133 lines
4.7 KiB
Plaintext
133 lines
4.7 KiB
Plaintext
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.\" 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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.\"
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\"
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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 25, 2020
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.Dt CRYPTO_FROM_EDDSA_PRIVATE 3MONOCYPHER
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.Os
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.Sh NAME
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.Nm crypto_from_eddsa_private ,
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.Nm crypto_from_eddsa_public
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.Nd conversion of key pairs for EdDSA with BLAKE2b to X25519 key pairs
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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_from_eddsa_private
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.Fa "uint8_t x25519[32]"
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.Fa "const uint8_t eddsa[32]"
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.Fc
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.Ft void
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.Fo crypto_from_eddsa_public
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.Fa "uint8_t x25519[32]"
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.Fa "const uint8_t eddsa[32]"
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.Fc
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.Sh DESCRIPTION
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These functions convert keys for use with
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.Xr crypto_sign 3monocypher
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(EdDSA with the BLAKE2b hash function)
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to keys for use with
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.Xr crypto_key_exchange 3monocypher
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and
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.Xr crypto_x25519 3monocypher .
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This may be useful in some resource-constrained contexts or when no
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other key is available (for example, when retrieving SSH public keys
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from GitHub and reusing the SSH public keys as X25519 public keys).
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.Pp
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The
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.Fn crypto_from_eddsa_private
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function converts an EdDSA (with BLAKE2b) private key to an X25519
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private key.
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The
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.Fn crypto_from_eddsa_public
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function converts an EdDSA public key to an X25519 public key.
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.Pp
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X25519 key pairs cannot be converted back to EdDSA key pairs.
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The conversion of private keys is specific to EdDSA with BLAKE2b because
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of the way EdDSA works.
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In particular, this means that the output of
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.Fn crypto_from_eddsa_private
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differs from
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.Xr crypto_from_ed25519_private 3monocypher
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in the optional code.
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However, the output of
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.Fn crypto_from_eddsa_public
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is identical to
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.Xr crypto_from_ed25519_public 3monocypher .
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.Pp
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The arguments are:
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.Bl -tag -width Ds
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.It Fa eddsa
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The signing public key or private key to convert to a X25519 public key
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or private key, respectively.
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.It Fa x25519
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The converted private key or public key.
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.El
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.Pp
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The arguments may overlap or point at the same buffer.
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.Sh RETURN VALUES
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These functions return nothing.
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They cannot fail.
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.Sh SEE ALSO
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.Xr crypto_key_exchange_public_key 3monocypher ,
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.Xr crypto_sign_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_from_eddsa_private
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and
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.Fn crypto_from_eddsa_public
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functions first appeared in Monocypher 3.1.0.
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.Sh SECURITY CONSIDERATIONS
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It is generally considered poor form to reuse the same key for different
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purposes.
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While this conversion is technically safe,
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avoid these functions nonetheless unless you are particularly
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resource-constrained or have some other kind of hard requirement.
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It is otherwise an unnecessary risk factor.
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