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.IX Title &quot;BIO_F_MD 3&quot;
.TH BIO_F_MD 3 &quot;2023-09-11&quot; &quot;1.1.1w&quot; &quot;OpenSSL&quot;
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.SH &quot;NAME&quot;
BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx \- message digest BIO filter
.SH &quot;SYNOPSIS&quot;
.IX Header &quot;SYNOPSIS&quot;
.Vb 2
\&amp; #include &lt;openssl/bio.h&gt;
\&amp; #include &lt;openssl/evp.h&gt;
\&amp;
\&amp; const BIO_METHOD *BIO_f_md(void);
\&amp; int BIO_set_md(BIO *b, EVP_MD *md);
\&amp; int BIO_get_md(BIO *b, EVP_MD **mdp);
\&amp; int BIO_get_md_ctx(BIO *b, EVP_MD_CTX **mdcp);
.Ve
.SH &quot;DESCRIPTION&quot;
.IX Header &quot;DESCRIPTION&quot;
\&amp;\fIBIO_f_md()\fR returns the message digest \s-1BIO\s0 method. This is a filter
\&amp;\s-1BIO\s0 that digests any data passed through it, it is a \s-1BIO\s0 wrapper
for the digest routines \fIEVP_DigestInit()\fR, \fIEVP_DigestUpdate()\fR
and \fIEVP_DigestFinal()\fR.
.PP
Any data written or read through a digest \s-1BIO\s0 using \fIBIO_read_ex()\fR and
\&amp;\fIBIO_write_ex()\fR is digested.
.PP
\&amp;\fIBIO_gets()\fR, if its \fBsize\fR parameter is large enough finishes the
digest calculation and returns the digest value. \fIBIO_puts()\fR is
not supported.
.PP
\&amp;\fIBIO_reset()\fR reinitialises a digest \s-1BIO.\s0
.PP
\&amp;\fIBIO_set_md()\fR sets the message digest of \s-1BIO \s0\fBb\fR to \fBmd\fR: this
must be called to initialize a digest \s-1BIO\s0 before any data is
passed through it. It is a \fIBIO_ctrl()\fR macro.
.PP
\&amp;\fIBIO_get_md()\fR places the a pointer to the digest BIOs digest method
in \fBmdp\fR, it is a \fIBIO_ctrl()\fR macro.
.PP
\&amp;\fIBIO_get_md_ctx()\fR returns the digest BIOs context into \fBmdcp\fR.
.SH &quot;NOTES&quot;
.IX Header &quot;NOTES&quot;
The context returned by \fIBIO_get_md_ctx()\fR can be used in calls
to \fIEVP_DigestFinal()\fR and also the signature routines \fIEVP_SignFinal()\fR
and \fIEVP_VerifyFinal()\fR.
.PP
The context returned by \fIBIO_get_md_ctx()\fR is an internal context
structure. Changes made to this context will affect the digest
\&amp;\s-1BIO\s0 itself and the context pointer will become invalid when the digest
\&amp;\s-1BIO\s0 is freed.
.PP
After the digest has been retrieved from a digest \s-1BIO\s0 it must be
reinitialized by calling \fIBIO_reset()\fR, or \fIBIO_set_md()\fR before any more
data is passed through it.
.PP
If an application needs to call \fIBIO_gets()\fR or \fIBIO_puts()\fR through
a chain containing digest BIOs then this can be done by prepending
a buffering \s-1BIO.\s0
.PP
Calling \fIBIO_get_md_ctx()\fR will return the context and initialize the \s-1BIO\s0
state. This allows applications to initialize the context externally
if the standard calls such as \fIBIO_set_md()\fR are not sufficiently flexible.
.SH &quot;RETURN VALUES&quot;
.IX Header &quot;RETURN VALUES&quot;
\&amp;\fIBIO_f_md()\fR returns the digest \s-1BIO\s0 method.
.PP
\&amp;\fIBIO_set_md()\fR, \fIBIO_get_md()\fR and \fIBIO_md_ctx()\fR return 1 for success and
0 for failure.
.SH &quot;EXAMPLES&quot;
.IX Header &quot;EXAMPLES&quot;
The following example creates a \s-1BIO\s0 chain containing an \s-1SHA1\s0 and \s-1MD5\s0
digest \s-1BIO\s0 and passes the string \*(L&quot;Hello World\*(R&quot; through it. Error
checking has been omitted for clarity.
