BIO_f_md(3)                 OpenSSL                 BIO_f_md(3)





NAME
       BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx - mes-
       sage 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_Diges-
       tInit(), 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 fin-
       ishes 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 rou-
       tines 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()
       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.

        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



