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Quote from: Geezer on 06/02/2011 04:01:45Encryption is over rated. If the transmission of the key could have been intercepted by anyone other than the transmitter and the receiver, it's pretty much worthless.You should read about Bob and Alice ... QuotePublic-key cryptography ... does not require a secure initial exchange of one or more secret keys to both sender and receiver.http://en.wikipedia.org/wiki/Public-key_cryptography [Links inactive - To make links active and clickable, login or click here to register]

Encryption is over rated. If the transmission of the key could have been intercepted by anyone other than the transmitter and the receiver, it's pretty much worthless.

Public-key cryptography ... does not require a secure initial exchange of one or more secret keys to both sender and receiver.

Bob and Alice have separate padlocks. First, Alice puts the secret message in a box, and locks the box using a padlock to which only she has a key. She then sends the box to Bob through regular mail. When Bob receives the box, he adds his own padlock to the box, and sends it back to Alice. When Alice receives the box with the two padlocks, she removes her padlock and sends it back to Bob. When Bob receives the box with only his padlock on it, Bob can then unlock the box with his key and read the message from Alice. Note that in this scheme the order of Decryption is the same as the order of encryption; this is only possible if commutative ciphers are used.

No keys sent in this method ...QuoteBob and Alice have separate padlocks. First, Alice puts the secret message in a box, and locks the box using a padlock to which only she has a key. She then sends the box to Bob through regular mail. When Bob receives the box, he adds his own padlock to the box, and sends it back to Alice. When Alice receives the box with the two padlocks, she removes her padlock and sends it back to Bob. When Bob receives the box with only his padlock on it, Bob can then unlock the box with his key and read the message from Alice. Note that in this scheme the order of Decryption is the same as the order of encryption; this is only possible if commutative ciphers are used. http://en.wikipedia.org/wiki/Public-key_cryptography#A_postal_analogy [Links inactive - To make links active and clickable, login or click here to register]

The key to Bob's lock can be deduced from the difference between the versions of the message that he did and didn't lock.

Vladimir Romanov has released source code for an algorithm which he claims can solve 3-SAT problems. The 3-SAT problem is NP-complete - and Romanov claims his algorithm will solve in polynomial time, this would prove that P==NP as all NP problems can be mapped within polynomial time to the satisfiability problem. With such a seemingly easily falsifiable claim Romanov might be proved wrong quite quickly - for any with the maths and compsci skill here is http://romvf.wordpress.com/2011/01/19/open-letter/ [Links inactive - To make links active and clickable, login or click here to register] Romanov's announcement and links to the source code and article - and for those who need a bit more background http://science.slashdot.org/story/11/01/20/1546206/Polynomial-Time-Code-For-3-SAT-Released-PNP [Links inactive - To make links active and clickable, login or click here to register] is a link to a long slashdot ramble that has some good stuff in it

Even if you determine the key Bob used you still can’t read the message Alice sent him.

Quote from: Geezer on 06/02/2011 19:54:45The key to Bob's lock can be deduced from the difference between the versions of the message that he did and didn't lock.Encryption algorithms are very carefully designed so that, while this can be done in theory, in practice it would take far too long (hundreds or thousands of years).

It depends on the cryptographic system; that's known as a 'known plaintext attack' but knowing a single message that is encrypted isn't, for many modern crypto systems, going to allow you to determine the decryption key, but there are some for which that is sufficient or at least necessary, for example the world war II Enigma was fairly vulnerable to it.Used correctly, the RSA public key encryption systems isn't vulnerable to it; although there are attacks known as 'chosen plaintext attack's which can permit you to crack it.

Well, if you read the book, you'll find that your point is incorrect.

Ah, right, thanks!So what happens if you have two versions of the message - one that is encoded with a key, and one that is not?