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✏️ Editing: BIO_get_md_ctx.3

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.\" ========================================================================
.\"
.IX Title "BIO_F_MD 3"
.TH BIO_F_MD 3 "2023-09-11" "1.1.1w" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx \- message digest BIO filter
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\& #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);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBBIO_f_md()\fR returns the message digest \s-1BIO\s0 method. This is a filter
\&\s-1BIO\s0 that digests any data passed through it, it is a \s-1BIO\s0 wrapper
for the digest routines \fBEVP_DigestInit()\fR, \fBEVP_DigestUpdate()\fR
and \fBEVP_DigestFinal()\fR.
.PP
Any data written or read through a digest \s-1BIO\s0 using \fBBIO_read_ex()\fR and
\&\fBBIO_write_ex()\fR is digested.
.PP
\&\fBBIO_gets()\fR, if its \fBsize\fR parameter is large enough finishes the
digest calculation and returns the digest value. \fBBIO_puts()\fR is
not supported.
.PP
\&\fBBIO_reset()\fR reinitialises a digest \s-1BIO.\s0
.PP
\&\fBBIO_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 \fBBIO_ctrl()\fR macro.
.PP
\&\fBBIO_get_md()\fR places the a pointer to the digest BIOs digest method
in \fBmdp\fR, it is a \fBBIO_ctrl()\fR macro.
.PP
\&\fBBIO_get_md_ctx()\fR returns the digest BIOs context into \fBmdcp\fR.
.SH "NOTES"
.IX Header "NOTES"
The context returned by \fBBIO_get_md_ctx()\fR can be used in calls
to \fBEVP_DigestFinal()\fR and also the signature routines \fBEVP_SignFinal()\fR
and \fBEVP_VerifyFinal()\fR.
.PP
The context returned by \fBBIO_get_md_ctx()\fR is an internal context
structure. Changes made to this context will affect the digest
\&\s-1BIO\s0 itself and the context pointer will become invalid when the digest
\&\s-1BIO\s0 is freed.
.PP
After the digest has been retrieved from a digest \s-1BIO\s0 it must be
reinitialized by calling \fBBIO_reset()\fR, or \fBBIO_set_md()\fR before any more
data is passed through it.
.PP
If an application needs to call \fBBIO_gets()\fR or \fBBIO_puts()\fR through
a chain containing digest BIOs then this can be done by prepending
a buffering \s-1BIO.\s0
.PP
Calling \fBBIO_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 \fBBIO_set_md()\fR are not sufficiently flexible.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBBIO_f_md()\fR returns the digest \s-1BIO\s0 method.
.PP
\&\fBBIO_set_md()\fR, \fBBIO_get_md()\fR and \fBBIO_md_ctx()\fR return 1 for success and
0 for failure.
.SH "EXAMPLES"
.IX Header "EXAMPLES"
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"Hello World\*(R" through it. Error
checking has been omitted for clarity.
.PP
.Vb 2
\& 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));
.Ve
.PP
The next example digests data by reading through a chain instead:
.PP
.Vb 3
\& 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);
.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
\& 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("\en");
\&     mdtmp = BIO_next(mdtmp);
\& } while (mdtmp);
\&
\& BIO_free_all(bio);
.Ve
.SH "BUGS"
.IX Header "BUGS"
The lack of support for \fBBIO_puts()\fR and the non standard behaviour of
\&\fBBIO_gets()\fR could be regarded as anomalous. It could be argued that \fBBIO_gets()\fR
and \fBBIO_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 \fBBIO_ctrl()\fR call.
.SH "HISTORY"
.IX Header "HISTORY"
Before OpenSSL 1.0.0., the call to \fBBIO_get_md_ctx()\fR would only work if the
\&\s-1BIO\s0 was initialized first.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2016 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R").  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
<https://www.openssl.org/source/license.html>.

MR SKB