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134 .IX Title "BN_mod_mul_reciprocal 3"
135 .TH BN_mod_mul_reciprocal 3 "2010-02-27" "0.9.8m" "OpenSSL"
136 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
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141 BN_mod_mul_reciprocal, BN_div_recp, BN_RECP_CTX_new, BN_RECP_CTX_init,
142 BN_RECP_CTX_free, BN_RECP_CTX_set \- modular multiplication using
145 .IX Header "SYNOPSIS"
147 \& #include <openssl/bn.h>
149 \& BN_RECP_CTX *BN_RECP_CTX_new(void);
150 \& void BN_RECP_CTX_init(BN_RECP_CTX *recp);
151 \& void BN_RECP_CTX_free(BN_RECP_CTX *recp);
153 \& int BN_RECP_CTX_set(BN_RECP_CTX *recp, const BIGNUM *m, BN_CTX *ctx);
155 \& int BN_div_recp(BIGNUM *dv, BIGNUM *rem, BIGNUM *a, BN_RECP_CTX *recp,
158 \& int BN_mod_mul_reciprocal(BIGNUM *r, BIGNUM *a, BIGNUM *b,
159 \& BN_RECP_CTX *recp, BN_CTX *ctx);
162 .IX Header "DESCRIPTION"
163 \&\fIBN_mod_mul_reciprocal()\fR can be used to perform an efficient
164 \&\fIBN_mod_mul\fR\|(3) operation when the operation will be performed
165 repeatedly with the same modulus. It computes \fBr\fR=(\fBa\fR*\fBb\fR)%\fBm\fR
166 using \fBrecp\fR=1/\fBm\fR, which is set as described below. \fBctx\fR is a
167 previously allocated \fB\s-1BN_CTX\s0\fR used for temporary variables.
169 \&\fIBN_RECP_CTX_new()\fR allocates and initializes a \fB\s-1BN_RECP\s0\fR structure.
170 \&\fIBN_RECP_CTX_init()\fR initializes an existing uninitialized \fB\s-1BN_RECP\s0\fR.
172 \&\fIBN_RECP_CTX_free()\fR frees the components of the \fB\s-1BN_RECP\s0\fR, and, if it
173 was created by \fIBN_RECP_CTX_new()\fR, also the structure itself.
175 \&\fIBN_RECP_CTX_set()\fR stores \fBm\fR in \fBrecp\fR and sets it up for computing
176 1/\fBm\fR and shifting it left by BN_num_bits(\fBm\fR)+1 to make it an
177 integer. The result and the number of bits it was shifted left will
178 later be stored in \fBrecp\fR.
180 \&\fIBN_div_recp()\fR divides \fBa\fR by \fBm\fR using \fBrecp\fR. It places the quotient
181 in \fBdv\fR and the remainder in \fBrem\fR.
183 The \fB\s-1BN_RECP_CTX\s0\fR structure is defined as follows:
186 \& typedef struct bn_recp_ctx_st
188 \& BIGNUM N; /* the divisor */
189 \& BIGNUM Nr; /* the reciprocal */
196 It cannot be shared between threads.
198 .IX Header "RETURN VALUES"
199 \&\fIBN_RECP_CTX_new()\fR returns the newly allocated \fB\s-1BN_RECP_CTX\s0\fR, and \s-1NULL\s0
202 \&\fIBN_RECP_CTX_init()\fR and \fIBN_RECP_CTX_free()\fR have no return values.
204 For the other functions, 1 is returned for success, 0 on error.
205 The error codes can be obtained by \fIERR_get_error\fR\|(3).
207 .IX Header "SEE ALSO"
208 \&\fIbn\fR\|(3), \fIERR_get_error\fR\|(3), \fIBN_add\fR\|(3),
209 \&\fIBN_CTX_new\fR\|(3)
212 \&\fB\s-1BN_RECP_CTX\s0\fR was added in SSLeay 0.9.0. Before that, the function
213 \&\fIBN_reciprocal()\fR was used instead, and the \fIBN_mod_mul_reciprocal()\fR
214 arguments were different.