- NAME
- SYNOPSIS
- DESCRIPTION
- NOTES
- RETURN VALUES
- EXAMPLES
- SEE ALSO
- HISTORY
- COPYRIGHT

# NAME

EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover_init_ex, EVP_PKEY_verify_recover_init_ex2, 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_init_ex2(EVP_PKEY_CTX *ctx, EVP_SIGNATURE *algo,
                                          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 provider(7)/Implicit fetch 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.

EVP_PKEY_verify_recover_init_ex2() is the same as `EVP_PKEY_verify_recover_init_ex()`, but works with an explicitly fetched EVP_SIGNATURE algo. A context ctx without a pre-loaded key cannot be used with this function. Depending on what algorithm was fetched, certain details revolving around the treatment of the input to `EVP_PKEY_verify()` may be pre-determined, and in that case, those details may normally not be changed. See NOTES below for a deeper explanation.

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.

After the call to EVP_PKEY_verify_recover_init_ex2(), algorithm specific control operations may not be needed if the chosen algorithm implies that those controls pre-set (and immutable).

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 2013-2024 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.