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Merge pull request #32 from pdsinterop/fix/jkt-check
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Fix for JKT check
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@ylebre
ylebre authored Sep 30, 2022
2 parents e07c22d + bc9e227 commit ea830b6
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Showing 2 changed files with 201 additions and 44 deletions.
151 changes: 122 additions & 29 deletions src/Utils/DPop.php
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Original file line number Diff line number Diff line change
Expand Up @@ -59,13 +59,7 @@ public function getWebId($request) {

//@FIXME: check that there is just one DPoP token in the request
try {
$dpopKey = $this->getDpopKey($dpop, $request);
} catch (InvalidTokenStructure $e) {
throw new InvalidTokenException("Invalid JWT token: {$e->getMessage()}", 0, $e);
}

try {
$this->validateJwtDpop($jwt, $dpopKey);
$this->validateJwtDpop($jwt, $dpop, $request);
} catch (RequiredConstraintsViolated $e) {
throw new InvalidTokenException($e->getMessage(), 0, $e);
}
Expand All @@ -76,20 +70,16 @@ public function getWebId($request) {
}

/**
* Returns the "kid" from the "jwk" header in the DPoP token.
* The DPoP token must be valid.
*
* @param string $dpop The DPoP token
* @param ServerRequestInterface $request Server Request
*
* @return string the "kid" from the "jwk" header in the DPoP token.
*
* @throws RequiredConstraintsViolated
* kept for backwards compatability
* note: the "kid" value is not guaranteed to be a hash of the jwk
* so to compare a jkt, calculate the jwk thumbprint instead
* @param string $dpop The DPoP token, raw
* @param ServerRequestInterface $request Server Request
* @return string The "kid" from the "jwk" header
*/
public function getDpopKey($dpop, $request) {
$this->validateDpop($dpop, $request);

// 1. the string value is a well-formed JWT,
$jwtConfig = Configuration::forUnsecuredSigner();
$dpop = $jwtConfig->parser()->parse($dpop);
$jwk = $dpop->headers()->get("jwk");
Expand All @@ -101,25 +91,126 @@ public function getDpopKey($dpop, $request) {
return $jwk['kid'];
}

private function validateJwtDpop($jwt, $dpopKey) {
/**
* RFC7638 defines a method for computing the hash value (or "digest") of a JSON Web Key (JWK).
*
* The resulting hash value can be used for identifying the key represented by the JWK
* that is the subject of the thumbprint.
*
* For instance by using the base64url-encoded JWK Thumbprint value as a key ID (or "kid") value.
*
* @see https://www.rfc-editor.org/rfc/rfc7638
*
* The thumbprint of a JWK is created by:
*
* 1. Constructing a JSON string (without whitespaces) with the required keys in alphabetical order.
* 2. Hashing the JSON string using SHA-256 (or another hash function)
*
* @param string $jwk The JWK key to thumbprint
* @return string the thumbprint
* @throws InvalidTokenException
*/
public function makeJwkThumbprint($jwk) {
if (!$jwk || !isset($jwk['kty'])) {
throw new InvalidTokenException('JWK has no "kty" key type');
}
// https://www.rfc-editor.org/rfc/rfc7517.html#section-4.1
// and https://www.rfc-editor.org/rfc/rfc7518.html#section-6.1
if (!in_array($jwk['kty'], ['RSA','EC'])) {
throw new InvalidTokenException('JWK "kty" key type value must be one of "RSA" or "EC", got "'.$jwk['kty'].'" instead.');
}
if ($jwk['kty']=='RSA') { // used with RS256 alg
if (!isset($jwk['e'], $jwk['n'])) {
throw new InvalidTokenException('JWK values do not match "RSA" key type');
}
$json = vsprintf('{"e":"%s","kty":"%s","n":"%s"}', [
$jwk['e'],
$jwk['kty'],
$jwk['n'],
]);
} else { // EC used with ES256 alg
if (!isset($jwk['crv'], $jwk['x'], $jwk['y'])) {
throw new InvalidTokenException('JWK values doe not match "EC" key type');
}
//crv, kty, x, y
$json = vsprintf('{"crv":"%s","kty":"%s","x":"%s","y":"%s"}', [
$jwk['crv'],
$jwk['kty'],
$jwk['x'],
$jwk['y']
]);
}
$hash = hash('sha256', $json);
$encoded = Base64Url::encode($hash);
return $encoded;
}

