# RSA message limitation (with C++ source code)

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I read somewhere that if used incorrectly, RSA has the security of the lowly Caesar cipher. To get around this, one is supposed to add a pad of (pseudo-)random numbers to the message. Do you put the pad at the beginning of the message, or at the end? Or both?

Edited by taby

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Is it a good idea to make the encryption key e large?

What I mean to ask is:

6831636268916828328318892898989685198399119653965311896986632969623229621083238916933629968153681965328969839888866484783

better than:

17?

Both primes p and q are two-hundred digits in length (much larger than e).

Edited by taby

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I am not aware of an attack that could exploit e being a small number. But what's wrong with making e be a random number between 0 and p*q? Technically you would have to verify that you didn't pick a multiple of p or q, but let's not worry about things that will not happen. :) Edited by Álvaro

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The two books I have bought recently were "Applied Cryptography" by Bruce Schneier and "Mastering Algorithms with C" by Kyle Loudon. Both books say that e must be coprime with (p - 1)(q - 1). Same with https://en.wikipedia.org/wiki/RSA_(cryptosystem)#Key_generation

Yeah, I made that large e by somewhat randomly banging on the keyboard.  :)

In the end I used e = 65537. I tried 65536 for fun, and the program died with a segfault LOL (I'm using someone else's bigint library).

I didn't do the optimal padding; I used 512 pseudorandom bits to salt the message before encryption. I didn't use a cryptographic-quality pseudorandom number generator.

... so there is room for improvement.

Edited by taby

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