BIO_F_MD(3) OpenSSL BIO_F_MD(3)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);
DESCRIPTIONBIO_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_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 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 con-
text 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.
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BIO_F_MD(3) OpenSSL BIO_F_MD(3)
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 VALUESBIO_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.
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BIO_F_MD(3) OpenSSL BIO_F_MD(3)
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
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