/**
* https://datatracker.ietf.org/doc/html/draft-ietf-oauth-dpop#section-4.2
* When the DPoP proof is used in conjunction with the presentation of
* an access token in protected resource access, see Section 7, the DPoP
* proof MUST also contain the following claim:
* ath: hash of the access token. The value MUST be the result of a
* base64url encoding (as defined in Section 2 of [RFC7515]) the
* SHA-256 [SHS] hash of the ASCII encoding of the associated access
* token's value.
* See also: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-dpop#section-7
*
* Validates the above part of the oauth dpop specification
* @param string $jwt JWT access token, raw
* @param string $dpop DPoP token, raw
* @param ServerRequestInterface $request Server Request
* @return bool true, if the dpop token "ath" claim matches the access token
*/
public function validateJwtDpop($jwt, $dpop, $request) {
$this->validateDpop($dpop, $request);
$jwtConfig = Configuration::forUnsecuredSigner();
$jwt = $jwtConfig->parser()->parse($jwt);
$dpopJWT = $jwtConfig->parser()->parse($dpop);

$ath = $dpopJWT->claims()->get('ath');
if ($ath === null) {
throw new InvalidTokenException('DPoP "ath" claim is missing');
}

$hash = hash('sha256', $jwt);
$encoded = Base64Url::encode($hash);
return ($ath === $encoded);
}

/**
* https://solidproject.org/TR/oidc#tokens-id
* validates that the provided OIDC ID Token matches the DPoP header
* @param string $token The OIDS ID Token (raw)
* @param string $dpop The DPoP Token (raw)
* @param ServerRequestInterface $request Server Request
* @return bool True if the id token jkt matches the dpop token jwk
* @throws InvalidTokenException when the tokens do not match
*/
public function validateIdTokenDpop($token, $dpop, $request) {
$this->validateDpop($dpop, $request);
$jwtConfig = Configuration::forUnsecuredSigner();
$jwt = $jwtConfig->parser()->parse($token);
$cnf = $jwt->claims()->get("cnf");

if ($cnf === null) {
throw new InvalidTokenException('JWT Confirmation claim (cnf) is missing');
}

if (isset($cnf['jkt']) === false) {
if (!isset($cnf['jkt'])) {
throw new InvalidTokenException('JWT Confirmation claim (cnf) is missing Thumbprint (jkt)');
}

if ($cnf['jkt'] !== $dpopKey) {
throw new InvalidTokenException('JWT Confirmation claim (cnf) provided Thumbprint (jkt) does not match Key ID from JWK header');
$jkt = $cnf['jkt'];

$dpopJwt = $jwtConfig->parser()->parse($dpop);
$jwk = $dpopJwt->headers()->get('jwk');

$jwkThumbprint = $this->makeJwkThumbprint($jwk);
if ($jwkThumbprint !== $jkt) {
throw new InvalidTokenException('ID Token JWK Thumbprint (jkt) does not match the JWK from DPoP header');
}

//@FIXME: add check for "ath" claim in DPoP token, per https://datatracker.ietf.org/doc/html/draft-ietf-oauth-dpop#section-7
return false;
return true;
}

/**
Expand All @@ -129,9 +220,10 @@ private function validateJwtDpop($jwt, $dpopKey) {
* @param string $dpop The DPOP token
* @param ServerRequestInterface $request Server Request
*
* @return bool True if the DPOP token is valid, false otherwise
* @return bool True if the DPOP token is valid
*
* @throws RequiredConstraintsViolated
* @throws InvalidTokenException
*/
public function validateDpop($dpop, $request) {
/*
Expand Down Expand Up @@ -161,7 +253,11 @@ public function validateDpop($dpop, $request) {
*/
// 1. the string value is a well-formed JWT,
$jwtConfig = Configuration::forUnsecuredSigner();
$dpop = $jwtConfig->parser()->parse($dpop);
try {
$dpop = $jwtConfig->parser()->parse($dpop);
} catch(\Exception $e) {
throw new InvalidTokenException('Invalid DPoP token', 400, $e);
}

// 2. all required claims are contained in the JWT,
$htm = $dpop->claims()->get("htm"); // http method
Expand Down Expand Up @@ -251,9 +347,6 @@ public function validateDpop($dpop, $request) {
throw new InvalidTokenException("jti is invalid");
}

// 10. that, if used with an access token, it also contains the 'ath' claim, with a hash of the access token
// TODO: implement

return true;
}

Expand Down
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