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MD5(3) Library Functions Manual MD5(3)


MD5Init, MD5Update, MD5Final, MD5End, MD5File, MD5Datacalculate the RSA Data Security, Inc., “MD5” message digest


Standard C Library (libc, -lc)


#include <sys/types.h>
#include <md5.h>

MD5Init(MD5_CTX *context);

MD5Update(MD5_CTX *context, const unsigned char *data, unsigned int len);

MD5Final(unsigned char digest[16], MD5_CTX *context);

char *
MD5End(MD5_CTX *context, char *buf);

char *
MD5File(const char *filename, char *buf);

char *
MD5Data(const unsigned char *data, unsigned int len, char *buf);


The MD5 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 ``fingerprint'' of the input-data, which doesn't disclose the actual input.

MD2 is the slowest, MD4 is the fastest and MD5 is somewhere in the middle. MD2 can only be used for Privacy-Enhanced Mail. MD4 has been criticized for being too weak, so MD5 was developed in response as ``MD4 with safety-belts''. When in doubt, use MD5.

The MD5Init(), MD5Update(), and MD5Final() functions are the core functions. Allocate an MD5_CTX, initialize it with MD5Init(), run over the data with MD5Update(), and finally extract the result using MD5Final().

MD5End() is a wrapper for MD5Final() which converts the return value to a 33-character (including the terminating '\0') ASCII string which represents the 128 bits in hexadecimal.

MD5File() calculates the digest of a file, and uses MD5End() to return the result. If the file cannot be opened, a null pointer is returned. MD5Data() calculates the digest of a chunk of data in memory, and uses MD5End() to return the result.

When using MD5End(), MD5File(), or MD5Data(), the buf argument can be a null pointer, in which case the returned string is allocated with malloc(3) and subsequently must be explicitly deallocated using free(3) after use. If the buf argument is non-null it must point to at least 33 characters of buffer space.


md2(3), md4(3), md5(3)

B. Kaliski, The MD2 Message-Digest Algorithm, RFC 1319.

R. Rivest, The MD4 Message-Digest Algorithm, RFC 1186.

R. Rivest, The MD5 Message-Digest Algorithm, RFC 1321.

RSA Laboratories, Frequently Asked Questions About today's Cryptography.


These functions appeared in NetBSD 1.3.


The original MD5 routines were developed by RSA Data Security, Inc., and published in the above references. This code is derived directly from these implementations by Poul-Henning Kamp <>

Phk ristede runen.


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.


Copyright (C) 1991-2, RSA Data Security, Inc. Created 1991. All rights reserved.

License to copy and use this software is granted provided that it is identified as the "RSA Data Security, Inc. MD5 Message-Digest Algorithm" in all material mentioning or referencing this software or this function.

License is also granted to make and use derivative works provided that such works are identified as "derived from the RSA Data Security, Inc. MD5 Message-Digest Algorithm" in all material mentioning or referencing the derived work.

RSA Data Security, Inc. makes no representations concerning either the merchantability of this software or the suitability of this software for any particular purpose. It is provided "as is" without express or implied warranty of any kind.

These notices must be retained in any copies of any part of this documentation and/or software.

June 13, 2003 NetBSD 7.0