BIO_F_MD(3ossl)

💅 BIO_F_MD — the message digest BIO filter ✨

okay so this is the BIO_F_MD(3ossl) page from the OpenSSL fam, and it's genuinely one of those "small but mighty" ones. bestie, if you've ever wanted to run data through a BIO chain and get a digest out the other end like a little ✨digest machine✨, this is your girl. let's get into it 🫶

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🎀 NAME 🎀

BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx — the message digest BIO filter, aka the glow-up filter for your data stream 💖

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✨ 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);

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💫 DESCRIPTION 💫

BIO_f_md() hands you back the message digest BIO method. it's a filter BIO — meaning it sits in the middle of your chain and digests literally any data that passes through it. think of it as a BIO wrapper around the digest routines EVP_DigestInit(), EVP_DigestUpdate() and EVP_DigestFinal(). iconic behavior tbh 👑

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

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

BIO_reset() reinitialises a digest BIO. fresh start energy 🌸

BIO_set_md() sets the message digest of BIO b to md. this is a must — you have to call it to initialize a digest BIO before any data is passed through it, otherwise nothing works and you'll be sad. 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. slay.

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

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🌈 NOTES 🌈

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

⚠️ bestie listen — 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… handle with care, she's fragile 😭

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. no skipping this step, fr fr.

if an application needs to call BIO_gets() or BIO_puts() through a chain containing digest BIOs, then this can be done by prepending a buffering BIO. that's the move. 🧚

calling BIO_get_md_ctx() will return the context and initialize the BIO state. this is the loophole that lets applications initialize the context externally if the standard calls such as BIO_set_md() are not sufficiently flexible. main character workaround energy ⭐

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🔥 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.

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🦋 EXAMPLES 🦋

okay the examples are giving "let me show you exactly how it's done" and honestly? ate. 💖

this first one 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 (iykyk, we're keeping it cute).

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));

the next example digests data by reading through a chain instead. same energy, opposite direction 🌀

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);

and this next one retrieves the message digests from a BIO chain and outputs them. it could be used with the examples above. look at her go 🎀

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);

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💀 BUGS 💀

the lack of support for BIO_puts() and the non standard behaviour of BIO_gets() could be regarded as 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 discourse, and we're just reporting it, not settling it 🙈

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🌸 HISTORY 🌸

before OpenSSL 1.0.0., the call to BIO_get_md_ctx() would only work if the BIO was initialized first. glow-up happened, we love growth 💜

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👑 COPYRIGHT 👑

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>.

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and that's the whole page bestie!! 🫶 this was BIO_F_MD out of OpenSSL 3.5.6, dated 2026-04-07 — go forth and digest, you absolute icon 💖✨🎀