292 lines
12 KiB
Plaintext
292 lines
12 KiB
Plaintext
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=pod
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=head1 NAME
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SSL_extension_supported,
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SSL_CTX_add_custom_ext,
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SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext,
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custom_ext_add_cb, custom_ext_free_cb, custom_ext_parse_cb
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- custom TLS extension handling
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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typedef int (*SSL_custom_ext_add_cb_ex) (SSL *s, unsigned int ext_type,
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unsigned int context,
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const unsigned char **out,
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size_t *outlen, X509 *x,
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size_t chainidx, int *al,
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void *add_arg);
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typedef void (*SSL_custom_ext_free_cb_ex) (SSL *s, unsigned int ext_type,
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unsigned int context,
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const unsigned char *out,
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void *add_arg);
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typedef int (*SSL_custom_ext_parse_cb_ex) (SSL *s, unsigned int ext_type,
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unsigned int context,
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const unsigned char *in,
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size_t inlen, X509 *x,
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size_t chainidx, int *al,
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void *parse_arg);
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int SSL_CTX_add_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
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unsigned int context,
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SSL_custom_ext_add_cb_ex add_cb,
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SSL_custom_ext_free_cb_ex free_cb,
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void *add_arg,
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SSL_custom_ext_parse_cb_ex parse_cb,
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void *parse_arg);
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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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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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=head1 DESCRIPTION
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SSL_CTX_add_custom_ext() adds a custom extension for a TLS/DTLS client or server
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for all supported protocol versions with extension type B<ext_type> and
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callbacks B<add_cb>, B<free_cb> and B<parse_cb> (see the
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L</EXTENSION CALLBACKS> section below). The B<context> value determines
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which messages and under what conditions the extension will be added/parsed (see
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the L</EXTENSION CONTEXTS> section below).
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SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS/DTLS 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>. This function is similar to SSL_CTX_add_custom_ext() except it only
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applies to clients, uses the older style of callbacks, and implicitly sets the
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B<context> value to:
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SSL_EXT_TLS1_2_AND_BELOW_ONLY | SSL_EXT_CLIENT_HELLO
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| SSL_EXT_TLS1_2_SERVER_HELLO | SSL_EXT_IGNORE_ON_RESUMPTION
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SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS/DTLS 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>. This function is similar to SSL_CTX_add_custom_ext() except it
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only applies to servers, uses the older style of callbacks, and implicitly sets
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the B<context> value to the same as for SSL_CTX_add_client_custom_ext() above.
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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. In all cases the extension type must not be
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handled by OpenSSL internally 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 various TLS messages. The B<ext_type> parameter is set to the
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extension type which will be added and B<add_arg> to the value set when the
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extension handler was added. When using the new style callbacks the B<context>
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parameter will indicate which message is currently being constructed e.g. for
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the ClientHello it will be set to B<SSL_EXT_CLIENT_HELLO>.
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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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When constructing the ClientHello, if B<add_cb> is set to NULL a zero length
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extension is added for B<ext_type>. For all other messages if B<add_cb> is set
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to NULL then no extension is added.
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When constructing a Certificate message the callback will be called for each
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certificate in the message. The B<x> parameter will indicate the
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current certificate and the B<chainidx> parameter will indicate the position
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of the certificate in the message. The first certificate is always the end
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entity certificate and has a B<chainidx> value of 0. The certificates are in the
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order that they were received in the Certificate message.
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For all messages except the ServerHello and EncryptedExtensions every
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registered B<add_cb> is always called to see if the application wishes to add an
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extension (as long as all requirements of the specified B<context> are met).
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For the ServerHello and EncryptedExtension messages every registered B<add_cb>
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is called once if and only if the requirements of the specified B<context> are
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met and the corresponding extension was received in the ClientHello. That is, if
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no corresponding extension was received in the ClientHello then B<add_cb> will
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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. The callback is only
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called if the extension is present and relevant for the context (see
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L</EXTENSION CONTEXTS> below).
