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- =pod
- =head1 NAME
- EVP_PKEY_verifyrecover_init, EVP_PKEY_verifyrecover - recover signature using a public key algorithm
- =head1 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);
- =head1 DESCRIPTION
- The EVP_PKEY_verifyrecover_init() function initializes a public key algorithm
- context using key B<pkey> for a verify recover operation.
- The EVP_PKEY_verifyrecover() function recovers signed data
- using B<ctx>. The signature is specified using the B<sig> and
- B<siglen> parameters. If B<rout> is B<NULL> then the maximum size of the output
- buffer is written to the B<routlen> parameter. If B<rout> is not B<NULL> then
- before the call the B<routlen> parameter should contain the length of the
- B<rout> buffer, if the call is successful recovered data is written to
- B<rout> and the amount of data written to B<routlen>.
- =head1 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.
- =head1 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.
- =head1 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, NULL, &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 */
- =head1 SEE ALSO
- L<EVP_PKEY_CTX_new(3)|EVP_PKEY_CTX_new(3)>,
- L<EVP_PKEY_encrypt(3)|EVP_PKEY_encrypt(3)>,
- L<EVP_PKEY_decrypt(3)|EVP_PKEY_decrypt(3)>,
- L<EVP_PKEY_sign(3)|EVP_PKEY_sign(3)>,
- L<EVP_PKEY_verify(3)|EVP_PKEY_verify(3)>,
- L<EVP_PKEY_derive(3)|EVP_PKEY_derive(3)>
- =head1 HISTORY
- These functions were first added to OpenSSL 1.0.0.
- =cut
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