GNURLOPT_PINNEDPUBLICKEY(3)

okay bestie, welcome to GNURLOPT_PINNEDPUBLICKEY(3) — the manual page for CURLOPT_PINNEDPUBLICKEY. we are pinning public keys today and it is giving security main character energy. let's go 💅

✨ NAME ✨

• CURLOPT_PINNEDPUBLICKEY — set pinned public key

🎀 SYNOPSIS 🎀

       #include <gnurl/curl.h>

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

🌸 DESCRIPTION 🌸

okay so picture this: you pass a pointer to a null-terminated string as the parameter. that string can be the file name of your pinned public key, and the file format expected is "PEM" or "DER". it can also be any number of base64 encoded sha256 hashes preceded by "sha256//" and separated by ";" — yes bestie, you can stack them like little security charms.

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 will abort the connection before sending or receiving any data. like, full stop, no data goes anywhere.

on mismatch, CURLE_SSL_PINNEDPUBKEYNOTMATCH is returned.

the application does not have to keep the string around after setting this option — set it and live your life, bestie 🫶

💖 DEFAULT 💖

NULL

🌈 PROTOCOLS 🌈

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

🔥 EXAMPLE 🔥

       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//YhKJKSzoTt2b5FP18fvpHo7fJYqQCjAa3HWY3tvRMwE=;sha256//t62CeU2tQiqkexU74Gxa2eg7fRbEgoChTociMee9wno=");
         */

         /* 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. no cap, here's the whole little ritual:

       # retrieve the server's certificate if you don't 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 don't 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
• 7.39.0-7.48.0,7.58.1+: GSKit
• 7.43.0: NSS and wolfSSL
• 7.47.0: mbedtls
• 7.54.1: SecureTransport on macOS 10.7+/iOS 10+
• 7.58.1: SChannel

sha256 support:

• 7.44.0: OpenSSL, GnuTLS, NSS and wolfSSL
• 7.47.0: mbedtls
• 7.54.1: SecureTransport on macOS 10.7+/iOS 10+
• 7.58.1: SChannel Windows XP SP3+

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.

🫶 SEE ALSO 🫶

CURLOPT_SSL_VERIFYPEER(3), CURLOPT_SSL_VERIFYHOST(3), CURLOPT_CAINFO(3), CURLOPT_CAPATH(3),

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mwah 💋 — this cutie page is from libcurl 7.38.0, dated 27 Aug 2014, and yes bestie it's GNURLOPT_PINNEDPUBLICKEY(3). bye, go pin those keys 🫶