PHP OpenSSL cannot read public key in PEM format - php

Question

I have a NodeJS application generating JSON Web Tokens with the PS256 algorithm. I want to try and verify the signatures in these tokens in a PHP application.
So far I've got the following:
My JWT:
eyJhbGciOiJQUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIwMTBiYjllYS00YTg0LTQ1ZTMtOTg5My0wYzNhNDYxZmQzMGUiLCJpYXQiOjE2MDU4OTI5NjcsImV4cCI6MTYwNjQ5Nzc2NywiYXVkIjoiNzBiYzcxMTQ1MWM2NDBjOTVlZjgzYjdhNDliMWE0MWIiLCJpc3MiOiIyM2ZhYTRiNC0wNmVlLTRlNGEtYTVjZC05NjJmOTRhMjEzYmYiLCJqdGkiOiI1MTNiYjczZC0zOTY3LTQxYzUtODMwOS00Yjc1ZDI4ZGU3NTIifQ.kLtaSYKyhqzx7Dc7UIz7tqU8TsXabRLxGiaqw21lgCcuf_eBvpiLkFOuXpUs-V8XQunQg8jV-bKlKUIb0pLvipjhRP50IwKDClQgNtIwn4yyX5RyDNGJur0qHNnkHMLaF11NsXGPyhvh-6ogSZjWgyZnkQJkXpz4jggBetwqz1hnicapGfNb6C-UdRcOLyCaiMD4OmvniFVCY6YoKlC6eHdwxsgHAxOSgD1QKiiQX_yAe39ja_axZD2Ii3QaNgO0WXzfWMbqRg_yl0y3kjQFys9iXGvQ1JIKDMLffR3rKVL5PgKSU3e472xcPKf6PNSJzphPi1G_xH2gqg1VVXo3Lg
Decoded:
Header:
(
[alg] => PS256
[typ] => JWT
)
Body:
(
[sub] => 010bb9ea-4a84-45e3-9893-0c3a461fd30e
[iat] => 1605892967
[exp] => 1606497767
[aud] => 70bc711451c640c95ef83b7a49b1a41b
[iss] => 23faa4b4-06ee-4e4a-a5cd-962f94a213bf
[jti] => 513bb73d-3967-41c5-8309-4b75d28de752
)
sub is a GUID user ID (we utilize GUIDs so that if a user's ID is leaked no information can be extrapolated, like the number of users in our system or when a user signed up)
iat is the epoch time that the token was issued (UTC)
exp is the epoch time that the token will expire (UTC)
aud doesn't conform to the JWT spec. I abused this claim to mitigate the effects of stolen tokens. It's the MD5 hash of data sent with every client request that would be difficult for someone to guess. So if someone were to steal this token and use it without sending the appropriate passphrase, the token would be automatically revoked
iss also doesn't conform to the JWT spec. I abused this claim to list the ID of the key used for signing the JWT. This way I can rotate my public-private key pair and know which key to use when validating signatures
jti is a GUID uniquely identifying the JWT. Compared against an in-memory store of revoked tokens
I went with the PS256 algorithm over RS256 because I read on a blog post that it's more secure. Honestly I don't know the difference.
I went with the PS256 algorithm over ES256 because upon testing I found that while ES256 generated smaller signatures (and therefore smaller tokens), it took about 3x longer to compute. My goal is to make this app as scalable as possible, so long computation time is to be avoided.
My public key:
-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEA0wO7By66n38BCOOPqxnj78gj8jMKACO5APe+M9wbwemOoipeC9DR
CGxC9q+n/Ki0lNKSujjCpZfnKe5xepL2klyKF7dGQwecgf3So7bve8vvb+vD8C6l
oqbCYEOHdLCDoC2UXEWIVRcV5H+Ahawym+OcE/0pzWlNV9asowIFWj/IXgDVKCFQ
nj164UFlxW1ITqLOQK1WlxqHAIoh20RzpeJTlX9PYx3DDja1Pw7TPomHChMeRNsw
Z7zJiavYrBCTvYE+tm7JrPfbIfc1a9fCY3LlwCTvaBkL2F5yeKdH7FMAlvsvBwCm
QhPE4jcDINUds8bHu2on5NU5VmwHjQ46xwIDAQAB
-----END RSA PUBLIC KEY-----
Using jsonwebtoken for NodeJS I can verify this token and authorize requests made using it. So all of the data seems good, the key works, and the math checks out.
However I've run into two problems when trying to verify the token in PHP:
1. The public key doesn't seem to be valid?
$key = openssl_pkey_get_public($pem);
print_r($key);
die();
This code prints out "false" - suggesting that the key could not be read from the PEM text posted above. Googling around I found this comment in the PHP manual which provided a solution. I did as instructed (removed new-lines from my key, prepended MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8A, then wrapped to 64 characters) and for some reason openssl_pkey_get_public($pem) actually returned an OpenSSL Public Key now. I'm not really keen on using copy/paste solutions I don't understand, though, and the comment mentioned that this will only work for 2048-bit keys, which concerns me if we ever want to upgrade our security in the future.
After making the changes suggested to my key the new key looks like this:
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA0wO7By66n38BCOOPqxnj
