add support for update_into on CipherContext (#3190)

* add support for update_into on CipherContext

This allows you to provide your own buffer (like recv_into) to improve
performance when repeatedly calling encrypt/decrypt on large payloads.

* another skip_if

* more skip_if complexity

* maybe do this right

* correct number of args

* coverage for the coverage gods

* add a cffi minimum test tox target and travis builder

This tests against macOS so we capture some commoncrypto branches

* extra arg

* need to actually install py35

* fix

* coverage for GCM decrypt in CC

* no longer relevant

* 1.8 now

* pep8

* dramatically simplify

* update docs

* remove unneeded test

* changelog entry

* test improvements

* coverage fix

* add some comments to example

* move the comments to their own line

* fix and move comment

4 files changed, 99 insertions, 2 deletions

diff --git a/src/cryptography/hazmat/backends/commoncrypto/ciphers.py b/src/cryptography/hazmat/backends/commoncrypto/ciphers.py
index 1ce8aec5..b59381cb 100644
--- a/src/cryptography/hazmat/backends/commoncrypto/ciphers.py
+++ b/src/cryptography/hazmat/backends/commoncrypto/ciphers.py
@@ -86,6 +86,24 @@ class _CipherContext(object):
         self._backend._check_cipher_response(res)
         return self._backend._ffi.buffer(buf)[:outlen[0]]
 
+    def update_into(self, data, buf):
+        if len(buf) < (len(data) + self._byte_block_size - 1):
+            raise ValueError(
+                "buffer must be at least {0} bytes for this "
+                "payload".format(len(data) + self._byte_block_size - 1)
+            )
+        # Count bytes processed to handle block alignment.
+        self._bytes_processed += len(data)
+        outlen = self._backend._ffi.new("size_t *")
+        buf = self._backend._ffi.cast(
+            "unsigned char *", self._backend._ffi.from_buffer(buf)
+        )
+        res = self._backend._lib.CCCryptorUpdate(
+            self._ctx[0], data, len(data), buf,
+            len(data) + self._byte_block_size - 1, outlen)
+        self._backend._check_cipher_response(res)
+        return outlen[0]
+
     def finalize(self):
         # Raise error if block alignment is wrong.
         if self._bytes_processed % self._byte_block_size:
@@ -161,6 +179,24 @@ class _GCMCipherContext(object):
         self._backend._check_cipher_response(res)
         return self._backend._ffi.buffer(buf)[:]
 
+    def update_into(self, data, buf):
+        if len(buf) < len(data):
+            raise ValueError(
+                "buffer must be at least {0} bytes".format(len(data))
+            )
+
+        buf = self._backend._ffi.cast(
+            "unsigned char *", self._backend._ffi.from_buffer(buf)
+        )
+        args = (self._ctx[0], data, len(data), buf)
+        if self._operation == self._backend._lib.kCCEncrypt:
+            res = self._backend._lib.CCCryptorGCMEncrypt(*args)
+        else:
+            res = self._backend._lib.CCCryptorGCMDecrypt(*args)
+
+        self._backend._check_cipher_response(res)
+        return len(data)
+
     def finalize(self):
         # CommonCrypto has a yet another bug where you must make at least one
         # call to update. If you pass just AAD and call finalize without a call
diff --git a/src/cryptography/hazmat/backends/openssl/ciphers.py b/src/cryptography/hazmat/backends/openssl/ciphers.py
index 898b3497..0e0918af 100644
--- a/src/cryptography/hazmat/backends/openssl/ciphers.py
+++ b/src/cryptography/hazmat/backends/openssl/ciphers.py
@@ -109,6 +109,22 @@ class _CipherContext(object):
         self._backend.openssl_assert(res != 0)
         return self._backend._ffi.buffer(buf)[:outlen[0]]
 
