path: root/lib/libc/hash/sha2.3

.\"	$OpenBSD: sha2.3,v 1.1 2003/05/08 23:34:55 millert Exp $
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.\" Copyright (c) 2003 Todd C. Miller <Todd.Miller@courtesan.com>
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.\" See http://www.nist.gov/sha/ for the detailed standard
.\"
.Dd April 24, 2003
.Dt SHA2 3
.Os
.Sh NAME
.Nm SHA256_Init ,
.Nm SHA256_Update ,
.Nm SHA256_Final ,
.Nm SHA256_End ,
.Nm SHA256_File ,
.Nm SHA256_Data
.Nd calculate the NIST Secure Hash Standard (version 2)
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sha2.h>
.Ft void
.Fn SHA256_Init "SHA256_CTX *context"
.Ft void
.Fn SHA256_Update "SHA256_CTX *context" "const u_int8_t *data" "size_t len"
.Ft void
.Fn SHA256_Final "u_int8_t digest[SHA256_DIGEST_LENGTH]" "SHA256_CTX *context"
.Ft "char *"
.Fn SHA256_End "SHA256_CTX *context" "char buf[SHA256_DIGEST_STRING_LENGTH]"
.Ft "char *"
.Fn SHA256_File "char *filename" "char buf[SHA256_DIGEST_STRING_LENGTH]"
.Ft "char *"
.Fn SHA256_Data "u_int8_t *data" "u_int len" "char buf[SHA256_DIGEST_STRING_LENGTH]"
.Ft void
.Fn SHA384_Init "SHA384_CTX *context"
.Ft void
.Fn SHA384_Update "SHA384_CTX *context" "const u_int8_t *data" "size_t len"
.Ft void
.Fn SHA384_Final "u_int8_t digest[SHA384_DIGEST_LENGTH]" "SHA384_CTX *context"
.Ft "char *"
.Fn SHA384_End "SHA384_CTX *context" "char buf[SHA384_DIGEST_STRING_LENGTH]"
.Ft "char *"
.Fn SHA384_File "char *filename" "char buf[SHA384_DIGEST_STRING_LENGTH]"
.Ft "char *"
.Fn SHA384_Data "u_int8_t *data" "size_t len" "char buf[SHA384_DIGEST_STRING_LENGTH]"
.Ft void
.Fn SHA512_Init "SHA512_CTX *context"
.Ft void
.Fn SHA512_Update "SHA512_CTX *context" "const u_int8_t *data" "size_t len"
.Ft void
.Fn SHA512_Final "u_int8_t digest[SHA512_DIGEST_LENGTH]" "SHA512_CTX *context"
.Ft "char *"
.Fn SHA512_End "SHA512_CTX *context" "char buf[SHA512_DIGEST_STRING_LENGTH]"
.Ft "char *"
.Fn SHA512_File "char *filename" "char buf[SHA512_DIGEST_STRING_LENGTH]"
.Ft "char *"
.Fn SHA512_Data "u_int8_t *data" "size_t len" "char buf[SHA512_DIGEST_STRING_LENGTH]"
.Sh DESCRIPTION
The SHA2 functions implement the NIST Secure Hash Standard,
FIPS PUB 180-2.
The SHA2 functions are used to generate a condensed representation of a
message called a message digest, suitable for use as a digital signature.
There are three families of functions, with names corresponding to 
the number of bits in the resulting message digest.
The SHA-256 functions are limited to processing a message of less
than 2^64 bits as input.
The SHA-384 and SHA-512 functions can process a message of at most 2^128 - 1
bits as input.
.Pp
The SHA2 functions are considered to be more secure than the
.Xr sha1 3
functions with which they share a similar interface.
The 256, 384, and 512 bit versions of SHA2 share the same interface.
For brevity, only the 256 bit variants are described below.
.Pp
The
.Fn SHA256_Init
function initializes a SHA256_CTX
.Ar context
for use with
.Fn SHA256_Update ,
and
.Fn SHA256_Final .
The
.Fn SHA256_Update
function adds
.Ar data
of length
.Ar len
to the SHA256_CTX specified by
.Ar context .
.Fn SHA256_Final
is called when all data has been added via
.Fn SHA256_Update
and stores a message digest in the
.Ar digest
parameter.
When a null pointer is passed to
.Fn SHA256_Final
as first argument only the final padding will be applied and the
current context can still be used with
.Fn SHA256_Update .
.Pp
The
.Fn SHA256_End
function is a front end for
.Fn SHA256_Final
which converts the digest into an
.Tn ASCII
representation of the digest in hexadecimal.
.Pp
The
.Fn SHA256_File
function calculates the digest for a file and returns the result via
.Fn SHA256_End .
If
.Fn SHA256_File
is unable to open the file a NULL pointer is returned.
.Pp
The
.Fn SHA256_Data
function
calculates the digest of an arbitrary string and returns the result via
.Fn SHA256_End .
.Pp
For each of the
.Fn SHA256_End ,
.Fn SHA256_File ,
and
.Fn SHA256_Data
functions the
.Ar buf
parameter should either be a string large enough to hold the resulting digest
(e.g.
.Ev SHA256_DIGEST_STRING_LENGTH ,
.Ev SHA384_DIGEST_STRING_LENGTH
or
.Ev SHA512_DIGEST_STRING_LENGTH ,
depending on the function being used)
or a 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 follow code fragment will calculate the digest for
the string "abc" which is ``0xa9993e36476816aba3e25717850c26c9cd0d89d''.
.Bd -literal -offset indent
SHA256_CTX ctx;
u_int8_t results[SHA256_DIGEST_LENGTH];
char *buf;
int n;

buf = "abc";
n = strlen(buf);
SHA256_Init(&ctx);
SHA256_Update(&ctx, (u_int8_t *)buf, n);
SHA256_Final(results, &ctx);

/* Print the digest as one long hex value */
printf("0x");
for (n = 0; n < SHA256_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
SHA256_CTX ctx;
u_int8_t output[SHA256_DIGEST_STRING_LENGTH];
char *buf = "abc";

printf("0x%s\en", SHA256_Data(buf, strlen(buf), output));
.Ed
.Sh CAVEATS
This implementation of the Secure Hash Standard has not been validated by
NIST and as such is not in official compliance with the standard.
.Pp
If a message digest is to be copied to a multi-byte type (ie:
an array of five 32-bit integers) it will be necessary to
perform byte swapping on little endian machines such as the i386, alpha,
and vax.
.Sh AUTHORS
This implementation of the SHA fucntions was written by Aaron D. Gifford.
.Pp
The
.Fn SHA256_End ,
.Fn SHA256_File ,
and
.Fn SHA256_Data
helper functions are derived from code written by Poul-Henning Kamp.
.Sh SEE ALSO
.Xr md4 3 ,
.Xr md5 3 ,
.Xr rmd160 3 ,
.Xr sha1 3
.Rs
.%T Secure Hash Standard
.%O FIPS PUB 180-2
.Re
.Sh HISTORY
The SHA2 functions appeared in
.Ox 3.4 .