Google Drive Auth：从JSON版本的私钥创建JWT签名

I am having trouble creating a JWT for Google Drive authorization. The instruction is given in this link.

<?php
//helper function
function base64url_encode($data) { 
return rtrim(strtr(base64_encode($data), '+/', '-_'), '='); 
}

//Google's Documentation of Creating a JWT https://developers.google.com/identity/protocols/OAuth2ServiceAccount#authorizingrequests

$raw_key_data="-----BEGIN PRIVATE KEY-----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-----END PRIVATE KEY-----
";   

$private_key = openssl_get_privatekey($raw_key_data);
if ($private_key === FALSE)
{
    error_log ("Unable to extract private key from raw key data: " . openssl_error_string());
};

print("$private_key
");

//{Base64url encoded JSON header}

$jwtHeader = base64url_encode(json_encode(array(
"alg" => "RS256",
"typ" => "JWT"
)));

//{Base64url encoded JSON claim set}
$now = time();
$jwtClaim = base64url_encode(json_encode(array(
"iss" => "myService-account-id@mySite.iam.gserviceaccount.com",
"scope" => "https://www.googleapis.com/auth/drive.file",
"aud" => "https://www.googleapis.com/oauth2/v4/token",
"exp" => $now + 3600,
"iat" => $now
)));

//The base string for the signature: {Base64url encoded JSON header}.    {Base64url encoded JSON claim set}

openssl_sign(
$jwtHeader.".".$jwtClaim,
$jwtSig,
$private_key,
"SHA256"
);

$jwtSign = base64url_encode($jwtSig);

//{Base64url encoded JSON header}.{Base64url encoded JSON claim set}.    {Base64url encoded signature}
$jwtAssertion = $jwtHeader.".".$jwtClaim.".".$jwtSig;
print("$jwtAssertion
");

?>

When I run this code, I get Resource id #4 followed by a nonsense signature. I even tried putting the private key in a file and reading from the file in which there is multiple lines (private key separated at ) but no success. I can't generate meaningful JWT signature from the private key. Can anyone help me with this issue?

Thanks,

I have managed to solve this issue. Below is the updated code that reads the JSON file downloaded from Google and creates an assertion with which one can then get access token from Google:

<?php
//helper function
function base64url_encode($data) { 
return rtrim(strtr(base64_encode($data), '+/', '-_'), '='); 
}

// Read the JSON credential file my-private-key.json download from Google
$private_key_file="my-private-key.json";
$json_file = file_get_contents($private_key_file);

$info = json_decode($json_file);
$private_key = $info->{'private_key'};

//{Base64url encoded JSON header}
$jwtHeader = base64url_encode(json_encode(array(
"alg" => "RS256",
"typ" => "JWT"
)));

//{Base64url encoded JSON claim set}
$now = time();
$jwtClaim = base64url_encode(json_encode(array(
"iss" => $info->{'client_email'},
"scope" => "https://www.googleapis.com/auth/drive.file",
"aud" => "https://www.googleapis.com/oauth2/v4/token",
"exp" => $now + 3600,
"iat" => $now
)));

$data = $jwtHeader.".".$jwtClaim;

// Signature
$Sig = '';
openssl_sign($data,$Sig,$private_key,'SHA256');
$jwtSign = base64url_encode( $Sig  );


//{Base64url encoded JSON header}.{Base64url encoded JSON claim set}.{Base64url encoded signature}

$jwtAssertion = $data.".".$jwtSign;
echo "$jwtAssertion
";

You may then test your code by communicating with Google Drive for example:

curl -d 'grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=YOUR-JWT-ASSERTION-BUILD-ABOVE' https://www.googleapis.com/oauth2/v4/token