MirBSD manpage: BIO_f_md(3), BIO_get_md(3), BIO_get_md_ctx(3), BIO_set_md(3)

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 filter


      #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);


     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.


     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 con-
     text 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.

MirBSD #10-current         2005-02-05                           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.


     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

      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

MirBSD #10-current         2005-02-05                           2

BIO_F_MD(3)                  OpenSSL                  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]);
             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.



MirBSD #10-current         2005-02-05                           3

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