1=pod
2
3=head1 NAME
4
5EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover - recover signature using a public key algorithm
6
7=head1 SYNOPSIS
8
9 #include <openssl/evp.h>
10
11 int EVP_PKEY_verify_recover_init(EVP_PKEY_CTX *ctx);
12 int EVP_PKEY_verify_recover(EVP_PKEY_CTX *ctx,
13                             unsigned char *rout, size_t *routlen,
14                             const unsigned char *sig, size_t siglen);
15
16=head1 DESCRIPTION
17
18The EVP_PKEY_verify_recover_init() function initializes a public key algorithm
19context using key B<pkey> for a verify recover operation.
20
21The EVP_PKEY_verify_recover() function recovers signed data
22using B<ctx>. The signature is specified using the B<sig> and
23B<siglen> parameters. If B<rout> is B<NULL> then the maximum size of the output
24buffer is written to the B<routlen> parameter. If B<rout> is not B<NULL> then
25before the call the B<routlen> parameter should contain the length of the
26B<rout> buffer, if the call is successful recovered data is written to
27B<rout> and the amount of data written to B<routlen>.
28
29=head1 NOTES
30
31Normally an application is only interested in whether a signature verification
32operation is successful in those cases the EVP_verify() function should be
33used.
34
35Sometimes however it is useful to obtain the data originally signed using a
36signing operation. Only certain public key algorithms can recover a signature
37in this way (for example RSA in PKCS padding mode).
38
39After the call to EVP_PKEY_verify_recover_init() algorithm specific control
40operations can be performed to set any appropriate parameters for the
41operation.
42
43The function EVP_PKEY_verify_recover() can be called more than once on the same
44context if several operations are performed using the same parameters.
45
46=head1 RETURN VALUES
47
48EVP_PKEY_verify_recover_init() and EVP_PKEY_verify_recover() return 1 for success
49and 0 or a negative value for failure. In particular a return value of -2
50indicates the operation is not supported by the public key algorithm.
51
52=head1 EXAMPLE
53
54Recover digest originally signed using PKCS#1 and SHA256 digest:
55
56 #include <openssl/evp.h>
57 #include <openssl/rsa.h>
58
59 EVP_PKEY_CTX *ctx;
60 unsigned char *rout, *sig;
61 size_t routlen, siglen;
62 EVP_PKEY *verify_key;
63
64 /*
65  * NB: assumes verify_key, sig and siglen are already set up
66  * and that verify_key is an RSA public key
67  */
68 ctx = EVP_PKEY_CTX_new(verify_key, NULL /* no engine */);
69 if (!ctx)
70     /* Error occurred */
71 if (EVP_PKEY_verify_recover_init(ctx) <= 0)
72     /* Error */
73 if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PADDING) <= 0)
74     /* Error */
75 if (EVP_PKEY_CTX_set_signature_md(ctx, EVP_sha256()) <= 0)
76     /* Error */
77
78 /* Determine buffer length */
79 if (EVP_PKEY_verify_recover(ctx, NULL, &routlen, sig, siglen) <= 0)
80     /* Error */
81
82 rout = OPENSSL_malloc(routlen);
83
84 if (!rout)
85     /* malloc failure */
86
87 if (EVP_PKEY_verify_recover(ctx, rout, &routlen, sig, siglen) <= 0)
88     /* Error */
89
90 /* Recovered data is routlen bytes written to buffer rout */
91
92=head1 SEE ALSO
93
94L<EVP_PKEY_CTX_new(3)>,
95L<EVP_PKEY_encrypt(3)>,
96L<EVP_PKEY_decrypt(3)>,
97L<EVP_PKEY_sign(3)>,
98L<EVP_PKEY_verify(3)>,
99L<EVP_PKEY_derive(3)>
100
101=head1 HISTORY
102
103These functions were added in OpenSSL 1.0.0.
104
105=head1 COPYRIGHT
106
107Copyright 2013-2018 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