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.\"
.IX Title "EVP_DigestVerifyInit 3"
.TH EVP_DigestVerifyInit 3 "2012-01-04" "1.0.0f" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH "NAME"
EVP_DigestVerifyInit, EVP_DigestVerifyUpdate, EVP_DigestVerifyFinal \- EVP signature verification functions
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/evp.h>
\&
\& int EVP_DigestVerifyInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
\&                        const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
\& int EVP_DigestVerifyUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
\& int EVP_DigestVerifyFinal(EVP_MD_CTX *ctx, unsigned char *sig, size_t siglen);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \s-1EVP\s0 signature routines are a high level interface to digital signatures.
.PP
\&\fIEVP_DigestVerifyInit()\fR sets up verification context \fBctx\fR to use digest
\&\fBtype\fR from \s-1ENGINE\s0 \fBimpl\fR and public key \fBpkey\fR. \fBctx\fR must be initialized
with \fIEVP_MD_CTX_init()\fR before calling this function. If \fBpctx\fR is not \s-1NULL\s0 the
\&\s-1EVP_PKEY_CTX\s0 of the verification operation will be written to \fB*pctx\fR: this
can be used to set alternative verification options.
.PP
\&\fIEVP_DigestVerifyUpdate()\fR hashes \fBcnt\fR bytes of data at \fBd\fR into the
verification context \fBctx\fR. This function can be called several times on the
same \fBctx\fR to include additional data. This function is currently implemented
using a macro.
.PP
\&\fIEVP_DigestVerifyFinal()\fR verifies the data in \fBctx\fR against the signature in
\&\fBsig\fR of length \fBsiglen\fR.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fIEVP_DigestVerifyInit()\fR and \fIEVP_DigestVerifyUpdate()\fR return 1 for success and 0
or a negative value for failure. In particular a return value of \-2 indicates
the operation is not supported by the public key algorithm.
.PP
Unlike other functions the return value 0 from \fIEVP_DigestVerifyFinal()\fR only
indicates that the signature did not not verify successfully (that is tbs did
not match the original data or the signature was of invalid form) it is not an
indication of a more serious error.
.PP
The error codes can be obtained from \fIERR_get_error\fR\|(3).
.SH "NOTES"
.IX Header "NOTES"
The \fB\s-1EVP\s0\fR interface to digital signatures should almost always be used in
preference to the low level interfaces. This is because the code then becomes
transparent to the algorithm used and much more flexible.
.PP
In previous versions of OpenSSL there was a link between message digest types
and public key algorithms. This meant that \*(L"clone\*(R" digests such as \fIEVP_dss1()\fR
needed to be used to sign using \s-1SHA1\s0 and \s-1DSA\s0. This is no longer necessary and
the use of clone digest is now discouraged.
.PP
For some key types and parameters the random number generator must be seeded
or the operation will fail.
.PP
The call to \fIEVP_DigestVerifyFinal()\fR internally finalizes a copy of the digest
context. This means that calls to \fIEVP_VerifyUpdate()\fR and \fIEVP_VerifyFinal()\fR can
be called later to digest and verify additional data.
.PP
Since only a copy of the digest context is ever finalized the context must
be cleaned up after use by calling \fIEVP_MD_CTX_cleanup()\fR or a memory leak
will occur.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIEVP_DigestSignInit\fR\|(3),
\&\fIEVP_DigestInit\fR\|(3), \fIerr\fR\|(3),
\&\fIevp\fR\|(3), \fIhmac\fR\|(3), \fImd2\fR\|(3),
\&\fImd5\fR\|(3), \fImdc2\fR\|(3), \fIripemd\fR\|(3),
\&\fIsha\fR\|(3), \fIdgst\fR\|(1)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIEVP_DigestVerifyInit()\fR, \fIEVP_DigestVerifyUpdate()\fR and \fIEVP_DigestVerifyFinal()\fR 
were first added to OpenSSL 1.0.0.