如何在PHP上使用hash_hmac（）和“SHA256withRSA”？

Question

I'm trying to get PayPal Webhooks to work with my PHP app. The problem is the hashing algorithm they send via headers, that i must use to verify if the request is valid.

When I try to use it, I get this error:

hash_hmac(): Unknown hashing algorithm: SHA256withRSA

I have tried hash_hmac using just the "sha256" algo and it worked, so I think the problem must be with the one they want me to use.

Here is the code I use to process the Webhook:

$headers = apache_request_headers();

$body = @file_get_contents('php://input');
$json = json_decode($body);

// Concatanate the reqired strings values
$sigString = $headers['PAYPAL-TRANSMISSION-ID'].'|'.$headers['PAYPAL-TRANSMISSION-TIME'].'|'.$json->id.'|'.crc32($body);

// Get the certificate file and read the key
$pub_key = openssl_pkey_get_public(file_get_contents($headers['PAYPAL-CERT-URL']));
$keyData = openssl_pkey_get_details($pub_key);

// check signature
if ($headers['PAYPAL-TRANSMISSION-SIG'] != hash_hmac($headers['PAYPAL-AUTH-ALGO'],$sigString,$keyData['key'])) {
    //invalid
}

Answer 1

Here is the code that worked in the end:

// Get the certificate file and read the key
$pubKey = openssl_pkey_get_public(file_get_contents($headers['PAYPAL-CERT-URL']));
$details = openssl_pkey_get_details($pubKey);

$verifyResult = openssl_verify($sigString, base64_decode($headers['PAYPAL-TRANSMISSION-SIG']), $details['key'], 'sha256WithRSAEncryption');

if ($verifyResult === 0) {
    throw new Exception('signature incorrect');
} elseif ($verifyResult === -1) {
    throw new Exception('error checking signature');
}

//rest of the code when signature is correct...

I needed to decode the signature PayPal sent me with base64_decode() and for some reason the key worked only when I used openssl_pkey_get_details()