.\"	$OpenBSD: SHA1Init.3,v 1.2 2019/12/05 21:45:05 jmc Exp $
.\"
.\" Copyright (c) 1997, 2004 Todd C. Miller <millert@openbsd.org>
.\"
.\" 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.
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\" See http://csrc.nist.gov/publications/fips/fips180-1/fip180-1.txt
.\" for the detailed standard
.\"
.Dd $Mdocdate: December 5 2019 $
.Dt SHA1INIT 3
.Os
.Sh NAME
.Nm SHA1Init ,
.Nm SHA1Update ,
.Nm SHA1Pad ,
.Nm SHA1Final ,
.Nm SHA1Transform ,
.Nm SHA1End ,
.Nm SHA1File ,
.Nm SHA1FileChunk ,
.Nm SHA1Data
.Nd calculate the NIST Secure Hash Algorithm
.Sh SYNOPSIS
.In sys/types.h
.In sha1.h
.Ft void
.Fn SHA1Init "SHA1_CTX *context"
.Ft void
.Fn SHA1Update "SHA1_CTX *context" "const u_int8_t *data" "size_t len"
.Ft void
.Fn SHA1Pad "SHA1_CTX *context"
.Ft void
.Fn SHA1Final "u_int8_t digest[SHA1_DIGEST_LENGTH]" "SHA1_CTX *context"
.Ft void
.Fn SHA1Transform "u_int32_t state[5]" "const u_int8_t buffer[SHA1_BLOCK_LENGTH]"
.Ft "char *"
.Fn SHA1End "SHA1_CTX *context" "char *buf"
.Ft "char *"
.Fn SHA1File "const char *filename" "char *buf"
.Ft "char *"
.Fn SHA1FileChunk "const char *filename" "char *buf" "off_t offset" "off_t length"
.Ft "char *"
.Fn SHA1Data "const u_int8_t *data" "size_t len" "char *buf"
.Sh DESCRIPTION
The SHA1 functions implement the NIST Secure Hash Algorithm (SHA-1),
FIPS PUB 180-1.
SHA-1 is used to generate a condensed representation
of a message called a message digest.
The algorithm takes a
message less than 2^64 bits as input and produces a 160-bit digest
suitable for use as a digital signature.
.Pp
SHA-1 has been broken; it should only be used where necessary for
backward compatibility.
The attack on SHA-1 is in the nature of finding
.Dq collisions
\(em that is, multiple inputs which hash to the same value.
It is still unlikely for an attacker to be able to determine the exact
original input given a hash value.
.Pp
The
.Fn SHA1Init
function initializes a SHA1_CTX
.Fa context
for use with
.Fn SHA1Update ,
and
.Fn SHA1Final .
The
.Fn SHA1Update
function adds
.Fa data
of length
.Fa len
to the SHA1_CTX specified by
.Fa context .
.Fn SHA1Final
is called when all data has been added via
.Fn SHA1Update
and stores a message digest in the
.Fa digest
parameter.
.Pp
The
.Fn SHA1Pad
function can be used to apply padding to the message digest as in
.Fn SHA1Final ,
but the current context can still be used with
.Fn SHA1Update .
.Pp
The
.Fn SHA1Transform
function is used by
.Fn SHA1Update
to hash 512-bit blocks and forms the core of the algorithm.
Most programs should use the interface provided by
.Fn SHA1Init ,
.Fn SHA1Update
and
.Fn SHA1Final
instead of calling
.Fn SHA1Transform
directly.
.Pp
The
.Fn SHA1End
function is a front end for
.Fn SHA1Final
which converts the digest into an ASCII representation
of the 160 bit digest in hexadecimal.
.Pp
The
.Fn SHA1File
function calculates the digest for a file and returns the result via
.Fn SHA1End .
If
.Fn SHA1File
is unable to open the file a
.Dv NULL
pointer is returned.
.Pp
.Fn SHA1FileChunk
behaves like
.Fn SHA1File
but calculates the digest only for that portion of the file starting at
.Fa offset
and continuing for
.Fa length
bytes or until end of file is reached, whichever comes first.
A zero
.Fa length
can be specified to read until end of file.
A negative
.Fa length
or
.Fa offset
will be ignored.
.Pp
The
.Fn SHA1Data
function
calculates the digest of an arbitrary string and returns the result via
.Fn SHA1End .
.Pp
For each of the
.Fn SHA1End ,
.Fn SHA1File ,
and
.Fn SHA1Data
functions the
.Fa buf
parameter should either be a string of at least 41 characters in
size or a
.Dv NULL
pointer.
In the latter case, space will be dynamically allocated via
.Xr malloc 3
and should be freed using
.Xr free 3
when it is no longer needed.
.Sh EXAMPLES
The following code fragment will calculate the digest for
the string
.Qq abc
which is
.Dq 0xa9993e364706816aba3e25717850c26c9cd0d89d .
.Bd -literal -offset indent
SHA1_CTX sha;
u_int8_t results[SHA1_DIGEST_LENGTH];
char *buf;
int n;

buf = "abc";
n = strlen(buf);
SHA1Init(&sha);
SHA1Update(&sha, (u_int8_t *)buf, n);
SHA1Final(results, &sha);

/* Print the digest as one long hex value */
printf("0x");
for (n = 0; n < SHA1_DIGEST_LENGTH; n++)
	printf("%02x", results[n]);
putchar('\en');
.Ed
.Pp
Alternately, the helper functions could be used in the following way:
.Bd -literal -offset indent
u_int8_t output[SHA1_DIGEST_STRING_LENGTH];
char *buf = "abc";

printf("0x%s\en", SHA1Data(buf, strlen(buf), output));
.Ed
.Sh SEE ALSO
.Xr cksum 1 ,
.Xr sha1 1 ,
.Xr MD5Init 3 ,
.Xr RMD160Init 3 ,
.Xr SHA256Init 3
.Sh STANDARDS
.Rs
.%A J. Burrows
.%R FIPS PUB 180-1
.%T The Secure Hash Standard
.Re
.Pp
.Rs
.%A D. Eastlake
.%A P. Jones
.%D September 2001
.%R RFC 3174
.%T US Secure Hash Algorithm 1 (SHA1)
.Re
.Sh HISTORY
The SHA-1 functions appeared in
.Ox 2.0 .
.Sh AUTHORS
.An -nosplit
This implementation of SHA-1 was written by
.An Steve Reid .
.Pp
The
.Fn SHA1End ,
.Fn SHA1File ,
.Fn SHA1FileChunk ,
and
.Fn SHA1Data
helper functions are derived from code written by
.An Poul-Henning Kamp .