xref: /aosp_15_r20/external/curl/docs/libcurl/opts/CURLOPT_ECH.md (revision 6236dae45794135f37c4eb022389c904c8b0090d)

---
c: Copyright (C) Daniel Stenberg, <[email protected]>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_ECH
Section: 3
Source: libcurl
See-also:
  - CURLOPT_DOH_URL (3)
Protocol:
  - TLS
TLS-backend:
  - OpenSSL
  - wolfSSL
Added-in: 8.8.0
---

# NAME

CURLOPT_ECH - configuration for Encrypted Client Hello

# SYNOPSIS

~~~c
#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ECH, char *config);
~~~

# DESCRIPTION

ECH is only compatible with TLSv1.3.

This experimental feature requires a special build of OpenSSL, as ECH is not
yet supported in OpenSSL releases. In contrast ECH is supported by the latest
BoringSSL and wolfSSL releases.

There is also a known issue with using wolfSSL which does not support ECH when
the HelloRetryRequest mechanism is used.

Pass a string that specifies configuration details for ECH. In all cases, if
ECH is attempted, it may fail for various reasons. The keywords supported are:

## false

Turns off ECH.

## grease

Instructs client to emit a GREASE ECH extension. (The connection fails if ECH
is attempted but fails.)

## true

Instructs client to attempt ECH, if possible, but to not fail if attempting
ECH is not possible.

## hard

Instructs client to attempt ECH and fail if attempting ECH is not possible.

## ecl:\<base64-value\>

If the string starts with `ecl:` then the remainder of the string should be a
base64-encoded ECHConfigList that is used for ECH rather than attempting to
download such a value from the DNS.

## pn:\<name\>

If the string starts with `pn:` then the remainder of the string should be a
DNS/hostname that is used to over-ride the public_name field of the
ECHConfigList that is used for ECH.

##

The application does not have to keep the string around after setting this
option.

Using this option multiple times makes the last set string override the
previous ones. Set it to NULL or "false" to disable its use again.

# DEFAULT

NULL, meaning ECH is disabled.

# %PROTOCOLS%

# EXAMPLE

~~~c
int main(void)
{
  CURL *curl = curl_easy_init();

  const char *config = \
    "ecl:AED+DQA87wAgACB/RuzUCsW3uBbSFI7mzD63TUXpI8sGDTnFTbFCDpa+" \
    "CAAEAAEAAQANY292ZXIuZGVmby5pZQAA";
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_ECH, config);
    curl_easy_perform(curl);
  }
}
~~~
# %AVAILABILITY%

# RETURN VALUE

Returns CURLE_OK on success or CURLE_OUT_OF_MEMORY if there was insufficient
heap space.