203 lines
4.9 KiB
Groff
203 lines
4.9 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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.\" $FreeBSD$
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.\"
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.Dd February 11, 1999
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.Dt MDX 3
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.Os
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.Sh NAME
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.Nm MDXInit ,
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.Nm MDXUpdate ,
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.Nm MDXPad ,
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.Nm MDXFinal ,
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.Nm MDXEnd ,
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.Nm MDXFile ,
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.Nm MDXFileChunk ,
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.Nm MDXData
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.Nd calculate the RSA Data Security, Inc., ``MDX'' 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 mdX.h
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.Ft void
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.Fn MDXInit "MDX_CTX *context"
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.Ft void
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.Fn MDXUpdate "MDX_CTX *context" "const void *data" "unsigned int len"
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.Ft void
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.Fn MDXPad "MDX_CTX *context"
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.Ft void
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.Fn MDXFinal "unsigned char digest[16]" "MDX_CTX *context"
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.Ft "char *"
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.Fn MDXEnd "MDX_CTX *context" "char *buf"
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.Ft "char *"
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.Fn MDXFile "const char *filename" "char *buf"
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.Ft "char *"
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.Fn MDXFileChunk "const char *filename" "char *buf" "off_t offset" "off_t length"
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.Ft "char *"
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.Fn MDXData "const void *data" "unsigned int len" "char *buf"
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.Sh DESCRIPTION
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The MDX functions calculate a 128-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, you cannot find (except by exhaustive search)
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the input corresponding to a particular output.
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This net result is 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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MD2 is the slowest, MD4 is the fastest and MD5 is somewhere in the middle.
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MD2 can only be used for Privacy-Enhanced Mail.
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MD4 has now been broken; it should only be used where necessary for
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backward compatibility.
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MD5 has not yet (1999-02-11) been broken, but sufficient attacks have been
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made that its security is in some doubt.
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The attacks on both MD4 and MD5
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are both in the nature of finding
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.Dq collisions
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\[en]
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that is, multiple
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inputs which hash to the same value; it is still unlikely for an attacker
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to be able to determine the exact original input given a hash value.
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.Pp
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The
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.Fn MDXInit ,
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.Fn MDXUpdate ,
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and
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.Fn MDXFinal
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functions are the core functions.
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Allocate an
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.Vt MDX_CTX ,
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initialize it with
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.Fn MDXInit ,
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run over the data with
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.Fn MDXUpdate ,
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and finally extract the result using
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.Fn MDXFinal .
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.Pp
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The
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.Fn MDXPad
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function can be used to pad message data in same way
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as done by
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.Fn MDXFinal
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without terminating calculation.
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.Pp
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The
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.Fn MDXEnd
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function is a wrapper for
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.Fn MDXFinal
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which converts the return value to a 33-character
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(including the terminating '\e0')
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.Tn ASCII
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string which represents the 128 bits in hexadecimal.
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.Pp
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The
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.Fn MDXFile
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function calculates the digest of a file, and uses
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.Fn MDXEnd
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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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The
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.Fn MDXFileChunk
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function is similar to
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.Fn MDXFile ,
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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 MDXFileChunk
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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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The
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.Fn MDXData
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function calculates the digest of a chunk of data in memory, and uses
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.Fn MDXEnd
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to return the result.
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.Pp
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When using
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.Fn MDXEnd ,
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.Fn MDXFile ,
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or
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.Fn MDXData ,
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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 33 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 sha 3
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.Rs
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.%A B. Kaliski
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.%T The MD2 Message-Digest Algorithm
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.%O RFC 1319
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.Re
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.Rs
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.%A R. Rivest
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.%T The MD4 Message-Digest Algorithm
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.%O RFC 1186
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.Re
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.Rs
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.%A R. Rivest
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.%T The MD5 Message-Digest Algorithm
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.%O RFC 1321
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.Re
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.Rs
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.%A RSA Laboratories
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.%T Frequently Asked Questions About today's Cryptography
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.%O \&<http://www.rsa.com/rsalabs/faq/>
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.Re
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.Rs
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.%A H. Dobbertin
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.%T Alf Swindles Ann
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.%J CryptoBytes
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.%N 1(3):5
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.%D 1995
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.Re
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.Rs
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.%A MJ. B. Robshaw
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.%T On Recent Results for MD2, MD4 and MD5
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.%J RSA Laboratories Bulletin
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.%N 4
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.%D November 12, 1996
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.Re
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.Sh HISTORY
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These functions appeared in
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.Fx 2.0 .
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.Sh AUTHORS
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The original MDX routines were developed by
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.Tn RSA
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Data Security, Inc., and published in the above references.
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This code is derived directly from these implementations by
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.An Poul-Henning Kamp Aq phk@FreeBSD.org
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.Pp
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Phk ristede runen.
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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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.Pp
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MD2 has only been licensed for use in Privacy Enhanced Mail.
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Use MD4 or MD5 if that is not what you are doing.
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