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+%%% -*-latex-*-
+
+\section{Modular arithmetic}
+
+Many public key cryptographic algorithms require modular arithmetic of
+various kinds.  Modular exponentiation (calculation of $g^x \bmod n$) is
+particularly important.  Catacomb provides two efficient methods for
+performing modular arithmetic.
+
+
+\subsection{Barrett reduction}
+\label{sec:mp-barrett}
+
+P.~Barrett invented an elegant method for computing modular reductions.  It
+requires a small amount of precomputation, depending only on the modulus $m$.
+Thereafter, it can compute residues mod $m$ using only multiplication and
+arithmetic shifts.
+
+Montgomery's method (section~\ref{sec:mp-mont}, page~\pageref{sec:mp-mont})
+is similar, and more efficient for heavy use, but only works with odd moduli.
+Barrett reduction works with an arbitrary modulus.
+
+The declarations required to use Barrett reduction are in
+\hdr{<catacomb/mpbarrett.h>}.  The precomuted values required are stored in a
+\emph{Barrett reduction context} of type \code{mpbarrett}.  A context can be
+initialized by calling \code{mpbarrett_create}
+(page~\pageref{fn:mpbarrett-create}) and disposed of by
+\code{mpbarrett_destroy} (page~\pageref{fn:mpbarrett-destroy}).  A number
+less then $m^2$ may then be reduced modulo $m$ by passing it to
+\code{mpbarrett_reduce} (page~\pageref{fn:mpbarrett-reduce}) along with the
+appropriate reduction context.
+
+Since modular exponentiation is such a common operation in cryptography, a
+function \code{mpbarrett_exp} (page~\pageref{fn:mpbarrett-exp}) is provided
+which uses a simple square-and-multiply method combined with Barrett
+reduction.
+
+\subsubsection{The \code{mpbarrett_create} function}
+\label{fn:mpbarrett-create}
+
+\fsec{Synopsis}
+
+\begin{listinglist}
+|#include <catacomb/mpbarrett.h>| \\
+|void mpbarrett_create(mpbarrett *|$b$|, mp *|$m$|);|
+\end{listinglist}
+
+\fsec{Description}
+
+Initializes a Barrett reduction context $b$ suitable for performing
+reductions modulo $m$.  The memory used by the context block must be provided
+by the caller, and must not be discarded until the context is destroyed.
+
+The computation performed is as follows.  Let $k = 2 \cdot |MP_LEN(|m|)|$.
+Calculate $\mu = \lfloor 2^{\code{MPW_BITS} \cdot k} / m \rfloor$.  The
+values $k$, $\mu$ and $m$ are stored in the context block for later use.
+
+\subsubsection{The \code{mpbarrett_destroy} function}
+\label{fn:mpbarrett-destroy}
+
+\fsec{Synopsis}
+
+\begin{listinglist}
+|#include <catacomb/mpbarrett.h>| \\
+|void mpbarrett_destroy(mpbarrett *|$b$|);|
+\end{listinglist}
+
+\fsec{Description}
+
+Disposes of a Barrett reduction context.  Resources associated with the
+context block are freed.  The memory used to hold the context may safely be
+discarded.
+
+\subsubsection{The \code{mpbarrett_reduce} function}
+\label{fn:mpbarrett-reduce}
+
+\fsec{Synopsis}
+
+\begin{listinglist}
+|#include <catacomb/mpbarrett.h>| \\
+|mp *mpbarrett_reduce(mpbarrett *|$b$|, mp *|$d$|, mp *|$x$|);|
+\end{listinglist}
+
+\fsec{Description}
+
+Calculates $x \bmod m$, where $m$ is the modulus configured into the Barrett
+reduction context $b$.  The argument $d$ is a suggested destination.
+
+Note that not all values of $x$ are suitable for reduction: in particular,
+negative values are improperly handled.  Let $k = 2 \cdot |MP_LEN(|m|)|$;
+then $x$ must be in the interval $[\,0, 2^{\code{MPW_BITS} \cdot k})$.  It's
+probably safer (and eqsier) to ensure that $x < m^2$.
+
+For reference, the calculation works as follows:
+\begin{itemize}
+\item To simplify the notation, let $b = 2^{\code{MPW_BITS}}$.
