Authentication in the Iconomi API in PHP - php

Question

I am developing a PHP script to consume the Iconomi API, but I have problems with authentication.
According to the documentation, you have to create a signature and attach it in the header. However the curl_exec function returns false, which means it has failed.
These are the instructions:
Creating a request
All REST requests must contain the following headers:
ICN-API-KEY - The api key as a string.
ICN-SIGN - The base64-encoded signature (see Signing a Message).
ICN-TIMESTAMP - A timestamp for your request in epoch milliseconds.
Signing a Message
You generate the ICN-SIGN header by creating a sha512 HMAC using the base64-decoded secret key on the > prehash string timestamp + method + requestPath + body (where + represents string concatenation) and base64-encode the output, where:
the timestamp value is the same as the ICN-TIMESTAMP header.
the body is the request body string or omitted if there is no request body (typically for GET requests).
method must always be in upper case
And this is my code:
function call_iconomi_api()
{
$url = 'https://api.iconomi.com';
$api_key = get_field('iconomi_api_key', 'option');
$api_secret = get_field('iconomi_api_secret', 'option');
$timestamp = round(microtime(true) * 1000);
$request_path = '/v1/assets';
$message = $timestamp . 'GET' . $request_path;
$signature = base64_encode(hash_hmac('sha512', $message, $api_secret));
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $url . $request_path);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($curl, CURLOPT_HTTPHEADER, array(
'ICN-API-KEY: ' . $api_key,
'ICN-SIGN: ' . $signature,
'ICN-TIMESTAMP: ' . $timestamp
));
$result = curl_exec($curl);
curl_close($curl);
return $result;
}

Answer 1

Try to use
$signature = base64_encode(hash_hmac('sha512', $message, $api_secret, true));
If you look into https://iconomi-ag.github.io/iconomi-api/#section/Introduction/Authentication under section "Signing a Message" (Example of GET with query parameters) this function with last param as true returns identical string as base64 encoded signature