106 lines
3.9 KiB
Plaintext
106 lines
3.9 KiB
Plaintext
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.\" This file is dual-licensed. Choose whichever you want.
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.\" The first licence is a regular 2-clause BSD licence. The second licence
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.\" to the public domain. The BSD licence serves as a fallback option.
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.\" ----------------------------------------------------------------------------
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.\"
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.\" Copyright (c) 2020 Fabio Scotoni
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.\" ----------------------------------------------------------------------------
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.\"
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.\" Written in 2020 by Fabio Scotoni
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.\"
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.\" To the extent possible under law, the author(s) have dedicated all copyright
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.\" worldwide. This software is distributed without any warranty.
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.\" You should have received a copy of the CC0 Public Domain Dedication along
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.\" with this software. If not, see
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.\" <https://creativecommons.org/publicdomain/zero/1.0/>
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.\"
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.Dd March 24, 2020
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.Dt CRYPTO_X25519_INVERSE 3MONOCYPHER
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.Os
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.Sh NAME
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.Nm crypto_x25519_inverse
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.Nd X25519 scalar multiplication with the multiplicative inverse of a scalar
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.Sh SYNOPSIS
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.In monocypher.h
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.Ft void
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.Fo crypto_x25519_inverse
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.Fa "uint8_t blind_salt[32]"
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.Fa "const uint8_t private_key[32]"
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.Fa "const uint8_t curve_point"
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.Fc
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.Sh DESCRIPTION
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The
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.Fn crypto_x25519_inverse
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function performs scalar multiplication of the multiplicative inverse
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of a scalar for X25519.
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.Sy This is a highly advanced, specialized feature ;
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unless you are implementing a protocol that requires this specifically,
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.Sy you likely have no reason to be using these functions
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and are probably looking for
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.Xr crypto_key_exchange 3monocypher
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or
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.Xr crypto_x25519 3monocypher
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instead.
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Expect elliptic curve jargon on this page.
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.Pp
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This function is used, for example, with exponential blinding in
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oblivious pseudo-random functions (OPRFs).
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The arguments are:
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.Bl -tag -width Ds
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.It Fa blind_salt
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The output point.
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.It Fa private_key
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The private key (scalar) to use.
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First, the value is clamped;
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then the clamped value's multiplicative inverse (modulo the curve order)
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is determined;
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the clamped value's multiplicative inverse then has its cofactor
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cleared,
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and that final value is then used for scalar multiplication.
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.It Fa curve_point
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The curve point on X25519 to multiply with the multiplicative inverse
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(modulo the curve order) of
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.Fa private_key .
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.El
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.Sh SEE ALSO
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.Xr crypto_x25519 3monocypher ,
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.Xr intro 3monocypher
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.Sh HISTORY
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The
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.Fn crypto_x25519_inverse
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function first appeared in Monocypher 3.1.0.
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