+\item Let $q_0 = \lfloor x / b^{k - 1} \rfloor$.  Let
+  $q = \lfloor q_0 \mu / b^{k + 1} \rfloor$.
+\item Let $r = (x - q m) \bmod b^{k + 1}$.
+\item At this point, $r \equiv x \pmod m$, but $r$ may be too large.  It can
+  be reduced by repeated subtraction until it's in range.
+\end{itemize}
+
+\fsec{Returns}
+
+A value $d$ such that $0 \le d < m$ and $d \equiv x \pmod m$.
+
+\subsubsection{The \code{mpbarrett_exp} function}
+\label{fn:mpbarrett-exp}
+
+\fsec{Synopsis}
+
+\begin{listinglist}
+|#include <catacomb/mpbarrett.h>| \\
+|mp *mpbarrett_exp(mpbarrett *|$b$|, mp *|$d$|, mp *|$x$|, mp *|$e$|);|
+\end{listinglist}
+
+\fsec{Description}
+
+Computes $x^e \bmod m$, where $m$ is the modulus configured into the Barrett
+reduction context $b$.  The argument $d$ is a suggested destination.
+
+\fsec{Returns}
+
+A value $d$ such that $0 \le d < m$ and $d \equiv x^e \pmod m$.
+
+
+\subsection{Montgomery reduction}
+\label{sec:mp-mont}
+
+Peter Montgomery has invented an ingenious method for computing modular
+reductions.  The method requires a number of values to be precomputed.
+\begin{itemize}
+\item Let $m$ be the modulus.  Let $b$ be the radix used to represent
+  multiprecision integers.  (In Catacomb, then, $b = 2^{\code{MPW_BITS}}$.)
+  For Montgomery reduction to work, $m$ and $b$ must have no common factors.
+\item Let $R = b^n > m$ be the smallest power of $b$ which exceeds $m$.
+\item Precompute $m'$ such that $m m' \equiv -1 \pmod b$.
+\item Precompute $R \bmod m$ and $R^2 \bmod m$, since these are generally
+  useful when using Montgomery's method.
+\end{itemize}
+Given $x < R^2$ and the above values, Montgomery's algorithm computes $\mont
+x = x R^{-1} \bmod m$ efficiently, using only multiplications, additions and
+word-sized shifts.  The quantity $\mont x$ is called the \emph{Montgomery
+reduction of $x$}.
+
+At first sight, this isn't particularly useful.  However, an interesting
+thing happens if you run an extra factor of $R$ all the way through your
+calculation.  Note for example that the Montgomery reduction of $x R \cdot y
+R$ is $xy R^2 R^{-1} \bmod m = x y R \bmod m$ -- there's still only a single
+factor of $R$ in the result.  This can be removed finally by an additional
+reduction step.
+
+Catacomb provides a function for performing a simple Montgomery reduction,
+and for calculating Montgomery multiplication: the Montgomery reduction of
+the product of two integers.
+
+Multiplying in the extra factor of $R$ can be done efficiently by multiplying
+by $R^2 \bmod m$ and reducing.\footnote{%
+  Some of the Catacomb comments refer to values with a factor of $R$ as
+  having been `Montgomerized'.  While this is an ugly word, it does describe
+  a useful concept.} %
+This is the reason that $R^2 \bmod m$ is precomputed.  The value $R \bmod m$
+is the Montgomery multiplicative identity and is often useful as an initial
+value when forming products.
+
+\subsubsection{Overview of functions provided}
+
+All of the types and declarations needed for Montgomery reduction are in the
+header file \hdr{<catacomb/mpmont.h>}.
+
+Catacomb maintains precomputed values in a \emph{Montgomery reduction
+context}, which has type \code{mpmont}.  A reduction context is initialized
+by calling \code{mpmont_create} (page~\pageref{fn:mpmont-create}) and
+disposed of by \code{mpmont_destroy} (page~\pageref{fn:mpmont-destroy}).
+
+The Montgomery reduction context is a structure containing the following
+members:
+\begin{description}
+  \def\makelabel#1{\code{#1}\hfil}
+\item[mp *m] The modulus $m$ with which the reduction context is working.
+\item[mp *mi] The quantity $m'$ where $m m' \equiv -1
+  \pmod{ 2^{\code{MPW_BITS}}}$.
