Add support for ChaCha20 in LibreSSL

CHANGELOG.rst

@@ -38,7 +38,9 @@ Changelog
   :meth:`~cryptography.x509.CertificateRevocationList.next_update`,
   :meth:`~cryptography.x509.CertificateRevocationList.last_update`
   in favor of the new timezone-aware variants mentioned above.
-
+* Added support for
+  :class:`~cryptography.hazmat.primitives.ciphers.algorithms.ChaCha20`
+  on LibreSSL.
 
 .. _v41-0-4:
 

src/cryptography/hazmat/backends/openssl/backend.py

@@ -264,8 +264,14 @@ def _register_default_ciphers(self) -> None:
         self.register_cipher_adapter(
             TripleDES, ECB, GetCipherByName("des-ede3")
         )
+        # ChaCha20 uses the Short Name "chacha20" in OpenSSL, but in LibreSSL
+        # it uses "chacha"
         self.register_cipher_adapter(
-            ChaCha20, type(None), GetCipherByName("chacha20")
+            ChaCha20,
+            type(None),
+            GetCipherByName(
+                "chacha" if self._lib.CRYPTOGRAPHY_IS_LIBRESSL else "chacha20"
+            ),
         )
         self.register_cipher_adapter(AES, XTS, _get_xts_cipher)
         for mode_cls in [ECB, CBC, OFB, CFB, CTR]:

tests/hazmat/primitives/test_chacha20.py

@@ -69,3 +69,24 @@ def test_invalid_nonce(self):
     def test_invalid_key_type(self):
         with pytest.raises(TypeError, match="key must be bytes"):
             algorithms.ChaCha20("0" * 32, b"0" * 16)  # type:ignore[arg-type]
+
+    def test_partial_blocks(self, backend):
+        # Test that partial blocks and counter increments are handled
+        # correctly. Successive calls to update should return the same
+        # as if the entire input was passed in a single call:
+        # update(pt[0:n]) + update(pt[n:m]) + update(pt[m:]) == update(pt)
+        key = bytearray(os.urandom(32))
+        nonce = bytearray(os.urandom(16))
+        cipher = Cipher(algorithms.ChaCha20(key, nonce), None, backend)
+        pt = bytearray(os.urandom(96 * 3))
+
+        enc_full = cipher.encryptor()
+        ct_full = enc_full.update(pt)
+
+        enc_partial = cipher.encryptor()
+        len_partial = len(pt) // 3
+        ct_partial_1 = enc_partial.update(pt[:len_partial])
+        ct_partial_2 = enc_partial.update(pt[len_partial : len_partial * 2])
+        ct_partial_3 = enc_partial.update(pt[len_partial * 2 :])
+
+        assert ct_full == ct_partial_1 + ct_partial_2 + ct_partial_3