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POUnderstanding RSA cell padding from textbook explaination
 

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  1. POUnderstanding RSA cell padding from textbook explaination
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    <p>I've been doing an RSA encryption/decryption assignment for a school assignment, and I've actually gotten the whole thing working. The one thing I want to make sure I understand is the padding. The book states that after we turn the string of characters into a string of digits (A = 00, and Z = 25) we then need to determine the size of the blocks and add dummy characters to the end.</p> <p>The book states:</p> <blockquote> <p>Next, we divide this string into equally sized blocks of 2N digits, where 2N is the largest even number such that the number 2525 ... 25 with 2N digits does not exceed n.</p> </blockquote> <p>It doesn't tell me where it gets 25 from, so I deduced that it was the index of the last character (Z in this case) of the our actual key of characters.</p> <p>So here is my Python3 implementation (fair warning it is somewhat cringe-worthy):</p> <pre><code>alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ" def __determineSize__(message, n): if (n &lt; len(alphabet) - 1): raise Exception("n is not sufficiently large") buffer = "" for i in range(0, n, 2): buffer += str(len(alphabet) - 1) #+= "25" in this case if (int(buffer) &gt; n): groupSize = len(buffer) - 2 return groupSize </code></pre> <p>It starts with 25 ( len(alphabet) = 26, 26 - 1 = 25), if it is not larger than n we increase it to 2525. If it larger at this point we stop because we know we've gone to far and we return the length 2, because the length 4 is too large.</p> <p>This is how I understood it, and it works but it doesn't seem right. Did I interpret this correctly or am I completely off base? If I am can someone set me straight? (I'm not asking for code, because it's for an assignment I don't want to plagerise anyone so if anyone could just tell me what I'm supposed to do in simple English or show me in pseudo code that would be great.)</p> <p>Like always, thanks to everyone in advance!</p>
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    1. USPaul Nelson Baker
    1. USPaul Nelson Baker
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    1. COIt actually looks quite OK to me. Note that the textbook does not promote secure RSA. RSA is normally not chained, key sizes should be adequate and the correct padding mode should be used.
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      1. POUnderstanding RSA cell padding from textbook explaination
      1. USMaarten Bodewes
    2. COThanks, that actually puts me somewhat at ease. And I was aware that the key sizes were academically small too, but what do you mean "not normally chained", is that where it steps through and checks 25, then 2525, then 252525 and so on? What is the "correct padding mode"?
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      1. POUnderstanding RSA cell padding from textbook explaination
      1. USPaul Nelson Baker
    3. COFor RSA encryption, the best padding mode is OAEP. It is defined in the rather readable PKCS#1 standards (2.0/2.1). Normally a symmetric (AES) random data or session key is encrypted. This symmetric key is then used in turn to encrypt the data. AES is much faster than RSA (for secure key sizes anyway). But that may be the next lesson :)
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      1. POUnderstanding RSA cell padding from textbook explaination
      1. USMaarten Bodewes
 

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