BIO_f_md(3)

     BIO_f_md(3)            0.9.6h (2000-09-16)            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_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(),
          EVP_DigestUpdate() and EVP_DigestFinal().

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

     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.

          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()

     Page 1                                         (printed 1/19/103)

     BIO_f_md(3)            0.9.6h (2000-09-16)            BIO_f_md(3)

          through a chain containing digest BIOs then this can be done
          by prepending a buffering BIO.

     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.

     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.

     Page 2                                         (printed 1/19/103)

     BIO_f_md(3)            0.9.6h (2000-09-16)            BIO_f_md(3)

           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);

     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.

     SEE ALSO
          TBA

     Page 3                                         (printed 1/19/103)