diff --git a/server-ca/chocolate.py b/server-ca/chocolate.py
index 8132be60f..0a7996530 100755
--- a/server-ca/chocolate.py
+++ b/server-ca/chocolate.py
@@ -300,7 +300,8 @@ class session(object):
             chall.succeeded = (c["satisfied"] == "True")   # TODO: this contradicts comment in protocol about meaning of "succeeded"
             # Calculate y
             dvsni_r = c["dvsni:r"]
-            y = RSA.importKey(self.pubkey()).encrypt(dvsni_r, None)[0]
+            pubkey = M2Crypto.RSA.load_key_string(self.pubkey())
+            y = pubkey.public_encrypt(dvsni_r, M2Crypto.RSA.pkcs1_oaep_padding)
             # In dvsni, we send nonce, y, ext
             chall.data.append(c["dvsni:nonce"])
             chall.data.append(y)