Document the BN library.

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diff --git a/doc/crypto/BN_add.pod b/doc/crypto/BN_add.pod
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+=pod
+
+=head1 NAME
+
+BN_add, BN_sub, BN_mul, BN_div, BN_sqr, BN_mod, BN_mod_mul, BN_exp,
+BN_mod_exp, BN_gcd - Arithmetic operations on BIGNUMs
+
+=head1 SYNOPSIS
+
+ #include <openssl/bn.h>
+
+ int BN_add(BIGNUM *r, const BIGNUM *a, const BIGNUM *b);
+
+ int BN_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b);
+
+ int BN_mul(BIGNUM *r, BIGNUM *a, BIGNUM *b, BN_CTX *ctx);
+
+ int BN_div(BIGNUM *dv, BIGNUM *rem, const BIGNUM *a, const BIGNUM *d,
+         BN_CTX *ctx);
+
+ int BN_sqr(BIGNUM *r, BIGNUM *a, BN_CTX *ctx);
+
+ int BN_mod(BIGNUM *rem, const BIGNUM *a, const BIGNUM *m, BN_CTX *ctx);
+
+ int BN_mod_mul(BIGNUM *ret, BIGNUM *a, BIGNUM *b, const BIGNUM *m,
+         BN_CTX *ctx);
+
+ int BN_exp(BIGNUM *r, BIGNUM *a, BIGNUM *p, BN_CTX *ctx);
+
+ int BN_mod_exp(BIGNUM *r, BIGNUM *a, const BIGNUM *p,
+         const BIGNUM *m, BN_CTX *ctx);
+
+ int BN_gcd(BIGNUM *r, BIGNUM *a, BIGNUM *b, BN_CTX *ctx);
+
+=head1 DESCRIPTION
+
+BN_add() adds B<a> and B<b> and places the result in B<r> (C<r=a+b>).
+B<r> may be the same B<BIGNUM> as B<a> or B<b>.
+
+BN_sub() substracts B<b> from B<a> and places the result in B<r> (C<r=a-b>).
+
+BN_mul() multiplies B<a> and B<b> and places the result in B<r> (C<r=a*b>).
+
+BN_div() divides B<a> by B<d> and places the result in B<dv> and the
+remainder in B<rem> (C<dv=a/d, rem=a%d>). Either of B<dv> and B<rem> may
+be NULL, in which case the respective value is not returned.
+
+BN_sqr() takes the square of B<a> and places the result in B<r>
+(C<r=a^2>). B<r> and B<a> may be the same B<BIGNUM>.
+This function is faster than BN_mul(r,a,a).
+
+BN_mod() find the remainder of B<a> divided by B<m> and places it in
+B<rem> (C<rem=a%m>).
+
+BN_mod_mul() multiplies B<a> by B<b> and finds the remainder when
+divided by B<m> (C<r=(a*b)%m>). B<r> may be the same B<BIGNUM> as B<a>
+or B<b>. For a more efficient algorithm, see
+L<BN_mod_mul_montgomery(3)>; for repeated computations using the same
+modulus, see L<BN_mod_mul_reciprocal(3)>.
+
+BN_exp() raises B<a> to the B<p>-th power and places the result in B<r>
+(C<r=a^p>). This function is faster than repeated applications of
+BN_mul().
+
+BN_mod_exp() computes B<a> to the B<p>-th power modulo B<m> (C<r=a^p %
+m>). This function uses less time and space than BN_exp().
+
+BN_gcd() computes the greatest common divisor of B<a> and B<b> and
+places the result in B<r>. B<r> may be the same B<BIGNUM> as B<a> or
+B<b>.
+
+For all functions, B<ctx> is a previously allocated B<BN_CTX> used for
+temporary variables; see L<BN_CTX_new(3)>.
+
+Unless noted otherwise, the result B<BIGNUM> must be different from
+the arguments.
+
+=head1 RETURN VALUES
+
+For all functions, 1 is returned for success, 0 on error. The return
+value should always be checked (e.g., C<if (!BN_add(r,a,b)) goto err;>).
+The error codes can be obtained by ERR_get_error(3).
+
+=head1 SEE ALSO
+
+bn(3), err(3), BN_CTX_new(3), BN_add_word(3), BN_set_bit(3)
+
+=head1 HISTORY
+
+BN_add(), BN_sub(), BN_div(), BN_sqr(), BN_mod(), BN_mod_mul(),
+BN_mod_exp() and BN_gcd() are available in all versions of SSLeay and
+OpenSSL. The B<ctx> argument to BN_mul() was added in SSLeay
+0.9.1b. BN_exp() appeared in SSLeay 0.9.0.
+
+=cut