.PP
.Vb 2
\&amp; BIO *bio, *mdtmp;
\&amp; char message[] = &quot;Hello World&quot;;
\&amp;
\&amp; bio = BIO_new(BIO_s_null());
\&amp; mdtmp = BIO_new(BIO_f_md());
\&amp; BIO_set_md(mdtmp, EVP_sha1());
\&amp; /*
\&amp;  * For BIO_push() we want to append the sink BIO and keep a note of
\&amp;  * the start of the chain.
\&amp;  */
\&amp; bio = BIO_push(mdtmp, bio);
\&amp; mdtmp = BIO_new(BIO_f_md());
\&amp; BIO_set_md(mdtmp, EVP_md5());
\&amp; bio = BIO_push(mdtmp, bio);
\&amp; /* Note: mdtmp can now be discarded */
\&amp; BIO_write(bio, message, strlen(message));
.Ve
.PP
The next example digests data by reading through a chain instead:
.PP
.Vb 3
\&amp; BIO *bio, *mdtmp;
\&amp; char buf[1024];
\&amp; int rdlen;
\&amp;
\&amp; bio = BIO_new_file(file, &quot;rb&quot;);
\&amp; mdtmp = BIO_new(BIO_f_md());
\&amp; BIO_set_md(mdtmp, EVP_sha1());
\&amp; bio = BIO_push(mdtmp, bio);
\&amp; mdtmp = BIO_new(BIO_f_md());
\&amp; BIO_set_md(mdtmp, EVP_md5());
\&amp; bio = BIO_push(mdtmp, bio);
\&amp; do {
\&amp;     rdlen = BIO_read(bio, buf, sizeof(buf));
\&amp;     /* Might want to do something with the data here */
\&amp; } while (rdlen &gt; 0);
.Ve
.PP
This next example retrieves the message digests from a \s-1BIO\s0 chain and
outputs them. This could be used with the examples above.
.PP
.Vb 4
\&amp; BIO *mdtmp;
\&amp; unsigned char mdbuf[EVP_MAX_MD_SIZE];
\&amp; int mdlen;
\&amp; int i;
\&amp;
\&amp; mdtmp = bio;   /* Assume bio has previously been set up */
\&amp; do {
\&amp;     EVP_MD *md;
\&amp;
\&amp;     mdtmp = BIO_find_type(mdtmp, BIO_TYPE_MD);
\&amp;     if (!mdtmp)
\&amp;         break;
\&amp;     BIO_get_md(mdtmp, &amp;md);
\&amp;     printf(&quot;%s digest&quot;, OBJ_nid2sn(EVP_MD_type(md)));
\&amp;     mdlen = BIO_gets(mdtmp, mdbuf, EVP_MAX_MD_SIZE);
\&amp;     for (i = 0; i &lt; mdlen; i++) printf(&quot;:%02X&quot;, mdbuf[i]);
\&amp;     printf(&quot;\en&quot;);
\&amp;     mdtmp = BIO_next(mdtmp);
\&amp; } while (mdtmp);
\&amp;
\&amp; BIO_free_all(bio);
.Ve
.SH &quot;BUGS&quot;
.IX Header &quot;BUGS&quot;
The lack of support for \fIBIO_puts()\fR and the non standard behaviour of
\&amp;\fIBIO_gets()\fR could be regarded as anomalous. It could be argued that \fIBIO_gets()\fR
and \fIBIO_puts()\fR should be passed to the next \s-1BIO\s0 in the chain and digest
the data passed through and that digests should be retrieved using a
separate \fIBIO_ctrl()\fR call.
.SH &quot;HISTORY&quot;
.IX Header &quot;HISTORY&quot;
Before OpenSSL 1.0.0., the call to \fIBIO_get_md_ctx()\fR would only work if the
\&amp;\s-1BIO\s0 was initialized first.
.SH &quot;COPYRIGHT&quot;
.IX Header &quot;COPYRIGHT&quot;
Copyright 2000\-2016 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L&quot;License\*(R&quot;).  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
&lt;https://www.openssl.org/source/license.html&gt;.
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