add ChaCha20 support (#3919)

* add ChaCha20 support

* review feedback

* 256 divided by 8 is what again?

* ...

3 files changed, 32 insertions, 1 deletions

diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index ede35ec0..2cbfca2c 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -58,7 +58,7 @@ from cryptography.hazmat.primitives.asymmetric.padding import (
     MGF1, OAEP, PKCS1v15, PSS
 )
 from cryptography.hazmat.primitives.ciphers.algorithms import (
-    AES, ARC4, Blowfish, CAST5, Camellia, IDEA, SEED, TripleDES
+    AES, ARC4, Blowfish, CAST5, Camellia, ChaCha20, IDEA, SEED, TripleDES
 )
 from cryptography.hazmat.primitives.ciphers.modes import (
     CBC, CFB, CFB8, CTR, ECB, GCM, OFB
@@ -258,6 +258,11 @@ class Backend(object):
             type(None),
             GetCipherByName("rc4")
         )
+        self.register_cipher_adapter(
+            ChaCha20,
+            type(None),
+            GetCipherByName("chacha20")
+        )
 
     def create_symmetric_encryption_ctx(self, cipher, mode):
         return _CipherContext(self, cipher, mode, _CipherContext._ENCRYPT)
diff --git a/src/cryptography/hazmat/backends/openssl/ciphers.py b/src/cryptography/hazmat/backends/openssl/ciphers.py
index e141e8ec..dfb33a07 100644
--- a/src/cryptography/hazmat/backends/openssl/ciphers.py
+++ b/src/cryptography/hazmat/backends/openssl/ciphers.py
@@ -59,6 +59,8 @@ class _CipherContext(object):
             iv_nonce = mode.initialization_vector
         elif isinstance(mode, modes.ModeWithNonce):
             iv_nonce = mode.nonce
+        elif isinstance(cipher, modes.ModeWithNonce):
+            iv_nonce = cipher.nonce
         else:
             iv_nonce = self._backend._ffi.NULL
         # begin init with cipher and operation type
diff --git a/src/cryptography/hazmat/primitives/ciphers/algorithms.py b/src/cryptography/hazmat/primitives/ciphers/algorithms.py
index c193f797..6e5eb313 100644
--- a/src/cryptography/hazmat/primitives/ciphers/algorithms.py
+++ b/src/cryptography/hazmat/primitives/ciphers/algorithms.py
@@ -8,6 +8,7 @@ from cryptography import utils
 from cryptography.hazmat.primitives.ciphers import (
     BlockCipherAlgorithm, CipherAlgorithm
 )
+from cryptography.hazmat.primitives.ciphers.modes import ModeWithNonce
 
 
 def _verify_key_size(algorithm, key):
@@ -138,3 +139,26 @@ class SEED(object):
     @property
     def key_size(self):
         return len(self.key) * 8
+
+
+@utils.register_interface(CipherAlgorithm)
+@utils.register_interface(ModeWithNonce)
+class ChaCha20(object):
+    name = "ChaCha20"
+    key_sizes = frozenset([256])
+
+    def __init__(self, key, nonce):
+        self.key = _verify_key_size(self, key)
+        if not isinstance(nonce, bytes):
+            raise TypeError("nonce must be bytes")
+
+        if len(nonce) != 16:
+            raise ValueError("nonce must be 128-bits (16 bytes)")
+
+        self._nonce = nonce
+
+    nonce = utils.read_only_property("_nonce")
+
+    @property
+    def key_size(self):
+        return len(self.key) * 8