214 lines
6.5 KiB
Plaintext
214 lines
6.5 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) 2019-2020 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 2019-2020 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 February 5, 2020
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.Dt CRYPTO_SHA512 3MONOCYPHER
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.Os
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.Sh NAME
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.Nm crypto_sha512 ,
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.Nm crypto_sha512_init ,
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.Nm crypto_sha512_update ,
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.Nm crypto_sha512_final
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.Nd cryptographic hashing with the SHA-512 algorithm
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.Sh SYNOPSIS
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.In monocypher-ed25519.h
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.Ft void
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.Fo crypto_sha512
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.Fa "uint8_t hash[64]"
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.Fa "const uint8_t *message"
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.Fa "size_t message_size"
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.Fc
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.Ft void
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.Fo crypto_sha512_init
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.Fa "crypto_sha512_ctx *ctx"
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.Fc
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.Ft void
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.Fo crypto_sha512_update
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.Fa "crypto_sha512_ctx *ctx"
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.Fa "const uint8_t *message"
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.Fa "size_t message_size"
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.Fc
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.Ft void
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.Fo crypto_sha512_final
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.Fa "crypto_sha512_ctx *ctx"
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.Fa "uint8_t hash[64]"
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.Fc
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.Sh DESCRIPTION
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SHA-512 is a cryptographically secure hash,
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provided to enable compatibility with other cryptographic systems.
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It is generally recommended to use
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.Xr crypto_blake2b 3monocypher
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instead,
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as it both performs faster on x86_64 CPUs and
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lacks many of the pitfalls of SHA-512.
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.Pp
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Note that SHA-512 itself is not suitable for hashing passwords and
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deriving keys from them;
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use the
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.Xr crypto_argon2i 3monocypher
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family of functions for that purpose instead.
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.Pp
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SHA-512 is
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.Em vulnerable to length extension attacks ;
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using it as a message authentication code (MAC) algorithm or keyed hash
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requires precautions.
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The
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.Xr crypto_hmac_sha512 3monocypher
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family of functions provides HMAC with SHA-512.
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Use
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.Xr crypto_verify64 3monocypher
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to compare MACs created this way.
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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 hash
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The output hash,
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which is always 64 bytes long.
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.It Fa message
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The message to hash.
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May overlap with
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.Fa hash .
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May be
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.Dv NULL
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if
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.Fa message_size
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is 0.
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.It Fa message_size
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Length of
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.Fa message ,
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in bytes.
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.El
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.Pp
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An incremental interface is provided.
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It is useful for handling streams of data or
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large files without using too much memory.
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This interface uses three steps:
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.Bl -bullet
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.It
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initialisation with
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.Fn crypto_sha512_init ,
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which sets up a context with the hashing parameters;
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.It
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update with
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.Fn crypto_sha512_update ,
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which hashes the message chunk by chunk, and keep the intermediary
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result in the context;
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.It
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and finalisation with
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.Fn crypto_sha512_final ,
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which produces the final hash.
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The
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.Ft crypto_sha512_ctx
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is automatically wiped upon finalisation.
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.El
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.Pp
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.Fn crypto_sha512
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is a convenience function that
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performs
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.Fn crypto_sha512_init ,
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.Fn crypto_sha512_update ,
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and
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.Fn crypto_sha512_final .
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.Sh RETURN VALUES
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These functions return nothing.
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.Sh EXAMPLES
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Hashing a message all at once:
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.Bd -literal -offset indent
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uint8_t hash [64]; /* Output hash (64 bytes) */
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uint8_t message[12] = "Lorem ipsum"; /* Message to hash */
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crypto_sha512(hash, message, 12);
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.Ed
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.Pp
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Hashing a message incrementally:
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.Bd -literal -offset indent
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uint8_t hash [ 64]; /* Output hash (64 bytes) */
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uint8_t message[500] = {1}; /* Message to hash */
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crypto_sha512_ctx ctx;
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crypto_sha512_init(&ctx);
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for (size_t i = 0; i < 500; i += 100) {
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crypto_sha512_update(&ctx, message + i, 100);
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}
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crypto_sha512_final(&ctx, hash);
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.Ed
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.Sh SEE ALSO
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.Xr crypto_blake2b 3monocypher ,
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.Xr crypto_hmac_sha512 3monocypher ,
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.Xr crypto_lock 3monocypher ,
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.Xr intro 3monocypher
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.Sh STANDARDS
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These functions implement SHA-512, described in RFC 6234 and
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the Federal Information Processing Standard (FIPS) 180-4.
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.Sh HISTORY
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The
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.Fn crypto_sha512 ,
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.Fn crypto_sha512_init ,
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.Fn crypto_sha512_update ,
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and
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.Fn crypto_sha512_final
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functions first appeared in Monocypher 0.3;
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they were not intended for use outside Monocypher itself and thus
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undocumented.
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They became part of the official API in Monocypher 3.0.0.
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.Sh SECURITY CONSIDERATIONS
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SHA-512 is a general-purpose cryptographic hash function;
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this means that it is not suited for hashing passwords and deriving
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cryptographic keys from passwords in particular.
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While cryptographic keys usually have hundreds of bits of entropy,
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passwords are often much less complex.
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When storing passwords as hashes or when deriving keys from them,
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the goal is normally to prevent attackers from quickly iterating all
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possible passwords.
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Because passwords tend to be simple,
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it is important to artificially slow down attackers by using especially
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computationally difficult hashing algorithms.
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Monocypher therefore provides
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.Xr crypto_argon2i 3monocypher
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for password hashing and deriving keys from passwords.
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