.\" $OpenBSD: BN_mod_mul_montgomery.3,v 1.14 2021/12/21 11:14:07 schwarze Exp $ .\" full merge up to: OpenSSL 6859cf74 Sep 25 13:33:28 2002 +0000 .\" selective merge up to: OpenSSL 24a535ea Sep 22 13:14:20 2020 +0100 .\" .\" This file is a derived work. .\" The changes are covered by the following Copyright and license: .\" .\" Copyright (c) 2021 Ingo Schwarze .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. 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Redistributions of any form whatsoever must retain the following .\" acknowledgment: .\" "This product includes software developed by the OpenSSL Project .\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" .\" .\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY .\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: December 21 2021 $ .Dt BN_MOD_MUL_MONTGOMERY 3 .Os .Sh NAME .Nm BN_MONT_CTX_new , .Nm BN_MONT_CTX_init , .Nm BN_MONT_CTX_free , .Nm BN_MONT_CTX_set , .Nm BN_MONT_CTX_set_locked , .Nm BN_MONT_CTX_copy , .Nm BN_mod_mul_montgomery , .Nm BN_from_montgomery , .Nm BN_to_montgomery .Nd Montgomery multiplication .Sh SYNOPSIS .In openssl/bn.h .Ft BN_MONT_CTX * .Fo BN_MONT_CTX_new .Fa void .Fc .Ft void .Fo BN_MONT_CTX_init .Fa "BN_MONT_CTX *ctx" .Fc .Ft void .Fo BN_MONT_CTX_free .Fa "BN_MONT_CTX *mont" .Fc .Ft int .Fo BN_MONT_CTX_set .Fa "BN_MONT_CTX *mont" .Fa "const BIGNUM *m" .Fa "BN_CTX *ctx" .Fc .Ft BN_MONT_CTX * .Fo BN_MONT_CTX_set_locked .Fa "BN_MONT_CTX **pmont" .Fa "int lock" .Fa "const BIGNUM *m" .Fa "BN_CTX *ctx" .Fc .Ft BN_MONT_CTX * .Fo BN_MONT_CTX_copy .Fa "BN_MONT_CTX *to" .Fa "BN_MONT_CTX *from" .Fc .Ft int .Fo BN_mod_mul_montgomery .Fa "BIGNUM *r" .Fa "const BIGNUM *a" .Fa "const BIGNUM *b" .Fa "BN_MONT_CTX *mont" .Fa "BN_CTX *ctx" .Fc .Ft int .Fo BN_from_montgomery .Fa "BIGNUM *r" .Fa "const BIGNUM *a" .Fa "BN_MONT_CTX *mont" .Fa "BN_CTX *ctx" .Fc .Ft int .Fo BN_to_montgomery .Fa "BIGNUM *r" .Fa "const BIGNUM *a" .Fa "BN_MONT_CTX *mont" .Fa "BN_CTX *ctx" .Fc .Sh DESCRIPTION These functions implement Montgomery multiplication. They are used automatically when .Xr BN_mod_exp 3 is called with suitable input, but they may be useful when several operations are to be performed using the same modulus. .Pp .Fn BN_MONT_CTX_new allocates and initializes a .Vt BN_MONT_CTX structure. .Pp .Fn BN_MONT_CTX_init initializes an existing uninitialized .Vt BN_MONT_CTX . It is deprecated and dangerous: see .Sx CAVEATS . .Pp .Fn BN_MONT_CTX_set sets up the .Fa mont structure from the modulus .Fa m by precomputing its inverse and a value R. .Pp .Fn BN_MONT_CTX_set_locked is a wrapper around .Fn BN_MONT_CTX_new and .Fn BN_MONT_CTX_set that is useful if more than one thread intends to use the same .Vt BN_MONT_CTX and none of these threads is exclusively responsible for creating and initializing the context. .Fn BN_MONT_CTX_set_locked first acquires the specified .Fa lock using .Xr CRYPTO_lock 3 . If .Pf * Fa pmont already differs from .Dv NULL , no action occurs. Otherwise, a new .Vt BN_MONT_CTX is allocated with .Fn BN_MONT_CTX_new , set up with .Fn BN_MONT_CTX_set , and a pointer to it is stored in .Pf * Fa pmont . Finally, the .Fa lock is released. .Pp .Fn BN_MONT_CTX_copy copies the .Vt BN_MONT_CTX .Fa from to .Fa to . .Pp .Fn BN_MONT_CTX_free frees the components of the .Vt BN_MONT_CTX , and, if it was created by .Fn BN_MONT_CTX_new , also the structure itself. If .Fa mont is a .Dv NULL pointer, no action occurs. .Pp .Fn BN_mod_mul_montgomery computes .Pp .D1 Mont Ns Po Fa a , Fa b Pc := Fa a No * Fa b No * R^-1 .Pp and places the result in .Fa r . .Pp .Fn BN_from_montgomery performs the Montgomery reduction .Pp .D1 Fa r No = Fa a No * R^-1 .Pp .Fn BN_to_montgomery computes .Pp .D1 Mont Ns Po Fa a , No R^2 Pc = Fa a No * R .Pp Note that .Fa a must be non-negative and smaller than the modulus. .Pp For all functions, .Fa ctx is a previously allocated .Vt BN_CTX used for temporary variables. .Pp The .Vt BN_MONT_CTX structure is defined as follows: .Bd -literal typedef struct bn_mont_ctx_st { int ri; /* number of bits in R */ BIGNUM RR; /* R^2 (used to convert to Montgomery form) */ BIGNUM N; /* The modulus */ BIGNUM Ni; /* R*(1/R mod N) - N*Ni = 1 * (Ni is only stored for bignum algorithm) */ BN_ULONG n0; /* least significant word of Ni */ int flags; } BN_MONT_CTX; .Ed .Pp .Sy Warning : The inputs must be reduced modulo .Fa m , otherwise the result will be outside the expected range. .Sh RETURN VALUES .Fn BN_MONT_CTX_new returns the newly allocated .Vt BN_MONT_CTX or .Dv NULL on error. .Pp .Fn BN_MONT_CTX_set_locked returns a pointer to the existing or newly created context or .Dv NULL on error. .Pp For the other functions, 1 is returned for success or 0 on error. The error codes can be obtained by .Xr ERR_get_error 3 . .Sh SEE ALSO .Xr BN_add 3 , .Xr BN_CTX_new 3 , .Xr BN_new 3 , .Xr CRYPTO_lock 3 .Sh HISTORY .Fn BN_MONT_CTX_new , .Fn BN_MONT_CTX_free , .Fn BN_MONT_CTX_set , .Fn BN_mod_mul_montgomery , .Fn BN_from_montgomery , and .Fn BN_to_montgomery first appeared in SSLeay 0.6.1 and have been available since .Ox 2.4 . .Pp .Fn BN_MONT_CTX_init and .Fn BN_MONT_CTX_copy first appeared in SSLeay 0.9.1 and have been available since .Ox 2.6 . .Pp .Fn BN_MONT_CTX_set_locked first appeared in OpenSSL 0.9.8 and has been available since .Ox 4.0 . .Sh CAVEATS .Fn BN_MONT_CTX_init must not be called on a context that was used previously, or memory used by the embedded .Vt BIGNUM structures is leaked immediately. Besides, it must not be called on a context created with .Fn BN_MONT_CTX_new , or the context itself will likely be leaked later. It can only be used on a static .Vt BN_MONT_CTX structure, on one located on the stack, or on one .Xr malloc 3 Ap ed manually, but all these options are discouraged because they will no longer work once .Vt BN_MONT_CTX is made opaque.