Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.

SSL_read_early_data(3)              OpenSSL             SSL_read_early_data(3)

       SSL_set_max_early_data, SSL_CTX_set_max_early_data,
       SSL_get_max_early_data, SSL_CTX_get_max_early_data,
       SSL_set_recv_max_early_data, SSL_CTX_set_recv_max_early_data,
       SSL_get_recv_max_early_data, SSL_CTX_get_recv_max_early_data,
       SSL_SESSION_get_max_early_data, SSL_SESSION_set_max_early_data,
       SSL_write_early_data, SSL_read_early_data, SSL_get_early_data_status,
       SSL_allow_early_data_cb_fn, SSL_CTX_set_allow_early_data_cb,
       SSL_set_allow_early_data_cb - functions for sending and receiving early

       libcrypto, -lcrypto

        #include <openssl/ssl.h>

        int SSL_CTX_set_max_early_data(SSL_CTX *ctx, uint32_t max_early_data);
        uint32_t SSL_CTX_get_max_early_data(const SSL_CTX *ctx);
        int SSL_set_max_early_data(SSL *s, uint32_t max_early_data);
        uint32_t SSL_get_max_early_data(const SSL *s);

        int SSL_CTX_set_recv_max_early_data(SSL_CTX *ctx, uint32_t recv_max_early_data);
        uint32_t SSL_CTX_get_recv_max_early_data(const SSL_CTX *ctx);
        int SSL_set_recv_max_early_data(SSL *s, uint32_t recv_max_early_data);
        uint32_t SSL_get_recv_max_early_data(const SSL *s);

        uint32_t SSL_SESSION_get_max_early_data(const SSL_SESSION *s);
        int SSL_SESSION_set_max_early_data(SSL_SESSION *s, uint32_t max_early_data);

        int SSL_write_early_data(SSL *s, const void *buf, size_t num, size_t *written);

        int SSL_read_early_data(SSL *s, void *buf, size_t num, size_t *readbytes);

        int SSL_get_early_data_status(const SSL *s);

        typedef int (*SSL_allow_early_data_cb_fn)(SSL *s, void *arg);

        void SSL_CTX_set_allow_early_data_cb(SSL_CTX *ctx,
                                             SSL_allow_early_data_cb_fn cb,
                                             void *arg);
        void SSL_set_allow_early_data_cb(SSL *s,
                                         SSL_allow_early_data_cb_fn cb,
                                         void *arg);

       These functions are used to send and receive early data where TLSv1.3
       has been negotiated. Early data can be sent by the client immediately
       after its initial ClientHello without having to wait for the server to
       complete the handshake.  Early data can be sent if a session has
       previously been established with the server or when establishing a new
       session using an out-of-band PSK, and only when the server is known to
       support it. Additionally these functions can be used to send data from
       the server to the client when the client has not yet completed the
       authentication stage of the handshake.

       Early data has weaker security properties than other data sent over an
       SSL/TLS connection. In particular the data does not have forward
       secrecy. There are also additional considerations around replay attacks
       (see "REPLAY PROTECTION" below). For these reasons extreme care should
       be exercised when using early data. For specific details, consult the
       TLS 1.3 specification.

       When a server receives early data it may opt to immediately respond by
       sending application data back to the client. Data sent by the server at
       this stage is done before the full handshake has been completed.
       Specifically the client's authentication messages have not yet been
       received, i.e. the client is unauthenticated at this point and care
       should be taken when using this capability.

       A server or client can determine whether the full handshake has been
       completed or not by calling SSL_is_init_finished(3).

       On the client side, the function SSL_SESSION_get_max_early_data() can
       be used to determine if a session established with a server can be used
       to send early data.  If the session cannot be used then this function
       will return 0. Otherwise it will return the maximum number of early
       data bytes that can be sent.

       The function SSL_SESSION_set_max_early_data() sets the maximum number
       of early data bytes that can be sent for a session. This would
       typically be used when creating a PSK session file (see
       SSL_CTX_set_psk_use_session_callback(3)). If using a ticket based PSK
       then this is set automatically to the value provided by the server.

       A client uses the function SSL_write_early_data() to send early data.
       This function is similar to the SSL_write_ex(3) function, but with the
       following differences. See SSL_write_ex(3) for information on how to
       write bytes to the underlying connection, and how to handle any errors
       that may arise. This page describes the differences between
       SSL_write_early_data() and SSL_write_ex(3).

