Initial Commit

openssl-1.0.2f/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod

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+=pod
+
+=head1 NAME
+
+SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh - handle DH keys for ephemeral key exchange
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ 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,
+            DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
+ long SSL_set_tmp_dh(SSL *ssl, DH *dh)
+
+=head1 DESCRIPTION
+
+SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be
+used when a DH parameters are required to B<tmp_dh_callback>.
+The callback is inherited by all B<ssl> objects created from B<ctx>.
+
+SSL_CTX_set_tmp_dh() sets DH parameters to be used to be B<dh>.
+The key is inherited by all B<ssl> objects created from B<ctx>.
+
+SSL_set_tmp_dh_callback() sets the callback only for B<ssl>.
+
+SSL_set_tmp_dh() sets the parameters only for B<ssl>.
+
+These functions apply to SSL/TLS servers only.
+
+=head1 NOTES
+
+When using a cipher with RSA authentication, an ephemeral DH key exchange
+can take place. Ciphers with DSA keys always use ephemeral DH keys as well.
+In these cases, the session data are negotiated using the
+ephemeral/temporary DH key and the key supplied and certified
+by the certificate chain is only used for signing.
+Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.
+
+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.
+
+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 L<dhparam(1)|dhparam(1)> application. This application
+guarantees that "strong" primes are used.
+
+Files dh2048.pem, and dh4096.pem in the 'apps' directory of the 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 B<-C> option of the
+L<dhparam(1)|dhparam(1)> application. Generation of custom DH
+parameters during installation should still be preferred to stop an
+attacker from specializing on a commonly used group. Files dh1024.pem
+and dh512.pem contain old parameters that must not be used by
+applications.
+
+An application may either directly specify the DH parameters or
+can supply the DH parameters via a callback function.
+
+Previous versions of the callback used B<is_export> and B<keylength>
+parameters to control parameter generation for export and non-export
+cipher suites. Modern servers that do not support export ciphersuites
+are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
+the callback but ignore B<keylength> and B<is_export> and simply
+supply at least 2048-bit parameters in the callback.
+
+=head1 EXAMPLES
+
+Setup DH parameters with a key length of 2048 bits. (Error handling
+partly left out.)
+
+ Command-line parameter generation:
+ $ openssl dhparam -out dh_param_2048.pem 2048
+
+ Code for setting up parameters during server initialization:
+
+ ...
+ SSL_CTX ctx = SSL_CTX_new();
+ ...
+
+ /* Set up ephemeral DH parameters. */
+ DH *dh_2048 = NULL;
+ FILE *paramfile;
+ paramfile = fopen("dh_param_2048.pem", "r");
+ if (paramfile) {
+   dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
+   fclose(paramfile);
+ } else {
+   /* Error. */
+ }
+ if (dh_2048 == NULL) {
+  /* Error. */
+ }
+ if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
+   /* Error. */
+ }
+ ...
+
+=head1 RETURN VALUES
+
+SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return
+diagnostic output.
+
+SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
+on failure. Check the error queue to find out the reason of failure.
+
+=head1 SEE ALSO
+
+L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
+L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
+L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>,
+L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)>
+
+=cut