curl/docs/libcurl/opts/CURLOPT_PINNEDPUBLICKEY.md

---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_PINNEDPUBLICKEY
Section: 3
Source: libcurl
See-also:
  - CURLOPT_CAINFO (3)
  - CURLOPT_CAPATH (3)
  - CURLOPT_SSL_VERIFYHOST (3)
  - CURLOPT_SSL_VERIFYPEER (3)
Protocol:
  - TLS
TLS-backend:
  - OpenSSL
  - GnuTLS
  - wolfSSL
  - mbedTLS
  - Secure Transport
  - Schannel
---

# NAME

CURLOPT_PINNEDPUBLICKEY - pinned public key

# SYNOPSIS

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

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PINNEDPUBLICKEY,
                          char *pinnedpubkey);
~~~

# DESCRIPTION

Pass a pointer to a null-terminated string as parameter. The string can be the
filename of your pinned public key. The file format expected is "PEM" or
"DER". The string can also be any number of base64 encoded sha256 hashes
preceded by "sha256//" and separated by ";"

When negotiating a TLS or SSL connection, the server sends a certificate
indicating its identity. A public key is extracted from this certificate and
if it does not exactly match the public key provided to this option, curl
aborts the connection before sending or receiving any data.

This option is independent of option CURLOPT_SSL_VERIFYPEER(3). If you turn
off that option then the peer is still verified by public key.

On mismatch, *CURLE_SSL_PINNEDPUBKEYNOTMATCH* is returned.

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

# DEFAULT

NULL

# EXAMPLE

~~~c
int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
    curl_easy_setopt(curl, CURLOPT_PINNEDPUBLICKEY, "/etc/publickey.der");
    /* OR
    curl_easy_setopt(curl, CURLOPT_PINNEDPUBLICKEY,
                     "sha256//YhKJKSzoTt2b5FP18fvpHo7fJYqQCjAa3HWY3"
                     "tvRMwE=;sha256//t62CeU2tQiqkexU74Gxa2eg7fRbEg"
                     "oChTociMee9wno=");
    */

    /* Perform the request */
    curl_easy_perform(curl);
  }
}
~~~

# PUBLIC KEY EXTRACTION

If you do not have the server's public key file you can extract it from the
server's certificate.
~~~
# retrieve the server's certificate if you do not already have it
#
# be sure to examine the certificate to see if it is what you expected
#
# Windows-specific:
# - Use NUL instead of /dev/null.
# - OpenSSL may wait for input instead of disconnecting. Hit enter.
# - If you do not have sed, then just copy the certificate into a file:
#   Lines from -----BEGIN CERTIFICATE----- to -----END CERTIFICATE-----.
#
openssl s_client -servername www.example.com -connect www.example.com:443 \
  < /dev/null | sed -n "/-----BEGIN/,/-----END/p" > www.example.com.pem

# extract public key in pem format from certificate
openssl x509 -in www.example.com.pem -pubkey -noout > www.example.com.pubkey.pem

# convert public key from pem to der
openssl asn1parse -noout -inform pem -in www.example.com.pubkey.pem \
  -out www.example.com.pubkey.der

# sha256 hash and base64 encode der to string for use
openssl dgst -sha256 -binary www.example.com.pubkey.der | openssl base64
~~~

The public key in PEM format contains a header, base64 data and a
footer:
~~~
-----BEGIN PUBLIC KEY-----
[BASE 64 DATA]
-----END PUBLIC KEY-----
~~~

# AVAILABILITY

## PEM/DER support

7.39.0: OpenSSL, GnuTLS

7.43.0: wolfSSL

7.47.0: mbedTLS

7.54.1: Secure Transport on macOS 10.7+/iOS 10+

7.58.1: Schannel

## sha256 support

7.44.0: OpenSSL, GnuTLS and wolfSSL

7.47.0: mbedTLS

7.54.1: Secure Transport on macOS 10.7+/iOS 10+

7.58.1: Schannel

Other SSL backends not supported.

# RETURN VALUE

Returns CURLE_OK if TLS enabled, CURLE_UNKNOWN_OPTION if not, or
CURLE_OUT_OF_MEMORY if there was insufficient heap space.