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MD5INIT(3) Library Functions Manual MD5INIT(3)

MD5Init, MD5Update, MD5Pad, MD5Final, MD5Transform, MD5End, MD5File, MD5FileChunk, MD5Data — calculate MD5 message digest

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

void
MD5Init(MD5_CTX *context);

void
MD5Update(MD5_CTX *context, const u_int8_t *data, size_t len);

void
MD5Pad(MD5_CTX *context);

void
MD5Final(u_int8_t digest[MD5_DIGEST_LENGTH], MD5_CTX *context);

void
MD5Transform(u_int32_t state[4], u_int8_t block[MD5_BLOCK_LENGTH]);

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

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

char *
MD5FileChunk(const char *filename, char *buf, off_t offset, off_t length);

char *
MD5Data(const u_int8_t *data, size_t 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.

MD5 has been broken; it should only be used where necessary for backward compatibility. The attack on MD5 is in the nature of finding “collisions” — 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.

The (), (), 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().

The () function can be used to apply padding to the message digest as in (), but the current context can still be used with MD5Update().

The () function is used by () to hash 512-bit blocks and forms the core of the algorithm. Most programs should use the interface provided by MD5Init(), MD5Update() and MD5Final() instead of calling MD5Transform() directly.

() is a wrapper for () which converts the return value to an MD5_DIGEST_STRING_LENGTH-character (including the terminating '\0') ASCII string which represents the 128 bits in hexadecimal.

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

() behaves like MD5File() but calculates the digest only for that portion of the file starting at offset and continuing for length bytes or until end of file is reached, whichever comes first. A zero length can be specified to read until end of file. A negative length or offset will be ignored. () calculates the digest of a chunk of data in memory, and uses MD5End() to return the result.

When using (), MD5File(), MD5FileChunk(), 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 MD5_DIGEST_STRING_LENGTH characters of buffer space.

cksum(1), md5(1), RMD160Init(3), SHA1Init(3), SHA256Init(3)

H. Dobbertin, Alf Swindles Ann, CryptoBytes, 1(3):5, 1995.

MJ. B. Robshaw, On Recent Results for MD4 and MD5, RSA Laboratories Bulletin, 4, November 12, 1996.

Hans Dobbertin, Cryptanalysis of MD5 Compress.

R. Rivest, The MD5 Message Digest Algorithm, RFC 1321, April 1992.

These functions appeared in OpenBSD 2.0.

The original MD5 routines were developed by RSA Data Security, Inc., and published in the above references. This code is derived from a public domain implementation written by Colin Plumb.

The MD5End(), MD5File(), MD5FileChunk(), and MD5Data() helper functions are derived from code written by Poul-Henning Kamp.

Collisions have been found for the full version of MD5. The use of the SHA2 functions is recommended instead.

March 31, 2022 OpenBSD-current