BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx - message digest BIO filter
#include <openssl/bio.h>
#include <openssl/evp.h>
const 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_DigestFinal().
Any data written or read through a digest BIO using
BIO_read_ex() and
BIO_write_ex() 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 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
EVP_VerifyFinal().
The context returned by
BIO_get_md_ctx() is an internal context
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.
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.
Calling
BIO_get_md_ctx() will return the context and initialize the BIO
state. This allows applications to initialize the context externally if the
standard calls such as
BIO_set_md() are not sufficiently flexible.
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 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.
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_get_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);
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.
Before OpenSSL 1.0.0., the call to
BIO_get_md_ctx() would only work if
the BIO was initialized first.
Copyright 2000-2021 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy in the
file LICENSE in the source distribution or at
<
https://www.openssl.org/source/license.html>.