Security Basics mailing list archives
RE: Encryption question
From: Hollis Johnson <hollis () cisco com>
Date: Wed, 25 Feb 2004 16:23:29 -0800
Tony. One more point. The public/private keys are a pair. Defining one defines the other. Or rather, as you defined the pair, there is a relationship.
I don't have the algorithm memorized (although many on this alias might) -- and no ref. books at hand. But it has to do with the relationship of 2 "very large" (I recall 100 digits) primes to which you ..... I don't remember.
But that's the answer to your last sentence -- they are a pair -- I msg encrypted with one part (public or private) can only be decrypted via the compliment.
encrypt(public) -> decrypt(private) assures confidentiality encrypt(private) -> decrypt(public) verifies the sender (authentication) - both assuming the private key is not compromised. I refer you to: http://oregonstate.edu/dept/honors/makmur/The challenge of public-key cryptography is developing a system in which it is impossible to determine the private key. This is accomplished through the use of a one-way function. With a one-way function, it is relatively easy to compute a result given some input values. However, it is extremely difficult, nearly impossible, to determine the original values if you start with the result. In mathematical terms, given x, computing f(x) is easy, but given f(x), computing x is nearly impossible. The one-way function used in RSA is multiplication of prime numbers. It is easy to multiply two big prime numbers, but for most very large primes, it is exremely time-consuming to factor them. Public-key cryptography uses this function by building a cryptosystem which uses two large primes to build the private key and the product of those primes to build the public key.
If you want to know more there are a number of lovely books you can spend your saturday nites reading :-)
Hope this helps as well. Hollis At 12:45 PM 2/25/2004 -0500, Gene LeDuc wrote:
Alice encrypts the message to Bob using Bob's public key and then signs it using her private key. Bob verifies that the message is from Alice by using her public key to check the signature and then decrypts the message with his private key. The encryption only hides the contents, it does not authenticate the message. The signing authenticates the message but does not hide the contents. You need both if you want to have a secure conversation. -----Original Message----- From: Preston, Tony [mailto:Tony.Preston () acs-inc com] Sent: Tuesday, February 24, 2004 11:01 AM To: security-basics () securityfocus com Subject: Encryption question Tony Preston Systems Engineer, AS&T Inc. Division of L3 Corporation (609) 485-0205 x 181 I have what is a rather basic question... I probably am missing something so I thought I would ask here. Alice and Bob both have a public and private key. Alice encrypts her email to Bob using his public key. Sends the email and Bob decrypts it using his keys.. Since both Bob and Alice's public keys are known, Why can't I take Alice's public key and create a key pair using any other private key. Now, I fake an electronic signature from Alice using the pair I created and send a bogus encrypted message to Bob with my "fake" Alice signature. Bob checks the signature by using the public key and it is valid. Bob assumes the message is from Alice... What prevents me from spoofing someone's electronic signature this way? --------------------------------------------------------------------------- ---------------------------------------------------------------------------- --------------------------------------------------------------------------- ----------------------------------------------------------------------------
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Current thread:
- Re: Encryption question, (continued)
- Re: Encryption question Lars Georg Paulsen (Feb 26)
- Re: Encryption question Jamie Pratt (Feb 25)
- RE: Encryption question Burton M. Strauss III (Feb 25)
- Re: Encryption question Aaron Keck (Feb 25)
- Re: Encryption question Theo Chaojareon (Feb 25)
- Re: Encryption question Raghu Chinthoju (Feb 27)
- RE: Encryption question Gene LeDuc (Feb 25)
- Re: Encryption question SERGIO OTERO (Feb 25)
- RE: Encryption question Jordan, Jason D. "Dallas" (Feb 25)
- RE: Encryption question Prasad S. Athawale (Feb 26)
- RE: Encryption question Hollis Johnson (Feb 26)