm implementation is relatively frugal, but 
the derivation of one key from the other is sufficiently 
expensive to be impractical.  It's success at making this 
operation expensive that allows one of the keys to be made
public.
  Cryptographers are gradually finding or building better
and better candidate algorithms -- and, to keep themselves
honest, better and better key-derivation systems.

David Gillett


> -----Original Message-----
> From: Roger A. Grimes [mailto:rogerg () cox net]
> Sent: October 7, 2003 15:43
> To: security-basics () securityfocus com
> Subject: Basic Questions about PKI
>
>
> Can someone that knows PKI cold confirm my knowledge of PKI?
>
> Here's what I think I know about PKI (accurate or not I'm not sure):
>
> a.  People ENCRYPT messages to me with my PUBLIC key and send 
> the encrypted
> message to me, and only I can open the encrypted 
> message...because ONLY my
> PRIVATE key can decrypt messages encrypted with my PUBLIC key.
>
> b.  If I want to SIGN a message, I use my private key to sign 
> the message
> digest (ENCRYPTING the hash result).  The receiver who wants 
> to rely on my
> signed message uses my PUBLIC key to DECRYPT my encrypted 
> message digest.
>
> c.  Both private and public keys can decrypt, and both 
> private and public
> keys can encrypt.  It just depends on the situation of what 
> we use when.
>
> Is that logic correct?
>
> Could we encrypt messages that we want to send to others with 
> our private
> key (but don't because if we did anyone with our public key 
> could read) the
> seemingly private message?
>
> Roger
>
>
> **************************************************************
> **************
> ****
> *Roger A. Grimes, Computer Security Consultant
> *CPA, MCSE (NT/2000), CNE (3/4), A+
> *email: rogerg () cox net
> *cell: 757-615-3355
> *Author of Malicious Mobile Code:  Virus Protection for 
> Windows by O'Reilly
> *http://www.oreilly.com/catalog/malmobcode
> *Author of upcoming Honeypots for Windows (Apress)
> **************************************************************
> **************
> *****
>
>
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