+    def update_into(self, data, buf):
+        if len(buf) < (len(data) + self._block_size_bytes - 1):
+            raise ValueError(
+                "buffer must be at least {0} bytes for this "
+                "payload".format(len(data) + self._block_size_bytes - 1)
+            )
+
+        buf = self._backend._ffi.cast(
+            "unsigned char *", self._backend._ffi.from_buffer(buf)
+        )
+        outlen = self._backend._ffi.new("int *")
+        res = self._backend._lib.EVP_CipherUpdate(self._ctx, buf, outlen,
+                                                  data, len(data))
+        self._backend.openssl_assert(res != 0)
+        return outlen[0]
+
     def finalize(self):
         # OpenSSL 1.0.1 on Ubuntu 12.04 (and possibly other distributions)
         # appears to have a bug where you must make at least one call to update
diff --git a/src/cryptography/hazmat/primitives/ciphers/base.py b/src/cryptography/hazmat/primitives/ciphers/base.py
index 496975ae..502d9804 100644
--- a/src/cryptography/hazmat/primitives/ciphers/base.py
+++ b/src/cryptography/hazmat/primitives/ciphers/base.py
@@ -6,6 +6,8 @@ from __future__ import absolute_import, division, print_function
 
 import abc
 
+import cffi
+
 import six
 
 from cryptography import utils
@@ -51,6 +53,13 @@ class CipherContext(object):
         """
 
     @abc.abstractmethod
+    def update_into(self, data, buf):
+        """
+        Processes the provided bytes and writes the resulting data into the
+        provided buffer. Returns the number of bytes written.
+        """
+
+    @abc.abstractmethod
     def finalize(self):
         """
         Returns the results of processing the final block as bytes.
@@ -136,6 +145,20 @@ class _CipherContext(object):
             raise AlreadyFinalized("Context was already finalized.")
         return self._ctx.update(data)
 
+    # cffi 1.7 supports from_buffer on bytearray, which is required. We can
+    # remove this check in the future when we raise our minimum PyPy version.
+    if utils._version_check(cffi.__version__, "1.7"):
+        def update_into(self, data, buf):
+            if self._ctx is None:
+                raise AlreadyFinalized("Context was already finalized.")
+            return self._ctx.update_into(data, buf)
+    else:
+        def update_into(self, data, buf):
+            raise NotImplementedError(
+                "update_into requires cffi 1.7+. To use this method please "
+                "update cffi."
+            )
+
     def finalize(self):
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
@@ -154,11 +177,11 @@ class _AEADCipherContext(object):
         self._tag = None
         self._updated = False
 
-    def update(self, data):
+    def _check_limit(self, data_size):
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
         self._updated = True
-        self._bytes_processed += len(data)
+        self._bytes_processed += data_size
         if self._bytes_processed > self._ctx._mode._MAX_ENCRYPTED_BYTES:
             raise ValueError(
                 "{0} has a maximum encrypted byte limit of {1}".format(
@@ -166,8 +189,23 @@ class _AEADCipherContext(object):
                 )
             )
 
+    def update(self, data):
+        self._check_limit(len(data))
         return self._ctx.update(data)
 
+    # cffi 1.7 supports from_buffer on bytearray, which is required. We can
+    # remove this check in the future when we raise our minimum PyPy version.
+    if utils._version_check(cffi.__version__, "1.7"):
+        def update_into(self, data, buf):
+            self._check_limit(len(data))
+            return self._ctx.update_into(data, buf)
+    else:
+        def update_into(self, data, buf):
+            raise NotImplementedError(
+                "update_into requires cffi 1.7+. To use this method please "
+                "update cffi."
+            )
+
     def finalize(self):
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
diff --git a/src/cryptography/utils.py b/src/cryptography/utils.py
index f16b7efa..8183bdaf 100644
--- a/src/cryptography/utils.py
+++ b/src/cryptography/utils.py
@@ -10,6 +10,8 @@ import inspect
 import sys
 import warnings
 
+from packaging.version import parse
+
 
 # the functions deprecated in 1.0 and 1.4 are on an arbitrarily extended
 # deprecation cycle and should not be removed until we agree on when that cycle
@@ -98,6 +100,11 @@ else:
         return len(bin(x)) - (2 + (x <= 0))
 
 
+def _version_check(version, required_version):
+    # This is used to check if we support update_into on CipherContext.
+    return parse(version) >= parse(required_version)
+
+
 class _DeprecatedValue(object):
     def __init__(self, value, message, warning_class):
         self.value = value