Accept PKCS11 uri as key or certificate in sslOptions

[martxel]

Is your feature request related to a problem?

curl very recently (8.12.0) added support to use keys and certificates with PKCS#11 provider (a way to access keys and certificates in a secure element, instead of files).
curl/curl#15587

If you pass a valid pkcs11 uri in the --key or --cert argument, it will use them with the provider configured in OpenSSL.

They also added a new --key-type / --cert-type named "PROV", and if not explicitly set it will take the "PROV" value if a pkcs11 uri is detected in the --key-type / --cert-type arguments.

An example curl command that uses pkcs11 for the key and a file for the cert:

curl --key-type PROV --key "pkcs11:token=mytoken;object=mykey;type=private;pin-value=111111" --cert cert.pem --cacert ca_cert.pem https://localhost/

With cpr, it is not possible to currently use this feature because even if you pass a pkcs11 uri as an argument to KeyFile, it will explicitly set the key type to "PEM".

Possible Solution

One easy solution is to create a new KeyProv class, almost the same as KeyFile, but that returns "PROV" as the key type:

class KeyProv {
  public:
    // NOLINTNEXTLINE(google-explicit-constructor, hicpp-explicit-conversions)
    KeyProv(fs::path&& p_filename) : filename(std::move(p_filename)) {}

    template <typename FileType, typename PassType>
    KeyProv(FileType&& p_filename, PassType p_password) : filename(std::forward<FileType>(p_filename)), password(std::move(p_password)) {}

    virtual ~KeyProv() {
        util::secureStringClear(password);
    }

    fs::path filename;
    std::string password;

    virtual const char* GetKeyType() const {
        return "PROV";
    }
};

Same with CertFile:

class CertProv {
  public:
    // NOLINTNEXTLINE(google-explicit-constructor, hicpp-explicit-conversions)
    CertProv(fs::path&& p_filename) : filename(std::move(p_filename)) {}

    virtual ~CertProv() = default;

    const fs::path filename;

    virtual const char* GetCertType() const {
        return "PROV";
    }
};

And the new SetOption:

void SetOption(const ssl::KeyProv& opt) {
    key_file = opt.filename.string();
    key_type = opt.GetKeyType();
    key_pass = opt.password;
}

void SetOption(const ssl::CertProv& opt) {
    cert_file = opt.filename.string();
    cert_type = opt.GetCertType();
}

I have tried this approach and works corrently with libcurl 8.12.1 with the key stored in the TPM:

  // key_file contains a pkcs11 uri
  cpr::SslOptions sslOpts =
      cpr::Ssl(cpr::ssl::TLSv1_2{}, cpr::ssl::CertFile{cert_file},
               cpr::ssl::KeyProv{key_file}, cpr::ssl::CaInfo{ca_file},
               cpr::ssl::VerifyPeer{true}, cpr::ssl::VerifyHost{true});

  cpr::Response r = cpr::Get(cpr::Url{"https://10.0.0.10"}, sslOpts);

Alternatives

Another alternative, and maybe more direct option would be to add another constructor to KeyFile and CertFile to accept the key type as a parameter, than then gets returned by GetCertType or CetKeyType. This way, it would be possible to just use KeyFile("pkcs11:my-uri", "PROV");

Additional Context

I can provide a pull request with either approach.

[COM8]

@martxel thanks for reporting!
I would love to see this in cpr.

I'd prefer going with the CertProv class approach since it is more verbose and aligns better with the current coding style of cpr.

[martxel]

Ok, I went with that approach in my project. I will provide a merge request in a few days.

Thanks!