NAME

       EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover_init_ex, EVP_PKEY_verify_recover -
       recover signature using a public key algorithm

SYNOPSIS

        #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);

DESCRIPTION

       EVP_PKEY_verify_recover_init() initializes a public key algorithm context ctx for signing
       using the algorithm given when the context was created using EVP_PKEY_CTX_new(3) or
       variants thereof.  The algorithm is used to fetch a EVP_SIGNATURE method implicitly, see
       "Implicit fetch" in provider(7) for more information about implicit fetches.

       EVP_PKEY_verify_recover_init_ex() is the same as EVP_PKEY_verify_recover_init() but
       additionally sets the passed parameters params on the context before returning.

       The EVP_PKEY_verify_recover() function recovers signed data using ctx. The signature is
       specified using the sig and siglen parameters. If rout is NULL then the maximum size of
       the output buffer is written to the routlen parameter. If rout is not NULL then before the
       call the routlen parameter should contain the length of the rout buffer, if the call is
       successful recovered data is written to rout and the amount of data written to routlen.

NOTES

       Normally an application is only interested in whether a signature verification operation
       is successful in those cases the EVP_verify() function should be used.

       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).

       After the call to EVP_PKEY_verify_recover_init() algorithm specific control operations can
       be performed to set any appropriate parameters for the operation.

       The function EVP_PKEY_verify_recover() can be called more than once on the same context if
       several operations are performed using the same parameters.

RETURN VALUES

       EVP_PKEY_verify_recover_init() and EVP_PKEY_verify_recover() 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.

EXAMPLES

       Recover digest originally signed using PKCS#1 and SHA256 digest:

        #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 */

SEE ALSO

       EVP_PKEY_CTX_new(3), EVP_PKEY_encrypt(3), EVP_PKEY_decrypt(3), EVP_PKEY_sign(3),
       EVP_PKEY_verify(3), EVP_PKEY_derive(3)

HISTORY

       The EVP_PKEY_verify_recover_init() and EVP_PKEY_verify_recover() functions were added in
       OpenSSL 1.0.0.

       The EVP_PKEY_verify_recover_init_ex() function was added in OpenSSL 3.0.

COPYRIGHT

       Copyright 2013-2021 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (the "License").  You may not use this file except
       in compliance with the License.  You can obtain a copy in the file LICENSE in the source
       distribution or at <https://www.openssl.org/source/license.html>.