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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>

  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(
	  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
  EVP_DigestInit(), EVP_DigestUpdate(), and EVP_DigestFinal() digest
  functions.

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

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

  The BIO_reset() function reinitializes a digest BIO.

  The BIO_set_md() function 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.

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

  The BIO_get_md_ctx() function returns the digest BIOs context into mdcp.

NOTES
  The context returned by the BIO_get_md_ctx() function can be used in calls
  to the EVP_DigestFinal(), EVP_SignFinal(), and EVP_VerifyFinal() functions.

  The context returned by the BIO_get_md_ctx() function 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.

RESTRICTIONS
  The lack of support for BIO_puts() and the nonstandard behavior 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.

RETURN VALUES
  The BIO_f_md() function returns the digest BIO method.

  The BIO_set_md(), BIO_get_md(), and BIO_md_ctx() functions return 1 for
  success and 0 for failure.

EXAMPLES
  Create a BIO chain containing an SHA1 and MD5 digest BIO and pass 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));

  Digest data by reading through a chain:

	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);

  Retrieve the message digests from a BIO chain and output them. This could
  be used with the previous examples:

	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
  TBA

Index for Section 3

Alphabetical listing for B

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