Add registration

captcha.py

@@ -0,0 +1,20 @@
+from random import randint
+import hashlib, base64
+
+# FIXME hash can be reused
+def generate(key):
+    '''
+    Generate a simple CAPTCHA.
+    It is based on a simple math expression which stops the simplest of bots.
+    '''
+    # The parameters are chosen such that they are simple to solve on paper.
+    a = randint(1, 10)
+    b = randint(1, 10)
+    c = randint(10, 20)
+    return f'{a} * {b} + {c} = ', _hash_answer(key, str(a * b + c))
+
+def verify(key, answer, hash):
+    return _hash_answer(key, answer) == hash
+
+def _hash_answer(key, answer):
+    return base64.b64encode(hashlib.sha256((key + answer).encode('utf-8')).digest()).decode('ascii')