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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<ext_type>.
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If the message being parsed is a TLSv1.3 compatible Certificate message then
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B<parse_cb> will be called for each certificate contained within the message.
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The B<x> parameter will indicate the current certificate and the B<chainidx>
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parameter will indicate the position of the certificate in the message. The
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first certificate is always the end entity certificate and has a B<chainidx>
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value of 0.
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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 EXTENSION CONTEXTS
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An extension context defines which messages and under which conditions an
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extension should be added or expected. The context is built up by performing
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a bitwise OR of multiple pre-defined values together. The valid context values
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are:
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=over 4
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=item SSL_EXT_TLS_ONLY
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The extension is only allowed in TLS
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=item SSL_EXT_DTLS_ONLY
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The extension is only allowed in DTLS
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=item SSL_EXT_TLS_IMPLEMENTATION_ONLY
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The extension is allowed in DTLS, but there is only a TLS implementation
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available (so it is ignored in DTLS).
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=item SSL_EXT_SSL3_ALLOWED
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Extensions are not typically defined for SSLv3. Setting this value will allow
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the extension in SSLv3. Applications will not typically need to use this.
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=item SSL_EXT_TLS1_2_AND_BELOW_ONLY
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The extension is only defined for TLSv1.2/DTLSv1.2 and below. Servers will
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ignore this extension if it is present in the ClientHello and TLSv1.3 is
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negotiated.
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=item SSL_EXT_TLS1_3_ONLY
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The extension is only defined for TLS1.3 and above. Servers will ignore this
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extension if it is present in the ClientHello and TLSv1.2 or below is
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negotiated.
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=item SSL_EXT_IGNORE_ON_RESUMPTION
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The extension will be ignored during parsing if a previous session is being
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successfully resumed.
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=item SSL_EXT_CLIENT_HELLO
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The extension may be present in the ClientHello message.
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=item SSL_EXT_TLS1_2_SERVER_HELLO
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The extension may be present in a TLSv1.2 or below compatible ServerHello
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message.
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=item SSL_EXT_TLS1_3_SERVER_HELLO
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The extension may be present in a TLSv1.3 compatible ServerHello message.
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=item SSL_EXT_TLS1_3_ENCRYPTED_EXTENSIONS
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The extension may be present in an EncryptedExtensions message.
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=item SSL_EXT_TLS1_3_HELLO_RETRY_REQUEST
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The extension may be present in a HelloRetryRequest message.
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=item SSL_EXT_TLS1_3_CERTIFICATE
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The extension may be present in a TLSv1.3 compatible Certificate message.
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=item SSL_EXT_TLS1_3_NEW_SESSION_TICKET
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The extension may be present in a TLSv1.3 compatible NewSessionTicket message.
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=item SSL_EXT_TLS1_3_CERTIFICATE_REQUEST
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The extension may be present in a TLSv1.3 compatible CertificateRequest message.
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=back
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The context must include at least one message value (otherwise the extension
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will never be used).
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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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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 a ServerHello/EncryptedExtensions message which was not sent in
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the ClientHello a fatal B<unsupported_extension> alert is sent and the
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handshake is aborted. The ServerHello/EncryptedExtensions B<add_cb> callback is
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only called if the corresponding extension was received in the ClientHello. This
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is compliant with the TLS specifications. This behaviour ensures that each
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callback is called at most once and that an application can never send
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unsolicited extensions.
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=head1 RETURN VALUES
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SSL_CTX_add_custom_ext(), SSL_CTX_add_client_custom_ext() and
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SSL_CTX_add_server_custom_ext() return 1 for success and 0 for failure. A
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failure can occur if an attempt is made to add the same B<ext_type> more than
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once, if an attempt is made to use an extension type handled internally by
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OpenSSL or if an internal error occurs (for example a memory allocation
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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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=head1 HISTORY
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The SSL_CTX_add_custom_ext() function was added in OpenSSL 1.1.1.
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=head1 COPYRIGHT
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Copyright 2014-2017 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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