78gj8jMKACO5APe+M9wbwemOoipeC9DRCGxC9q+n/Ki0lNKSujjCpZfnKe5xepL2
klyKF7dGQwecgf3So7bve8vvb+vD8C6loqbCYEOHdLCDoC2UXEWIVRcV5H+Ahawy
m+OcE/0pzWlNV9asowIFWj/IXgDVKCFQnj164UFlxW1ITqLOQK1WlxqHAIoh20Rz
peJTlX9PYx3DDja1Pw7TPomHChMeRNswZ7zJiavYrBCTvYE+tm7JrPfbIfc1a9fC
Y3LlwCTvaBkL2F5yeKdH7FMAlvsvBwCmQhPE4jcDINUds8bHu2on5NU5VmwHjQ46
xwIDAQAB
-----END PUBLIC KEY-----
(note that this is the same key, just with 32 magic bytes prepended to the beginning of it and "BEGIN RSA PUBLIC KEY" replaced with "BEGIN PUBLIC KEY")
2. The signature fails to verify (possibly because I'm using PS256 and not RS256)
Ignoring the issues with #1 for now and moving on to the next step, I tried to verify my signature like so:
$success = openssl_verify($jwtHeader . "." . $jwtBody, $jwtSignature, $key, OPENSSL_ALGO_SHA256);
This returned false. Meaning the signature was not valid. But I know the signature was valid because it worked fine in NodeJS. So I suspect the issue here revolves around my choice of algorithm.
How do I get PHP to properly verify this token?
Update 1
Here's the code that I'm using to verify my tokens in NodeJS. This is a HapiJS + TypeScript project, but you should be able to make sense of it. jwt is just defined as import * as jwt from "jsonwebtoken";:
jwt.verify(
token,
server.plugins["bf-jwtAuth"].publicKeys[tokenData.iss].key,
{
algorithms: [options.algorithm],
audience: userHash,
maxAge: options.tokenMaxAge
},
err =>
{
// We can disregard the "decoded" parameter
// because we already decoded it earlier
// We're just interested in the error
// (implying a bad signature)
if (err !== null)
{
request.log([], err);
return reject(Boom.unauthorized());
}
return resolve(h.authenticated({
credentials: {
user: {
id: tokenData.sub
}
}
}));
}
);
There's not too much to see here, because I just relied on a third-party tool to do all of the validation for me. jwt.verify(...) and it worked like magic.
Update 2
Assuming that my issue lie in the algorithm being used (PS256 vs RS256) I started searching around and found this StackOverflow post which pointed me at phpseclib
We actually coincidentally already had phpseclib installed via Composer as a dependency of Google's auth SDK, so I bumped it up to a top-level dependency and gave it a try. Unfortunately I still ran into an issue. Here's my code so far:
use phpseclib\Crypt\RSA;
// Setup:
$rsa = new RSA();
$rsa->setHash("sha256");
$rsa->setMGFHash("sha256");
$rsa->setSignatureMode(RSA::SIGNATURE_PSS);
// The variables I'm working with:
$jwt = explode(".", "..."); // [Header, Body, Signature]
$key = "..."; // This is my PEM-encoded string, from above
// Attempting to verify:
$rsa->loadKey($key);
$valid = $rsa->verify($jwt[0] . "." . $jwt[1], base64_decode($jwt[2]));
if ($valid) { die("Valid"); } else { die("Invalid"); }
Neither die() statement is reached as I hit an error on the $rsa->verify() line with the following:
ErrorException: Invalid signature
at
/app/vendor/phpseclib/phpseclib/phpseclib/Crypt/RSA.php(2693)
Looking at this line in the library, it looks like it's failing at the "length checking" step:
if (strlen($s) != $this->k) {
user_error("Invalid signature");
}
I'm not sure what length it's expecting, though. I passed the raw signature directly from the JWT

Answer 1

After messing with this all day I finally figured out the missing piece.
First, some quick notes on the original question (already touched on in the updates):
To do RSA signatures with PSS padding ("PS256") you will need a third-party library, because the OpenSSL functions in PHP don't support this. A common recommendation is phpseclib
The 32 magic bytes I had to add to the key were only a quirk of PHP's OpenSSL functions and don't need to be utilized with phpseclib
With that said, my final problem (with the signature being "invalid") was:
JWT signatures are base64URL encoded, not base64 encoded
I wasn't even aware there was an RFC specification for base64URL encoding. It turns out you just replace every + with a - and every / with an _. So instead of:
$signature = base64_decode($jwt[2]);
It should be:
$signature = base64_decode(strtr($jwt[2], "-_", "+/"));
This works and my signature finally validates!