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.Dd $Mdocdate: November 3 2016 $
.Dt EVP_DIGESTVERIFYINIT 3
.Os
.Sh NAME
.Nm EVP_DigestVerifyInit ,
.Nm EVP_DigestVerifyUpdate ,
.Nm EVP_DigestVerifyFinal
.Nd EVP signature verification functions
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_DigestVerifyInit
.Fa "EVP_MD_CTX *ctx"
.Fa "EVP_PKEY_CTX **pctx"
.Fa "const EVP_MD *type"
.Fa "ENGINE *e"
.Fa "EVP_PKEY *pkey"
.Fc
.Ft int
.Fo EVP_DigestVerifyUpdate
.Fa "EVP_MD_CTX *ctx"
.Fa "const void *d"
.Fa "unsigned int cnt"
.Fc
.Ft int
.Fo EVP_DigestVerifyFinal
.Fa "EVP_MD_CTX *ctx"
.Fa "unsigned char *sig"
.Fa "size_t siglen"
.Fc
.Sh DESCRIPTION
The EVP signature routines are a high level interface to digital
signatures.
.Pp
.Fn EVP_DigestVerifyInit
sets up verification context
.Fa ctx
to use digest
.Fa type
from
.Vt ENGINE
.Fa e
and public key
.Fa pkey .
.Fa ctx
must be initialized with
.Xr EVP_MD_CTX_init 3
before calling this function.
If
.Fa pctx
is not
.Dv NULL ,
the
.Vt EVP_PKEY_CTX
of the verification operation will be written to
.Pf * Fa pctx :
this can be used to set alternative verification options.
.Pp
.Fn EVP_DigestVerifyUpdate
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.
This function is currently implemented using a macro.
.Pp
.Fn EVP_DigestVerifyFinal
verifies the data in
.Fa ctx
against the signature in
.Fa sig
of length
.Fa siglen .
.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
In previous versions of OpenSSL, there was a link between message digest
types and public key algorithms.
This meant that "clone" digests such as
.Xr EVP_dss1 3
needed to be used to sign using SHA1 and DSA.
This is no longer necessary and the use of clone digest is now
discouraged.
.Pp
The call to
.Fn EVP_DigestVerifyFinal
internally finalizes a copy of the digest context.
This means that calls to
.Xr EVP_VerifyUpdate 3
and
.Xr EVP_VerifyFinal 3
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_DigestVerifyInit
and
.Fn EVP_DigestVerifyUpdate
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
.Fn EVP_DigestVerifyFinal
only indicates that the signature did not verify successfully.
That is it 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
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr ERR 3 ,
.Xr evp 3 ,
.Xr EVP_DigestInit 3 ,
.Xr EVP_DigestSignInit 3
.Sh HISTORY
.Fn EVP_DigestVerifyInit ,
.Fn EVP_DigestVerifyUpdate ,
and
.Fn EVP_DigestVerifyFinal
were first added to OpenSSL 1.0.0.
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