mirror of
https://github.com/openssl/openssl.git
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134 lines
5.1 KiB
Plaintext
134 lines
5.1 KiB
Plaintext
=pod
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=head1 NAME
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SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext - custom TLS extension handling
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
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custom_ext_add_cb add_cb,
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custom_ext_free_cb free_cb, void *add_arg,
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custom_ext_parse_cb parse_cb,
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void *parse_arg);
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int SSL_CTX_add_server_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
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custom_ext_add_cb add_cb,
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custom_ext_free_cb free_cb, void *add_arg,
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custom_ext_parse_cb parse_cb,
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void *parse_arg);
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int SSL_extension_supported(unsigned int ext_type);
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typedef int (*custom_ext_add_cb)(SSL *s, unsigned int ext_type,
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const unsigned char **out,
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size_t *outlen, int *al,
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void *add_arg);
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typedef void (*custom_ext_free_cb)(SSL *s, unsigned int ext_type,
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const unsigned char *out,
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void *add_arg);
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typedef int (*custom_ext_parse_cb)(SSL *s, unsigned int ext_type,
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const unsigned char *in,
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size_t inlen, int *al,
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void *parse_arg);
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=head1 DESCRIPTION
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SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS client
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with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and
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B<parse_cb>.
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SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS server
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with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and
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B<parse_cb>.
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In both cases the extension type must not be handled by OpenSSL internally
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or an error occurs.
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SSL_extension_supported() returns 1 if the extension B<ext_type> is handled
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internally by OpenSSL and 0 otherwise.
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=head1 EXTENSION CALLBACKS
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The callback B<add_cb> is called to send custom extension data to be
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included in ClientHello for TLS clients or ServerHello for servers. The
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B<ext_type> parameter is set to the extension type which will be added and
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B<add_arg> to the value set when the extension handler was added.
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If the application wishes to include the extension B<ext_type> it should
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set B<*out> to the extension data, set B<*outlen> to the length of the
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extension data and return 1.
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If the B<add_cb> does not wish to include the extension it must return 0.
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If B<add_cb> returns -1 a fatal handshake error occurs using the TLS
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alert value specified in B<*al>.
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For clients (but not servers) if B<add_cb> is set to NULL a zero length
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extension is added for B<ext_type>.
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For clients every registered B<add_cb> is always called to see if the
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application wishes to add an extension to ClientHello.
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For servers every registered B<add_cb> is called once if and only if the
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corresponding extension was received in ClientHello to see if the application
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wishes to add the extension to ServerHello. That is, if no corresponding extension
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was received in ClientHello then B<add_cb> will not be called.
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If an extension is added (that is B<add_cb> returns 1) B<free_cb> is called
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(if it is set) with the value of B<out> set by the add callback. It can be
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used to free up any dynamic extension data set by B<add_cb>. Since B<out> is
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constant (to permit use of constant data in B<add_cb>) applications may need to
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cast away const to free the data.
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The callback B<parse_cb> receives data for TLS extensions. For TLS clients
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the extension data will come from ServerHello and for TLS servers it will
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come from ClientHello.
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The extension data consists of B<inlen> bytes in the buffer B<in> for the
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extension B<extension_type>.
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If the B<parse_cb> considers the extension data acceptable it must return
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1. If it returns 0 or a negative value a fatal handshake error occurs
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using the TLS alert value specified in B<*al>.
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The buffer B<in> is a temporary internal buffer which will not be valid after
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the callback returns.
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=head1 NOTES
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The B<add_arg> and B<parse_arg> parameters can be set to arbitrary values
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which will be passed to the corresponding callbacks. They can, for example,
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be used to store the extension data received in a convenient structure or
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pass the extension data to be added or freed when adding extensions.
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The B<ext_type> parameter corresponds to the B<extension_type> field of
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RFC5246 et al. It is B<not> a NID.
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If the same custom extension type is received multiple times a fatal
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B<decode_error> alert is sent and the handshake aborts. If a custom extension
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is received in ServerHello which was not sent in ClientHello a fatal
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B<unsupported_extension> alert is sent and the handshake is aborted. The
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ServerHello B<add_cb> callback is only called if the corresponding extension
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was received in ClientHello. This is compliant with the TLS specifications.
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This behaviour ensures that each callback is called at most once and that
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an application can never send unsolicited extensions.
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=head1 RETURN VALUES
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SSL_CTX_add_client_custom_ext() and SSL_CTX_add_server_custom_ext() return 1 for
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success and 0 for failure. A failure can occur if an attempt is made to
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add the same B<ext_type> more than once, if an attempt is made to use an
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extension type handled internally by OpenSSL or if an internal error occurs
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(for example a memory allocation failure).
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SSL_extension_supported() returns 1 if the extension B<ext_type> is handled
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internally by OpenSSL and 0 otherwise.
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=cut
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