BIO_F_MD(3SSL)

💅 BIO_F_MD — the digest BIO filter, bestie edition

okay so this is from the OpenSSL manual, and it's the BIO_F_MD page, aka everything you ever wanted to know about the message digest BIO filter. sit down, grab a strawberry 🍓, we're doing this.

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✨ NAME ✨

BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx — it's giving message digest BIO filter, and honestly? iconic. 👑

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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);

no cap, that's the whole cast of characters 💖

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🦋 DESCRIPTION 🦋

BIO_f_md() hands you the message digest BIO method. it's a filter BIO, which means it digests every lil byte of data you push through it. think of it as a glow-up wrapper around the digest routines EVP_DigestInit(), EVP_DigestUpdate() and EVP_DigestFinal(). 💅

any data you write or read through a digest BIO using BIO_read_ex() and BIO_write_ex() gets digested. periodt.

BIO_gets(), if its size parameter is big enough, finishes the digest calculation and returns the digest value. BIO_puts()? not supported. we do not do that here 🙈

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

BIO_set_md() sets the message digest of BIO b to md. you must call this to initialize a digest BIO before any data goes through it, fr fr. 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.

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🌸 NOTES 🌸

the context that BIO_get_md_ctx() gives you can be used in calls to EVP_DigestFinal() and also the signature routines EVP_SignFinal() and EVP_VerifyFinal(). versatile queen.

careful bestie: the context returned by BIO_get_md_ctx() is an internal context structure. any changes you make to it will affect the digest BIO itself, and the context pointer becomes invalid when the digest BIO is freed. do not be out here using a freed pointer, i am begging 😭

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, it's mandatory bestie.

if an app needs to call BIO_gets() or BIO_puts() through a chain that contains digest BIOs, you can do that by prepending a buffering BIO. cute lil workaround 💅

calling BIO_get_md_ctx() will return the context and initialize the BIO state. this lets applications initialize the context externally when the standard calls like BIO_set_md() just aren't flexible enough for what they're trying to do. main character flexibility. 👑

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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. so like… check your return values, bestie, they're literally telling you things. 💀

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🔥 EXAMPLES 🔥

ex. 1 — writing through a chain

this one creates a BIO chain with an SHA1 and an MD5 digest BIO and passes the string "Hello World" through it. error checking has been omitted for clarity ✨

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));

ex. 2 — reading through a chain

the next one 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);

ex. 3 — retrieving all the digests

this next example retrieves the message digests from a BIO chain and outputs them. it could be used together 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);

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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, digest the data passed through, and that digests should be retrieved using a separate BIO_ctrl() call. but here we are 🙈 not my design choices, i'm just the messenger.

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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 achieved since then ✨

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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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okay and that's the page, bestie 🎀 this was BIO_F_MD(3SSL) from OpenSSL, and the version info at the bottom was 4.0.3, dated 2026-09-29. you're welcome, go digest something. 💖🦋✨