MD2(3)

💖 MD2 — the girlies' guide to the RSA Data Security, Inc. "MD2" message digest 💖

okay bestie, sit down, grab a strawberry 🍓, because we're about to talk about MD2Init, MD2Update, MD2Pad, MD2Final, MD2Transform, MD2End, MD2File, MD2FileChunk, and MD2Data — the whole little family that calculates the RSA Data Security, Inc., "MD2" message digest. yes that's a mouthful. we love her anyway 💅

🎀 LIBRARY 🎀

she lives in the Message Digest (MD4, MD5, etc.) Support Library — that's libmd, and you link her with -lmd. periodt.

✨ SYNOPSIS ✨

#include <sys/types.h>
#include <md2.h>

void
MD2Init(MD2_CTX *context);

void
MD2Update(MD2_CTX *context, const uint8_t *data, size_t len);

void
MD2Pad(MD2_CTX *context);

void
MD2Final(uint8_t digest[MD2_DIGEST_LENGTH], MD2_CTX *context);

void
MD2Transform(uint32_t state[4], uint8_t block[MD2_BLOCK_LENGTH]);

char *
MD2End(MD2_CTX *context, char *buf);

char *
MD2File(const char *filename, char *buf);

char *
MD2FileChunk(const char *filename, char *buf, off_t offset,
    off_t length);

char *
MD2Data(const uint8_t *data, size_t len, char *buf);

look at her. iconic. 🫶

🦋 DESCRIPTION 🦋

so the MD2 functions calculate a 128-bit cryptographic checksum (aka a digest) for any number of input bytes. and what's a cryptographic checksum, you ask? it's a one-way hash-function, meaning — and I need you to hear me on this — you literally cannot find the input that corresponds to a particular output, except by exhaustive search. that's the whole vibe. the net result is a "fingerprint" of the input-data, which doesn't disclose the actual input. so it's like, giving you the vibe of the data without spilling the data. 🔥

now let's talk the family drama:

• MD2 is the slowest
• MD4 is the fastest
• 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''. and here's the part where I hold your hand, bestie: MD4 and MD5 have been broken 😭 — they should only be used where necessary for backward compatibility. 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. but good news: it is still unlikely for an attacker to be able to determine the exact original input given a hash value. so, like, not great, but not the full apocalypse either.

💜 the core trio

MD2Init(), MD2Update(), and MD2Final() are the core functions. the whole routine is:

1. Allocate an MD2_CTX
2. initialize it with MD2Init()
3. run over the data with MD2Update()
4. and finally extract the result using MD2Final()

that's it. that's the ritual. 🕯️

🌸 the supporting cast

MD2Pad() can be used to apply padding to the message digest as in MD2Final(), but here's the cute part — the current context can still be used with MD2Update() afterward. flexible queen.

MD2Transform() is used by MD2Update() to hash 512-bit blocks and forms the core of the algorithm. but listen — most programs should use the interface provided by MD2Init(), MD2Update() and MD2Final() instead of calling MD2Transform() directly. like, you can text him directly, but maybe go through the group chat. 🙈

MD2End() is a wrapper for MD2Final() which converts the return value to an MD2_DIGEST_STRING_LENGTH-character (including the terminating '\0') ASCII string which represents the 128 bits in hexadecimal. slay.

MD2File() calculates the digest of a file, and uses MD2End() to return the result. if the file cannot be opened, a null pointer is returned. rude but fair.

MD2FileChunk() behaves like MD2File() 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 MD2Data() calculates the digest of a chunk of data in memory, and uses MD2End() to return the result. memory girlies, this one's for you.

⭐ the buf situation (important!!)

When using MD2End(), MD2File(), MD2FileChunk(), or MD2Data(), the buf argument can be a null pointer. in that case the returned string is allocated with malloc(3) and subsequently must be explicitly deallocated using free(3) after use. don't forget. I will know. 👀

If the buf argument is non-null, it must point to at least MD2_DIGEST_STRING_LENGTH characters of buffer space. that's the minimum. don't skimp.

🌈 SEE ALSO 🌈

• md2(3)
• md4(3)
• md5(3)
• rmd160(3)
• sha1(3)
• sha2(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, <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 and NetBSD 1.3. elder icons. 👑

👑 AUTHORS 👑

The original MD2 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 MD2End(), MD2File(), MD2FileChunk(), and MD2Data() 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. The use of sha2(3) is recommended instead.

like. say it louder for the people in the back. use sha2(3). 🚨

and that's the whole page, bestie!! mwah 💋 — this little love letter comes to you from Debian, dated July 13, 2010, and it's filed under MD2(3) in the Library Functions Manual. stay hashing, stay hydrated, and please free your buffers. 💖✨🎀