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126 .IX Title "BN_mod_mul_reciprocal 3"
127 .TH BN_mod_mul_reciprocal 3 "2007-10-24" "0.9.8g" "OpenSSL"
128 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
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133 BN_mod_mul_reciprocal, BN_div_recp, BN_RECP_CTX_new, BN_RECP_CTX_init,
134 BN_RECP_CTX_free, BN_RECP_CTX_set \- modular multiplication using
137 .IX Header "SYNOPSIS"
139 \& #include <openssl/bn.h>
141 \& BN_RECP_CTX *BN_RECP_CTX_new(void);
142 \& void BN_RECP_CTX_init(BN_RECP_CTX *recp);
143 \& void BN_RECP_CTX_free(BN_RECP_CTX *recp);
145 \& int BN_RECP_CTX_set(BN_RECP_CTX *recp, const BIGNUM *m, BN_CTX *ctx);
147 \& int BN_div_recp(BIGNUM *dv, BIGNUM *rem, BIGNUM *a, BN_RECP_CTX *recp,
150 \& int BN_mod_mul_reciprocal(BIGNUM *r, BIGNUM *a, BIGNUM *b,
151 \& BN_RECP_CTX *recp, BN_CTX *ctx);
154 .IX Header "DESCRIPTION"
155 \&\fIBN_mod_mul_reciprocal()\fR can be used to perform an efficient
156 \&\fIBN_mod_mul\fR\|(3) operation when the operation will be performed
157 repeatedly with the same modulus. It computes \fBr\fR=(\fBa\fR*\fBb\fR)%\fBm\fR
158 using \fBrecp\fR=1/\fBm\fR, which is set as described below. \fBctx\fR is a
159 previously allocated \fB\s-1BN_CTX\s0\fR used for temporary variables.
161 \&\fIBN_RECP_CTX_new()\fR allocates and initializes a \fB\s-1BN_RECP\s0\fR structure.
162 \&\fIBN_RECP_CTX_init()\fR initializes an existing uninitialized \fB\s-1BN_RECP\s0\fR.
164 \&\fIBN_RECP_CTX_free()\fR frees the components of the \fB\s-1BN_RECP\s0\fR, and, if it
165 was created by \fIBN_RECP_CTX_new()\fR, also the structure itself.
167 \&\fIBN_RECP_CTX_set()\fR stores \fBm\fR in \fBrecp\fR and sets it up for computing
168 1/\fBm\fR and shifting it left by BN_num_bits(\fBm\fR)+1 to make it an
169 integer. The result and the number of bits it was shifted left will
170 later be stored in \fBrecp\fR.
172 \&\fIBN_div_recp()\fR divides \fBa\fR by \fBm\fR using \fBrecp\fR. It places the quotient
173 in \fBdv\fR and the remainder in \fBrem\fR.
175 The \fB\s-1BN_RECP_CTX\s0\fR structure is defined as follows:
178 \& typedef struct bn_recp_ctx_st
180 \& BIGNUM N; /* the divisor */
181 \& BIGNUM Nr; /* the reciprocal */
188 It cannot be shared between threads.
190 .IX Header "RETURN VALUES"
191 \&\fIBN_RECP_CTX_new()\fR returns the newly allocated \fB\s-1BN_RECP_CTX\s0\fR, and \s-1NULL\s0
194 \&\fIBN_RECP_CTX_init()\fR and \fIBN_RECP_CTX_free()\fR have no return values.
196 For the other functions, 1 is returned for success, 0 on error.
197 The error codes can be obtained by \fIERR_get_error\fR\|(3).
199 .IX Header "SEE ALSO"
200 \&\fIbn\fR\|(3), \fIERR_get_error\fR\|(3), \fIBN_add\fR\|(3),
201 \&\fIBN_CTX_new\fR\|(3)
204 \&\fB\s-1BN_RECP_CTX\s0\fR was added in SSLeay 0.9.0. Before that, the function
205 \&\fIBN_reciprocal()\fR was used instead, and the \fIBN_mod_mul_reciprocal()\fR
206 arguments were different.