BIO_F_MD(3ossl)

🎀 NAME 🎀

BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx — the message digest BIO filter 💅

okay bestie, you're reading the BIO_F_MD(3ossl) page, and it's literally about wrapping a BIO around a digest so everything that flows through it gets hashed. iconic behavior tbh.

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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, which means it digests any data you pass through it. Basically it's a glow-up wrapper for the digest routines EVP_DigestInit(), EVP_DigestUpdate() and EVP_DigestFinal().

Any data written 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. we don't do that here 🙈

BIO_reset() reinitialises a digest BIO.

• BIO_set_md() — sets the message digest of BIO b to md. You must call this to initialize a digest BIO before any data is passed through it. 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 returned by BIO_get_md_ctx() can be used in calls to EVP_DigestFinal() and also the signature routines EVP_SignFinal() and EVP_VerifyFinal().

The context returned by BIO_get_md_ctx() is an internal context structure. Changes made to this context will affect the digest BIO itself, and the context pointer will become invalid when the digest BIO is freed. ⚠️ so like, don't hang onto it, fr fr.

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.

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.

Calling BIO_get_md_ctx() will return the context and initialize the BIO state. This allows applications to initialize the context externally if the standard calls such as BIO_set_md() are not sufficiently flexible.

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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 🌈

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.

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:

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);

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);

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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.

like yeah it's a little quirky, we said what we said 🫶

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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. ancient lore ✨

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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 one shipped with OpenSSL 3.5.9, dated 2026-09-29 — slay, go digest something 💜