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Document some SSL DH related functions/macros
Reviewed-by: Richard Levitte <levitte@openssl.org> (Merged from https://github.com/openssl/openssl/pull/13368)
This commit is contained in:
Matt Caswell
parent
1072041b17
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0437309fdf
@ -2,7 +2,8 @@
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=head1 NAME
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SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh,
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SSL_CTX_set_dh_auto, SSL_set_dh_auto, SSL_CTX_set0_tmp_dh_pkey,
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SSL_set0_tmp_dh_pkey, SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh,
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SSL_set_tmp_dh_callback, SSL_set_tmp_dh
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- handle DH keys for ephemeral key exchange
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@ -10,6 +11,15 @@ SSL_set_tmp_dh_callback, SSL_set_tmp_dh
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#include <openssl/ssl.h>
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long SSL_CTX_set_dh_auto(SSL *s, int onoff);
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long SSL_set_dh_auto(SSL *s, int onoff);
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int SSL_CTX_set0_tmp_dh_pkey(SSL_CTX *ctx, EVP_PKEY *dhpkey);
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int SSL_set0_tmp_dh_pkey(SSL *s, EVP_PKEY *dhpkey);
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Deprecated since OpenSSL 3.0, can be hidden entirely by defining
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B<OPENSSL_API_COMPAT> with a suitable version value, see
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L<openssl_user_macros(7)>:
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void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
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DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
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int keylength));
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@ -22,33 +32,19 @@ SSL_set_tmp_dh_callback, SSL_set_tmp_dh
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=head1 DESCRIPTION
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SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be
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used when a DH parameters are required to B<tmp_dh_callback>.
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The callback is inherited by all B<ssl> objects created from B<ctx>.
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The functions described on this page are relevant for servers only.
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SSL_CTX_set_tmp_dh() sets DH parameters to be used to be B<dh>.
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The key is inherited by all B<ssl> objects created from B<ctx>.
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Some ciphersuites may use ephemeral Diffie-Hellman (DH) key exchange. In these
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cases, the session data is negotiated using the ephemeral/temporary DH key and
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the key supplied and certified by the certificate chain is only used for
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signing. Anonymous ciphers (without a permanent server key) also use ephemeral
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DH keys.
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SSL_set_tmp_dh_callback() sets the callback only for B<ssl>.
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SSL_set_tmp_dh() sets the parameters only for B<ssl>.
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These functions apply to SSL/TLS servers only.
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=head1 NOTES
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When using a cipher with RSA authentication, an ephemeral DH key exchange
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can take place. Ciphers with DSA keys always use ephemeral DH keys as well.
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In these cases, the session data are negotiated using the
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ephemeral/temporary DH key and the key supplied and certified
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by the certificate chain is only used for signing.
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Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.
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Using ephemeral DH key exchange yields forward secrecy, as the connection
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can only be decrypted, when the DH key is known. By generating a temporary
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Using ephemeral DH key exchange yields forward secrecy as the connection
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can only be decrypted when the DH key is known. By generating a temporary
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DH key inside the server application that is lost when the application
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is left, it becomes impossible for an attacker to decrypt past sessions,
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even if he gets hold of the normal (certified) key, as this key was
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even if they get hold of the normal (certified) key, as this key was
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only used for signing.
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In order to perform a DH key exchange the server must use a DH group
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@ -56,59 +52,57 @@ In order to perform a DH key exchange the server must use a DH group
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a new DH key during the negotiation.
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As generating DH parameters is extremely time consuming, an application
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should not generate the parameters on the fly but supply the parameters.
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DH parameters can be reused, as the actual key is newly generated during
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the negotiation. The risk in reusing DH parameters is that an attacker
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may specialize on a very often used DH group. Applications should therefore
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generate their own DH parameters during the installation process using the
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openssl L<openssl-dhparam(1)> application. This application
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guarantees that "strong" primes are used.
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should not generate the parameters on the fly. DH parameters can be reused, as
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the actual key is newly generated during the negotiation.
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An application may either directly specify the DH parameters or
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can supply the DH parameters via a callback function.
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Typically applications should use well know DH parameters that have built-in
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support in OpenSSL. The macros SSL_CTX_set_dh_auto() and SSL_set_dh_auto()
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configure OpenSSL to use the default built-in DH parameters for the B<SSL_CTX>
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and B<SSL> objects respectively. Passing a value of 1 in the I<onoff> parameter
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switches the feature on, and passing a value of 0 switches it off. The default
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setting is off.
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Previous versions of the callback used B<is_export> and B<keylength>
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parameters to control parameter generation for export and non-export
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cipher suites. Modern servers that do not support export cipher suites
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are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
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the callback but ignore B<keylength> and B<is_export> and simply
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supply at least 2048-bit parameters in the callback.
