DMT DMT - 2 months ago 16
PHP Question

How do I verify a Google idToken in PHP?

I have been working on this for far too long. I am looking for a working example as of September 2016 for verifying a Google idToken such as

eyJhbGciOiJSUzI1NiIsImtpZCI6IjZjNzgxOTQyZDg0OWJhMmVjZGE4Y2VkYjcyZDM0MzU3ZmM5NWIzMjcifQ.eyJpc3MiOiJodHRwczovL2FjY291bnRzLmdvb2dsZS5jb20iLCJhdWQiOiIxMDQ5MTQ4MTU2NTQ2LTk2YjFxcTJsNTJtODVtODB0ZHVoZHVma2RwODRtN2tuLmFwcHMuZ29vZ2xldXNlcmNvbnRlbnQuY29tIiwic3ViIjoiMTEyNTk4NDgzNjQ2MjY1OTYxNTQwIiwiZW1haWxfdmVyaWZpZWQiOnRydWUsImF6cCI6IjEwNDkxNDgxNTY1NDYtdjJwZjRlbGhzOGNwcXBlcWZkMzU5am5nOWs5aW5kcTQuYXBwcy5nb29nbGV1c2VyY29udGVudC5jb20iLCJlbWFpbCI6InRlc3R1c2VydGh4QGdtYWlsLmNvbSIsImlhdCI6MTQ3NDc1NDMzMiwiZXhwIjoxNDc0NzU3OTMyLCJuYW1lIjoiVGVzdCBVc2VyIiwicGljdHVyZSI6Imh0dHBzOi8vbGg0Lmdvb2dsZXVzZXJjb250ZW50LmNvbS8tU0dldkZZRDlaWFEvQUFBQUFBQUFBQUkvQUFBQUFBQUFBQUEvQVBhWEhoUmtuX1hEaEhNLTEzeVMwTUtBcFNrZG1zVEdYdy9zOTYtYy9waG90by5qcGciLCJnaXZlbl9uYW1lIjoiVGVzdCIsImZhbWlseV9uYW1lIjoiVXNlciIsImxvY2FsZSI6ImVuIn0.btukbBvhek6w14CrBVTGs8X9_IXIHZKpV1NzJ3OgbGUfmoRMirNGzZiFAgrR7COTeDJTamxRzojxxmXx6EEkQqNQcbyN8dO0PTuNt9pujQjLbFw_HBhIFJQaJSR3-tYPN-UtHGQ5JAAySsvCPapXbxyiKzTyvGYRSU65LmyNuiGxe6RQe1zHjq2ABJ4IPRqKPuFupnGRPWYyBSTPU7XQvtfhgyqA0BWZUfmCIFyDxQhvMaXNLTs01gnGVhcUDWZLi9vuUiKUlz3-aSSbwdfCMAljhBHnjpYO6341k5-qmgKkWawv8DX_nMEzntsCMCr664rP4wFEbsRB5ledM9Pc9Q


Using Google's recommended way and pulling "accounts.google.com/.well-known/openid-configuration" for the jwks_uri and pulling that "www.googleapis.com/oauth2/v3/certs", yielding a relevant entry for

{
"kty": "RSA",
"alg": "RS256",
"use": "sig",
"kid": "6c781942d849ba2ecda8cedb72d34357fc95b327",
"n": "s1dt5wFFaYl-Bt7Yb7QgWEatLJfxwWDhbd5yvm2Z4d1PRgNVQa9kwOArQNoOJ-b-oZnXLVFsVASUXEAumGf1ip5TVCQmMBKqlchSDNuoZfoWdpCCX7jx4gNuS43pS6VqV3QDjWnoXRTHaUi5pZEbpAmWpOeG_CfmewNVwBXPFx8-mtvEdtxIrspX4ayXTViR4vHc7MhQhUxllFbocxMjJysDQuZV9wN3MI0lVtQdf52SKJwF3lhvWA9-WAEZ1q8wq-I93Sfte95RaFjDqCH--Sh-8DjhK4OvgItcEGd5QRHjdLvrayPwaDQbpMRN2n3BkVWIxKJubtRiSeWbawCklQ",
"e": "AQAB"
}


Verification happens if I pass the token to https://www.googleapis.com/oauth2/v3/tokeninfo?id_token=TOKEN, but this is not a real answer, since they don't change that often but doing an extra web call every time is just asking for trouble.

So can someone point me to a working example? I've tried phpseclib, but it never verifies. I've probably looked for about 40 hours at this point, and I'm at my wits' end.

