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Thread: Finding RSA Constants for keygenning...

  1. #1
    foxthree
    Guest

    Finding RSA Constants for keygenning...

    Hi All:

    I just unpacked a "famous" packer and analyzed its keygen scheme. It is using RSA. I'm pretty sure of this as it is using constants like A, E, D, N etc. in its keygen code.

    My question here is that, if an application packed with this packer is shipped and uses the keygen mechanism provided by this packer for registering the software, then:

    (1) What constants must be encoded into the binary so that I can fish it out for a keygen?
    (2) What constants must be factored by me to complete the keygen?

    BTW, i thought I knew RSA by heart, alas, that thought is short-lived after I reversed this code. I'm off to study RSA again but wanted your opinions on this one.

    Signed,
    -- Foxthree
    I promise that I have read the FAQ and tried to use the Search to answer my question.

  2. #2
    The constants E and N will be in the target in some form. You must factor N to get it's two prime factors P and Q. The decryption exponent D is calculated from P and Q using D = E^-1 % (P-1)(Q-1).

    Hope that helps!
    Kythen

  3. #3
    הבּרוּ נשׂאי כּלי יהוה mike's Avatar
    Join Date
    Mar 2001
    Posts
    491

    not sure?

    If you're not sure it's RSA, can you post what he's calculating?

  4. #4
    foxthree
    Guest

    Thanks and I will...

    Hi Kythen:

    Thanks for your post.

    Mike: Yes, I will. Let me first complete my analysis.

    Signed,
    -- FoxThree
    I promise that I have read the FAQ and tried to use the Search to answer my question.

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