=pod =head1 NAME EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover - recover signature using a public key algorithm =head1 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); =head1 DESCRIPTION The EVP_PKEY_verify_recover_init() function initializes a public key algorithm context using key B<pkey> for a verify recover operation. The EVP_PKEY_verify_recover() 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_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. =head1 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. =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_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 */ =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