1=pod
2
3=head1 NAME
4
5EVP_DigestVerifyInit, EVP_DigestVerifyUpdate, EVP_DigestVerifyFinal,
6EVP_DigestVerify - EVP signature verification functions
7
8=head1 SYNOPSIS
9
10 #include <openssl/evp.h>
11
12 int EVP_DigestVerifyInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
13                          const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
14 int EVP_DigestVerifyUpdate(EVP_MD_CTX *ctx, const void *d, size_t cnt);
15 int EVP_DigestVerifyFinal(EVP_MD_CTX *ctx, const unsigned char *sig,
16                           size_t siglen);
17 int EVP_DigestVerify(EVP_MD_CTX *ctx, const unsigned char *sigret,
18                      size_t siglen, const unsigned char *tbs, size_t tbslen);
19
20=head1 DESCRIPTION
21
22The EVP signature routines are a high-level interface to digital signatures.
23
24EVP_DigestVerifyInit() sets up verification context B<ctx> to use digest
25B<type> from ENGINE B<e> and public key B<pkey>. B<ctx> must be created
26with EVP_MD_CTX_new() before calling this function. If B<pctx> is not NULL, the
27EVP_PKEY_CTX of the verification operation will be written to B<*pctx>: this
28can be used to set alternative verification options. Note that any existing
29value in B<*pctx> is overwritten. The EVP_PKEY_CTX value returned must not be freed
30directly by the application if B<ctx> is not assigned an EVP_PKEY_CTX value before
31being passed to EVP_DigestVerifyInit() (which means the EVP_PKEY_CTX is created
32inside EVP_DigestVerifyInit() and it will be freed automatically when the
33EVP_MD_CTX is freed).
34
35No B<EVP_PKEY_CTX> will be created by EVP_DigestSignInit() if the passed B<ctx>
36has already been assigned one via L<EVP_MD_CTX_set_pkey_ctx(3)>. See also L<SM2(7)>.
37
38EVP_DigestVerifyUpdate() hashes B<cnt> bytes of data at B<d> into the
39verification context B<ctx>. This function can be called several times on the
40same B<ctx> to include additional data. This function is currently implemented
41using a macro.
42
43EVP_DigestVerifyFinal() verifies the data in B<ctx> against the signature in
44B<sig> of length B<siglen>.
45
46EVP_DigestVerify() verifies B<tbslen> bytes at B<tbs> against the signature
47in B<sig> of length B<siglen>.
48
49=head1 RETURN VALUES
50
51EVP_DigestVerifyInit() and EVP_DigestVerifyUpdate() return 1 for success and 0
52for failure.
53
54EVP_DigestVerifyFinal() and EVP_DigestVerify() return 1 for success; any other
55value indicates failure.  A return value of zero indicates that the signature
56did not verify successfully (that is, B<tbs> did not match the original data or
57the signature had an invalid form), while other values indicate a more serious
58error (and sometimes also indicate an invalid signature form).
59
60The error codes can be obtained from L<ERR_get_error(3)>.
61
62=head1 NOTES
63
64The B<EVP> interface to digital signatures should almost always be used in
65preference to the low-level interfaces. This is because the code then becomes
66transparent to the algorithm used and much more flexible.
67
68EVP_DigestVerify() is a one shot operation which verifies a single block of
69data in one function. For algorithms that support streaming it is equivalent
70to calling EVP_DigestVerifyUpdate() and EVP_DigestVerifyFinal(). For
71algorithms which do not support streaming (e.g. PureEdDSA) it is the only way
72to verify data.
73
74In previous versions of OpenSSL there was a link between message digest types
75and public key algorithms. This meant that "clone" digests such as EVP_dss1()
76needed to be used to sign using SHA1 and DSA. This is no longer necessary and
77the use of clone digest is now discouraged.
78
79For some key types and parameters the random number generator must be seeded.
80If the automatic seeding or reseeding of the OpenSSL CSPRNG fails due to
81external circumstances (see L<RAND(7)>), the operation will fail.
82
83The call to EVP_DigestVerifyFinal() internally finalizes a copy of the digest
84context. This means that EVP_VerifyUpdate() and EVP_VerifyFinal() can
85be called later to digest and verify additional data.
86
87Since only a copy of the digest context is ever finalized, the context must
88be cleaned up after use by calling EVP_MD_CTX_free() or a memory leak
89will occur.
90
91=head1 SEE ALSO
92
93L<EVP_DigestSignInit(3)>,
94L<EVP_DigestInit(3)>,
95L<evp(7)>, L<HMAC(3)>, L<MD2(3)>,
96L<MD5(3)>, L<MDC2(3)>, L<RIPEMD160(3)>,
97L<SHA1(3)>, L<dgst(1)>,
98L<RAND(7)>
99
100=head1 HISTORY
101
102EVP_DigestVerifyInit(), EVP_DigestVerifyUpdate() and EVP_DigestVerifyFinal()
103were added in OpenSSL 1.0.0.
104
105=head1 COPYRIGHT
106
107Copyright 2006-2020 The OpenSSL Project Authors. All Rights Reserved.
108
109Licensed under the OpenSSL license (the "License").  You may not use
110this file except in compliance with the License.  You can obtain a copy
111in the file LICENSE in the source distribution or at
112L<https://www.openssl.org/source/license.html>.
113
114=cut
115