Any help is appreciated.

My relevant code:

$modulus = "";
$exponent = "";
$token = $_POST['token'];
$pieces = explode(".", $token);
$header = json_decode(base64_decode(str_replace(['-','_'], ['+','/'], $pieces[0])), true);
$alg = $header['alg'];
$kid = $header['kid'];
$payload = base64_decode(str_replace(['-','_'], ['+','/'], $pieces[1]));
$signature = str_replace(['-','_'], ['+','/'], $pieces[2]);
//$signature = base64_decode(str_replace(['-','_'], ['+','/'], $pieces[2]));

if (testGoogleList($alg, $kid, $modulus, $exponent))
{

echo "Found in list: kid=".$kid."\n";
echo "n: (base64URL)".$modulus."\n";
echo "e: (base64URL)".$exponent."\n";
$modulus = str_replace(['-','_'], ['+','/'], $modulus);
$exponent = str_replace(['-','_'], ['+','/'], $exponent);
echo "n: (base64)".$modulus."\n";
echo "e: (base64)".$exponent."\n";
$rsa = new Crypt_RSA();
$rsa->setHash("sha256");
$rsa->setSignatureMode(CRYPT_RSA_SIGNATURE_PKCS1);
$modulus = new Math_BigInteger($modulus, 256);
$exponent = new Math_BigInteger($exponent, 256);
echo "n: (BigInteger)".$modulus."\n";
echo "e: (BigInteger)".$exponent."\n";
$rsa->loadKey(array('n' => $modulus, 'e' => $exponent));
$rsa->setPublicKey();
$pubKey = $rsa->getPublicKey();
echo "Public Key from phpseclib\n".$pubKey."\n";
echo "--First openSSL error check--\n";
while ($msg = openssl_error_string())
echo $msg . "<br />\n";
echo "--After First Error Check, before Verify--\n";
$res = $rsa->verify($pieces[0].".".$pieces[1], $signature);
while ($msg = openssl_error_string())
echo $msg . "<br />\n";
echo "--Verify result: ".var_export($res, true)."--\n";
}


Output

Found in list:  kid=6c781942d849ba2ecda8cedb72d34357fc95b327
n: (base64URL)s1dt5wFFaYl-Bt7Yb7QgWEatLJfxwWDhbd5yvm2Z4d1PRgNVQa9kwOArQNoOJ-b-oZnXLVFsVASUXEAumGf1ip5TVCQmMBKqlchSDNuoZfoWdpCCX7jx4gNuS43pS6VqV3QDjWnoXRTHaUi5pZEbpAmWpOeG_CfmewNVwBXPFx8-mtvEdtxIrspX4ayXTViR4vHc7MhQhUxllFbocxMjJysDQuZV9wN3MI0lVtQdf52SKJwF3lhvWA9-WAEZ1q8wq-I93Sfte95RaFjDqCH--Sh-8DjhK4OvgItcEGd5QRHjdLvrayPwaDQbpMRN2n3BkVWIxKJubtRiSeWbawCklQ
e: (base64URL)AQAB
n: (base64)s1dt5wFFaYl+Bt7Yb7QgWEatLJfxwWDhbd5yvm2Z4d1PRgNVQa9kwOArQNoOJ+b+oZnXLVFsVASUXEAumGf1ip5TVCQmMBKqlchSDNuoZfoWdpCCX7jx4gNuS43pS6VqV3QDjWnoXRTHaUi5pZEbpAmWpOeG/CfmewNVwBXPFx8+mtvEdtxIrspX4ayXTViR4vHc7MhQhUxllFbocxMjJysDQuZV9wN3MI0lVtQdf52SKJwF3lhvWA9+WAEZ1q8wq+I93Sfte95RaFjDqCH++Sh+8DjhK4OvgItcEGd5QRHjdLvrayPwaDQbpMRN2n3BkVWIxKJubtRiSeWbawCklQ
e: (base64)AQAB
n: (BigInteger)18674717054764783973087488855176842456138281065703345249166514684640666364313492818979675328236363014396820758462507776710767978395332237045824933690552916871072924852353561300648679961653291310130667565640227949181785672954620248276915721938277908962537175894062430220752771265500386404609948390377043762106166027544443459977210114747088393335234720657330424186435226141073425445733987857419933850994487913462193466159335385639996611717486282518255208499657362420183528330692236194252505592468150318350852955051377118157817611947817677975817359347998935961426571802421142861030565807099600656362069178972477827638867161671399657071319083914500667014214521757304661303525496653078786180348831678824969667950119891369610525474165187687495455755684504105433077872587114630537058768184460798470456362909589578101896361255070801
e: (BigInteger)1095844162
Public Key from phpseclib
-----BEGIN PUBLIC KEY-----
MIIBeDANBgkqhkiG9w0BAQEFAAOCAWUAMIIBYAKCAVZzMWR0NXdGRmFZbCtCdDdZ
YjdRZ1dFYXRMSmZ4d1dEaGJkNXl2bTJaNGQxUFJnTlZRYTlrd09BclFOb09KK2Ir
b1puWExWRnNWQVNVWEVBdW1HZjFpcDVUVkNRbU1CS3FsY2hTRE51b1pmb1dkcEND
WDdqeDRnTnVTNDNwUzZWcVYzUURqV25vWFJUSGFVaTVwWkVicEFtV3BPZUcvQ2Zt
ZXdOVndCWFBGeDgrbXR2RWR0eElyc3BYNGF5WFRWaVI0dkhjN01oUWhVeGxsRmJv
Y3hNakp5c0RRdVpWOXdOM01JMGxWdFFkZjUyU0tKd0YzbGh2V0E5K1dBRVoxcTh3
cStJOTNTZnRlOTVSYUZqRHFDSCsrU2grOERqaEs0T3ZnSXRjRUdkNVFSSGpkTHZy
YXlQd2FEUWJwTVJOMm4zQmtWV0l4S0p1YnRSaVNlV2Jhd0NrbFECBEFRQUI=
-----END PUBLIC KEY-----
--First openSSL error check--
--After First Error Check, before Verify--
error:0906D06C:PEM routines:PEM_read_bio:no start line

