BIO_F_MD(3)

💅 BIO_F_MD(3) — bestie's guide to the message digest BIO filter 💅

okay so this is a page from the libcrypto library (you'll want #include <openssl/bio.h> and #include <openssl/evp.h>), and it's all about the message digest BIO filter. it's giving cryptography but make it a filter. let's go 🫶

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✨ SYNOPSIS ✨

const BIO_METHOD *
BIO_f_md(void);

long
BIO_set_md(BIO *b, EVP_MD *md);

long
BIO_get_md(BIO *b, EVP_MD **mdp);

long
BIO_get_md_ctx(BIO *b, EVP_MD_CTX **mdcp);

long
BIO_set_md_ctx(BIO *b, EVP_MD_CTX *mdc);

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🌸 DESCRIPTION 🌸

BIO_f_md() returns the message digest BIO method. it's a filter BIO, which means it digests literally any data you pass through it. think of it as a little wrapper around the digest routines EVP_DigestInit(3), EVP_DigestUpdate(3), and EVP_DigestFinal(3). iconic behavior honestly 👑

BIO_set_md() sets the message digest of b to md and initializes it using EVP_DigestInit_ex(3). and bestie listen — calling this function is required before any data is passed through b. no skipping, fr fr.

BIO_get_md() places a pointer to the digest method of b into *mdp.

any data written or read through a digest BIO using BIO_read(3) and BIO_write(3) gets digested. that's the whole point 💅

BIO_gets(3), if its size parameter is large enough, finishes the digest calculation and returns the digest value. BIO_puts(3)? not supported. nope. if an application needs to call BIO_gets(3) or BIO_puts(3) through a chain containing digest BIOs, you can do it by prepending a buffering BIO. that's the workaround, you're welcome 🎀

⚠️ after the digest has been retrieved from a digest BIO, call BIO_reset(3) to reinitialize it and any BIOs following it in its chain before passing any more data through it. if no subsequent BIOs require reinitialization, BIO_set_md() can be used instead of BIO_reset(3). don't skip this step, it's lowkey important 😭

BIO_get_md_ctx() places a pointer to the digest context of b into *mdcp and marks the BIO as initialized without actually initializing it. sneaky! unless BIO_set_md() was already called on b, the caller becomes responsible for initializing the digest context with EVP_DigestInit_ex(3). you own that now, congrats 🫶

the context returned by BIO_get_md_ctx() can be used in calls to EVP_DigestFinal(3) and also in the signature routines EVP_SignFinal(3) and EVP_VerifyFinal(3).

💀 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. handle with care bestie, that pointer does NOT stay valid.

BIO_set_md_ctx() replaces the digest context of b with mdc. calling this function is usually not necessary because creating a digest BIO with BIO_new(3) automatically creates a digest context and stores it internally. before calling BIO_set_md_ctx(), the caller has to retrieve the old context using BIO_get_md_ctx(), and the caller also becomes responsible for calling EVP_MD_CTX_free(3) on the old context. and unless mdc is already initialized, the caller needs to initialize it after calling BIO_set_md_ctx() using either BIO_set_md() or EVP_DigestInit(3). it's giving responsibility chain 🌈

when a chain containing a message digest BIO is copied with BIO_dup_chain(3), EVP_MD_CTX_copy_ex(3) is called internally to automatically copy the message digest context from the existing BIO object to the new one, and the init flag that can be retrieved with BIO_get_init(3) is set to 1. cute and automatic ✨

BIO_ctrl(3) cmd arguments correspond to macros as follows:

• BIO_C_GET_MD — corresponds to BIO_get_md()
• BIO_C_GET_MD_CTX — corresponds to BIO_get_md_ctx()
• BIO_C_SET_MD — corresponds to BIO_set_md()
• BIO_C_SET_MD_CTX — corresponds to BIO_set_md_ctx()
• BIO_CTRL_RESET — corresponds to BIO_reset(3)

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🔥 RETURN VALUES 🔥

BIO_f_md() returns the digest BIO method.

when called on a message digest BIO object, BIO_method_type(3) returns the constant BIO_TYPE_MD and BIO_method_name(3) returns a pointer to the static string "message digest".

BIO_set_md() returns 1 on success or 0 if EVP_DigestInit_ex(3) fails.

BIO_get_md() and BIO_set_md_ctx() return 1 on success or 0 if b is not initialized.

BIO_get_md_ctx() returns 1 on success or 0 on failure, but the current implementation cannot actually fail. it literally cannot lose 💀✨

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🦋 EXAMPLES 🦋

the following example creates a BIO chain containing a SHA-1 and MD5 digest BIO and passes the string "Hello World" through it. error checking has been omitted for clarity. (ngl, don't omit error checking in real code, but this is a demo, periodt)

BIO *bio, *mdtmp;
const 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. main character energy honestly 👑

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_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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🌈 SEE ALSO 🌈

BIO_new(3), EVP_DigestInit(3)

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💜 HISTORY 💜

BIO_f_md(), BIO_set_md(), and BIO_get_md() first appeared in SSLeay 0.6.0. BIO_get_md_ctx() first appeared in SSLeay 0.8.1. these functions have been available since OpenBSD 2.4. old money ✨

BIO_set_md_ctx() first appeared in OpenSSL 0.9.7e and has been available since OpenBSD 3.8.

🕰️ before OpenSSL 1.0.0, the call to BIO_get_md_ctx() would only work if the BIO had been initialized, for example by calling BIO_set_md(). ancient behavior, we don't stan but we respect the lore.

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💀 BUGS 💀

the lack of support for BIO_puts(3) and the non-standard behaviour of BIO_gets(3) could be regarded as anomalous. it could be argued that BIO_gets(3) and BIO_puts(3) 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(3) call. that's the discourse, anyway 🙈

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mwah — this page came to you from Debian, dated June 8, 2025, and it's BIO_F_MD(3) in the Library Functions Manual. go digest something, bestie 🍓💖