+\item[size_t shift] The smallest integer $n$ such that
+  $R = 2^{\code{MPW_BITS} \cdot n} > m$.
+\item[mp *r] The quantity $R \bmod m$.  The Montgomery multiplicative
+  identity.
+\item[mp *r2] The quantity $R^2 \bmod m$.
+\end{description}
+All of these are set up by \code{mpmont_create} and should not be changed.
+
+Montgomery reduction can be performed by passing a Montgomery reduction
+context and a number to be reduced to \code{mpmont_reduce}
+(page~\pageref{fn:mpmont-reduce}).  Simultaneous multiplication and reduction
+are performed by \code{mpmont_mul} (page~\pageref{fn:mpmont-mul}).
+
+Catacomb can perform modular exponentiation using Montgomery reduction.  The
+function \code{mpmont_exp} (page~\pageref{fn:mpmont-exp}) performs a standard
+modular exponentiation; \code{mpmont_expr} (page~\pageref{fn:mpmont-expr})
+does the same but doesn't remove the factor of $R$ from the result, which can
+be useful if you want to perform further computations.
+
+Catacomb can also perform multiple simultaneous modular exponentations,
+multiplying the results together, without much of a performance penalty over
+a single exponentation.  The function \code{mpmont_mexp}
+(page~\pageref{fn:mpmont-mexp}) calculates the product $x_0^{e_0} x_1^{e_1}
+\ldots x_{k - 1}^{e_{k - 1}} \bmod m$ given an array of $x_i$ and $e_i$;
+\code{mpmont_mexpr} (page~\pageref{fn:mpmont-mexpr}) does the same but leaves
+a factor of $R$ in the result.  These functions are particularly useful for
+verifying discrete-logarith-based digital signatures.
+
+\subsubsection{Calculation using Montgomery reduction}
+
+Before any reduction work can be done, a Montgomery context must be
+initialized.  Suppose $m$ is the modulus we need to work with:
+\begin{listing}
+mpmont mm;
+mpmont_create(&mm, m);
+\end{listing}
+The next step is usually to multiply all of the inputs to the calculation by
+$R$.  This is usually best done like this:
+\begin{listing}
+xr = mpmont_mul(&mm, MP_NEW, x, mm.r2);
+\end{listing}
+Because multiplication is distributive over addition, addition and
+subtraction work normally, although it can be worth doing a little repeated
+subtraction in case the result ends up larger than the modulus.
+\begin{listing}
+a = mp_add(a, a, b);
+a = mp_add(a, a, c);
+a = mp_add(a, a, d);
+while (MP_CMP(a, >=, mm.m))
+  a = mp_sub(a, a, mm.m);
+\end{listing}
+Multiplication of different numbers is best done with the supplied
+multiply-and-reduce operation.
+\begin{listing}
+ab = mpmont_mul(&mm, MP_NEW, a, b);
+\end{listing}
+However, squaring is best done using separate square and reduce steps.
+\begin{listing}
+a2 = mp_sqr(MP_NEW, a);
+a2 = mpmont_reduce(&mm, a2, a2);
+\end{listing}
+When the computation has finished, the results can be read out using
+straightforward Montgomery reduction.  Don't forget to dispose of the
+reduction context.
+\begin{listing}
+x = mpmont_reduce(&mm, x, xr);
+mpmont_destroy(&mm);
+\end{listing}
+
+For particularly simple calculations, it can save a multiplication if you
+reduce first and correct afterwards.
+\begin{listing}
+ab = mpmont_mul(&mm, MP_NEW, a, b);
+ab = mpmont_mul(&mm, ab, ab, mm.r2);
+\end{listing}
+The first stage computes $ab R^{-1} \bmod m$.  The second computes $abR^{-1}
+\cdot R^2 \cdot R^{-1} \bmod m = ab \bmod m$.  Doing this the usual way is
+nowhere near as efficient:
+\begin{listing}
+ar = mpmont_mul(&mm, MP_NEW, a, mm.r2);
+br = mpmont_mul(&mm, MP_NEW, b, mm.r2);
+ab = mpmont_mul(&mm, ar, ar, br);
+ab = mpmont_reduce(&mm, ab, ab);
+mp_drop(br);
+\end{listing}
+
+Remember that for all of this magic to work, $m$ and $b$ must have no common
+factors.  Since in Catacomb $b$ is a power of 2, this means simply that
+\emph{$m$ must be odd}.  If you want to work with an even modulus, you'll
+have to use Barrett reduction instead (section~\ref{sec:mp-barrett},
+page~\pageref{sec:mp-barrett}).
