curl/docs/cmdline-opts/tls-earlydata.md

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c: Copyright (C) Daniel Stenberg, daniel@haxx.se, et al. SPDX-License-Identifier: curl Long: tls-earlydata Help: Allow use of TLSv1.3 early data (0RTT) Protocols: TLS Added: 8.11.0 Category: tls Multi: boolean See-also:

• tlsv1.3
• tls-max Example:
• --tls-earlydata $URL ---

--tls-earlydata

Enable the use of TLSv1.3 early data, also known as '0RTT' where possible. This has security implications for the requests sent that way.

This option is used when curl is built to use GnuTLS.

If a server supports this TLSv1.3 feature, and to what extent, is announced as part of the TLS "session" sent back to curl. Until curl has seen such a session in a previous request, early data cannot be used.

When a new connection is initiated with a known TLSv1.3 session, and that session announced early data support, the first request on this connection is sent before the TLS handshake is complete. While the early data is also encrypted, it is not protected against replays. An attacker can send your early data to the server again and the server would accept it.

If your request contacts a public server and only retrieves a file, there may be no harm in that. If the first request orders a refrigerator for you, it is probably not a good idea to use early data for it. curl cannot deduce what the security implications of your requests actually are and make this decision for you.

WARNING: this option has security implications. See above for more details.