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    Assymetric Crypto Question

    Hi all, first post.

    I am trying to find a little info behind the background workings of typical Assymetric Encryption (EG rsa and PGP).

    As I am currently picking up on crypyto theory prior to deploying it in an app, I was wondering if someone had any answers to the following questions:

    1) Do small changes to the pre-encrypted message result in large changes in the cyphertext a la hashing, or are the changes in cyphertext limited to only part of the cyphertext? I am unsure as to how the algorithms work, do they encrypt an entire message or do they encrypt a part of the message limited to the assymetric key length?


    longstring(bitlength of 3072) * F(Public key encryption bitlength 1024) = cyphertext(3072)

    Or does it result in this

    longstring(bitlength of 3072) * F(Public key encryption bitlength 1024) = Cyphertext(1024) & Cyphertext(1024) & Cyphertext(1024)?

    assuming the second example above is valid,


    IF you had hold of both the cyphertext and the part of the message corresponding to the cyphertext, how easily would it be to generate the private key from this information and are there any equations out there that can rapidly generate private keys from knowing the cyphertext and the original message from which the cyphertext was generated?

    Thanks again in advance.
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    Devshed Specialist (4000 - 4499 posts)

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    1. Depends on the mode of operation of your cipher.

    2. What you're describing is a known plaintext attack (or even chosen plaintext attack)
    Good algorithms are highly resistant to such attempts.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper

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