NAME    [Toc]    [Back]

       BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx - Message
       digest BIO filter

SYNOPSIS    [Toc]    [Back]

       #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    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       TBA



                                                      BIO_f_md(3)