freebsd-nq/lib/libmd/sha.3

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.\" 	From: Id: mdX.3,v 1.14 1999/02/11 20:31:49 wollman Exp
.\" $FreeBSD$
.\"
.Dd February 25, 1999
.Dt SHA 3
.Os FreeBSD
.Sh NAME
.Nm SHA_Init ,
.Nm SHA_Update ,
.Nm SHA_Final ,
.Nm SHA_End ,
.Nm SHA_File ,
.Nm SHA_Data ,
.Nm SHA1_Init ,
.Nm SHA1_Update ,
.Nm SHA1_Final ,
.Nm SHA1_End ,
.Nm SHA1_File ,
.Nm SHA1_Data
.Nd calculate the FIPS 160 and 160-1 ``SHA'' message digests
.Sh LIBRARY
.Lb libmd
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sha.h>
.Ft void
.Fn SHA_Init "SHA_CTX *context"
.Ft void
.Fn SHA_Update "SHA_CTX *context" "const unsigned char *data" "unsigned int len"
.Ft void
.Fn SHA_Final "unsigned char digest[20]" "SHA_CTX *context"
.Ft "char *"
.Fn SHA_End "SHA_CTX *context" "char *buf"
.Ft "char *"
.Fn SHA_File "const char *filename" "char *buf"
.Ft "char *"
.Fn SHA_Data "const unsigned char *data" "unsigned int len" "char *buf"
.Ft void
.Fn SHA1_Init "SHA_CTX *context"
.Ft void
.Fn SHA1_Update "SHA_CTX *context" "const unsigned char *data" "unsigned int len"
.Ft void
.Fn SHA1_Final "unsigned char digest[20]" "SHA_CTX *context"
.Ft "char *"
.Fn SHA1_End "SHA_CTX *context" "char *buf"
.Ft "char *"
.Fn SHA1_File "const char *filename" "char *buf"
.Ft "char *"
.Fn SHA1_Data "const unsigned char *data" "unsigned int len" "char *buf"
.Sh DESCRIPTION
The
.Li SHA_
and
.Li SHA1_
functions calculate a 160-bit cryptographic checksum (digest)
for any number of input bytes.  A cryptographic checksum is a one-way
hash function; that is, it is computationally impractical to find
the input corresponding to a particular output.  This net result is 
a ``fingerprint'' of the input-data, which doesn't disclose the actual
input.
.Pp
.Tn SHA
.Pq \&or Tn SHA-0
is the original Secure Hash Algorithm specified in
.Tn FIPS
160.  It was quickly proven insecure, and has been superseded by
.Tn SHA-1 .
.Tn SHA-0
is included for compatibility purposes only.
.Pp
The
.Fn SHA1_Init  ,
.Fn SHA1_Update ,
and
.Fn SHA1_Final
functions are the core functions.  Allocate an SHA_CTX, initialize it with
.Fn SHA1_Init ,
run over the data with
.Fn SHA1_Update ,
and finally extract the result using
.Fn SHA1_Final .
.Pp
.Fn SHA1_End
is a wrapper for
.Fn SHA1_Final
which converts the return value to a 41-character
(including the terminating '\e0')
.Tn ASCII
string which represents the 160 bits in hexadecimal.
.Pp
.Fn SHA1_File
calculates the digest of a file, and uses 
.Fn SHA1_End
to return the result.
If the file cannot be opened, a null pointer is returned.
.Fn SHA1_Data
calculates the digest of a chunk of data in memory, and uses
.Fn SHA1_End
to return the result.
.Pp
When using
.Fn SHA1_End ,
.Fn SHA1_File ,
or
.Fn SHA1_Data ,
the 
.Ar buf
argument can be a null pointer, in which case the returned string
is allocated with
.Xr malloc 3
and subsequently must be explicitly deallocated using
.Xr free 3
after use.
If the 
.Ar buf
argument is non-null it must point to at least 41 characters of buffer space.
.Sh SEE ALSO
.Xr md2 3 ,
.Xr md4 3 ,
.Xr md5 3 ,
.Xr ripemd 3
.Sh AUTHORS
The core hash routines were implemented by Eric Young based on the
published
.Tn FIPS
standards.
.Sh HISTORY
These functions appeared in
.Fx 4.0 .
.Sh BUGS
No method is known to exist which finds two files having the same hash value,
nor to find a file with a specific hash value.
There is on the other hand no guarantee that such a method doesn't exist.
.Pp
The
.Tn IA32
(Intel) implementation of
.Tn SHA-1
makes heavy use of the
.Ql bswapl
instruction, which is not present on the original 80386.  Attempts
to use
.Tn SHA-1
on those processors will cause an illegal instruction trap.
(Arguably, the kernel should simply emulate this instruction.)