SSL_CTX_set_psk_client_callback.pod 7.2 KB

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  1. =pod
  2. =head1 NAME
  3. SSL_psk_client_cb_func,
  4. SSL_psk_use_session_cb_func,
  5. SSL_CTX_set_psk_client_callback,
  6. SSL_set_psk_client_callback,
  7. SSL_CTX_set_psk_use_session_callback,
  8. SSL_set_psk_use_session_callback
  9. - set PSK client callback
  10. =head1 SYNOPSIS
  11. #include <openssl/ssl.h>
  12. typedef int (*SSL_psk_use_session_cb_func)(SSL *ssl, const EVP_MD *md,
  13. const unsigned char **id,
  14. size_t *idlen,
  15. SSL_SESSION **sess);
  16. void SSL_CTX_set_psk_use_session_callback(SSL_CTX *ctx,
  17. SSL_psk_use_session_cb_func cb);
  18. void SSL_set_psk_use_session_callback(SSL *s, SSL_psk_use_session_cb_func cb);
  19. typedef unsigned int (*SSL_psk_client_cb_func)(SSL *ssl,
  20. const char *hint,
  21. char *identity,
  22. unsigned int max_identity_len,
  23. unsigned char *psk,
  24. unsigned int max_psk_len);
  25. void SSL_CTX_set_psk_client_callback(SSL_CTX *ctx, SSL_psk_client_cb_func cb);
  26. void SSL_set_psk_client_callback(SSL *ssl, SSL_psk_client_cb_func cb);
  27. =head1 DESCRIPTION
  28. A client application wishing to use TLSv1.3 PSKs should use either
  29. SSL_CTX_set_psk_use_session_callback() or SSL_set_psk_use_session_callback() as
  30. appropriate. These functions cannot be used for TLSv1.2 and below PSKs.
  31. The callback function is given a pointer to the SSL connection in B<ssl>.
  32. The first time the callback is called for a connection the B<md> parameter is
  33. NULL. In some circumstances the callback will be called a second time. In that
  34. case the server will have specified a ciphersuite to use already and the PSK
  35. must be compatible with the digest for that ciphersuite. The digest will be
  36. given in B<md>. The PSK returned by the callback is allowed to be different
  37. between the first and second time it is called.
  38. On successful completion the callback must store a pointer to an identifier for
  39. the PSK in B<*id>. The identifier length in bytes should be stored in B<*idlen>.
  40. The memory pointed to by B<*id> remains owned by the application and should
  41. be freed by it as required at any point after the handshake is complete.
  42. Additionally the callback should store a pointer to an SSL_SESSION object in
  43. B<*sess>. This is used as the basis for the PSK, and should, at a minimum, have
  44. the following fields set:
  45. =over 4
  46. =item The master key
  47. This can be set via a call to L<SSL_SESSION_set1_master_key(3)>.
  48. =item A ciphersuite
  49. Only the handshake digest associated with the ciphersuite is relevant for the
  50. PSK (the server may go on to negotiate any ciphersuite which is compatible with
  51. the digest). The application can use any TLSv1.3 ciphersuite. If B<md> is
  52. not NULL the handshake digest for the ciphersuite should be the same.
  53. The ciphersuite can be set via a call to <SSL_SESSION_set_cipher(3)>. The
  54. handshake digest of an SSL_CIPHER object can be checked using
  55. <SSL_CIPHER_get_handshake_digest(3)>.
  56. =item The protocol version
  57. This can be set via a call to L<SSL_SESSION_set_protocol_version(3)> and should
  58. be TLS1_3_VERSION.
  59. =back
  60. Additionally the maximum early data value should be set via a call to
  61. L<SSL_SESSION_set_max_early_data(3)> if the PSK will be used for sending early
  62. data.
  63. Alternatively an SSL_SESSION created from a previous non-PSK handshake may also
  64. be used as the basis for a PSK.
  65. Ownership of the SSL_SESSION object is passed to the OpenSSL library and so it
  66. should not be freed by the application.
