BIO_F_MD(3)

NAME
BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx - message digest BIO filter

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);

DESCRIPTION
BIO_f_md() returns the message digest BIO method. This is a filter BIO that digests any data passed through it. It is a BIO wrapper for the digest routines EVP_DigestInit(3), EVP_DigestUpdate(3), and EVP_DigestFinal(3).

Any data written or read through a digest BIO using BIO_read(3) and BIO_write(3) is digested.

BIO_gets(3), if its size parameter is large enough, finishes the digest calculation and returns the digest value. BIO_puts(3) is not supported.

BIO_reset(3) reinitialises a digest BIO.

BIO_set_md() sets the message digest of BIO b to md: this must be called to initialize a digest BIO before any data is passed through it. It is a BIO_ctrl(3) macro.

BIO_get_md() places a pointer to the digest BIOs digest method in mdp. It is a BIO_ctrl(3) macro.

BIO_get_md_ctx() returns the digest BIOs context in mdcp.

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.

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.

After the digest has been retrieved from a digest BIO, it must be reinitialized by calling BIO_reset(3) or BIO_set_md() before any more data is passed through it.

If an application needs to call BIO_gets(3) or BIO_puts(3) 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.

RETURN VALUES
BIO_f_md() returns the digest BIO method.

BIO_set_md(), BIO_get_md(), and BIO_get_md_ctx() return 1 for success and 0 for failure.

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.

    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.

    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);

SEE ALSO
BIO_new(3), EVP_DigestInit(3)

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.

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().

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.