1 files changed, 170 insertions, 0 deletions
diff --git a/openssl/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod b/openssl/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod
new file mode 100644
index 000000000..29d1f8a6f
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+++ b/openssl/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod
@@ -0,0 +1,170 @@
+=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 *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));
+
+=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, when the DH parameters are supplied via
+callback and/or when the SSL_OP_SINGLE_DH_USE option of
+L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)> is set. It will
+immediately create a DH key, when DH parameters are supplied via
+SSL_CTX_set_tmp_dh() 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 L<dhparam(1)|dhparam(1)> application. In order to reduce the computer
+time needed for this generation, it is possible to use DSA parameters
+instead (see L<dhparam(1)|dhparam(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 B<-C> option of the
+L<dhparam(1)|dhparam(1)> application.
+Authors may also generate their own set of parameters using
+L<dhparam(1)|dhparam(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 B<tmp_dh_callback> is called with the B<keylength> needed and
+the B<is_export> information. The B<is_export> flag is set, when the
+ephemeral DH key exchange is performed with an export cipher.
+
+=head1 EXAMPLES
+
+Handle DH parameters for key lengths of 512 and 1024 bits. (Error handling
+partly left out.)
+
+ ...
+ /* 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);
+ }
+
+=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