diff --git a/oauth2client/crypt.py b/oauth2client/crypt.py
new file mode 100644
index 0000000..523c921
--- /dev/null
+++ b/oauth2client/crypt.py
@@ -0,0 +1,251 @@
+#!/usr/bin/python2.4
+# -*- coding: utf-8 -*-
+#
+# Copyright (C) 2011 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import base64
+import hashlib
+import logging
+import time
+
+from OpenSSL import crypto
+
+try:  # pragma: no cover
+  import simplejson
+except ImportError:  # pragma: no cover
+  try:
+    # Try to import from django, should work on App Engine
+    from django.utils import simplejson
+  except ImportError:
+    # Should work for Python2.6 and higher.
+    import json as simplejson
+
+
+CLOCK_SKEW_SECS = 300  # 5 minutes in seconds
+AUTH_TOKEN_LIFETIME_SECS = 300  # 5 minutes in seconds
+MAX_TOKEN_LIFETIME_SECS = 86400  # 1 day in seconds
+
+
+class AppIdentityError(Exception):
+  pass
+
+
+class Verifier(object):
+  """Verifies the signature on a message."""
+
+  def __init__(self, pubkey):
+    """Constructor.
+
+    Args:
+      pubkey, OpenSSL.crypto.PKey, The public key to verify with.
+    """
+    self._pubkey = pubkey
+
+  def verify(self, message, signature):
+    """Verifies a message against a signature.
+
+    Args:
+      message: string, The message to verify.
+      signature: string, The signature on the message.
+
+    Returns:
+      True if message was singed by the private key associated with the public
+      key that this object was constructed with.
+    """
+    try:
+      crypto.verify(self._pubkey, signature, message, 'sha256')
+      return True
+    except:
+      return False
+
+  @staticmethod
+  def from_string(key_pem, is_x509_cert):
+    """Construct a Verified instance from a string.
+
+    Args:
+      key_pem: string, public key in PEM format.
+      is_x509_cert: bool, True if key_pem is an X509 cert, otherwise it is
+        expected to be an RSA key in PEM format.
+
+    Returns:
+      Verifier instance.
+
+    Raises:
+      OpenSSL.crypto.Error if the key_pem can't be parsed.
+    """
+    if is_x509_cert:
+      pubkey = crypto.load_certificate(crypto.FILETYPE_PEM, key_pem)
+    else:
+      pubkey = crypto.load_privatekey(crypto.FILETYPE_PEM, key_pem)
+    return Verifier(pubkey)
+
+
+class Signer(object):
+  """Signs messages with a private key."""
+
+  def __init__(self, pkey):
+    """Constructor.
+
+    Args:
+      pkey, OpenSSL.crypto.PKey, The private key to sign with.
+    """
+    self._key = pkey
+
+  def sign(self, message):
+    """Signs a message.
+
+    Args:
+      message: string, Message to be signed.
+
+    Returns:
+      string, The signature of the message for the given key.
+    """
+    return crypto.sign(self._key, message, 'sha256')
+
+  @staticmethod
+  def from_string(key, password='notasecret'):
+    """Construct a Signer instance from a string.
+
+    Args:
+      key: string, private key in P12 format.
+      password: string, password for the private key file.
+
+    Returns:
+      Signer instance.
+
+    Raises:
+      OpenSSL.crypto.Error if the key can't be parsed.
+    """
+    pkey = crypto.load_pkcs12(key, password).get_privatekey()
+    return Signer(pkey)
+
+
+def _urlsafe_b64encode(raw_bytes):
+  return base64.urlsafe_b64encode(raw_bytes).rstrip('=')
+
+
+def _urlsafe_b64decode(b64string):
+  padded = b64string + '=' * (4 - len(b64string) % 4)
+  return base64.urlsafe_b64decode(padded)
+
+
+def _json_encode(data):
+  return simplejson.dumps(data, separators = (',', ':'))
+
+
+def make_signed_jwt(signer, payload):
+  """Make a signed JWT.
+
+  See http://self-issued.info/docs/draft-jones-json-web-token.html.
+
+  Args:
+    signer: crypt.Signer, Cryptographic signer.
+    payload: dict, Dictionary of data to convert to JSON and then sign.
+
+  Returns:
+    string, The JWT for the payload.
+  """
+  header = {'typ': 'JWT', 'alg': 'RS256'}
+
+  segments = [
+          _urlsafe_b64encode(_json_encode(header)),
+          _urlsafe_b64encode(_json_encode(payload)),
+  ]
+  signing_input = '.'.join(segments)
+
+  signature = signer.sign(signing_input)
+  segments.append(_urlsafe_b64encode(signature))
+
+  logging.debug(str(segments))
+
+  return '.'.join(segments)
+
+
+def verify_signed_jwt_with_certs(jwt, certs, audience):
+  """Verify a JWT against public certs.
+
+  See http://self-issued.info/docs/draft-jones-json-web-token.html.
+
+  Args:
+    jwt: string, A JWT.
+    certs: dict, Dictionary where values of public keys in PEM format.
+    audience: string, The audience, 'aud', that this JWT should contain. If
+      None then the JWT's 'aud' parameter is not verified.
+
+  Returns:
+    dict, The deserialized JSON payload in the JWT.
+
+  Raises:
+    AppIdentityError if any checks are failed.
+  """
+  segments = jwt.split('.')
+
+  if (len(segments) != 3):
+    raise AppIdentityError(
+      'Wrong number of segments in token: %s' % jwt)
+  signed = '%s.%s' % (segments[0], segments[1])
+
+  signature = _urlsafe_b64decode(segments[2])
+
+  # Parse token.
+  json_body = _urlsafe_b64decode(segments[1])
+  try:
+    parsed = simplejson.loads(json_body)
+  except:
+    raise AppIdentityError('Can\'t parse token: %s' % json_body)
+
+  # Check signature.
+  verified = False
+  for (keyname, pem) in certs.items():
+    verifier = Verifier.from_string(pem, True)
+    if (verifier.verify(signed, signature)):
+      verified = True
+      break
+  if not verified:
+    raise AppIdentityError('Invalid token signature: %s' % jwt)
+
+  # Check creation timestamp.
+  iat = parsed.get('iat')
+  if iat is None:
+    raise AppIdentityError('No iat field in token: %s' % json_body)
+  earliest = iat - CLOCK_SKEW_SECS
+
+  # Check expiration timestamp.
+  now = long(time.time())
+  exp = parsed.get('exp')
+  if exp is None:
+    raise AppIdentityError('No exp field in token: %s' % json_body)
+  if exp >= now + MAX_TOKEN_LIFETIME_SECS:
+    raise AppIdentityError(
+      'exp field too far in future: %s' % json_body)
+  latest = exp + CLOCK_SKEW_SECS
+
+  if now < earliest:
+    raise AppIdentityError('Token used too early, %d < %d: %s' %
+      (now, earliest, json_body))
+  if now > latest:
+    raise AppIdentityError('Token used too late, %d > %d: %s' %
+      (now, latest, json_body))
+
+  # Check audience.
+  if audience is not None:
+    aud = parsed.get('aud')
+    if aud is None:
+      raise AppIdentityError('No aud field in token: %s' % json_body)
+    if aud != audience:
+      raise AppIdentityError('Wrong recipient, %s != %s: %s' %
+          (aud, audience, json_body))
+
+  return parsed