commit eeb6956ed7f7ddd0f2592c17f4a5d58a580fb878 Author: Isis Lovecruft isis@torproject.org Date: Tue Mar 11 20:55:05 2014 +0000

Change captcha.GimpCaptcha check() and get() to use HMAC(encrypt(answer)).

This changes the way that ``captcha.GimpCaptcha``s work to use an asymmetric keypair to encrypt the CAPTCHA answer. Then, it creates an HMAC of that encrypted result, using an HMAC key which is derived from the client's IP address and a master HMAC key used for CAPTCHA challenge string verification.

TODO: We should possibly add a timestamp into the HMACed data so that we can timeout requests, and also to disallow adversarial bots which solved one CAPTCHA a year ago from reusing it over and over again. --- lib/bridgedb/captcha.py | 113 ++++++++++++++++++++++++++++++++++++----------- 1 file changed, 88 insertions(+), 25 deletions(-)

diff --git a/lib/bridgedb/captcha.py b/lib/bridgedb/captcha.py index 721f72e..73ed70f 100644 --- a/lib/bridgedb/captcha.py +++ b/lib/bridgedb/captcha.py @@ -32,7 +32,9 @@ gimp-captcha_ and then cached locally. .. _gimp-captcha: https://github.com/isislovecruft/gimp-captcha """

-import hashlib +from base64 import urlsafe_b64encode +from base64 import urlsafe_b64decode + import logging import random import os @@ -42,6 +44,8 @@ from BeautifulSoup import BeautifulSoup from recaptcha.client.captcha import API_SSL_SERVER from zope.interface import Interface, Attribute, implements

+from bridgedb import crypto +

class ReCaptchaKeyError(Exception): """Exception raised when recaptcha API keys are not supplied.""" @@ -49,6 +53,9 @@ class ReCaptchaKeyError(Exception): class GimpCaptchaError(Exception): """General exception raised when a Gimp CAPTCHA cannot be retrieved."""

+class GimpCaptchaKeyError(ValueError): + """Raised when there is a problem with one of the Gimp CAPTCHA keys.""" +

class ICaptcha(Interface): """Interface specification for CAPTCHAs.""" @@ -128,42 +135,96 @@ class ReCaptcha(Captcha): class GimpCaptcha(Captcha): """A cached CAPTCHA image which was created with Gimp."""

- def __init__(self, cacheDir=None, clientIP=None): + def __init__(self, secretKey=None, publicKey=None, hmacKey=None, + cacheDir=None): """Create a ``GimpCaptcha`` which retrieves images from **cacheDir**.

+ :param str secretkey: A PKCS#1 OAEP-padded, private RSA key, used for + verifying the client's solution to the CAPTCHA. + :param str publickey: A PKCS#1 OAEP-padded, public RSA key, used for + creating the ``captcha_challenge_field`` string to give to a + client. + :param bytes hmacKey: A client-specific HMAC secret key. + :param str cacheDir: The local directory which pre-generated CAPTCHA + images have been stored in. This can be set via the + ``GIMP_CAPTCHA_DIR`` setting in the config file. :raises GimpCaptchaError: if **cacheDir** is not a directory. + :raises GimpCaptchaKeyError: if any of **secretKey**, **publicKey**, + or **hmacKey** is invalid, or missing. """ super(GimpCaptcha, self).__init__() - if not os.path.isdir(cacheDir): + + if not cacheDir or not os.path.isdir(cacheDir): raise GimpCaptchaError("Gimp captcha cache isn't a directory: %r" % cacheDir) + if not (publicKey and secretKey and hmacKey): + raise GimpCaptchaKeyError( + "Invalid key supplied to GimpCaptcha: SK=%r PK=%r HMAC=%r" + % (secretKey, publicKey, hmacKey)) + + self.secretKey = secretKey + self.publicKey = publicKey self.cacheDir = cacheDir - self.clientIP = clientIP + self.hmacKey = hmacKey + self.answer = None

@classmethod - def check(cls, challenge, answer, clientIP=None): - """Check a client's CAPTCHA solution against the **challenge**. - + def check(cls, challenge, solution, secretKey, hmacKey): + """Check a client's CAPTCHA **solution** against the **challenge**. + + :param str challenge: The contents of the + ``'captcha_challenge_field'`` HTTP form field. + :param str solution: The client's proposed solution to the CAPTCHA + that they were presented with. + :param str secretkey: A PKCS#1 OAEP-padded, private RSA key, used for + verifying the client's solution to the CAPTCHA. + :param bytes hmacKey: A private key for generating HMACs. :rtype: bool :returns: True if the CAPTCHA solution was correct. """ + validHMAC = False + + if not solution: + return validHMAC + logging.debug("Checking CAPTCHA solution %r against challenge %r" - % (answer, challenge)) - solution = cls.createChallenge(answer, clientIP) - if (not challenge) or (challenge != solution): + % (solution, challenge)) + try: + decoded = urlsafe_b64decode(challenge) + hmac, original = decoded.split(';', 1) + verified = crypto.getHMAC(hmacKey, original) + validHMAC = verified == hmac + except Exception as error: + logging.exception(error) + finally: + if validHMAC: + decrypted = secretKey.decrypt(original) + if solution == decrypted: + return True return False - return True

- @classmethod - def createChallenge(cls, answer, clientIP=None): - """Hash a CAPTCHA answer together with a **clientIP**, if given. + def createChallenge(self, answer): + """Encrypt the CAPTCHA **answer** and HMAC the encrypted data. + + Take a string containing the answer to a CAPTCHA and encrypts it to + :attr:`publicKey`. The resulting encrypted blob is then HMACed with a + client-specific :attr:`hmacKey`. These two strings are then joined + together in the form:

- :param str answer: The answer (either actual, or a client's proposed - solution) to a CAPTCHA. - :param str clientIP: The client's IP address. + HMAC ";" ENCRYPTED_ANSWER + + and lastly base64-encoded (in a URL safe manner). + + :param str answer: The answer to a CAPTCHA. + :rtype: str + :returns: An HMAC of, as well as a string containing the URL-safe, + base64-encoded encrypted **answer**. """ - challenge = '\n'.join([answer, str(clientIP)]) - return hashlib.sha256(challenge).hexdigest() + encrypted = self.publicKey.encrypt(answer) + hmac = crypto.getHMAC(self.hmacKey, encrypted) + challenge = hmac + ';' + encrypted + encoded = urlsafe_b64encode(challenge) + return encoded

def get(self): """Get a random CAPTCHA from the cache directory. @@ -173,17 +234,19 @@ class GimpCaptcha(Captcha): :returns: A 2-tuple of ``(captcha, None)``, where ``captcha`` is the image file contents. """ - imageFilename = random.choice(os.listdir(self.cacheDir)) - imagePath = os.path.join(self.cacheDir, imageFilename) - try: + imageFilename = random.choice(os.listdir(self.cacheDir)) + imagePath = os.path.join(self.cacheDir, imageFilename) with open(imagePath) as imageFile: self.image = imageFile.read() - except (OSError, IOError) as err: + except IndexError: + raise GimpCaptchaError("CAPTCHA cache dir appears empty: %r" + % self.cacheDir) + except (OSError, IOError): raise GimpCaptchaError("Could not read Gimp captcha image file: %r" % imageFilename)

- captchaAnswer = imageFilename.rsplit(os.path.extsep, 1)[0] - self.challenge = self.createChallenge(captchaAnswer, self.clientIP) + self.answer = imageFilename.rsplit(os.path.extsep, 1)[0] + self.challenge = self.createChallenge(self.answer)

return (self.image, self.challenge)