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=head1 NAME

EVP_VerifyInit, EVP_VerifyUpdate, EVP_VerifyFinal - EVP signature verification functions

=head1 SYNOPSIS

 #include <openssl/evp.h>

 void EVP_VerifyInit(EVP_MD_CTX *ctx, const EVP_MD *type);
 void EVP_VerifyUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
 int EVP_VerifyFinal(EVP_MD_CTX *ctx,unsigned char *sigbuf, unsigned int siglen,EVP_PKEY *pkey);

=head1 DESCRIPTION

The EVP signature verification routines are a high level interface to digital
signatures.

EVP_VerifyInit() initialises a verification context B<ctx> to using digest
B<type>: this will typically be supplied by a function such as EVP_sha1().

EVP_VerifyUpdate() hashes B<cnt> bytes of data at B<d> into the
verification context B<ctx>. This funtion can be called several times on the
same B<ctx> to include additional data.

EVP_VerifyFinal() verifies the data in B<ctx> using the public key B<pkey>
and against the B<siglen> bytes at B<sigbuf>. After calling EVP_VerifyFinal()
no additional calls to EVP_VerifyUpdate() can be made, but EVP_VerifyInit()
can be called to initialiase a new verification operation.

=head1 RETURN VALUES

EVP_VerifyInit() and EVP_VerifyUpdate() do not return values.

EVP_VerifyFinal() returns 1 for a correct signature, 0 for failure and -1 if some
other error occurred.

The error codes can be obtained by L<ERR_get_error(3)|ERR_get_error(3)>.

=head1 NOTES

The B<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.

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 
L<EVP_DigestInit(3)|EVP_DigestInit(3)>.

=head1 BUGS

Several of the functions do not return values: maybe they should. Although the
internal digest operations will never fail some future hardware based operations
might.

=head1 SEE ALSO

L<EVP_SignInit(3)|EVP_SignInit(3)>,
L<EVP_DigestInit(3)|EVP_DigestInit(3)>, L<err(3)|err(3)>,
L<evp(3)|evp(3)>, L<hmac(3)|hmac(3)>, L<md2(3)|md2(3)>,
L<md5(3)|md5(3)>, L<mdc2(3)|mdc2(3)>, L<ripemd(3)|ripemd(3)>,
L<sha(3)|sha(3)>, L<digest(1)|digest(1)>

=head1 HISTORY

EVP_VerifyInit(), EVP_VerifyUpdate() and EVP_VerifyFinal() are
available in all versions of SSLeay and OpenSSL.

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