openssl/doc/crypto/BIO_f_md.pod

=pod

=head1 NAME

BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx - message digest BIO filter

=head1 SYNOPSIS

 #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);

=head1 DESCRIPTION

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() and
BIO_write() is digested.

BIO_gets(), if its B<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<b> to B<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 B<mdp>, it is a BIO_ctrl() macro.

BIO_get_md_ctx() returns the digest BIOs context into B<mdcp>.

=head1 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
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.

=head1 RETURN VALUES

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.

=head1 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.

 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);

=head1 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.

=head1 HISTORY

Before OpenSSL 1.0.0., the call to BIO_get_md_ctx() would only work if the
BIO was initialized first.

=head1 SEE ALSO

TBA

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