MD5(3)

💜 MD5(3) — MD4 & MD5 hash functions, bestie edition 💜

okay so this page lives in section 3 of the Library Functions Manual, and it's all about MD4 and MD5 — the OG cryptographic hash girlies. Let's get into it ✨

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🎀 NAME 🎀

MD4, MD5, MD4_Init, MD4_Update, MD4_Final, MD5_Init, MD5_Update, MD5_Final — MD4 and MD5 hash functions, aka the whole friend group 💅

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✨ SYNOPSIS ✨

library “libcrypto” #include <openssl/md4.h>

unsigned char *
MD4(const unsigned char *d, unsigned long n, unsigned char *md);

int
MD4_Init(MD4_CTX *c);

int
MD4_Update(MD4_CTX *c, const void *data, unsigned long len);

int
MD4_Final(unsigned char *md, MD4_CTX *c);

#include <openssl/md5.h>

unsigned char *
MD5(const unsigned char *d, unsigned long n, unsigned char *md);

int
MD5_Init(MD5_CTX *c);

int
MD5_Update(MD5_CTX *c, const void *data, unsigned long len);

int
MD5_Final(unsigned char *md, MD5_CTX *c);

like, literally just #include the right header and the loml libcrypto has you covered 🫶

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🌸 DESCRIPTION 🌸

So MD4 and MD5 are cryptographic hash functions with a 128-bit output. cute little 16-byte queens 👑

MD4() and MD5() compute the MD4 and MD5 message digest of the n bytes at d and drop it into md — and md must have space for MD4_DIGEST_LENGTH == MD5_DIGEST_LENGTH == 16 bytes of output. no negotiations on the size, bestie, that's the vibe check 🧚

But wait — what if your message isn't fully chilling in memory all at once? 👀 That's where the incremental crew comes in. The following functions may be used if the message is not completely stored in memory:

• MD5_Init() — initializes an MD5_CTX structure. setting the stage, main character energy ✨
• MD5_Update() — can be called repeatedly with chunks of the message to be hashed (len bytes at data). feed it piece by piece, she eats every time 🍓
• MD5_Final() — places the message digest in md, which must have space for MD5_DIGEST_LENGTH == 16 bytes of output, and erases the MD5_CTX. and that's a wrap, clean up your mess 💅

MD4_Init(), MD4_Update(), and MD4_Final() are analogous using an MD4_CTX structure — same energy, different struct 🦋

⚠️ okay important one: applications should use the higher level functions EVP_DigestInit(3) etc. instead of calling these hash functions directly. like, these are the direct-line friends, but the EVP layer is the one you bring to the function. no cap 🙈

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🔥 RETURN VALUES 🔥

• MD4() and MD5() return pointers to the hash value 💖
• MD4_Init(), MD4_Update(), MD4_Final(), MD5_Init(), MD5_Update(), and MD5_Final() return 1 for success or 0 otherwise. that's it, binary queen, no in-between ⭐

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🌈 SEE ALSO 🌈

• EVP_DigestInit(3) — go hang out with this one instead, fr fr

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👑 STANDARDS 👑

RFC 1320, RFC 1321 — the receipts, the lore, the founding documents 📜✨

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💀 HISTORY 💀

MD5(), MD5_Init(), MD5_Update(), and MD5_Final() appeared in SSLeay 0.4 or earlier and have been available since OpenBSD 2.4. ancient icon behavior, honestly 🏛️

MD4(), MD4_Init(), MD4_Update(), and MD4_Final() first appeared in OpenSSL 0.9.6 and have been available since OpenBSD 2.9. a slightly younger legend 🌱

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🙈 CAVEATS 🙈

okay so this is the messy part. Other implementations allow md in MD4() and MD5() to be NULL and return a static array — which is not thread safe. 🚨

Like... that's a real problem, bestie. Do not rely on that. It's giving undefined chaos and your threads will fight each other 😭 Different implementations, different drama — stay safe out there 💜

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that's the whole page, bestie — go hash something and slay ✨

mwah 💋 — this one came to you from Debian, dated August 25, 2026, page MD5(3).