#include
void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx, DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)); long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
void SSL_set_tmp_dh_callback(SSL_CTX *ctx, DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)); long SSL_set_tmp_dh(SSL *ssl, DH *dh)
DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
_S_S_L___C_T_X___s_e_t___t_m_p___d_h_(_) sets DH parameters to be used to be ddhh. The key is inherited by all ssssll objects created from ccttxx.
_S_S_L___s_e_t___t_m_p___d_h___c_a_l_l_b_a_c_k_(_) sets the callback only for ssssll.
_S_S_L___s_e_t___t_m_p___d_h_(_) sets the parameters only for ssssll.
These functions apply to SSL/TLS servers only.
Using ephemeral DH key exchange yields forward secrecy, as the connection can only be decrypted, when the DH key is known. By generating a temporary DH key inside the server application that is lost when the application is left, it becomes impossible for an attacker to decrypt past sessions, even if he gets hold of the normal (certified) key, as this key was only used for signing.
In order to perform a DH key exchange the server must use a DH group (DH parameters) and generate a DH key. The server will always generate a new DH key during the negotiation, when the DH parameters are supplied via callback and/or when the SSL_OP_SINGLE_DH_USE option of _S_S_L___C_T_X___s_e_t___o_p_t_i_o_n_s(3) is set. It will immediately create a DH key, when DH parameters are supplied via _S_S_L___C_T_X___s_e_t___t_m_p___d_h_(_) and SSL_OP_SINGLE_DH_USE is not set. In this case, it may happen that a key is generated on initialization without later being needed, while on the other hand the computer time during the negotiation is being saved.
If "strong" primes were used to generate the DH parameters, it is not strictly necessary to generate a new key for each handshake but it does improve forward secrecy. If it is not assured, that "strong" primes were used (see especially the section about DSA parameters below), SSL_OP_SINGLE_DH_USE must be used in order to prevent small subgroup attacks. Always using SSL_OP_SINGLE_DH_USE has an impact on the computer time needed during negotiation, but it is not very large, so application authors/users should consider to always enable this option.
As generating DH parameters is extremely time consuming, an application should not generate the parameters on the fly but supply the parameters. DH parameters can be reused, as the actual key is newly generated during the negotiation. The risk in reusing DH parameters is that an attacker may specialize on a very often used DH group. Applications should therefore generate their own DH parameters during the installation process using the openssl _o_p_e_n_s_s_l___d_h_p_a_r_a_m(1) application. In order to reduce the computer time needed for this generation, it is possible to use DSA parameters instead (see _o_p_e_n_s_s_l___d_h_p_a_r_a_m(1)), but in this case SSL_OP_SINGLE_DH_USE is mandatory.
Application authors may compile in DH parameters. Files dh512.pem, dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current version of the OpenSSL distribution contain the 'SKIP' DH parameters, which use safe primes and were generated verifiably pseudo-randomly. These files can be converted into C code using the --CC option of the _o_p_e_n_s_s_l___d_h_p_a_r_a_m(1) application. Authors may also generate their own set of parameters using _o_p_e_n_s_s_l___d_h_p_a_r_a_m(1), but a user may not be sure how the parameters were generated. The generation of DH parameters during installation is therefore recommended.
An application may either directly specify the DH parameters or can supply the DH parameters via a callback function. The callback approach has the advantage, that the callback may supply DH parameters for different key lengths.
The ttmmpp__ddhh__ccaallllbbaacckk is called with the kkeeyylleennggtthh needed and the iiss__eexxppoorrtt information. The iiss__eexxppoorrtt flag is set, when the ephemeral DH key exchange is performed with an export cipher.
... /* Set up ephemeral DH stuff */ DH *dh_512 = NULL; DH *dh_1024 = NULL; FILE *paramfile;
... /* "openssl dhparam -out dh_param_512.pem -2 512" */ paramfile = fopen("dh_param_512.pem", "r"); if (paramfile) { dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); fclose(paramfile); } /* "openssl dhparam -out dh_param_1024.pem -2 1024" */ paramfile = fopen("dh_param_1024.pem", "r"); if (paramfile) { dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); fclose(paramfile); } ...
/* "openssl dhparam -C -2 512" etc... */ DH *get_dh512() { ... } DH *get_dh1024() { ... }
DH *tmp_dh_callback(SSL *s, int is_export, int keylength) { DH *dh_tmp=NULL;
switch (keylength) { case 512: if (!dh_512) dh_512 = get_dh512(); dh_tmp = dh_512; break; case 1024: if (!dh_1024) dh_1024 = get_dh1024(); dh_tmp = dh_1024; break; default: /* Generating a key on the fly is very costly, so use what is there */ setup_dh_parameters_like_above(); } return(dh_tmp); }
_S_S_L___C_T_X___s_e_t___t_m_p___d_h_(_) and _S_S_L___s_e_t___t_m_p___d_h_(_) do return 1 on success and 0 on failure. Check the error queue to find out the reason of failure.