Added support for pure Python RSA library when present.

crypt.py

@@ -244,16 +244,86 @@ def from_string(key, password='notasecret'):
   PyCryptoVerifier = None
   PyCryptoSigner = None
 
+# Use pure Python RSA module if installed.
+# Dependencies:
+# For signing only: easy_install rsa
+# For X509 verifications: easy_install rsa pyasn1_modules
+try:
+  import rsa
+
+  def _BitStringToByteString(bs):
+    def BitsToInt(bits):
+      return sum(v * (2 ** (7 - j)) for j, v in enumerate(bits))
+    return str(bytearray([BitsToInt(bs[i:i + 8]) for i in range(0, len(bs), 8)]))
+
+  # Equivalent class to OpenSSLVerifier above
+  class RsaVerifier(object):
+    def __init__(self, pubkey):
+      self._pubkey = pubkey
+
+    def verify(self, message, signature):
+      try:
+        return rsa.pkcs1.verify(message, signature, self._pubkey)
+      except rsa.pkcs1.VerificationError:
+        return False
+
+    @staticmethod
+    def from_string(key_pem, is_x509_cert):
+      if is_x509_cert:
+        try:
+          from pyasn1.codec.der import decoder
+          from pyasn1_modules.rfc2459 import Certificate
+
+          der = rsa.pem.load_pem(key_pem, 'CERTIFICATE')
+          asn1_cert, _ = decoder.decode(der, asn1Spec=Certificate())
+
+          pubkey = rsa.PublicKey.load_pkcs1(_BitStringToByteString(
+              asn1_cert['tbsCertificate']['subjectPublicKeyInfo']['subjectPublicKey']), 'DER')
+        except ImportError:
+          raise NotImplementedError(
+              'Please install pyasn1 and pyasn1_modules to parse x509.')
+      else:
+        pubkey = rsa.PublicKey.load_pkcs1(key_pem, 'PEM')
+      return RsaVerifier(pubkey)
+
+
+  # Equivalent class to OpenSSLSigner above
+  class RsaSigner(object):
+    def __init__(self, pkey):
+       self._key = pkey
+
+    def sign(self, message):
+      return rsa.pkcs1.sign(message, self._key, 'SHA-256')
+
+    @staticmethod
+    def from_string(key, password='notasecret'):
+      if key.startswith('-----BEGIN '):
+        pkey = rsa.key.PrivateKey.load_pkcs1(key, 'PEM')
+      else:
+        raise NotImplementedError(
+            'PKCS12 format is not supported by the RSA library. '
+            'Either install PyOpenSSL, or '
+            'please convert .p12 format to .pem format: \n'
+            'cat key.p12 | openssl pkcs12 -nodes -nocerts -passin '
+            'pass:notasecret | openssl rsa > key.pem')
+      return RsaSigner(pkey)
+
+except ImportError:
+  RsaSigner = None
+  RsaVerifier = None
 
-if OpenSSLSigner:
+if OpenSSLSigner:  # Supports PEM, X509, PKCS12
   Signer = OpenSSLSigner
   Verifier = OpenSSLVerifier
-elif PyCryptoSigner:
+elif RsaSigner:  # Supports PEM, X509
+  Signer = RsaSigner
+  Verifier = RsaVerifier
+elif PyCryptoSigner:  # Supports PEM
   Signer = PyCryptoSigner
   Verifier = PyCryptoVerifier
 else:
   raise ImportError('No encryption library found. Please install either '
-                    'PyOpenSSL, or PyCrypto 2.6 or later')
+                    'PyOpenSSL, PyCrypto 2.6 or later, or rsa.')
 
 
 def _urlsafe_b64encode(raw_bytes):