--Verify result: false--

Answer

The problem is that you're not base64-decoding anything relevant.

This worked for me (told me that the signature was valid):

<?php
include('Crypt/RSA.php');

$data = 'eyJhbGciOiJSUzI1NiIsImtpZCI6IjZjNzgxOTQyZDg0OWJhMmVjZGE4Y2VkYjcyZDM0MzU3ZmM5NWIzMjcifQ.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';
$signature = 'btukbBvhek6w14CrBVTGs8X9_IXIHZKpV1NzJ3OgbGUfmoRMirNGzZiFAgrR7COTeDJTamxRzojxxmXx6EEkQqNQcbyN8dO0PTuNt9pujQjLbFw_HBhIFJQaJSR3-tYPN-UtHGQ5JAAySsvCPapXbxyiKzTyvGYRSU65LmyNuiGxe6RQe1zHjq2ABJ4IPRqKPuFupnGRPWYyBSTPU7XQvtfhgyqA0BWZUfmCIFyDxQhvMaXNLTs01gnGVhcUDWZLi9vuUiKUlz3-aSSbwdfCMAljhBHnjpYO6341k5-qmgKkWawv8DX_nMEzntsCMCr664rP4wFEbsRB5ledM9Pc9Q';
$signature = str_replace(['-','_'], ['+','/'], $signature);
$signature = base64_decode($signature);

$n = 's1dt5wFFaYl-Bt7Yb7QgWEatLJfxwWDhbd5yvm2Z4d1PRgNVQa9kwOArQNoOJ-b-oZnXLVFsVASUXEAumGf1ip5TVCQmMBKqlchSDNuoZfoWdpCCX7jx4gNuS43pS6VqV3QDjWnoXRTHaUi5pZEbpAmWpOeG_CfmewNVwBXPFx8-mtvEdtxIrspX4ayXTViR4vHc7MhQhUxllFbocxMjJysDQuZV9wN3MI0lVtQdf52SKJwF3lhvWA9-WAEZ1q8wq-I93Sfte95RaFjDqCH--Sh-8DjhK4OvgItcEGd5QRHjdLvrayPwaDQbpMRN2n3BkVWIxKJubtRiSeWbawCklQ';
$n = str_replace(['-','_'], ['+','/'], $n);
$n = base64_decode($n);

$e = 'AQAB';
$e = base64_decode($e);

$rsa = new Crypt_RSA();
$rsa->loadKey([
    'n' => new Math_BigInteger($n, 256),
    'e' => new Math_BigInteger($e, 256)
]);

$rsa->setHash('sha256');
$rsa->setSignatureMode(CRYPT_RSA_SIGNATURE_PKCS1);

echo $rsa->verify($data, $signature) ?
    'valid' :
    'invalid';