EVP_PKEY_GETTABLE_PARAMS(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💖 EVP_PKEY_GETTABLE_PARAMS 💖

okay so this is the OpenSSL manual page, bestie — specifically EVP_PKEY_GETTABLE_PARAMS(3ossl), and it's all about pulling parameter values out of a key. buckle up, it's actually so good 🎀

✨ NAME ✨

EVP_PKEY_gettable_params, EVP_PKEY_get_params, EVP_PKEY_get_int_param, EVP_PKEY_get_size_t_param, EVP_PKEY_get_bn_param, EVP_PKEY_get_utf8_string_param, EVP_PKEY_get_octet_string_param — retrieve key parameters from a key 👑

like, all seven of them. one big happy function family. periodt.

🦋 SYNOPSIS 🦋

#include <openssl/evp.h>

const OSSL_PARAM *EVP_PKEY_gettable_params(EVP_PKEY *pkey);
int EVP_PKEY_get_params(const EVP_PKEY *pkey, OSSL_PARAM params[]);
int EVP_PKEY_get_int_param(const EVP_PKEY *pkey, const char *key_name,
                           int *out);
int EVP_PKEY_get_size_t_param(const EVP_PKEY *pkey, const char *key_name,
                              size_t *out);
int EVP_PKEY_get_bn_param(const EVP_PKEY *pkey, const char *key_name,
                          BIGNUM **bn);
int EVP_PKEY_get_utf8_string_param(const EVP_PKEY *pkey, const char *key_name,
                                   char *str, size_t max_buf_sz,
                                   size_t *out_len);
int EVP_PKEY_get_octet_string_param(const EVP_PKEY *pkey, const char *key_name,
                                    unsigned char *buf, size_t max_buf_sz,
                                    size_t *out_len);

copy it exactly, bestie. no typos. the compiler is watching 👀

🌸 DESCRIPTION 🌸

first things first: go read OSSL_PARAM(3) for the whole tea on parameters. that's the lore. 📖

EVP_PKEY_get_params() retrieves parameters from the key pkey, according to the contents of params. it's giving "you tell me what you want, I'll go get it" energy.

EVP_PKEY_gettable_params() returns a constant list of params indicating the names and types of key parameters that can be retrieved. like a menu. a little parameter menu. iconic. 🍓

💅 the integer situation (important, ngl)

an OSSL_PARAM(3) of type OSSL_PARAM_INTEGER or OSSL_PARAM_UNSIGNED_INTEGER is of arbitrary length. yes, arbitrary. it can be a whole chonky number. 😭

such a parameter can be obtained using any of the functions EVP_PKEY_get_int_param(), EVP_PKEY_get_size_t_param() or EVP_PKEY_get_bn_param().

but bestie, listen: attempting to obtain an integer value that does not fit into a native C int type will cause EVP_PKEY_get_int_param() to fail. similarly, attempting to obtain an integer value that is negative or does not fit into a native C size_t type using EVP_PKEY_get_size_t_param() will also fail. the vibes will be off. the function will return sad. so know your sizes. 🫶

🧚 the individual girlies

  • EVP_PKEY_get_int_param() — retrieves a key pkey integer value *out associated with a name of key_name if it fits into "int" type. for parameters that do not fit into "int" use EVP_PKEY_get_bn_param().
  • EVP_PKEY_get_size_t_param() — retrieves a key pkey size_t value *out associated with a name of key_name if it fits into "size_t" type. for parameters that do not fit into "size_t" use EVP_PKEY_get_bn_param().
  • EVP_PKEY_get_bn_param() — retrieves a key pkey BIGNUM value **bn associated with a name of key_name. if *bn is NULL then the BIGNUM is allocated by the method. (so it's doing the work for you, slay 💅)
  • EVP_PKEY_get_utf8_string_param() — get a key pkey UTF8 string value into a buffer str of maximum size max_buf_sz associated with a name of key_name. the maximum size must be large enough to accommodate the string value including a terminating NUL byte, or this function will fail. if out_len is not NULL, *out_len is set to the length of the string not including the terminating NUL byte. the required buffer size not including the terminating NUL byte can be obtained from *out_len by calling the function with str set to NULL. (two-step glow-up move, iykyk 🌈)
  • EVP_PKEY_get_octet_string_param() — get a key pkey's octet string value into a buffer buf of maximum size max_buf_sz associated with a name of key_name. if out_len is not NULL, *out_len is set to the length of the contents. the required buffer size can be obtained from *out_len by calling the function with buf set to NULL.

🔥 NOTES 🔥

these functions only work for EVP_PKEYs that contain a provider side key. that's the whole entry requirement. no provider side key, no party. 🙈

⭐ RETURN VALUES ⭐

  • EVP_PKEY_gettable_params() returns NULL on error or if it is not supported. (not me getting NULL… anyway 💀)
  • all other methods return 1 if a value associated with the key's key_name was successfully returned, or 0 if there was an error.

an error may be returned by methods EVP_PKEY_get_utf8_string_param() and EVP_PKEY_get_octet_string_param() if max_buf_sz is not big enough to hold the value. and if out_len is not NULL, *out_len will be assigned the required buffer size to hold the value — so even when it flops, it tells you what size would have ate. considerate king behavior 👑

🎀 EXAMPLES 🎀

#include <openssl/evp.h>

char curve_name[64];
unsigned char pub[256];
BIGNUM *bn_priv = NULL;

/*
 * NB: assumes 'key' is set up before the next step. In this example the key
 * is an EC key.
 */

if (!EVP_PKEY_get_utf8_string_param(key, OSSL_PKEY_PARAM_GROUP_NAME,
                                    curve_name, sizeof(curve_name), &len)) {
  /* Error */
}
if (!EVP_PKEY_get_octet_string_param(key, OSSL_PKEY_PARAM_PUB_KEY,
                                     pub, sizeof(pub), &len)) {
    /* Error */
}
if (!EVP_PKEY_get_bn_param(key, OSSL_PKEY_PARAM_PRIV_KEY, &bn_priv)) {
    /* Error */
}

BN_clear_free(bn_priv);

and yes, BN_clear_free(bn_priv); at the end. clean up after yourself, bestie. 🧹✨

🌈 SEE ALSO 🌈

go read them. they're the supporting cast and they understood the assignment. 💜

👑 HISTORY 👑

these functions were added in OpenSSL 3.0. so if you're on some ancient version, that's why nothing's working, not you. 🫶

Copyright 2020-2022 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at https://www.openssl.org/source/license.html.

okay that's the whole page, you're officially smarter than you were three minutes ago 🧚💖

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