Cache pub/pri keys on retrieval

Pyca rightfully performs consistency checks when importing keys and
these operations are rather expensive. So cache keys once generated so
that repeated uses of the same JWK do not incur undue cost of reloading
the keys from scratch for each subsequent operation.

with a simple test by hand:
$ python
>>> from jwcrypto import jwk
>>> def test():
...     key = jwk.JWK.generate(kty='RSA', size=2048)
...     for i in range(1000):
...             k = key._get_private_key()
...
>>> import timeit

Before the patch:
>>> print(timeit.timeit("test()", setup="from __main__ import test", number=10))
35.80328264506534

After the patch:
>>> print(timeit.timeit("test()", setup="from __main__ import test", number=10))
0.9109518649056554

Resolves #243

Signed-off-by: Simo Sorce <simo@redhat.com>

jwcrypto/jwk.py

@@ -301,6 +301,8 @@ def __init__(self, **kwargs):
             are provided.
         """
         super(JWK, self).__init__()
+        self._cache_pub_k = None
+        self._cache_pri_k = None
 
         if 'generate' in kwargs:
             self.generate_key(**kwargs)
@@ -485,6 +487,8 @@ def _import_pyca_pub_okp(self, key, **params):
     def import_key(self, **kwargs):
         newkey = {}
         key_vals = 0
+        self._cache_pub_k = None
+        self._cache_pri_k = None
 
         names = list(kwargs.keys())
 
@@ -730,57 +734,93 @@ def _check_constraints(self, usage, operation):
     def _decode_int(self, n):
         return int(hexlify(base64url_decode(n)), 16)
 
-    def _rsa_pub(self):
+    def _rsa_pub_n(self):
         e = self._decode_int(self.get('e'))
         n = self._decode_int(self.get('n'))
         return rsa.RSAPublicNumbers(e, n)
 
-    def _rsa_pri(self):
+    def _rsa_pri_n(self):
         p = self._decode_int(self.get('p'))
         q = self._decode_int(self.get('q'))
         d = self._decode_int(self.get('d'))
         dp = self._decode_int(self.get('dp'))
         dq = self._decode_int(self.get('dq'))
         qi = self._decode_int(self.get('qi'))
-        return rsa.RSAPrivateNumbers(p, q, d, dp, dq, qi, self._rsa_pub())
+        return rsa.RSAPrivateNumbers(p, q, d, dp, dq, qi, self._rsa_pub_n())
 
-    def _ec_pub(self, curve):
+    def _rsa_pub(self):
+        k = self._cache_pub_k
+        if k is None:
+            k = self._rsa_pub_n().public_key(default_backend())
+            self._cache_pub_k = k
+        return k
+
+    def _rsa_pri(self):
+        k = self._cache_pri_k
+        if k is None:
+            k = self._rsa_pri_n().private_key(default_backend())
+            self._cache_pri_k = k
+        return k
+
+    def _ec_pub_n(self, curve):
         x = self._decode_int(self.get('x'))
         y = self._decode_int(self.get('y'))
         return ec.EllipticCurvePublicNumbers(x, y, self.get_curve(curve))
 
-    def _ec_pri(self, curve):
+    def _ec_pri_n(self, curve):
         d = self._decode_int(self.get('d'))
-        return ec.EllipticCurvePrivateNumbers(d, self._ec_pub(curve))
+        return ec.EllipticCurvePrivateNumbers(d, self._ec_pub_n(curve))
+
+    def _ec_pub(self, curve):
+        k = self._cache_pub_k
+        if k is None:
+            k = self._ec_pub_n(curve).public_key(default_backend())
+            self._cache_pub_k = k
+        return k
+
+    def _ec_pri(self, curve):
+        k = self._cache_pri_k
+        if k is None:
+            k = self._ec_pri_n(curve).private_key(default_backend())
+            self._cache_pri_k = k
+        return k
 
     def _okp_pub(self):
-        crv = self.get('crv')
-        try:
-            pubkey = _OKP_CURVES_TABLE[crv].pubkey
-        except KeyError as e:
-            raise InvalidJWKValue('Unknown curve "%s"' % crv) from e
+        k = self._cache_pub_k
+        if k is None:
+            crv = self.get('crv')
+            try:
+                pubkey = _OKP_CURVES_TABLE[crv].pubkey
+            except KeyError as e:
+                raise InvalidJWKValue('Unknown curve "%s"' % crv) from e
 
-        x = base64url_decode(self.get('x'))
-        return pubkey.from_public_bytes(x)
+            x = base64url_decode(self.get('x'))
+            k = pubkey.from_public_bytes(x)
+            self._cache_pub_k = k
+        return k
 
     def _okp_pri(self):
-        crv = self.get('crv')
-        try:
-            privkey = _OKP_CURVES_TABLE[crv].privkey
-        except KeyError as e:
-            raise InvalidJWKValue('Unknown curve "%s"' % crv) from e
+        k = self._cache_pri_k
+        if k is None:
+            crv = self.get('crv')
+            try:
+                privkey = _OKP_CURVES_TABLE[crv].privkey
+            except KeyError as e:
+                raise InvalidJWKValue('Unknown curve "%s"' % crv) from e
 
-        d = base64url_decode(self.get('d'))
-        return privkey.from_private_bytes(d)
+            d = base64url_decode(self.get('d'))
+            k = privkey.from_private_bytes(d)
+            self._cache_pri_k = k
+        return k
 
     def _get_public_key(self, arg=None):
         ktype = self.get('kty')
         if ktype == 'oct':
             return self.get('k')
         elif ktype == 'RSA':
-            return self._rsa_pub().public_key(default_backend())
+            return self._rsa_pub()
         elif ktype == 'EC':
-            return self._ec_pub(arg).public_key(default_backend())
+            return self._ec_pub(arg)
         elif ktype == 'OKP':
             return self._okp_pub()
         else:
@@ -791,9 +831,9 @@ def _get_private_key(self, arg=None):
         if ktype == 'oct':
             return self.get('k')
         elif ktype == 'RSA':
-            return self._rsa_pri().private_key(default_backend())
+            return self._rsa_pri()
         elif ktype == 'EC':
-            return self._ec_pri(arg).private_key(default_backend())
+            return self._ec_pri(arg)
         elif ktype == 'OKP':
             return self._okp_pri()
         else:
@@ -969,6 +1009,9 @@ def __setitem__(self, item, value):
 
         # Check if item is a key value and verify its format
         if item in list(JWKValuesRegistry[kty].keys()):
+            # Invalidate cached keys if any
+            self._cache_pub_k = None
+            self._cache_pri_k = None
             if JWKValuesRegistry[kty][item].type == ParmType.b64:
                 try:
                     v = base64url_decode(value)
@@ -1028,6 +1071,12 @@ def __delitem__(self, item):
                 if self.get(name) is not None:
                     raise KeyError("Cannot remove 'kty', values present")
 
+        kty = self.get('kty')
+        if kty is not None and item in list(JWKValuesRegistry[kty].keys()):
+            # Invalidate cached keys if any
+            self._cache_pub_k = None
+            self._cache_pri_k = None
+
         super(JWK, self).__delitem__(item)
 
     def __eq__(self, other):