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If "auto" DH parameters are switched on then the parameters will be selected to
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be consistent with the size of the key associated with the server's certificate.
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If there is no certificate (e.g. for PSK ciphersuites), then it it will be
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consistent with the size of the negotiated symmetric cipher key.
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Applications may supply their own DH parameters instead of using the built-in
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values. This approach is discouraged and applications should in preference use
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the built-in parameter support described above. Applications wishing to supply
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their own DH parameters should call SSL_CTX_set0_tmp_dh_pkey() or
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SSL_set0_tmp_dh_pkey() to supply the parameters for the B<SSL_CTX> or B<SSL>
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respectively. The parameters should be supplied in the I<dhpkey> argument as
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an B<EVP_PKEY> containg DH parameters. Ownership of the I<dhpkey> value is
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passed to the B<SSL_CTX> or B<SSL> object as a result of this call, and so the
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caller should not free it if the function call is succesful.
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The deprecated macros SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do the same
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thing as SSL_CTX_set0_tmp_dh_pkey() and SSL_set0_tmp_dh_pkey() except that the
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DH parameters are supplied in a B<DH> object instead in the I<dh> argument, and
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ownership of the B<DH> object is retained by the application. Applications
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should use "auto" parameters instead, or call SSL_CTX_set0_tmp_dh_pkey() or
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SSL_set0_tmp_dh_pkey() as appropriate.
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An application may instead specify the DH parameters via a callback function
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using the functions SSL_CTX_set_tmp_dh_callback() or SSL_set_tmp_dh_callback()
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to set the callback for the B<SSL_CTX> or B<SSL> object respectively. These
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functions are deprecated. Applications should instead use "auto" parameters, or
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specify the parameters via SSL_CTX_set0_tmp_dh_pkey() or SSL_set0_tmp_dh_pkey()
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as appropriate.
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The callback will be invoked during a connection when DH parameters are
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required. The B<SSL> object for the current connection is supplied as an
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argument. Previous versions of OpenSSL used the B<is_export> and B<keylength>
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arguments to control parameter generation for export and non-export
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cipher suites. Modern OpenSSL does not support export ciphersuites and so these
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arguments are unused and can be ignored by the callback. The callback should
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return the parameters to be used in a DH object. Ownership of the DH object is
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retained by the application and should later be freed.
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=head1 RETURN VALUES
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SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return
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diagnostic output.
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SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
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on failure. Check the error queue to find out the reason of failure.
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=head1 EXAMPLES
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Setup DH parameters with a key length of 2048 bits. (Error handling
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partly left out.)
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Command-line parameter generation:
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$ openssl dhparam -out dh_param_2048.pem 2048
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Code for setting up parameters during server initialization:
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SSL_CTX ctx = SSL_CTX_new();
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DH *dh_2048 = NULL;
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FILE *paramfile = fopen("dh_param_2048.pem", "r");
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if (paramfile) {
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dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
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fclose(paramfile);
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} else {
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/* Error. */
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}
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if (dh_2048 == NULL)
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/* Error. */
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if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1)
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/* Error. */
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...
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All of these functions/macros return 1 for success or 0 on error.
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=head1 SEE ALSO
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@ -143,8 +143,6 @@ DTLSv1_handle_timeout(3)
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SSL_num_renegotiations(3)
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SSL_clear_num_renegotiations(3)
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SSL_total_renegotiations(3)
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SSL_CTX_set_dh_auto(3)
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SSL_set_dh_auto(3)
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SSL_get0_certificate_types(3)
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SSL_CTX_set1_client_certificate_types(3)
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SSL_set1_client_certificate_types(3)
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@ -179,8 +179,6 @@ SSL_num_renegotiations(3)
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SSL_clear_num_renegotiations(3)
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SSL_total_renegotiations(3)
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SSL_CTX_set_tmp_ecdh(3)
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SSL_CTX_set_dh_auto(3)
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SSL_set_dh_auto(3)
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SSL_set_tmp_ecdh(3)
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SSL_CTX_get_extra_chain_certs(3)
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SSL_CTX_get_extra_chain_certs_only(3)
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@ -475,6 +475,7 @@ SSL_CTX_set1_sigalgs define
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SSL_CTX_set1_sigalgs_list define
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SSL_CTX_set1_verify_cert_store define
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SSL_CTX_set_current_cert define
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SSL_CTX_set_dh_auto define
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SSL_CTX_set_ecdh_auto define
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SSL_CTX_set_max_cert_list define
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SSL_CTX_set_max_pipelines define
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@ -548,6 +549,7 @@ SSL_set1_sigalgs define
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SSL_set1_sigalgs_list define
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SSL_set1_verify_cert_store define
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SSL_set_current_cert define
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SSL_set_dh_auto define
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SSL_set_ecdh_auto define
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SSL_set_max_cert_list define
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SSL_set_max_pipelines define
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