       When called by a client, SSL_write_early_data() must be the first IO
       function called on a new connection, i.e. it must occur before any
       calls to SSL_write_ex(3), SSL_read_ex(3), SSL_connect(3),
       SSL_do_handshake(3) or other similar functions. It may be called
       multiple times to stream data to the server, but the total number of
       bytes written must not exceed the value returned from
       SSL_SESSION_get_max_early_data(). Once the initial
       SSL_write_early_data() call has completed successfully the client may
       interleave calls to SSL_read_ex(3) and SSL_read(3) with calls to
       SSL_write_early_data() as required.

       If SSL_write_early_data() fails you should call SSL_get_error(3) to
       determine the correct course of action, as for SSL_write_ex(3).

       When the client no longer wishes to send any more early data then it
       should complete the handshake by calling a function such as
       SSL_connect(3) or SSL_do_handshake(3). Alternatively you can call a
       standard write function such as SSL_write_ex(3), which will
       transparently complete the connection and write the requested data.

       A server may choose to ignore early data that has been sent to it. Once
       the connection has been completed you can determine whether the server
       accepted or rejected the early data by calling
       SSL_get_early_data_status(). This will return SSL_EARLY_DATA_ACCEPTED
       if the data was accepted, SSL_EARLY_DATA_REJECTED if it was rejected or
       SSL_EARLY_DATA_NOT_SENT if no early data was sent. This function may be
       called by either the client or the server.

       A server uses the SSL_read_early_data() function to receive early data
       on a connection for which early data has been enabled using
       SSL_CTX_set_max_early_data() or SSL_set_max_early_data(). As for
       SSL_write_early_data(), this must be the first IO function called on a
       connection, i.e. it must occur before any calls to SSL_write_ex(3),
       SSL_read_ex(3), SSL_accept(3), SSL_do_handshake(3), or other similar

       SSL_read_early_data() is similar to SSL_read_ex(3) with the following
       differences. Refer to SSL_read_ex(3) for full details.

       SSL_read_early_data() may return 3 possible values:

           This indicates an IO or some other error occurred. This should be
           treated in the same way as a 0 return value from SSL_read_ex(3).

           This indicates that early data was successfully read. This should
           be treated in the same way as a 1 return value from SSL_read_ex(3).
           You should continue to call SSL_read_early_data() to read more

           This indicates that no more early data can be read. It may be
           returned on the first call to SSL_read_early_data() if the client
           has not sent any early data, or if the early data was rejected.

       Once the initial SSL_read_early_data() call has completed successfully
       (i.e. it has returned SSL_READ_EARLY_DATA_SUCCESS or
       SSL_READ_EARLY_DATA_FINISH) then the server may choose to write data
       immediately to the unauthenticated client using SSL_write_early_data().
       If SSL_read_early_data() returned SSL_READ_EARLY_DATA_FINISH then in
       some situations (e.g. if the client only supports TLSv1.2) the
       handshake may have already been completed and calls to
       SSL_write_early_data() are not allowed. Call SSL_is_init_finished(3) to
       determine whether the handshake has completed or not. If the handshake
       is still in progress then the server may interleave calls to
       SSL_write_early_data() with calls to SSL_read_early_data() as required.

       Servers must not call SSL_read_ex(3), SSL_read(3), SSL_write_ex(3) or
       SSL_write(3)  until SSL_read_early_data() has returned with
       SSL_READ_EARLY_DATA_FINISH. Once it has done so the connection to the
       client still needs to be completed. Complete the connection by calling
       a function such as SSL_accept(3) or SSL_do_handshake(3). Alternatively
       you can call a standard read function such as SSL_read_ex(3), which
       will transparently complete the connection and read the requested data.
       Note that it is an error to attempt to complete the connection before
       SSL_read_early_data() has returned SSL_READ_EARLY_DATA_FINISH.

       Only servers may call SSL_read_early_data().

       Calls to SSL_read_early_data() may, in certain circumstances, complete
       the connection immediately without further need to call a function such
       as SSL_accept(3). This can happen if the client is using a protocol
       version less than TLSv1.3. Applications can test for this by calling
       SSL_is_init_finished(3). Alternatively, applications may choose to call
       SSL_accept(3) anyway. Such a call will successfully return immediately
       with no further action taken.

