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1.\" $OpenBSD: EVP_PKEY_verify_recover.3,v 1.9 2018/03/23 04:34:23 schwarze Exp $
2.\" full merge up to: OpenSSL 48e5119a Jan 19 10:49:22 2018 +0100
3.\"
4.\" This file was written by Dr. Stephen Henson <steve@openssl.org>.
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51.\"
52.Dd $Mdocdate: March 23 2018 $
53.Dt EVP_PKEY_VERIFY_RECOVER 3
54.Os
55.Sh NAME
56.Nm EVP_PKEY_verify_recover_init ,
57.Nm EVP_PKEY_verify_recover
58.Nd recover signature using a public key algorithm
59.Sh SYNOPSIS
60.In openssl/evp.h
61.Ft int
62.Fo EVP_PKEY_verify_recover_init
63.Fa "EVP_PKEY_CTX *ctx"
64.Fc
65.Ft int
66.Fo EVP_PKEY_verify_recover
67.Fa "EVP_PKEY_CTX *ctx"
68.Fa "unsigned char *rout"
69.Fa "size_t *routlen"
70.Fa "const unsigned char *sig"
71.Fa "size_t siglen"
72.Fc
73.Sh DESCRIPTION
74The
75.Fn EVP_PKEY_verify_recover_init
76function initializes a public key algorithm context using key
77.Fa ctx->pkey
78for a verify recover operation.
79.Pp
80The
81.Fn EVP_PKEY_verify_recover
82function recovers signed data using
83.Fa ctx .
84The signature is specified using the
85.Fa sig
86and
87.Fa siglen
88parameters.
89If
90.Fa rout
91is
92.Dv NULL ,
93then the maximum size of the output buffer is written to the
94.Fa routlen
95parameter.
96If
97.Fa rout
98is not
99.Dv NULL ,
100then before the call the
101.Fa routlen
102parameter should contain the length of the
103.Fa rout
104buffer.
105If the call is successful, recovered data is written to
106.Fa rout
107and the amount of data written to
108.Fa routlen .
109.Pp
110Normally an application is only interested in whether a signature
111verification operation is successful.
112In those cases, the
113.Xr EVP_PKEY_verify 3
114function should be used.
115.Pp
116Sometimes however it is useful to obtain the data originally signed
117using a signing operation.
118Only certain public key algorithms can recover a signature in this way
119(for example RSA in PKCS padding mode).
120.Pp
121After the call to
122.Fn EVP_PKEY_verify_recover_init ,
123algorithm specific control operations can be performed to set any
124appropriate parameters for the operation.
125.Pp
126The function
127.Fn EVP_PKEY_verify_recover
128can be called more than once on the same context if several operations
129are performed using the same parameters.
130.Sh RETURN VALUES
131.Fn EVP_PKEY_verify_recover_init
132and
133.Fn EVP_PKEY_verify_recover
134return 1 for success and 0 or a negative value for failure.
135In particular, a return value of -2 indicates the operation is not
136supported by the public key algorithm.
137.Sh EXAMPLES
138Recover digest originally signed using PKCS#1 and SHA256 digest:
139.Bd -literal -offset indent
140#include <openssl/evp.h>
141#include <openssl/rsa.h>
142
143EVP_PKEY_CTX *ctx;
144unsigned char *rout, *sig;
145size_t routlen, siglen;
146EVP_PKEY *verify_key;
147
148/*
149 * Assumes that verify_key, sig, and siglen are already set up
150 * and that verify_key is an RSA public key.
151 */
152ctx = EVP_PKEY_CTX_new(verify_key, NULL);
153if (!ctx)
154 /* Error occurred */
155if (EVP_PKEY_verify_recover_init(ctx) <= 0)
156 /* Error */
157if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PADDING) <= 0)
158 /* Error */
159if (EVP_PKEY_CTX_set_signature_md(ctx, EVP_sha256()) <= 0)
160 /* Error */
161
162/* Determine buffer length */
163if (EVP_PKEY_verify_recover(ctx, NULL, &routlen, sig, siglen) <= 0)
164 /* Error */
165
166rout = malloc(routlen);
167
168if (!rout)
169 /* malloc failure */
170
171if (EVP_PKEY_verify_recover(ctx, rout, &routlen, sig, siglen) <= 0)
172 /* Error */
173
174/* Recovered data is routlen bytes written to buffer rout */
175.Ed
176.Sh SEE ALSO
177.Xr EVP_PKEY_CTX_new 3 ,
178.Xr EVP_PKEY_decrypt 3 ,
179.Xr EVP_PKEY_derive 3 ,
180.Xr EVP_PKEY_encrypt 3 ,
181.Xr EVP_PKEY_meth_set_verify_recover 3 ,
182.Xr EVP_PKEY_sign 3 ,
183.Xr EVP_PKEY_verify 3
184.Sh HISTORY
185.Fn EVP_PKEY_verify_recover_init
186and
187.Fn EVP_PKEY_verify_recover
188first appeared in OpenSSL 1.0.0 and have been available since
189.Ox 4.9 .
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