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.\" ========================================================================
.\"
.IX Title "EVP_PKEY_VERIFY_RECOVER 3ossl"
.TH EVP_PKEY_VERIFY_RECOVER 3ossl "2024-09-03" "3.1.7+quic" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH "NAME"
EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover_init_ex,
EVP_PKEY_verify_recover
\&\- recover signature using a public key algorithm
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/evp.h>
\&
\& int EVP_PKEY_verify_recover_init(EVP_PKEY_CTX *ctx);
\& int EVP_PKEY_verify_recover_init_ex(EVP_PKEY_CTX *ctx,
\&                                     const OSSL_PARAM params[]);
\& int EVP_PKEY_verify_recover(EVP_PKEY_CTX *ctx,
\&                             unsigned char *rout, size_t *routlen,
\&                             const unsigned char *sig, size_t siglen);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBEVP_PKEY_verify_recover_init()\fR initializes a public key algorithm context
\&\fIctx\fR for signing using the algorithm given when the context was created
using \fBEVP_PKEY_CTX_new\fR\|(3) or variants thereof.  The algorithm is used to
fetch a \fB\s-1EVP_SIGNATURE\s0\fR method implicitly, see \*(L"Implicit fetch\*(R" in \fBprovider\fR\|(7)
for more information about implicit fetches.
.PP
\&\fBEVP_PKEY_verify_recover_init_ex()\fR is the same as
\&\fBEVP_PKEY_verify_recover_init()\fR but additionally sets the passed parameters
\&\fIparams\fR on the context before returning.
.PP
The \fBEVP_PKEY_verify_recover()\fR function recovers signed data
using \fIctx\fR. The signature is specified using the \fIsig\fR and
\&\fIsiglen\fR parameters. If \fIrout\fR is \s-1NULL\s0 then the maximum size of the output
buffer is written to the \fIroutlen\fR parameter. If \fIrout\fR is not \s-1NULL\s0 then
before the call the \fIroutlen\fR parameter should contain the length of the
\&\fIrout\fR buffer, if the call is successful recovered data is written to
\&\fIrout\fR and the amount of data written to \fIroutlen\fR.
.SH "NOTES"
.IX Header "NOTES"
Normally an application is only interested in whether a signature verification
operation is successful in those cases the \fBEVP_verify()\fR function should be
used.
.PP
Sometimes however it is useful to obtain the data originally signed using a
signing operation. Only certain public key algorithms can recover a signature
in this way (for example \s-1RSA\s0 in \s-1PKCS\s0 padding mode).
.PP
After the call to \fBEVP_PKEY_verify_recover_init()\fR algorithm specific control
operations can be performed to set any appropriate parameters for the
operation.
.PP
The function \fBEVP_PKEY_verify_recover()\fR can be called more than once on the same
context if several operations are performed using the same parameters.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBEVP_PKEY_verify_recover_init()\fR and \fBEVP_PKEY_verify_recover()\fR return 1 for success
and 0 or a negative value for failure. In particular a return value of \-2
indicates the operation is not supported by the public key algorithm.
.SH "EXAMPLES"
.IX Header "EXAMPLES"
Recover digest originally signed using PKCS#1 and \s-1SHA256\s0 digest:
.PP
.Vb 2
\& #include <openssl/evp.h>
\& #include <openssl/rsa.h>
\&
\& EVP_PKEY_CTX *ctx;
\& unsigned char *rout, *sig;
\& size_t routlen, siglen;
\& EVP_PKEY *verify_key;
\&
\& /*
\&  * NB: assumes verify_key, sig and siglen are already set up
\&  * and that verify_key is an RSA public key
\&  */
\& ctx = EVP_PKEY_CTX_new(verify_key, NULL /* no engine */);
\& if (!ctx)
\&     /* Error occurred */
\& if (EVP_PKEY_verify_recover_init(ctx) <= 0)
\&     /* Error */
\& if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PADDING) <= 0)
\&     /* Error */
\& if (EVP_PKEY_CTX_set_signature_md(ctx, EVP_sha256()) <= 0)
\&     /* Error */
\&
\& /* Determine buffer length */
\& if (EVP_PKEY_verify_recover(ctx, NULL, &routlen, sig, siglen) <= 0)
\&     /* Error */
\&
\& rout = OPENSSL_malloc(routlen);
\&
\& if (!rout)
\&     /* malloc failure */
\&
\& if (EVP_PKEY_verify_recover(ctx, rout, &routlen, sig, siglen) <= 0)
\&     /* Error */
\&
\& /* Recovered data is routlen bytes written to buffer rout */
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBEVP_PKEY_CTX_new\fR\|(3),
\&\fBEVP_PKEY_encrypt\fR\|(3),
\&\fBEVP_PKEY_decrypt\fR\|(3),
\&\fBEVP_PKEY_sign\fR\|(3),
\&\fBEVP_PKEY_verify\fR\|(3),
\&\fBEVP_PKEY_derive\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
The \fBEVP_PKEY_verify_recover_init()\fR and \fBEVP_PKEY_verify_recover()\fR
functions were added in OpenSSL 1.0.0.
.PP
The \fBEVP_PKEY_verify_recover_init_ex()\fR function was added in OpenSSL 3.0.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2013\-2021 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the Apache License 2.0 (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.