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BIO_F_MD(3)			    OpenSSL			   BIO_F_MD(3)

       BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx	- message digest BIO

	#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);

       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(), EVP_DigestUpdate() and

       Any data	written	or read	through	a digest BIO using BIO_read_ex() and
       BIO_write_ex() is digested.

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

       BIO_reset() 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() macro.

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

       BIO_get_md_ctx()	returns	the digest BIOs	context	into mdcp.

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

       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.

       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.

       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.

       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)
	    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]);
	    mdtmp = BIO_next(mdtmp);
	} while	(mdtmp);


       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.

       Before OpenSSL 1.0.0., the call to BIO_get_md_ctx() would only work if
       the BIO was initialized first.

       Copyright 2000-2016 The OpenSSL Project Authors.	All Rights Reserved.

       Licensed	under the OpenSSL license (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

1.1.1h				  2020-09-22			   BIO_F_MD(3)


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