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I have a fair idea of what RSA encryption is and how to do it, but I don't quite understand the following bit given in my textbook:

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I have an exam tomorrow and I'm expected to encrypt a string of text (e.g., "meet me tomorrow.") using RSA encryption. If somebody could clarify what that bit means, I'll be good to go.

Rafay
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1 Answers1

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We divide the string into equal parts of size $2N$. Each "block" gives us an integer $M_i$. We encrypt each block using the function given. Then we concatenate all the encrypted blocks. The restriction that $2525...25$ should be less than $n$ is there so that each different block gives a unique integer modulo $n$. Otherwise say if $n = 11*13 = 143$ and we pick a block size of $4$ (that is $2N = 4$) then $1001$('kb') and $0000$('aa') both would be $0$ modulo $143$ and you couldn't tell whether $0$ means 'kb' or 'aa'. In this case we should pick a block size of $2$ so the largest integer we could get $25$('z') is less than $143$

Shalan
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  • I still don't get what 'N' is exactly. – Rafay Dec 31 '14 at 15:24
  • it is just the length of the string that which you used to make the number M. Like if we're encrypting one character at a time then N = 1. If we're encrypting two characters at a time then N = 2. Like in the example I gave in my post I used N = 2 and then N = 1. – Shalan Dec 31 '14 at 16:43
  • So N can be arbitrary? – Rafay Dec 31 '14 at 17:50
  • In principle, yes. But the book says that you should always pick the largest block size that you can. Like in my post the largest block size you could pick was 2. So (2N = 2) and N = 1. If I had picked a larger n, than I could've picked a larger block size. – Shalan Dec 31 '14 at 21:38
  • Thank you for your help! :) – Rafay Jan 01 '15 at 04:37