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_DigestFi-
       nal().

       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 struc-
       ture. 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 reini-
       tialized 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_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