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1 .\" $NetBSD: BN_mod_mul_reciprocal.3,v 1.14 2015/06/12 17:01:13 christos Exp $
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137 .IX Title "BN_mod_mul_reciprocal 3"
138 .TH BN_mod_mul_reciprocal 3 "2009-07-19" "1.0.1n" "OpenSSL"
139 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
140 .\" way too many mistakes in technical documents.
141 .if n .ad l
142 .nh
143 .SH "NAME"
144 BN_mod_mul_reciprocal, BN_div_recp, BN_RECP_CTX_new, BN_RECP_CTX_init,
145 BN_RECP_CTX_free, BN_RECP_CTX_set \- modular multiplication using
146 reciprocal
147 .SH "LIBRARY"
148 libcrypto, -lcrypto
149 .SH "SYNOPSIS"
150 .IX Header "SYNOPSIS"
151 .Vb 1
152 \& #include <openssl/bn.h>
153 \&
154 \& BN_RECP_CTX *BN_RECP_CTX_new(void);
155 \& void BN_RECP_CTX_init(BN_RECP_CTX *recp);
156 \& void BN_RECP_CTX_free(BN_RECP_CTX *recp);
157 \&
158 \& int BN_RECP_CTX_set(BN_RECP_CTX *recp, const BIGNUM *m, BN_CTX *ctx);
159 \&
160 \& int BN_div_recp(BIGNUM *dv, BIGNUM *rem, BIGNUM *a, BN_RECP_CTX *recp,
161 \& BN_CTX *ctx);
162 \&
163 \& int BN_mod_mul_reciprocal(BIGNUM *r, BIGNUM *a, BIGNUM *b,
164 \& BN_RECP_CTX *recp, BN_CTX *ctx);
165 .Ve
166 .SH "DESCRIPTION"
167 .IX Header "DESCRIPTION"
168 \&\fIBN_mod_mul_reciprocal()\fR can be used to perform an efficient
169 \&\fIBN_mod_mul\fR\|(3) operation when the operation will be performed
170 repeatedly with the same modulus. It computes \fBr\fR=(\fBa\fR*\fBb\fR)%\fBm\fR
171 using \fBrecp\fR=1/\fBm\fR, which is set as described below. \fBctx\fR is a
172 previously allocated \fB\s-1BN_CTX\s0\fR used for temporary variables.
173 .PP
174 \&\fIBN_RECP_CTX_new()\fR allocates and initializes a \fB\s-1BN_RECP\s0\fR structure.
175 \&\fIBN_RECP_CTX_init()\fR initializes an existing uninitialized \fB\s-1BN_RECP\s0\fR.
176 .PP
177 \&\fIBN_RECP_CTX_free()\fR frees the components of the \fB\s-1BN_RECP\s0\fR, and, if it
178 was created by \fIBN_RECP_CTX_new()\fR, also the structure itself.
179 .PP
180 \&\fIBN_RECP_CTX_set()\fR stores \fBm\fR in \fBrecp\fR and sets it up for computing
181 1/\fBm\fR and shifting it left by BN_num_bits(\fBm\fR)+1 to make it an
182 integer. The result and the number of bits it was shifted left will
183 later be stored in \fBrecp\fR.
184 .PP
185 \&\fIBN_div_recp()\fR divides \fBa\fR by \fBm\fR using \fBrecp\fR. It places the quotient
186 in \fBdv\fR and the remainder in \fBrem\fR.
187 .PP
188 The \fB\s-1BN_RECP_CTX\s0\fR structure is defined as follows:
189 .PP
190 .Vb 8
191 \& typedef struct bn_recp_ctx_st
192 \& {
193 \& BIGNUM N; /* the divisor */
194 \& BIGNUM Nr; /* the reciprocal */
195 \& int num_bits;
196 \& int shift;
197 \& int flags;
198 \& } BN_RECP_CTX;
199 .Ve
200 .PP
201 It cannot be shared between threads.
202 .SH "RETURN VALUES"
203 .IX Header "RETURN VALUES"
204 \&\fIBN_RECP_CTX_new()\fR returns the newly allocated \fB\s-1BN_RECP_CTX\s0\fR, and \s-1NULL\s0
205 on error.
206 .PP
207 \&\fIBN_RECP_CTX_init()\fR and \fIBN_RECP_CTX_free()\fR have no return values.
208 .PP
209 For the other functions, 1 is returned for success, 0 on error.
210 The error codes can be obtained by \fIERR_get_error\fR\|(3).
211 .SH "SEE ALSO"
212 .IX Header "SEE ALSO"
213 \&\fIopenssl_bn\fR\|(3), \fIERR_get_error\fR\|(3), \fIBN_add\fR\|(3),
214 \&\fIBN_CTX_new\fR\|(3)
215 .SH "HISTORY"
216 .IX Header "HISTORY"
217 \&\fB\s-1BN_RECP_CTX\s0\fR was added in SSLeay 0.9.0. Before that, the function
218 \&\fIBN_reciprocal()\fR was used instead, and the \fIBN_mod_mul_reciprocal()\fR
219 arguments were different.