CURLOPT_PROXY_PINNEDPUBLICKEY(3)

💅 CURLOPT_PROXY_PINNEDPUBLICKEY(3)

okay bestie, so this little gem lives in the Library Functions Manual, and it's literally the vibe check for your HTTPS proxy's identity. Buckle up 🎀

---

✨ NAME ✨

CURLOPT_PROXY_PINNEDPUBLICKEY — the pinned public key for your https proxy 🔑

✨ SYNOPSIS ✨

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXY_PINNEDPUBLICKEY,
                          char *pinnedpubkey);

✨ DESCRIPTION ✨

so you pass a pointer to a null-terminated string as the parameter, okay? 📌 That string can be the filename of your pinned public key — and the file format it expects is "PEM" or "DER". OR, plot twist, the string can also be any number of base64 encoded sha256 hashes, each one preceded by "sha256//" and separated by ";". iconic, truly.

Here's the tea ☕: when negotiating a TLS or SSL connection, the https proxy sends a certificate indicating its identity. A public key gets extracted from that certificate, and if it does not exactly match the public key you provided to this option, libcurl aborts the connection before sending or receiving any data. no cap, it just stops. 💀

On mismatch, CURLE_SSL_PINNEDPUBKEYNOTMATCH is returned.

Also super chill of it: 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 to disable its use again.

✨ DEFAULT ✨

NULL

🌈 PROTOCOLS 🌈

This functionality affects all TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.

This option works only with the following TLS backends: GnuTLS, OpenSSL, mbedTLS and wolfSSL

🦋 EXAMPLE 🦋

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
    curl_easy_setopt(curl, CURLOPT_PROXY, "https://proxy.example:443");
    curl_easy_setopt(curl, CURLOPT_PROXY_PINNEDPUBLICKEY,
                     "sha256//YhKJKSzoTt2b5FP18fvpHo7fJYqQCjA"
                     "a3HWY3tvRMwE=;sha256//t62CeU2tQiqkexU74"
                     "Gxa2eg7fRbEgoChTociMee9wno=");

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

🔍 PUBLIC KEY EXTRACTION 🔍

okay so if you do not have the https proxy server's public key file, don't panic — you can extract it from the https proxy server's certificate. slay 💅

# 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

ngl, the public key in PEM format contains a header, base64 data and a footer, like:

-----BEGIN PUBLIC KEY-----
[BASE 64 DATA]
-----END PUBLIC KEY-----

🕰️ HISTORY 🕰️

PEM/DER support:

• 7.52.0: GnuTLS, OpenSSL, mbedTLS, wolfSSL

sha256 support:

• 7.52.0: GnuTLS, OpenSSL, mbedTLS, wolfSSL

Other SSL backends not supported. 🙈

⭐ AVAILABILITY ⭐

Added in curl 7.52.0

💜 RETURN VALUE 💜

curl_easy_setopt(3) returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK — we love that for us 🥳 — and non-zero means an error occurred, see libcurl-errors(3).

👀 SEE ALSO 👀

• CURLOPT_PINNEDPUBLICKEY(3)
• CURLOPT_PROXY_CAINFO(3)
• CURLOPT_PROXY_CAPATH(3)
• CURLOPT_PROXY_SSL_VERIFYHOST(3)
• CURLOPT_PROXY_SSL_VERIFYPEER(3)

---

and that's the whole page, bestie — brought to you by libcurl, dated 2026-10-12, from the Library Functions Manual. go pin those keys 💖✨🎀🦋