MDC2_INIT(3ossl)

💖 MDC2_INIT — the MDC2 hash function, bestie edition 💖

okay so this page is MDC2_INIT(3ossl) from the OpenSSL manual, and it's literally all about MDC2, MDC2_Init, MDC2_Update and MDC2_Final — the MDC2 hash function. grab a snack, we're going in 🍓

---

✨ NAME ✨

MDC2, MDC2_Init, MDC2_Update, MDC2_Final — it's giving MDC2 hash function, periodt.

---

🌸 SYNOPSIS 🌸

first things first, you gotta pull in the header:

#include <openssl/mdc2.h>

now listen — the following functions have been deprecated since OpenSSL 3.0, and you can hide them entirely by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

unsigned char *MDC2(const unsigned char *d, unsigned long n,
                    unsigned char *md);

int MDC2_Init(MDC2_CTX *c);
int MDC2_Update(MDC2_CTX *c, const unsigned char *data,
                unsigned long len);
int MDC2_Final(unsigned char *md, MDC2_CTX *c);

not me copying those signatures character for character because they HAVE to be exact 😭

---

🦋 DESCRIPTION 🦋

okay so real talk: all of the functions described on this page are deprecated. like, every single one. applications should instead use EVP_DigestInit_ex(3), EVP_DigestUpdate(3) and EVP_DigestFinal_ex(3). that's the glow-up, bestie 💅

so what even is MDC2? MDC2 is a method to construct hash functions with 128 bit output from block ciphers. and these functions right here? they're an implementation of MDC2 with DES. iconic honestly 👑

• MDC2() computes the MDC2 message digest of the n bytes at d and places it in md (which must have space for MDC2_DIGEST_LENGTH == 16 bytes of output). and if md is NULL, the digest is placed in a static array. sneaky but okay 🙈

the following functions may be used if the message is not completely stored in memory:

• MDC2_Init() initializes a MDC2_CTX structure.
• MDC2_Update() can be called repeatedly with chunks of the message to be hashed (len bytes at data).
• MDC2_Final() places the message digest in md, which must have space for MDC2_DIGEST_LENGTH == 16 bytes of output, and erases the MDC2_CTX.

and one more time for the people in the back: applications should use the higher level functions EVP_DigestInit(3) etc. instead of calling the hash functions directly. no cap 🫶

---

🌈 RETURN VALUES 🌈

• MDC2() returns a pointer to the hash value.
• MDC2_Init(), MDC2_Update() and MDC2_Final() return 1 for success, 0 otherwise.

that's it, that's the whole vibe check ⭐

---

🎀 CONFORMING TO 🎀

ISO/IEC 10118-2:2000 Hash-Function 2, with DES as the underlying block cipher. certified, standardized, slay ✨

---

👑 SEE ALSO 👑

• EVP_DigestInit(3)

go read that one too, bestie, iykyk 💜

---

💀 HISTORY 💀

all of these functions were deprecated in OpenSSL 3.0. yeah. all of them. 💀

---

📜 COPYRIGHT 📜

Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

---

fr fr, don't use these in new code. use the EVP functions. your future self will thank you 🫶

and that's the whole page!! this was OpenSSL 3.5.9, dated 2026-09-29, coming to you straight from the MDC2_INIT(3ossl) manual. mwah 💋