+
+\begin{note}[Important]
+  Montgomery reduction only works when the modulus is an odd number.
+\end{note}
+
+\subsubsection{How it works}
+
+Let $x_0$ be the integer we wish to reduce.  Compute $u_1 = x_0 m' \bmod b$,
+and $v_1 = x_0 + u_1 m$.
+
+Obviously $u_1 = x_0 m' + bk$ for some integer $k$.  So $v_1 = x_0 + u_1 m =
+x_0 + x_0 m m' + b k m$.  Note that $m m' \equiv -1 \pmod b$; thus $m m' =
+b k' - 1$ for some $k'$, so $v_1$ then becomes $x_0(1 + bk' - 1) + b k m =
+b (k' x_0 + k m)$.  Let $x_1 = v_1 / b = k' x_0 + k m$.
+
+Now note that $b k' = m m' + 1$, so $b k' \equiv 1 \pmod m$; hence, $k'
+\equiv b^{-1} \pmod m$.  Thus $x_1 \equiv x_0 b^{-1} \pmod m$.
+
+
+
+\subsubsection{The \code{mpmont_create} function}
+\label{fn:mpmont-create}
+
+\fsec{Synopsis}
+
+\begin{listinglist}
+|#include <catacomb/mpmont.h>| \\
+|void mpmont_create(mpmont *|$\mm$|, mp *|$m$|);|
+\end{listinglist}
+
+\fsec{Description}
+
+Initializes a Montgomery reduction context $\mm$, using the given modulus
+$m$.  The caller must provide memory for the context, and must ensure that
+the memory remains available until the context is destroyed.
+
+If Catacomb is compiled with debugging support enabled, it will abort if $m$
+is negative or even.
+
+\subsubsection{The \code{mpmont_destroy} function}
+\label{fn:mpmont-destroy}
+
+\fsec{Synopsis}
+
+\begin{listinglist}
+|#include <catacomb/mpmont.h>| \\
+|void mpmont_destroy(mpmont *|$\mm$|);|
+\end{listinglist}
+
+\fsec{Description}
+
+Destroys the Montgomery reduction context $\mm$, releasing any resources it
+had acquired.
+
+\subsubsection{The \code{mpmont_reduce} function}
+\label{fn:mpmont-reduce}
+
+\fsec{Synopsis}
+
+\begin{listinglist}
+|#include <catacomb/mpmont.h>| \\
+|mp *mpmont_reduce(mpmont *|$\mm$|, mp *|$d$|, mp *|$x$|);|
+\end{listinglist}
+
+\fsec{Description}
+
+Performs a Montgomery reduction operation on the integer $x$, modulo $m$.
+The integer $d$ is a suggested destination.
+
+For Montgomery reduction to work, $x$ must be less than $R^2$.  In practice,
+it's probably safest to ensure that $x < m^2$.
+
+This function is particularly efficient if $d = x$, i.e., if you overwrite
+$x$ with its Montgomery reduction.
+
+\fsec{Returns}
+
+An integer $d$ such that $0 \le d < m$ and $d R \equiv x \pmod m$.
+
+\subsubsection{The \code{mpmont_mul} function}
+\label{fn:mpmont-mul}
+
+\fsec{Synopsis}
+
+\begin{listinglist}
+|#include <catacomb/mpmont.h>| \\
+|mp *mpmont_mul(mpmont *|$\mm$|, mp *|$d$|, mp *|$x$|, mp *|$y$|);|
+\end{listinglist}
+
+\fsec{Description}
+
+Multiplies $x$ and $y$, and returns the Montgomery reduction of the product
+modulo $m$.  The integer $d$ is a suggested destination.
+
+Both $x$ and $y$ must be less than $R$, and it's probably safer in practice
+if you ensure that $x, y < m$.
+
+\fsec{Returns}
+
+An integer $d$ such that $0 \le d < m$ and $d R \equiv x y \bmod m$.
+
+%%% Local Variables: 
+%%% mode: latex
+%%% TeX-master: "catacomb"
+%%% End: 
