.\"	$OpenBSD: EVP_VerifyInit.3,v 1.2 2016/11/06 15:52:50 jmc Exp $
.\"
.Dd $Mdocdate: November 6 2016 $
.Dt EVP_VERIFYINIT 3
.Os
.Sh NAME
.Nm EVP_VerifyInit ,
.Nm EVP_VerifyUpdate ,
.Nm EVP_VerifyFinal
.Nd EVP signature verification functions
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_VerifyInit_ex
.Fa "EVP_MD_CTX *ctx"
.Fa "const EVP_MD *type"
.Fa "ENGINE *impl"
.Fc
.Ft int
.Fo EVP_VerifyUpdate
.Fa "EVP_MD_CTX *ctx"
.Fa "const void *d"
.Fa "unsigned int cnt"
.Fc
.Ft int
.Fo EVP_VerifyFinal
.Fa "EVP_MD_CTX *ctx"
.Fa "unsigned char *sigbuf"
.Fa "unsigned int siglen"
.Fa "EVP_PKEY *pkey"
.Fc
.Ft int
.Fo EVP_VerifyInit
.Fa "EVP_MD_CTX *ctx"
.Fa "const EVP_MD *type"
.Fc
.Sh DESCRIPTION
The EVP signature verification routines are a high level interface to
digital signatures.
.Pp
.Fn EVP_VerifyInit_ex
sets up a verification context
.Fa ctx
to use the digest
.Fa type
from
.Vt ENGINE
.Fa impl .
.Fa ctx
must be initialized by calling
.Xr EVP_MD_CTX_init 3
before calling this function.
.Pp
.Fn EVP_VerifyUpdate
hashes
.Fa cnt
bytes of data at
.Fa d
into the verification context
.Fa ctx .
This function can be called several times on the same
.Fa ctx
to include additional data.
.Pp
.Fn EVP_VerifyFinal
verifies the data in
.Fa ctx
using the public key
.Fa pkey
and against the
.Fa siglen
bytes at
.Fa sigbuf .
.Pp
.Fn EVP_VerifyInit
initializes a verification context
.Fa ctx
to use the default implementation of digest
.Fa type .
.Pp
The EVP 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
Due to the link between message digests and public key algorithms, the
correct digest algorithm must be used with the correct public key type.
A list of algorithms and associated public key algorithms appears in
.Xr EVP_DigestInit 3 .
.Pp
The call to
.Fn EVP_VerifyFinal
internally finalizes a copy of the digest context.
This means that calls to
.Fn EVP_VerifyUpdate
and
.Fn EVP_VerifyFinal
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
.Xr EVP_MD_CTX_cleanup 3 ,
or a memory leak will occur.
.Sh RETURN VALUES
.Fn EVP_VerifyInit_ex
and
.Fn EVP_VerifyUpdate
return 1 for success and 0 for failure.
.Pp
.Fn EVP_VerifyFinal
returns 1 for a correct signature, 0 for failure, and -1 if some other
error occurred.
.Pp
The error codes can be obtained by
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr ERR 3 ,
.Xr evp 3 ,
.Xr EVP_DigestInit 3 ,
.Xr EVP_SignInit 3
.Sh HISTORY
.Fn EVP_VerifyInit ,
.Fn EVP_VerifyUpdate ,
and
.Fn EVP_VerifyFinal
are available in all versions of SSLeay and OpenSSL.
.Pp
.Fn EVP_VerifyInit_ex
was added in OpenSSL 0.9.7.
.Sh BUGS
Older versions of this documentation wrongly stated that calls to
.Fn EVP_VerifyUpdate
could not be made after calling
.Fn EVP_VerifyFinal .
.Pp
Since the public key is passed in the call to
.Xr EVP_SignFinal 3 ,
any error relating to the private key (for example an unsuitable key and
digest combination) will not be indicated until after potentially large
amounts of data have been passed through
.Xr EVP_SignUpdate 3 .
.Pp
It is not possible to change the signing parameters using these
functions.
.Pp
The previous two bugs are fixed in the newer functions of the
.Xr EVP_DigestVerifyInit 3
family.