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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

       libcrypto, -lcrypto

        #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_DigestFi-

       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

       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

       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.

       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 check-
       ing 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]);
               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.


3rd Berkeley Distribution           0.9.7d                         BIO_f_md(3)