EVP_DigestVerifyInit.pod revision 279264 
1=pod
2
3=head1 NAME
4
5EVP_DigestVerifyInit, EVP_DigestVerifyUpdate, EVP_DigestVerifyFinal - EVP signature verification functions
6
7=head1 SYNOPSIS
8
9 #include <openssl/evp.h>
10
11 int EVP_DigestVerifyInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
12			const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
13 int EVP_DigestVerifyUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
14 int EVP_DigestVerifyFinal(EVP_MD_CTX *ctx, unsigned char *sig, size_t siglen);
15
16=head1 DESCRIPTION
17
18The EVP signature routines are a high level interface to digital signatures.
19
20EVP_DigestVerifyInit() sets up verification context B<ctx> to use digest
21B<type> from ENGINE B<impl> and public key B<pkey>. B<ctx> must be initialized
22with EVP_MD_CTX_init() before calling this function. If B<pctx> is not NULL the
23EVP_PKEY_CTX of the verification operation will be written to B<*pctx>: this
24can be used to set alternative verification options.
25
26EVP_DigestVerifyUpdate() hashes B<cnt> bytes of data at B<d> into the
27verification context B<ctx>. This function can be called several times on the
28same B<ctx> to include additional data. This function is currently implemented
29using a macro.
30
31EVP_DigestVerifyFinal() verifies the data in B<ctx> against the signature in
32B<sig> of length B<siglen>.
33
34=head1 RETURN VALUES
35
36EVP_DigestVerifyInit() and EVP_DigestVerifyUpdate() return 1 for success and 0
37or a negative value for failure. In particular a return value of -2 indicates
38the operation is not supported by the public key algorithm.
39
40Unlike other functions the return value 0 from EVP_DigestVerifyFinal() only
41indicates that the signature did not verify successfully (that is tbs did
42not match the original data or the signature was of invalid form) it is not an
43indication of a more serious error.
44
45The error codes can be obtained from L<ERR_get_error(3)|ERR_get_error(3)>.
46
47=head1 NOTES
48
49The B<EVP> interface to digital signatures should almost always be used in
50preference to the low level interfaces. This is because the code then becomes
51transparent to the algorithm used and much more flexible.
52
53In previous versions of OpenSSL there was a link between message digest types
54and public key algorithms. This meant that "clone" digests such as EVP_dss1()
55needed to be used to sign using SHA1 and DSA. This is no longer necessary and
56the use of clone digest is now discouraged.
57
58For some key types and parameters the random number generator must be seeded
59or the operation will fail. 
60
61The call to EVP_DigestVerifyFinal() internally finalizes a copy of the digest
62context. This means that EVP_VerifyUpdate() and EVP_VerifyFinal() can
63be called later to digest and verify additional data.
64
65Since only a copy of the digest context is ever finalized the context must
66be cleaned up after use by calling EVP_MD_CTX_cleanup() or a memory leak
67will occur.
68
69=head1 SEE ALSO
70
71L<EVP_DigestSignInit(3)|EVP_DigestSignInit(3)>,
72L<EVP_DigestInit(3)|EVP_DigestInit(3)>, L<err(3)|err(3)>,
73L<evp(3)|evp(3)>, L<hmac(3)|hmac(3)>, L<md2(3)|md2(3)>,
74L<md5(3)|md5(3)>, L<mdc2(3)|mdc2(3)>, L<ripemd(3)|ripemd(3)>,
75L<sha(3)|sha(3)>, L<dgst(1)|dgst(1)>
76
77=head1 HISTORY
78
79EVP_DigestVerifyInit(), EVP_DigestVerifyUpdate() and EVP_DigestVerifyFinal() 
80were first added to OpenSSL 1.0.0.
81
82=cut
83