132 lines
4.7 KiB
Plaintext
132 lines
4.7 KiB
Plaintext
.\" This file is dual-licensed. Choose whichever you want.
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.\"
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.\" The first licence is a regular 2-clause BSD licence. The second licence
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.\" is the CC-0 from Creative Commons. It is intended to release Monocypher
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.\" to the public domain. The BSD licence serves as a fallback option.
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.\"
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.\" SPDX-License-Identifier: BSD-2-Clause OR CC0-1.0
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.\"
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.\" ----------------------------------------------------------------------------
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.\"
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.\" Copyright (c) 2020 Fabio Scotoni
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.\" All rights reserved.
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.\"
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions are
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.\" met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the
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.\" distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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.\" "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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.\" LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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.\" A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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.\" HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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.\" OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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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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.\" and related neighboring rights to this software to the public domain
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.\" worldwide. This software is distributed without any warranty.
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.\"
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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_DIRTY_FAST 3monocypher
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.Os
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.Sh NAME
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.Nm crypto_x25519_dirty_fast ,
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.Nm crypto_x25519_dirty_small
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.Nd generation of Curve25519 points with a low-order component
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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_dirty_fast
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.Fa "uint8_t pk[32]"
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.Fa "const uint8_t sk[32]"
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.Fc
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.Ft void
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.Fo crypto_x25519_dirty_small
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.Fa "uint8_t pk[32]"
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.Fa "const uint8_t sk[32]"
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.Fc
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.Sh DESCRIPTION
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These functions are used in public key generation for
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.Xr crypto_curve_to_hidden 3monocypher .
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.Sy This is a highly advanced feature ;
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unless you are reading this because you were referred here from
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.Xr crypto_curve_to_hidden 3monocypher ,
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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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Both functions generate a Curve25519 public key
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.Fa pk
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from the given secret key
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.Fa sk ;
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the public keys are on the
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.Em whole
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curve, rather than just the main prime-order subgroup.
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Both do the same with different code size and memory characteristics:
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.Fn crypto_x25519_dirty_fast
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uses multiple large temporary variables and uses functions that are
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normally used internally for
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.Xr crypto_sign 3monocypher ;
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accordingly, it uses both more memory (for the temporary variables) and
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more code size (unless the signing code is already compiled in
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elsewhere).
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.Fn crypto_x25519_dirty_small
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yields the same result, but does so using less code and memory at a
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large performance penalty compared to
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.Fn crypto_x25519_dirty_fast .
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.Pp
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The resulting public keys are to be used with
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.Xr crypto_x25519 3monocypher
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or
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.Xr crypto_key_exchange 3monocypher ,
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which clear the cofactor.
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.Sh RETURN VALUES
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These functions have no return value.
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They cannot fail.
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.Sh SEE ALSO
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.Xr crypto_curve_to_hidden 3monocypher ,
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.Xr crypto_key_exchange_public_key 3monocypher ,
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.Xr crypto_x25519_public_key 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_dirty_fast
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and
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.Fn crypto_x25519_dirty_small
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functions first appeared in Monocypher 3.1.0.
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.Sh IMPLEMENTATION DETAILS
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The slow variant is approximately an entire two times slower than the
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fast variant.
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When considering that, on average, two calls to this function will be
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required for obtaining a valid key pair for
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.Xr crypto_curve_to_hidden 3monocypher ,
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this adds up to an
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.Em average
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effective slowdown for key pair generation of a factor of four.
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