SSL_CTX_set_tmp_dh_callback.pod 5.0 KB

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  1. =pod
  2. =head1 NAME
  3. 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
  4. =head1 SYNOPSIS
  5. #include <openssl/ssl.h>
  6. void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
  7. DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
  8. int keylength));
  9. long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
  10. void SSL_set_tmp_dh_callback(SSL *ctx,
  11. DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
  12. int keylength));
  13. long SSL_set_tmp_dh(SSL *ssl, DH *dh)
  14. =head1 DESCRIPTION
  15. SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be
  16. used when a DH parameters are required to B<tmp_dh_callback>.
  17. The callback is inherited by all B<ssl> objects created from B<ctx>.
  18. SSL_CTX_set_tmp_dh() sets DH parameters to be used to be B<dh>.
  19. The key is inherited by all B<ssl> objects created from B<ctx>.
  20. SSL_set_tmp_dh_callback() sets the callback only for B<ssl>.
  21. SSL_set_tmp_dh() sets the parameters only for B<ssl>.
  22. These functions apply to SSL/TLS servers only.
  23. =head1 NOTES
  24. When using a cipher with RSA authentication, an ephemeral DH key exchange
  25. can take place. Ciphers with DSA keys always use ephemeral DH keys as well.
  26. In these cases, the session data are negotiated using the
  27. ephemeral/temporary DH key and the key supplied and certified
  28. by the certificate chain is only used for signing.
  29. Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.
  30. Using ephemeral DH key exchange yields forward secrecy, as the connection
  31. can only be decrypted, when the DH key is known. By generating a temporary
  32. DH key inside the server application that is lost when the application
  33. is left, it becomes impossible for an attacker to decrypt past sessions,
  34. even if he gets hold of the normal (certified) key, as this key was
  35. only used for signing.
  36. In order to perform a DH key exchange the server must use a DH group
  37. (DH parameters) and generate a DH key. The server will always generate
  38. a new DH key during the negotiation.
  39. As generating DH parameters is extremely time consuming, an application
  40. should not generate the parameters on the fly but supply the parameters.
  41. DH parameters can be reused, as the actual key is newly generated during
  42. the negotiation. The risk in reusing DH parameters is that an attacker
  43. may specialize on a very often used DH group. Applications should therefore
  44. generate their own DH parameters during the installation process using the
  45. openssl L<dhparam(1)> application. This application
  46. guarantees that "strong" primes are used.
  47. Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current
  48. version of the OpenSSL distribution contain the 'SKIP' DH parameters,
  49. which use safe primes and were generated verifiably pseudo-randomly.
  50. These files can be converted into C code using the B<-C> option of the
  51. L<dhparam(1)> application. Generation of custom DH
  52. parameters during installation should still be preferred to stop an
  53. attacker from specializing on a commonly used group. File dh1024.pem
  54. contains old parameters that must not be used by applications.
  55. An application may either directly specify the DH parameters or
  56. can supply the DH parameters via a callback function.
  57. Previous versions of the callback used B<is_export> and B<keylength>
  58. parameters to control parameter generation for export and non-export
  59. cipher suites. Modern servers that do not support export cipher suites
  60. are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
  61. the callback but ignore B<keylength> and B<is_export> and simply
  62. supply at least 2048-bit parameters in the callback.
  63. =head1 EXAMPLES
  64. Setup DH parameters with a key length of 2048 bits. (Error handling
  65. partly left out.)
  66. Command-line parameter generation:
  67. $ openssl dhparam -out dh_param_2048.pem 2048
  68. Code for setting up parameters during server initialization:
  69. SSL_CTX ctx = SSL_CTX_new();
  70. DH *dh_2048 = NULL;
  71. FILE *paramfile = fopen("dh_param_2048.pem", "r");
  72. if (paramfile) {
  73. dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
  74. fclose(paramfile);
  75. } else {
  76. /* Error. */
  77. }
  78. if (dh_2048 == NULL)
  79. /* Error. */
  80. if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1)
  81. /* Error. */
  82. ...
  83. =head1 RETURN VALUES
  84. SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return
  85. diagnostic output.
  86. SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
  87. on failure. Check the error queue to find out the reason of failure.
  88. =head1 SEE ALSO
  89. L<ssl(7)>, L<SSL_CTX_set_cipher_list(3)>,
  90. L<SSL_CTX_set_options(3)>,
  91. L<ciphers(1)>, L<dhparam(1)>
  92. =head1 COPYRIGHT
  93. Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved.
  94. Licensed under the OpenSSL license (the "License"). You may not use
  95. this file except in compliance with the License. You can obtain a copy
  96. in the file LICENSE in the source distribution or at
  97. L<https://www.openssl.org/source/license.html>.
  98. =cut