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# Public Key Encryption Question

Posted on 2006-10-29

I'm looking for confirmation of the following understanding of public key encryption and signing:

Given another party's public key, and your own public/private key pair, and a plain text message: Is it possible or impossible (or computationally infeasible), to generate the cipher text message/string that would decrypt into the intended plain text message?

That's confusing, I know. So here's a simplistic, concrete example:

Given Party A and Party B, each with a key pair and each other's public keys.

Suppose a plain text message of: 1234567890

And suppose that if Party A encrypted this message using his key pair and Party B's public key, he ended up with cipher text of "4tqgh9wetffeqwiu".

Could Party B, starting with the plain text of 1234567890, and using his own key pair and party A's public key, predict (or generate) the ciphered message that Party A would send (4tqgh9wetffeqwiu), which, when decrypted, would result in that unciphered message of 1234567890?

Intuition tells me no, that this would invalidate the security provided by signing a message, since someone could spoof a ciphered message and say it came from you. But I'm not clear on the difference between a ciphered message and the accompanying signature, so would like confirmation of the above.

Thanks!