/usr/man/cat.3/BIO_set_md.3

BIO_f_md(3)                  OpenSSL                  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() 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.

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.

1.0.2t               Last change: 2019-09-10                    1

BIO_f_md(3)                  OpenSSL                  BIO_f_md(3)

     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.

     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() ). In OpenSSL 1.0.0 and later the context is
     always returned and the BIO is state is set to initialized.
     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_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:

1.0.2t               Last change: 2019-09-10                    2

BIO_f_md(3)                  OpenSSL                  BIO_f_md(3)

      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);

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

1.0.2t               Last change: 2019-09-10                    3

See also BIO_f_md(3)
See also BIO_get_md(3)
See also BIO_get_md_ctx(3)