Add utility module to olm/python - for ed25519_verify

Signed-off-by: Alexander Maznev <alexander.maznev@gmail.com>

python/olm/utility.py

+from ._base import lib, c_void_p, c_size_t, c_char_p, \
+    create_string_buffer, ERR, OlmError
+
+lib.olm_utility_size.argtypes = []
+lib.olm_utility_size.restype = c_size_t
+
+lib.olm_utility.argtypes = [c_void_p]
+lib.olm_utility.restype = c_void_p
+
+lib.olm_utility_last_error.argtypes = [c_void_p]
+lib.olm_utility_last_error.restype = c_char_p
+
+
+def utility_errcheck(res, func, args):
+    if res == ERR:
+        raise OlmError("%s: %s" % (
+            func.__name__, lib.olm_utility_last_error(args[0])
+        ))
+    return res
+
+
+def utility_function(func, *types):
+    func.argtypes = (c_void_p,) + types
+    func.restypes = c_size_t
+    func.errcheck = utility_errcheck
+
+utility_function(
+    lib.olm_ed25519_verify,
+    c_void_p, c_size_t,  # key, key_length
+    c_void_p, c_size_t,  # message, message_length
+    c_void_p, c_size_t,  # signature, signature_length
+)
+
+
+class Utility(object):
+    def __init__(self):
+        self.buf = create_string_buffer(lib.olm_utility_size())
+        self.ptr = lib.olm_utility(self.buf)
+
+_utility = None
+
+
+def ed25519_verify(key, message, signature):
+    """ Verify an ed25519 signature. Raises an OlmError if verification fails.
+    Args:
+        key(bytes): The ed25519 public key used for signing.
+        message(bytes): The signed message.
+        signature(bytes): The message signature.
+    """
+    global _utility
+    if not _utility:
+        _utility = Utility()
+    lib.olm_ed25519_verify(_utility.ptr,
+                           key, len(key),
+                           message, len(message),
+                           signature, len(signature))