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"THE BEER-WARE LICENSE" (Revision 42):
<phk@FreeBSD.org> wrote this file. As long as you retain this notice you
can do whatever you want with this stuff. If we meet some day, and you think
this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
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$FreeBSD: head/lib/libmd/mdX.3 267773 2014-06-23 08:23:05Z bapt $
.Dd February 11, 1999 .Dt MDX 3 .Os .Sh NAME .Nm MDXInit , .Nm MDXUpdate , .Nm MDXPad , .Nm MDXFinal , .Nm MDXEnd , .Nm MDXFile , .Nm MDXFileChunk , .Nm MDXData .Nd calculate the RSA Data Security, Inc., ``MDX'' message digest .Sh LIBRARY .Lb libmd .Sh SYNOPSIS n sys/types.h n mdX.h .Ft void .Fn MDXInit "MDX_CTX *context" .Ft void .Fn MDXUpdate "MDX_CTX *context" "const void *data" "unsigned int len" .Ft void .Fn MDXPad "MDX_CTX *context" .Ft void .Fn MDXFinal "unsigned char digest[16]" "MDX_CTX *context" .Ft "char *" .Fn MDXEnd "MDX_CTX *context" "char *buf" .Ft "char *" .Fn MDXFile "const char *filename" "char *buf" .Ft "char *" .Fn MDXFileChunk "const char *filename" "char *buf" "off_t offset" "off_t length" .Ft "char *" .Fn MDXData "const void *data" "unsigned int len" "char *buf" .Sh DESCRIPTION The MDX functions calculate a 128-bit cryptographic checksum (digest) for any number of input bytes. A cryptographic checksum is a one-way hash-function, that is, you cannot find (except by exhaustive search) the input corresponding to a particular output. This net result is a .Dq fingerprint of the input-data, which does not disclose the actual input.
p MD4 is the fastest and MD5 is somewhat slower. MD4 has now been broken; it should only be used where necessary for backward compatibility. MD5 has not yet (1999-02-11) been broken, but sufficient attacks have been made that its security is in some doubt. The attacks on both MD4 and MD5 are both in the nature of finding .Dq collisions \[en] 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.
p The .Fn MDXInit , .Fn MDXUpdate , and .Fn MDXFinal functions are the core functions. Allocate an .Vt MDX_CTX , initialize it with .Fn MDXInit , run over the data with .Fn MDXUpdate , and finally extract the result using .Fn MDXFinal .
p The .Fn MDXPad function can be used to pad message data in same way as done by .Fn MDXFinal without terminating calculation.
p The .Fn MDXEnd function is a wrapper for .Fn MDXFinal which converts the return value to a 33-character (including the terminating '\e0') .Tn ASCII string which represents the 128 bits in hexadecimal.
p The .Fn MDXFile function calculates the digest of a file, and uses .Fn MDXEnd to return the result. If the file cannot be opened, a null pointer is returned. The .Fn MDXFileChunk function is similar to .Fn MDXFile , but it only calculates the digest over a byte-range of the file specified, starting at .Fa offset and spanning .Fa length bytes. If the .Fa length parameter is specified as 0, or more than the length of the remaining part of the file, .Fn MDXFileChunk calculates the digest from .Fa offset to the end of file. The .Fn MDXData function calculates the digest of a chunk of data in memory, and uses .Fn MDXEnd to return the result.
p When using .Fn MDXEnd , .Fn MDXFile , or .Fn MDXData , the .Fa 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 .Fa buf argument is non-null it must point to at least 33 characters of buffer space. .Sh SEE ALSO .Xr md4 3 , .Xr md5 3 , .Xr sha 3 .Rs .%A R. Rivest .%T The MD4 Message-Digest Algorithm .%O RFC 1186 .Re .Rs .%A R. Rivest .%T The MD5 Message-Digest Algorithm .%O RFC 1321 .Re .Rs .%A H. Dobbertin .%T Alf Swindles Ann .%J CryptoBytes .%N 1(3):5 .%D 1995 .Re .Rs .%A MJ. B. Robshaw .%T On Recent Results for MD2, MD4 and MD5 .%J RSA Laboratories Bulletin .%N 4 .%D November 12, 1996 .Re .Sh HISTORY These functions appeared in .Fx 2.0 . .Sh AUTHORS The original MDX routines were developed by .Tn RSA Data Security, Inc., and published in the above references. This code is derived directly from these implementations by .An Poul-Henning Kamp Aq Mt phk@FreeBSD.org .
p Phk ristede runen. .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 does not exist.