       When a session is created between a server and a client the server will
       specify the maximum amount of any early data that it will accept on any
       future connection attempt. By default the server does not accept early
       data; a server may indicate support for early data by calling
       SSL_CTX_set_max_early_data() or SSL_set_max_early_data() to set it for
       the whole SSL_CTX or an individual SSL object respectively. The
       max_early_data parameter specifies the maximum amount of early data in
       bytes that is permitted to be sent on a single connection. Similarly
       the SSL_CTX_get_max_early_data() and SSL_get_max_early_data() functions
       can be used to obtain the current maximum early data settings for the
       SSL_CTX and SSL objects respectively. Generally a server application
       will either use both of SSL_read_early_data() and
       SSL_CTX_set_max_early_data() (or SSL_set_max_early_data()), or neither
       of them, since there is no practical benefit from using only one of
       them. If the maximum early data setting for a server is nonzero then
       replay protection is automatically enabled (see "REPLAY PROTECTION"

       If the server rejects the early data sent by a client then it will skip
       over the data that is sent. The maximum amount of received early data
       that is skipped is controlled by the recv_max_early_data setting. If a
       client sends more than this then the connection will abort. This value
       can be set by calling SSL_CTX_set_recv_max_early_data() or
       SSL_set_recv_max_early_data(). The current value for this setting can
       be obtained by calling SSL_CTX_get_recv_max_early_data() or
       SSL_get_recv_max_early_data(). The default value for this setting is
       16,384 bytes.

       The recv_max_early_data value also has an impact on early data that is
       accepted.  The amount of data that is accepted will always be the lower
       of the max_early_data for the session and the recv_max_early_data
       setting for the server. If a client sends more data than this then the
       connection will abort.

       The configured value for max_early_data on a server may change over
       time as required. However, clients may have tickets containing the
       previously configured max_early_data value. The recv_max_early_data
       should always be equal to or higher than any recently configured
       max_early_data value in order to avoid aborted connections. The
       recv_max_early_data should never be set to less than the current
       configured max_early_data value.

       Some server applications may wish to have more control over whether
       early data is accepted or not, for example to mitigate replay risks
       (see "REPLAY PROTECTION" below) or to decline early_data when the
       server is heavily loaded. The functions
       SSL_CTX_set_allow_early_data_cb() and SSL_set_allow_early_data_cb() set
       a callback which is called at a point in the handshake immediately
       before a decision is made to accept or reject early data. The callback
       is provided with a pointer to the user data argument that was provided
       when the callback was first set. Returning 1 from the callback will
       allow early data and returning 0 will reject it. Note that the OpenSSL
       library may reject early data for other reasons in which case this
       callback will not get called. Notably, the built-in replay protection
       feature will still be used even if a callback is present unless it has
       been explicitly disabled using the SSL_OP_NO_ANTI_REPLAY option. See
       "REPLAY PROTECTION" below.

       The whole purpose of early data is to enable a client to start sending
       data to the server before a full round trip of network traffic has
       occurred. Application developers should ensure they consider
       optimisation of the underlying TCP socket to obtain a performant
       solution. For example Nagle's algorithm is commonly used by operating
       systems in an attempt to avoid lots of small TCP packets. In many
       scenarios this is beneficial for performance, but it does not work well
       with the early data solution as implemented in OpenSSL. In Nagle's
       algorithm the OS will buffer outgoing TCP data if a TCP packet has
       already been sent which we have not yet received an ACK for from the
       peer. The buffered data will only be transmitted if enough data to fill
       an entire TCP packet is accumulated, or if the ACK is received from the
       peer. The initial ClientHello will be sent in the first TCP packet
       along with any data from the first call to SSL_write_early_data(). If
       the amount of data written will exceed the size of a single TCP packet,
       or if there are more calls to SSL_write_early_data() then that
       additional data will be sent in subsequent TCP packets which will be
       buffered by the OS and not sent until an ACK is received for the first
       packet containing the ClientHello. This means the early data is not
       actually sent until a complete round trip with the server has occurred
       which defeats the objective of early data.

       In many operating systems the TCP_NODELAY socket option is available to
       disable Nagle's algorithm. If an application opts to disable Nagle's
       algorithm consideration should be given to turning it back on again
       after the handshake is complete if appropriate.

