ILIAS\Authentication\SessionReminder\TokenCipher Class Reference

Encodes the opaque token the session reminder sends to sessioncheck.php. More...

Collaboration diagram for ILIAS\Authentication\SessionReminder\TokenCipher:

Public Member Functions
	__construct (private string $key)
	encode (int $user_id, string $session_hash)
	decode (string $token)

Static Public Member Functions
static	fromKeyFile ()

Private Member Functions
	deriveSubkey (string $salt)

Private Attributes
const string	CIPHER = 'aes-256-gcm'
const int	KEY_LENGTH = 32
const int	SALT_LENGTH = 16
const int	NONCE_LENGTH = 12
const int	TAG_LENGTH = 16
const string	CONTEXT = 'ilias_session_reminder_v1'
	Binds the subkeys and the ciphertext to their purpose, a token of another context cannot be decrypted. More...
const string	USER_ID = 'u'
const string	SESSION_HASH = 'h'

Detailed Description

Encodes the opaque token the session reminder sends to sessioncheck.php.

The token is encrypted (authenticated encryption, AES-256-GCM), so the client can neither read nor modify the contained user id and session hash.

Each token is encrypted with its own subkey, derived from the secret key and a random salt (HKDF). This way the limited number of encryptions per key (because of the 96 bit nonce of GCM) does not apply to the secret key, which is never replaced.

Token format (base64url): salt (16 bytes) | nonce (12 bytes) | ciphertext | authentication tag (16 bytes)

Definition at line 36 of file TokenCipher.php.

Constructor & Destructor Documentation

__construct()

ILIAS\Authentication\SessionReminder\TokenCipher::__construct

(

private string

$key

)

Parameters

string

$key

Binary key

Definition at line 53 of file TokenCipher.php.

      {
        if (\strlen($this->key) !== self::KEY_LENGTH) {
            throw new \InvalidArgumentException('Invalid key length.');
        }
    }

Member Function Documentation

decode()

ILIAS\Authentication\SessionReminder\TokenCipher::decode

(

string

$token

)

Returns

array{user_id: int, session_hash: string}|null NULL if the token is invalid

Definition at line 113 of file TokenCipher.php.

                                         : ?array
    {
        if (preg_match('/^[A-Za-z0-9_-]+$/', $token) !== 1) {
            return null;
        }
 
        $raw = base64_decode(strtr($token, '-_', '+/'), true);
        if ($raw === false || \strlen($raw) <= self::SALT_LENGTH + self::NONCE_LENGTH + self::TAG_LENGTH) {
            return null;
        }
 
        $salt = substr($raw, 0, self::SALT_LENGTH);
        $nonce = substr($raw, self::SALT_LENGTH, self::NONCE_LENGTH);
        $ciphertext = substr($raw, self::SALT_LENGTH + self::NONCE_LENGTH, -self::TAG_LENGTH);
        $tag = substr($raw, -self::TAG_LENGTH);
 
        // Fails if the token has been modified (authentication tag)
        $plaintext = openssl_decrypt(
            $ciphertext,
            self::CIPHER,
            $this->deriveSubkey($salt),
            OPENSSL_RAW_DATA,
            $nonce,
            $tag,
            self::CONTEXT
        );
        if ($plaintext === false) {
            return null;
        }
 
        $data = json_decode($plaintext, true, 512, JSON_THROW_ON_ERROR);
        if (!\is_int($data[self::USER_ID] ?? null) || !\is_string($data[self::SESSION_HASH] ?? null)) {
            return null;
        }
 
        return [
            'user_id' => $data[self::USER_ID],
            'session_hash' => $data[self::SESSION_HASH],
        ];
    }

References $data, $token, ILIAS\Authentication\SessionReminder\TokenCipher\deriveSubkey(), ILIAS\Authentication\SessionReminder\TokenCipher\SESSION_HASH, and ILIAS\Authentication\SessionReminder\TokenCipher\USER_ID.

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deriveSubkey()

ILIAS\Authentication\SessionReminder\TokenCipher::deriveSubkey ( string  $salt )

private

Definition at line 154 of file TokenCipher.php.

                                               : string
    {
        return hash_hkdf('sha256', $this->key, self::KEY_LENGTH, self::CONTEXT, $salt);
    }

Referenced by ILIAS\Authentication\SessionReminder\TokenCipher\decode(), and ILIAS\Authentication\SessionReminder\TokenCipher\encode().

