140 lines
3.5 KiB
Groff
140 lines
3.5 KiB
Groff
.\"
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.\" ----------------------------------------------------------------------------
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.\" "THE BEER-WARE LICENSE" (Revision 42):
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.\" <phk@FreeBSD.org> wrote this file. As long as you retain this notice you
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.\" can do whatever you want with this stuff. If we meet some day, and you think
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.\" this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
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.\" ----------------------------------------------------------------------------
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.\"
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.\" From: Id: mdX.3,v 1.14 1999/02/11 20:31:49 wollman Exp
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.\" $FreeBSD$
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.\"
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.Dd September 14, 2005
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.Dt SHA256 3
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.Os
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.Sh NAME
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.Nm SHA256_Init ,
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.Nm SHA256_Update ,
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.Nm SHA256_Final ,
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.Nm SHA256_End ,
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.Nm SHA256_File ,
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.Nm SHA256_FileChunk ,
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.Nm SHA256_Data
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.Nd calculate the FIPS 180-2 ``SHA-256'' message digest
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.Sh LIBRARY
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.Lb libmd
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.Sh SYNOPSIS
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.In sys/types.h
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.In sha256.h
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.Ft void
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.Fn SHA256_Init "SHA256_CTX *context"
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.Ft void
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.Fn SHA256_Update "SHA256_CTX *context" "const unsigned char *data" "size_t len"
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.Ft void
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.Fn SHA256_Final "unsigned char digest[32]" "SHA256_CTX *context"
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.Ft "char *"
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.Fn SHA256_End "SHA256_CTX *context" "char *buf"
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.Ft "char *"
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.Fn SHA256_File "const char *filename" "char *buf"
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.Ft "char *"
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.Fn SHA256_FileChunk "const char *filename" "char *buf" "off_t offset" "off_t length"
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.Ft "char *"
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.Fn SHA256_Data "const unsigned char *data" "unsigned int len" "char *buf"
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.Sh DESCRIPTION
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The
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.Li SHA256_
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functions calculate a 256-bit cryptographic checksum (digest)
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for any number of input bytes.
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A cryptographic checksum is a one-way
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hash function; that is, it is computationally impractical to find
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the input corresponding to a particular output.
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This net result is
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a
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.Dq fingerprint
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of the input-data, which does not disclose the actual input.
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.Pp
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The
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.Fn SHA256_Init ,
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.Fn SHA256_Update ,
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and
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.Fn SHA256_Final
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functions are the core functions.
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Allocate an
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.Vt SHA256_CTX ,
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initialize it with
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.Fn SHA256_Init ,
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run over the data with
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.Fn SHA256_Update ,
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and finally extract the result using
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.Fn SHA256_Final .
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.Pp
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.Fn SHA256_End
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is a wrapper for
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.Fn SHA256_Final
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which converts the return value to a 65-character
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(including the terminating '\e0')
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.Tn ASCII
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string which represents the 256 bits in hexadecimal.
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.Pp
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.Fn SHA256_File
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calculates the digest of a file, and uses
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.Fn SHA256_End
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to return the result.
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If the file cannot be opened, a null pointer is returned.
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.Fn SHA256_FileChunk
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is similar to
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.Fn SHA256_File ,
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but it only calculates the digest over a byte-range of the file specified,
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starting at
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.Fa offset
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and spanning
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.Fa length
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bytes.
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If the
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.Fa length
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parameter is specified as 0, or more than the length of the remaining part
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of the file,
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.Fn SHA256_FileChunk
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calculates the digest from
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.Fa offset
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to the end of file.
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.Fn SHA256_Data
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calculates the digest of a chunk of data in memory, and uses
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.Fn SHA256_End
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to return the result.
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.Pp
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When using
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.Fn SHA256_End ,
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.Fn SHA256_File ,
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or
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.Fn SHA256_Data ,
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the
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.Fa buf
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argument can be a null pointer, in which case the returned string
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is allocated with
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.Xr malloc 3
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and subsequently must be explicitly deallocated using
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.Xr free 3
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after use.
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If the
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.Fa buf
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argument is non-null it must point to at least 65 characters of buffer space.
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.Sh SEE ALSO
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.Xr md2 3 ,
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.Xr md4 3 ,
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.Xr md5 3 ,
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.Xr ripemd 3 ,
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.Xr sha 3
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.Sh HISTORY
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These functions appeared in
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.Fx 6.0 .
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.Sh AUTHORS
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The core hash routines were implemented by Colin Percival based on
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the published
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.Tn FIPS 180-2
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standard.
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.Sh BUGS
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No method is known to exist which finds two files having the same hash value,
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nor to find a file with a specific hash value.
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There is on the other hand no guarantee that such a method does not exist.
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