MD5(3bsd)

💖 MD5 — the tea on MD5(3bsd) 💖

okay so bestie, welcome to MD5(3bsd) — this is a LOCAL man page, and we're about to get into it. no cap, this one's a classic. 🎀

✨ NAME ✨

MD5Init, MD5Update, MD5Pad, MD5Final, MD5Transform, MD5End, MD5File, MD5FileChunk, MD5Data — calculate the RSA Data Security, Inc., ``MD5'' message digest 💅

📚 LIBRARY 📚

Utility functions from BSD systems (libbsd, -lbsd) 🫶

🌸 SYNOPSIS 🌸

#include <sys/types.h>
#include <md5.h>
(See libbsd(7) for include usage.)

void
MD5Init(MD5_CTX *context);

void
MD5Update(MD5_CTX *context, const uint8_t *data, size_t len);

void
MD5Pad(MD5_CTX *context);

void
MD5Final(uint8_t digest[MD5_DIGEST_LENGTH], MD5_CTX *context);

void
MD5Transform(uint32_t state[4], uint8_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 uint8_t *data, size_t len, char *buf);

yes bestie, that's a whole lot of function signatures. it's giving ✨cryptography✨ and honestly? we're obsessed.

💅 DESCRIPTION 💅

The MD5 functions calculate a 128-bit cryptographic checksum (digest) for any number of input bytes. it's a one-way hash-function, meaning: you literally cannot find (except by exhaustive search, and good luck with that) the input corresponding to a particular output. the net result is a “fingerprint” of the input-data, which doesn't disclose the actual input. iconic behaviour, fr fr. 🦋

💀 okay so this part is important, don't skip it bestie

MD4 has been broken; it should only be used where necessary for backward compatibility. MD5 has not yet (as of 1999-02-11) been broken, but recent attacks have cast some doubt on its security properties. 😭 The attacks on both MD4 and MD5 are both 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. so like, it's not a full-on break, but the vibe check is... complicated. 🙈

🧚 the core functions

The MD5Init(), MD5Update(), and MD5Final() functions are the core functions. here's the whole routine, bestie:

1. Allocate an MD5_CTX.
2. initialize it with MD5Init().
3. run over the data with MD5Update().
4. and finally extract the result using MD5Final(). periodt. 💖

🎀 the padding one

The MD5Pad() function can be used to apply padding to the message digest as in MD5Final(), but the current context can still be used with MD5Update(). cute, right? 🍓

🌈 the low-level one

The MD5Transform() function is used by MD5Update() 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. like, don't touch the internals unless you know what you're doing, okay? 😌

⭐ the wrapper ones

• MD5End() is a wrapper for MD5Final() 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. ✨
• 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. 👑
• MD5FileChunk() 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. and MD5Data() calculates the digest of a chunk of data in memory, and uses MD5End() to return the result. 🦋

💜 the buf situation (read this, it matters)

When using MD5End(), 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.

🫶 so basically: pass NULL, you own the memory, you free it. pass a buffer, you already own it, no freeing needed. don't leak, bestie, it's not cute.

👀 SEE ALSO 👀

• cksum(1), md5(1), adler32(3), md4(3), rmd160(3), sfv(3), sha1(3), sha2(3), suma(3), tiger(3), whirlpool(3) 💖
• 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, <http://www.rsa.com/rsalabs/faq/>.
• 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.

📜 HISTORY 📜

These functions appeared in OpenBSD 2.0. 🌸

👑 AUTHORS 👑

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. ✨

💀 BUGS 💀

Collisions have been found for the full versions of both MD4 and MD5 as well as strong attacks against the SHA-0 and SHA-1 family. The use of sha2(3), or rmd160(3) is recommended instead.

😭 okay this is the real talk part: don't use this for anything security-critical in 2025. the man page literally says use sha2(3) or rmd160(3). listen to the man page. it's giving protective bestie energy.

and that's the whole page, bestie! 🎀✨

this little gem is from the Debian crew, dated April 29, 2004 — so yes, it's a vintage icon, and we love her for it. MD5(3bsd), signing off 💖🦋👑