       In rare circumstances, it may be possible for a client to have a
       session that reports a max early data value greater than 0, but where
       the server does not support this. For example, this can occur if a
       server has had its configuration changed to accept a lower max early
       data value such as by calling SSL_CTX_set_recv_max_early_data().
       Another example is if a server used to support TLSv1.3 but was later
       downgraded to TLSv1.2. Sending early data to such a server will cause
       the connection to abort. Clients that encounter an aborted connection
       while sending early data may want to retry the connection without
       sending early data as this does not happen automatically. A client will
       have to establish a new transport layer connection to the server and
       attempt the SSL/TLS connection again but without sending early data.
       Note that it is inadvisable to retry with a lower maximum protocol

       When early data is in use the TLS protocol provides no security
       guarantees that the same early data was not replayed across multiple
       connections. As a mitigation for this issue OpenSSL automatically
       enables replay protection if the server is configured with a nonzero
       max early data value. With replay protection enabled sessions are
       forced to be single use only. If a client attempts to reuse a session
       ticket more than once, then the second and subsequent attempts will
       fall back to a full handshake (and any early data that was submitted
       will be ignored). Note that single use tickets are enforced even if a
       client does not send any early data.

       The replay protection mechanism relies on the internal OpenSSL server
       session cache (see SSL_CTX_set_session_cache_mode(3)). When replay
       protection is being used the server will operate as if the
       SSL_OP_NO_TICKET option had been selected (see SSL_CTX_set_options(3)).
       Sessions will be added to the cache whenever a session ticket is
       issued. When a client attempts to resume the session, OpenSSL will
       check for its presence in the internal cache. If it exists then the
       resumption is allowed and the session is removed from the cache. If it
       does not exist then the resumption is not allowed and a full handshake
       will occur.

       Note that some applications may maintain an external cache of sessions
       (see SSL_CTX_sess_set_new_cb(3) and similar functions). It is the
       application's responsibility to ensure that any sessions in the
       external cache are also populated in the internal cache and that once
       removed from the internal cache they are similarly removed from the
       external cache. Failing to do this could result in an application
       becoming vulnerable to replay attacks. Note that OpenSSL will lock the
       internal cache while a session is removed but that lock is not held
       when the remove session callback (see SSL_CTX_sess_set_remove_cb(3)) is
       called. This could result in a small amount of time where the session
       has been removed from the internal cache but is still available in the
       external cache. Applications should be designed with this in mind in
       order to minimise the possibility of replay attacks.

       The OpenSSL replay protection does not apply to external Pre Shared
       Keys (PSKs) (e.g. see SSL_CTX_set_psk_find_session_callback(3)).
       Therefore, extreme caution should be applied when combining external
       PSKs with early data.

       Some applications may mitigate the replay risks in other ways. For
       those applications it is possible to turn off the built-in replay
       protection feature using the SSL_OP_NO_ANTI_REPLAY option. See
       SSL_CTX_set_options(3) for details. Applications can also set a
       callback to make decisions about accepting early data or not. See
       SSL_CTX_set_allow_early_data_cb() above for details.

       SSL_write_early_data() returns 1 for success or 0 for failure. In the
       event of a failure call SSL_get_error(3) to determine the correct
       course of action.

       SSL_read_early_data() returns SSL_READ_EARLY_DATA_ERROR for failure,
       SSL_READ_EARLY_DATA_SUCCESS for success with more data to read and
       SSL_READ_EARLY_DATA_FINISH for success with no more to data be read. In
       the event of a failure call SSL_get_error(3) to determine the correct
       course of action.

       SSL_get_max_early_data(), SSL_CTX_get_max_early_data() and
       SSL_SESSION_get_max_early_data() return the maximum number of early
       data bytes that may be sent.

       SSL_set_max_early_data(), SSL_CTX_set_max_early_data() and
       SSL_SESSION_set_max_early_data() return 1 for success or 0 for failure.

       SSL_get_early_data_status() returns SSL_EARLY_DATA_ACCEPTED if early
       data was accepted by the server, SSL_EARLY_DATA_REJECTED if early data
       was rejected by the server, or SSL_EARLY_DATA_NOT_SENT if no early data
       was sent.

       SSL_get_error(3), SSL_write_ex(3), SSL_read_ex(3), SSL_connect(3),
       SSL_accept(3), SSL_do_handshake(3),
       SSL_CTX_set_psk_use_session_callback(3), ssl(7)

       All of the functions described above were added in OpenSSL 1.1.1.

       Copyright 2017-2020 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

1.1.1i                            2020-12-10            SSL_read_early_data(3)