Reverted commit
https://github.com/ronf/asyncssh/commit/386432c9b698beb23ccb88b69f4023560b76750f

--- b/asyncssh/crypto/chacha.py
+++ a/asyncssh/crypto/chacha.py
@@ -51,13 +51,18 @@
     def poly1305(key, data, nonce):
         """Compute a Poly1305 tag for a block of data"""
 
+        poly = Poly1305(poly1305_key(key, nonce))
+        poly.update(data)
+        return poly.finalize()
-        return Poly1305.generate_tag(poly1305_key(key, nonce), data)
 
     def poly1305_verify(key, data, nonce, tag):
         """Verify a Poly1305 tag for a block of data"""
 
+        poly = Poly1305(poly1305_key(key, nonce))
+
         try:
+            poly.update(data)
+            poly.verify(tag)
-            Poly1305.verify_tag(poly1305_key(key, nonce), data, tag)
             return True
         except InvalidSignature:
             return False
--- b/setup.py
+++ a/setup.py
@@ -52,7 +52,7 @@
       long_description = long_description,
       platforms = 'Any',
       python_requires = '>= 3.6',
+      install_requires = ['cryptography >= 2.7'],
-      install_requires = ['cryptography >= 2.8'],
       extras_require = {
           'bcrypt':     ['bcrypt >= 3.1.3'],
           'gssapi':     ['gssapi >= 1.2.0'],