  67. It is also possible for the callback to succeed but not supply a PSK. In this
  68. case no PSK will be sent to the server but the handshake will continue. To do
  69. this the callback should return successfully and ensure that B<*sess> is
  70. NULL. The contents of B<*id> and B<*idlen> will be ignored.
  71. A client application wishing to use PSK ciphersuites for TLSv1.2 and below must
  72. provide a different callback function. This function will be called when the
  73. client is sending the ClientKeyExchange message to the server.
  74. The purpose of the callback function is to select the PSK identity and
  75. the pre-shared key to use during the connection setup phase.
  76. The callback is set using functions SSL_CTX_set_psk_client_callback()
  77. or SSL_set_psk_client_callback(). The callback function is given the
  78. connection in parameter B<ssl>, a B<NUL>-terminated PSK identity hint
  79. sent by the server in parameter B<hint>, a buffer B<identity> of
  80. length B<max_identity_len> bytes (including the B<NUL>-terminator) where the
  81. resulting B<NUL>-terminated identity is to be stored, and a buffer B<psk>
  82. of length B<max_psk_len> bytes where the resulting pre-shared key is to
  83. be stored.
  84. The callback for use in TLSv1.2 will also work in TLSv1.3 although it is
  85. recommended to use SSL_CTX_set_psk_use_session_callback()
  86. or SSL_set_psk_use_session_callback() for this purpose instead. If TLSv1.3 has
  87. been negotiated then OpenSSL will first check to see if a callback has been set
  88. via SSL_CTX_set_psk_use_session_callback() or SSL_set_psk_use_session_callback()
  89. and it will use that in preference. If no such callback is present then it will
  90. check to see if a callback has been set via SSL_CTX_set_psk_client_callback() or
  91. SSL_set_psk_client_callback() and use that. In this case the B<hint> value will
  92. always be NULL and the handshake digest will default to SHA-256 for any returned
  93. PSK. TLSv1.3 early data exchanges are possible in PSK connections only with the
  94. B<SSL_psk_use_session_cb_func> callback, and are not possible with the
  95. B<SSL_psk_client_cb_func> callback.
  96. =head1 NOTES
  97. Note that parameter B<hint> given to the callback may be B<NULL>.
  98. A connection established via a TLSv1.3 PSK will appear as if session resumption
  99. has occurred so that L<SSL_session_reused(3)> will return true.
  100. There are no known security issues with sharing the same PSK between TLSv1.2 (or
  101. below) and TLSv1.3. However, the RFC has this note of caution:
  102. "While there is no known way in which the same PSK might produce related output
  103. in both versions, only limited analysis has been done. Implementations can
  104. ensure safety from cross-protocol related output by not reusing PSKs between
  105. TLS 1.3 and TLS 1.2."
  106. =head1 RETURN VALUES
  107. Return values from the B<SSL_psk_client_cb_func> callback are interpreted as
  108. follows:
  109. On success (callback found a PSK identity and a pre-shared key to use)
  110. the length (> 0) of B<psk> in bytes is returned.
  111. Otherwise or on errors the callback should return 0. In this case
  112. the connection setup fails.
  113. The SSL_psk_use_session_cb_func callback should return 1 on success or 0 on
  114. failure. In the event of failure the connection setup fails.
  115. =head1 SEE ALSO
  116. L<ssl(7)>,
  117. L<SSL_CTX_set_psk_find_session_callback(3)>,
  118. L<SSL_set_psk_find_session_callback(3)>
  119. =head1 HISTORY
  120. SSL_CTX_set_psk_use_session_callback() and SSL_set_psk_use_session_callback()
  121. were added in OpenSSL 1.1.1.
  122. =head1 COPYRIGHT
  123. Copyright 2006-2020 The OpenSSL Project Authors. All Rights Reserved.
  124. Licensed under the Apache License 2.0 (the "License"). You may not use
  125. this file except in compliance with the License. You can obtain a copy
  126. in the file LICENSE in the source distribution or at
  127. L<https://www.openssl.org/source/license.html>.
  128. =cut