BIO_F_MD(3) BSD Library Functions Manual 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(3) and
BIO_write(3) is digested.
BIO_gets(3), if its size parameter is large enough, finishes the digest
calculation and returns the digest value. BIO_puts(3) is not supported.
BIO_reset(3) 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(3) macro.
BIO_get_md() places the a pointer to the digest BIOs digest method in
mdp. It is a BIO_ctrl(3) macro.
BIO_get_md_ctx() returns the digest BIOs context in mdcp.
NOTES
The context returned by BIO_get_md_ctx() can be used in calls to
EVP_DigestFinal(3) and also in the signature routines EVP_SignFinal(3)
and EVP_VerifyFinal(3).
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(3) or BIO_set_md() before any more data is
passed through it.
If an application needs to call BIO_gets(3) or BIO_puts(3) through a
chain containing digest BIOs, then this can be done by prepending a
buffering BIO.
Before OpenSSL 1.0.0 the call to BIO_get_md_ctx() would only work if the
BIO had been initialized for example by calling BIO_set_md(). In OpenSSL
1.0.0 and later the context is always returned and the BIO 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 suffi‐
ciently flexible.
RETURN VALUESBIO_f_md() returns the digest BIO method.
BIO_set_md(), BIO_get_md(), and BIO_get_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 check‐
ing has been omitted for clarity.
BIO *bio, *mdtmp;
const 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 out‐
puts 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(3) and the non standard behaviour of
BIO_gets(3) could be regarded as anomalous. It could be argued that
BIO_gets(3) and BIO_puts(3) 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(3) call.
BSD July 17, 2014 BSD
