diff options
Diffstat (limited to 'openssl-1.1.0h/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod')
| -rw-r--r-- | openssl-1.1.0h/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod | 137 |
1 files changed, 137 insertions, 0 deletions
diff --git a/openssl-1.1.0h/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod b/openssl-1.1.0h/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod new file mode 100644 index 0000000..fbfb8cb --- /dev/null +++ b/openssl-1.1.0h/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod @@ -0,0 +1,137 @@ +=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)> 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)> application. Generation of custom DH +parameters during installation should still be preferred to stop an +attacker from specializing on a commonly used group. File dh1024.pem +contains 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)>, L<SSL_CTX_set_cipher_list(3)>, +L<SSL_CTX_set_options(3)>, +L<ciphers(1)>, L<dhparam(1)> + +=head1 COPYRIGHT + +Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved. + +Licensed under the OpenSSL license (the "License"). You may not use +this file except in compliance with the License. You can obtain a copy +in the file LICENSE in the source distribution or at +L<https://www.openssl.org/source/license.html>. + +=cut |