fix encoding BMPString in x509 name entries (#4321)

Previously we encoded them as UTF-8, but as best I can tell in reality a
BMPString is fixed-width basic multilingual plane big endian encoding.
This is basically UCS-2 (aka original Unicode). However, Python doesn't
support UCS-2 encoding so we need to use utf_16_be. This means you can encode
surrogate code points that are invalid in the context of what a
BMPString is supposed to be, but in reality I strongly suspect the sane
encoding ship has sailed and dozens if not hundreds of implementations
both do this and expect other systems to handle their nonsense.

2 files changed, 25 insertions, 2 deletions

diff --git a/src/cryptography/hazmat/backends/openssl/encode_asn1.py b/src/cryptography/hazmat/backends/openssl/encode_asn1.py
index 4bea03e6..a2c7ed73 100644
--- a/src/cryptography/hazmat/backends/openssl/encode_asn1.py
+++ b/src/cryptography/hazmat/backends/openssl/encode_asn1.py
@@ -14,6 +14,7 @@ from cryptography.hazmat.backends.openssl.decode_asn1 import (
     _CRL_ENTRY_REASON_ENUM_TO_CODE, _DISTPOINT_TYPE_FULLNAME,
     _DISTPOINT_TYPE_RELATIVENAME
 )
+from cryptography.x509.name import _ASN1Type
 from cryptography.x509.oid import CRLEntryExtensionOID, ExtensionOID
 
 
@@ -116,11 +117,15 @@ def _encode_sk_name_entry(backend, attributes):
 
 
 def _encode_name_entry(backend, attribute):
-    value = attribute.value.encode('utf8')
+    if attribute._type is _ASN1Type.BMPString:
+        value = attribute.value.encode('utf_16_be')
+    else:
+        value = attribute.value.encode('utf8')
+
     obj = _txt2obj_gc(backend, attribute.oid.dotted_string)
 
     name_entry = backend._lib.X509_NAME_ENTRY_create_by_OBJ(
-        backend._ffi.NULL, obj, attribute._type.value, value, -1
+        backend._ffi.NULL, obj, attribute._type.value, value, len(value)
     )
     return name_entry
 
diff --git a/tests/x509/test_x509.py b/tests/x509/test_x509.py
index 7f9f1830..fe57784a 100644
--- a/tests/x509/test_x509.py
+++ b/tests/x509/test_x509.py
@@ -4092,6 +4092,24 @@ class TestName(object):
             b"b060355040a0c0450794341"
         )
 
+    @pytest.mark.requires_backend_interface(interface=X509Backend)
+    def test_bmpstring_bytes(self, backend):
+        # For this test we need an odd length string. BMPString is UCS-2
+        # encoded so it will always be even length and OpenSSL will error if
+        # you pass an odd length string without encoding it properly first.
+        name = x509.Name([
+            x509.NameAttribute(
+                NameOID.COMMON_NAME,
+                u'cryptography.io',
+                _ASN1Type.BMPString
+            ),
+            x509.NameAttribute(NameOID.ORGANIZATION_NAME, u'PyCA'),
+        ])
+        assert name.public_bytes(backend) == binascii.unhexlify(
+            b"30383127302506035504031e1e00630072007900700074006f00670072006100"
+            b"7000680079002e0069006f310d300b060355040a0c0450794341"
+        )
+
 
 def test_random_serial_number(monkeypatch):
     sample_data = os.urandom(20)