BIO_f_md (3)

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NAME

BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx - message 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 method. This is a filter that digests any data passed through it, it is a wrapper for the digest routines EVP_DigestInit(), EVP_DigestUpdate() and EVP_DigestFinal().

Any data written or read through a digest

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_set_md() sets the message digest of

b to md: this must be called to initialize a digest 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 structure. Changes made to this context will affect the digest

itself and the context pointer will become invalid when the digest is freed.

After the digest has been retrieved from a digest

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

Before OpenSSL 1.0.0 the call to BIO_get_md_ctx() would only work if the

had been initialized for example by calling BIO_set_md() ). In OpenSSL 1.0.0 and later the context is always returned and the is state is set to initialized. This allows applications to initialize the context externally if the standard calls such as BIO_set_md() are not sufficiently flexible.

RETURN VALUES

BIO_f_md() returns the digest 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 chain containing an and digest 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

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 in the chain and digest the data passed through and that digests should be retrieved using a separate BIO_ctrl() call.

SEE ALSO