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encode()

ILIAS\Authentication\SessionReminder\TokenCipher::encode	(	int	$user_id,
		string	$session_hash
	)

Definition at line 85 of file TokenCipher.php.

                                                              : string
    {
        $salt = random_bytes(self::SALT_LENGTH);
        $nonce = random_bytes(self::NONCE_LENGTH);
        $tag = '';
        $ciphertext = openssl_encrypt(
            json_encode([
                self::USER_ID => $user_id,
                self::SESSION_HASH => $session_hash,
            ], JSON_THROW_ON_ERROR),
            self::CIPHER,
            $this->deriveSubkey($salt),
            OPENSSL_RAW_DATA,
            $nonce,
            $tag,
            self::CONTEXT,
            self::TAG_LENGTH
        );
        if ($ciphertext === false) {
            throw new \RuntimeException('Could not encrypt the session reminder token.');
        }
 
        return rtrim(strtr(base64_encode($salt . $nonce . $ciphertext . $tag), '+/', '-_'), '=');
    }

References $user_id, and ILIAS\Authentication\SessionReminder\TokenCipher\deriveSubkey().

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fromKeyFile()

static ILIAS\Authentication\SessionReminder\TokenCipher::fromKeyFile ( )

static

Returns

self|null NULL if the key has not been created, yet (see: SessionReminderKeyCreatedObjective), or the cipher is not available

Definition at line 65 of file TokenCipher.php.

                                        : ?self
    {
        $path = SessionReminderKeyCreatedObjective::PATH();
        if (!\extension_loaded('openssl')
            || !is_readable($path)
            || !\in_array(self::CIPHER, openssl_get_cipher_methods(), true)) {
            return null;
        }
 
        $data = require $path;
        $key = \is_array($data) && \is_string($data[SessionReminderKeyCreatedObjective::KEY] ?? null)
            ? hex2bin($data[SessionReminderKeyCreatedObjective::KEY])
            : false;
        if ($key === false || \strlen($key) !== self::KEY_LENGTH) {
            return null;
        }
 
        return new self($key);
    }

References $data, ILIAS\Authentication\Setup\SessionReminderKeyCreatedObjective\KEY, and ILIAS\FileDelivery\Setup\BuildStaticConfigStoredObjective\PATH().

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Field Documentation

CIPHER

const string ILIAS\Authentication\SessionReminder\TokenCipher::CIPHER = 'aes-256-gcm'

private

Definition at line 38 of file TokenCipher.php.

CONTEXT

const string ILIAS\Authentication\SessionReminder\TokenCipher::CONTEXT = 'ilias_session_reminder_v1'

private

Binds the subkeys and the ciphertext to their purpose, a token of another context cannot be decrypted.

Definition at line 46 of file TokenCipher.php.

KEY_LENGTH

const int ILIAS\Authentication\SessionReminder\TokenCipher::KEY_LENGTH = 32

private

Definition at line 39 of file TokenCipher.php.

NONCE_LENGTH

const int ILIAS\Authentication\SessionReminder\TokenCipher::NONCE_LENGTH = 12

private

Definition at line 41 of file TokenCipher.php.

SALT_LENGTH

const int ILIAS\Authentication\SessionReminder\TokenCipher::SALT_LENGTH = 16

private

Definition at line 40 of file TokenCipher.php.

SESSION_HASH

const string ILIAS\Authentication\SessionReminder\TokenCipher::SESSION_HASH = 'h'

private

Definition at line 48 of file TokenCipher.php.

Referenced by ILIAS\Authentication\SessionReminder\TokenCipher\decode().

TAG_LENGTH

const int ILIAS\Authentication\SessionReminder\TokenCipher::TAG_LENGTH = 16

private

Definition at line 42 of file TokenCipher.php.

USER_ID

const string ILIAS\Authentication\SessionReminder\TokenCipher::USER_ID = 'u'

private

Definition at line 47 of file TokenCipher.php.

Referenced by ILIAS\Authentication\SessionReminder\TokenCipher\decode().

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The documentation for this class was generated from the following file:

• components/ILIAS/Authentication/src/SessionReminder/TokenCipher.php