diff --git a/proton/srp/_ctsrp.py b/proton/srp/_ctsrp.py
index 05c297f..e19f184 100644
--- a/proton/srp/_ctsrp.py
+++ b/proton/srp/_ctsrp.py
@@ -111,6 +111,10 @@ def new_bn():
     return bn
 
 
+def bn_num_bytes(a):
+    return ((BN_num_bits(a) + 7) // 8) # noqa
+
+
 def bn_mod(rem, m, d, ctx):
     return BN_div(None, rem, m, d, ctx) # noqa
 
@@ -120,9 +124,9 @@ def bn_is_zero(n):
 
 
 def bn_to_bytes(n, num_bytes):
-    b = ctypes.create_string_buffer(num_bytes)
+    b = ctypes.create_string_buffer(bn_num_bytes(n))
     BN_bn2bin(n, b) # noqa
-    return b.raw[::-1]
+    return b.raw[::-1].ljust(num_bytes, b'\0')
 
 
 def bytes_to_bn(dest_bn, bytes):
diff --git a/tests/testserver.py b/tests/testserver.py
index c6e4b8b..9f9a81c 100644
--- a/tests/testserver.py
+++ b/tests/testserver.py
@@ -18,7 +18,7 @@ class TestServer:
             self.calculate_k() * self.verifier + pow(
                 self.generator, self.b, self.modulus
             )
-        )
+        ) % self.modulus
 
         self.secret = None
         self.A = None