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.\" ========================================================================
.\"
.IX Title "EVP_PKEY_verify_recover 3"
.TH EVP_PKEY_verify_recover 3 "2019-12-20" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
EVP_PKEY_verify_recover_init, 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(EVP_PKEY_CTX *ctx,
\&                        unsigned char *rout, size_t *routlen,
\&                        const unsigned char *sig, size_t siglen);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \fBEVP_PKEY_verify_recover_init()\fR function initializes a public key algorithm
context using key \fBpkey\fR for a verify recover operation.
.PP
The \fBEVP_PKEY_verify_recover()\fR function recovers signed data
using \fBctx\fR. The signature is specified using the \fBsig\fR and
\&\fBsiglen\fR parameters. If \fBrout\fR is \fB\s-1NULL\s0\fR then the maximum size of the output
buffer is written to the \fBroutlen\fR parameter. If \fBrout\fR is not \fB\s-1NULL\s0\fR then
before the call the \fBroutlen\fR parameter should contain the length of the
\&\fBrout\fR buffer, if the call is successful recovered data is written to
\&\fBrout\fR and the amount of data written to \fBroutlen\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 "EXAMPLE"
.IX Header "EXAMPLE"
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);
\& 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"
These functions were first added to OpenSSL 1.0.0.