/usr/man/cat.3/EVP_PKEY_verify_recover_init.3

EVP_PKEY_verify_recover(3)   OpenSSL   EVP_PKEY_verify_recover(3)

NAME

     EVP_PKEY_verify_recover_init, 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(EVP_PKEY_CTX *ctx,
                             unsigned char *rout, size_t *routlen,
                             const unsigned char *sig, size_t siglen);

DESCRIPTION

     The EVP_PKEY_verify_recover_init() function initializes a
     public key algorithm context using key pkey for a verify
     recover operation.

     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.

EXAMPLE

     Recover digest originally signed using PKCS#1 and SHA256

1.0.2t               Last change: 2019-09-10                    1

EVP_PKEY_verify_recover(3)   OpenSSL   EVP_PKEY_verify_recover(3)

     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);
      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

     These functions were first added to OpenSSL 1.0.0.

1.0.2t               Last change: 2019-09-10                    2

See also EVP_PKEY_verify_recover(3)