CURLOPT_SSLKEYTYPE(3)

💅 CURLOPT_SSLKEYTYPE(3) 💅

okay so this is from the Library Functions Manual bestie, and it's literally all about telling curl what format your private key file is in 🔑

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✨ NAME ✨

CURLOPT_SSLKEYTYPE — the type of the private key file, like, what flavor your key is 🍓

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🎀 SYNOPSIS 🎀

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSLKEYTYPE, char *type);

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🦋 DESCRIPTION 🦋

You pass in a pointer to a null-terminated string as the parameter, and that string should be the format of your private key, no cap. The supported formats are "PEM", "DER", "ENG" and "PROV" ✨

The "ENG" format is what lets you load the private key from a crypto engine — very main character energy tbh. When you do that, CURLOPT_SSLKEY(3) gets used as an identifier that's passed to the engine, and you have to set the crypto engine with CURLOPT_SSLENGINE(3) 💅

The "PROV" format lets you load the private key from a crypto provider (this was added in 8.12.0, iykyk). Same deal here: CURLOPT_SSLKEY(3) is used as an identifier passed to the provider 🫶

⚠️ Heads up bestie: the "DER" format does not work with OpenSSL. Not me warning you twice, but like… it doesn't.

Good news though — your application does not have to keep the string around after setting this option, so you can yeet it as soon as you're done 😌

And if you use this option multiple times? The last set string overrides the previous ones, periodt. Set it to NULL to restore the internal default 💖

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👑 DEFAULT 👑

"PEM" — the standard, the icon, the moment ⭐

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🌈 PROTOCOLS 🌈

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

This option works only with the following TLS backends: OpenSSL and wolfSSL — that's the vibe check and those two passed 🧚

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🌸 EXAMPLE 🌸

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode result;
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
    curl_easy_setopt(curl, CURLOPT_SSLCERT, "client.pem");
    curl_easy_setopt(curl, CURLOPT_SSLKEY, "key.pem");
    curl_easy_setopt(curl, CURLOPT_SSLKEYTYPE, "PEM");
    curl_easy_setopt(curl, CURLOPT_KEYPASSWD, "s3cret");
    result = curl_easy_perform(curl);
    curl_easy_cleanup(curl);
  }
}

look at that little certified slay of a program 🔥

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⭐ AVAILABILITY ⭐

Added in curl 7.9.3 — she's been around for a minute, an absolute veteran 💜

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💀 RETURN VALUE 💀

curl_easy_setopt(3) returns a CURLcode that indicates success or error, that's it, that's the whole thing 🙈

CURLE_OK (0) means everything was OK 😭❤️, and non-zero means an error occurred — go check libcurl-errors(3) for the deets.

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🫶 SEE ALSO 🫶

• CURLOPT_PROXY_SSLKEYTYPE(3)
• CURLOPT_SSLCERT(3)
• CURLOPT_SSLKEY(3)

go read those next bestie, they're all connected, it's giving lore 🦋

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mwah 💋 that's the whole page — brought to you by libcurl, dated 2026-10-12, and yes that date is in the future, don't question it, she's iconic like that 🌟