EVP_PKEY_verify_recover(3) OpenSSL EVP_PKEY_verify_recover(3) NNAAMMEE EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover - recover signa- ture using a public key algorithm SSYYNNOOPPSSIISS #include 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); DDEESSCCRRIIPPTTIIOONN The _E_V_P___P_K_E_Y___v_e_r_i_f_y___r_e_c_o_v_e_r___i_n_i_t_(_) function initializes a public key algorithm context using key ppkkeeyy for a verify recover operation. The _E_V_P___P_K_E_Y___v_e_r_i_f_y___r_e_c_o_v_e_r_(_) function recovers signed data using ccttxx. The signature is specified using the ssiigg and ssiigglleenn parameters. If rroouutt is NNUULLLL then the maximum size of the output buffer is written to the rroouuttlleenn parameter. If rroouutt is not NNUULLLL then before the call the rroouuttlleenn parameter should contain the length of the rroouutt buffer, if the call is successful recovered data is written to rroouutt and the amount of data written to rroouuttlleenn. NNOOTTEESS Normally an application is only interested in whether a signature veri- fication operation is successful in those cases the _E_V_P___v_e_r_i_f_y_(_) func- tion 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 _E_V_P___P_K_E_Y___v_e_r_i_f_y___r_e_c_o_v_e_r___i_n_i_t_(_) algorithm specific control operations can be performed to set any appropriate parameters for the operation. The function _E_V_P___P_K_E_Y___v_e_r_i_f_y___r_e_c_o_v_e_r_(_) can be called more than once on the same context if several operations are performed using the same parameters. RREETTUURRNN VVAALLUUEESS _E_V_P___P_K_E_Y___v_e_r_i_f_y___r_e_c_o_v_e_r___i_n_i_t_(_) and _E_V_P___P_K_E_Y___v_e_r_i_f_y___r_e_c_o_v_e_r_(_) 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 pub- lic key algorithm. EEXXAAMMPPLLEE Recover digest originally signed using PKCS#1 and SHA256 digest: #include #include 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 */ SSEEEE AALLSSOO _E_V_P___P_K_E_Y___C_T_X___n_e_w(3), _E_V_P___P_K_E_Y___e_n_c_r_y_p_t(3), _E_V_P___P_K_E_Y___d_e_c_r_y_p_t(3), _E_V_P___P_K_E_Y___s_i_g_n(3), _E_V_P___P_K_E_Y___v_e_r_i_f_y(3), _E_V_P___P_K_E_Y___d_e_r_i_v_e(3) HHIISSTTOORRYY These functions were first added to OpenSSL 1.0.0. 1.0.2u 2019-12-20 EVP_PKEY_verify_recover(3)