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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 \fBNULL\fR then the maximum size of the output
buffer is written to the \fBroutlen\fR parameter. If \fBrout\fR is not \fBNULL\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 RSA in PKCS 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 SHA256 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.