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%%%% why-RSA-works/huge-integers.tex
%%%% Copyright 2012 Peter Franusic.
%%%% All rights reserved.
Figure \ref{block-diagram} shows an RSA cryptosystem.
It consists of a transmitter, a receiver, and a sniffer in between.
Tradition has it that \emph{Bob} is the transmitter, \emph{Alice} is the receiver,
and \emph{Eve} is the sniffer. (\emph{Eavesdropper}, get it?)
The twin engines of the system are the modular exponentiation (modex) functions.
RSA uses \emph{huge} integers.
By huge we mean integers with over 300 decimal digits.
It would take over four lines to print a 300 digit integer on this page.
Happily, we can represent huge integers with single letters.
Each of the letters in the figure represents a huge integer.
These are: \mbox{message $m$}, \mbox{modulus $n$}, \mbox{encryptor $e$},
\mbox{ciphertext $c$}, \mbox{decryptor $d$}, and \mbox{output $y$}.
%%%% Figure: Block diagram
\caption{An RSA cryptosystem}
Bob generates encrypted messages and transmits them to Alice.
He originates \mbox{message $m$},
computes the modex function using \mbox{modulus $n$} and \mbox{encryptor $e$},
then writes \mbox{ciphertext $c$} into the insecure channel.
The public \mbox{key $(n,e)$} was generated and published by Alice
prior to any transmission by Bob.
Alice receives encrypted messages from Bob and decrypts them.
She reads \mbox{ciphertext $c$} from the insecure channel,
computes the modex function using \mbox{modulus $n$} and \mbox{decryptor $d$},
then writes \mbox{output $y$}.
The magic of RSA is that \mbox{output $y$} is identical to \mbox{message $m$}.
I.e., \mbox{$y=m$}.
Alice generated and secured her private \mbox{key $(n,d)$} prior to
receiving any ciphertext from Bob.
Eve is the threat that exists on every insecure channel.
She attempts to read messages that are meant to be read only by Alice.
But Eve is defeated by RSA encryption.
She can intercept ciphertext $c$ but she won't be able to compute $y$
because she doesn't have access to decryptor $d$.
Only Alice has access to decryptor $d$.