BIO_F_MD(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💅 BIO_f_md — bestie, it's the message digest BIO filter

okay so this is a page from the OpenSSL manual (section 3ossl), and it's literally all about the message digest BIO filter — the lil' filter that sits in your BIO chain and digests everything that flows through it. pull up a chair, we're learning this together 🫶

✨ NAME ✨

BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx — the message digest BIO filter. that's the whole vibe. 🎀

🌸 SYNOPSIS 🌸

#include <openssl/bio.h>
#include <openssl/evp.h>

const BIO_METHOD *BIO_f_md(void);
int BIO_set_md(BIO *b, EVP_MD *md);
int BIO_get_md(BIO *b, EVP_MD **mdp);
int BIO_get_md_ctx(BIO *b, EVP_MD_CTX **mdcp);

like, four functions, that's it, that's the roster 👑

🦋 DESCRIPTION 🦋

BIO_f_md() hands you back the message digest BIO method. it's a filter BIO — meaning it digests any data passed through it. it's basically a glow-up wrapper around the digest routines EVP_DigestInit(), EVP_DigestUpdate() and EVP_DigestFinal(). iconic behavior honestly.

any data you write or read through a digest BIO using BIO_read_ex() and BIO_write_ex() gets digested. no cap. 🔥

BIO_gets(), if its size parameter is large enough, finishes the digest calculation and returns the digest value. BIO_puts()? not supported. it simply does not do that. 💀

BIO_reset() reinitialises a digest BIO — like a little factory reset moment ✨

BIO_set_md() sets the message digest of BIO b to md. and bestie listen: this must be called to initialize a digest BIO before any data is passed through it. no setup, no service. it's a BIO_ctrl() macro.

BIO_get_md() places a pointer to the digest BIO's digest method in mdp. also a BIO_ctrl() macro 🎀

BIO_get_md_ctx() returns the digest BIO's context into mdcp.

🌈 NOTES 🌈

the context returned by BIO_get_md_ctx() can be used in calls to EVP_DigestFinal(), and also with the signature routines EVP_SignFinal() and EVP_VerifyFinal(). multitasking queen 💅

⚠️ the context returned by BIO_get_md_ctx() is an internal context structure. changes you make to this context will affect the digest BIO itself, and the context pointer becomes invalid when the digest BIO is freed. so like. don't hang onto it, and don't poke it unless you mean it 🙈

after the digest has been retrieved from a digest BIO, it must be reinitialized by calling BIO_reset(), or BIO_set_md(), before any more data is passed through it. it's giving "one shot per setup" energy.

if an application needs to call BIO_gets() or BIO_puts() through a chain containing digest BIOs, you can do that by prepending a buffering BIO. lil' buffer friend to the rescue 🧚

calling BIO_get_md_ctx() returns the context and initializes the BIO state. the way this is useful: it lets applications initialize the context externally if the standard calls such as BIO_set_md() are not flexible enough for what they're doing. main character flexibility ✨

⭐ RETURN VALUES ⭐

  • BIO_f_md() returns the digest BIO method.
  • BIO_set_md(), BIO_get_md() and BIO_md_ctx() return 1 for success and <=0 for failure.
💀 yes, the page literally says BIO_md_ctx() there — that's what's written, we're not fixing it, we're just reporting it, iykyk

🍓 EXAMPLES 🍓

example one: writing through the chain

the following example creates a BIO chain containing an SHA1 and MD5 digest BIO and passes the string "Hello World" through it. error checking has been omitted for clarity (girl, we're keeping it readable).

BIO *bio, *mdtmp;
char message[] = "Hello World";

bio = BIO_new(BIO_s_null());
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_sha1());
/*
 * For BIO_push() we want to append the sink BIO and keep a note of
 * the start of the chain.
 */
bio = BIO_push(mdtmp, bio);
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_md5());
bio = BIO_push(mdtmp, bio);
/* Note: mdtmp can now be discarded */
BIO_write(bio, message, strlen(message));

example two: reading through the chain

the next example digests data by reading through a chain instead:

BIO *bio, *mdtmp;
char buf[1024];
int rdlen;

bio = BIO_new_file(file, "rb");
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_sha1());
bio = BIO_push(mdtmp, bio);
mdtmp = BIO_new(BIO_f_md());
BIO_set_md(mdtmp, EVP_md5());
bio = BIO_push(mdtmp, bio);
do {
    rdlen = BIO_read(bio, buf, sizeof(buf));
    /* Might want to do something with the data here */
} while (rdlen > 0);

example three: collecting the digests

this next example retrieves the message digests from a BIO chain and outputs them. this could be used with the examples above 👑

BIO *mdtmp;
unsigned char mdbuf[EVP_MAX_MD_SIZE];
int mdlen;
int i;

mdtmp = bio;   /* Assume bio has previously been set up */
do {
    EVP_MD *md;

    mdtmp = BIO_find_type(mdtmp, BIO_TYPE_MD);
    if (!mdtmp)
        break;
    BIO_get_md(mdtmp, &md);
    printf("%s digest", OBJ_nid2sn(EVP_MD_get_type(md)));
    mdlen = BIO_gets(mdtmp, mdbuf, EVP_MAX_MD_SIZE);
    for (i = 0; i < mdlen; i++) printf(":%02X", mdbuf[i]);
    printf("\n");
    mdtmp = BIO_next(mdtmp);
} while (mdtmp);

BIO_free_all(bio);

💀 BUGS 💀

so the lack of support for BIO_puts() and the non standard behaviour of BIO_gets() could be regarded as anomalous. that's the official word: anomalous. it could be argued that BIO_gets() and BIO_puts() should be passed to the next BIO in the chain, and digest the data passed through, and that digests should be retrieved using a separate BIO_ctrl() call. that's the argument. we're just relaying it, don't shoot the messenger 🙈

🎀 HISTORY 🎀

before OpenSSL 1.0.0., the call to BIO_get_md_ctx() would only work if the BIO was initialized first. glow-up achieved since then ✨

Copyright 2000-2021 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>.

and that's the page, bestie 🫶 this was OpenSSL 3.5.0, dated 2026-09-25, filed under BIO_F_MD(3ossl) — go forth and digest responsibly 💖🌸🦋

📦 where this page comes from