EVP_PKEY_verifyrecover_init, EVP_PKEY_verifyrecover -- recover signature using a public key algorithm
Synopsis
#include <openssl/evp.h>
int EVP_PKEY_verifyrecover_init(EVP_PKEY_CTX *ctx);
int EVP_PKEY_verifyrecover(EVP_PKEY_CTX *ctx,
unsigned char *rout, size_t *routlen,
const unsigned char *sig, size_t siglen);
DESCRIPTION
The EVP_PKEY_verifyrecover_init() function initializes a public key
algorithm context using key pkey for
a verify recover operation.
The EVP_PKEY_verifyrecover() 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_verifyrecover_init() algorithm specific
control operations can be performed to set any appropriate parameters
for the operation.
The function EVP_PKEY_verifyrecover() can be called more than once on
the same context if several operations are performed using the same
parameters.
RETURN VALUES
EVP_PKEY_verifyrecover_init() and EVP_PKEY_verifyrecover() 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 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_verifyrecover_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_verifyrecover(ctx, rout, &routlen, sig, siglen) <= 0)
/* Error */
rout = OPENSSL_malloc(routlen);
if (!rout)
/* malloc failure */
if (EVP_PKEY_verifyrecover(ctx, rout, &routlen, sig, siglen) <= 0)
/* Error */
/* Recovered data is routlen bytes written to buffer rout */
