add aurweb.captcha, a CAPTCHA utility module

This CAPTCHA workflow is the same workflow used by our current
PHP implementation of account registration.

Signed-off-by: Kevin Morris <kevr@0cost.org>

test/test_captcha.py

@@ -0,0 +1,60 @@
+import hashlib
+
+from aurweb import captcha
+
+
+def test_captcha_salts():
+    """ Make sure we can get some captcha salts. """
+    salts = captcha.get_captcha_salts()
+    assert len(salts) == 6
+
+
+def test_captcha_token():
+    """ Make sure getting a captcha salt's token matches up against
+    the first three digits of the md5 hash of the salt. """
+    salts = captcha.get_captcha_salts()
+    salt = salts[0]
+
+    token1 = captcha.get_captcha_token(salt)
+    token2 = hashlib.md5(salt.encode()).hexdigest()[:3]
+
+    assert token1 == token2
+
+
+def test_captcha_challenge_answer():
+    """ Make sure that executing the captcha challenge via shell
+    produces the correct result by comparing it against a straight
+    up token conversion. """
+    salts = captcha.get_captcha_salts()
+    salt = salts[0]
+
+    challenge = captcha.get_captcha_challenge(salt)
+
+    token = captcha.get_captcha_token(salt)
+    challenge2 = f"LC_ALL=C pacman -V|sed -r 's#[0-9]+#{token}#g'|md5sum|cut -c1-6"
+
+    assert challenge == challenge2
+
+
+def test_captcha_salt_filter():
+    """ Make sure captcha_salt_filter returns the first salt from
+    get_captcha_salts().
+
+    Example usage:
+        <input type="hidden" name="captcha_salt" value="{{ captcha_salt }}">
+    """
+    salt = captcha.captcha_salt_filter(None)
+    assert salt == captcha.get_captcha_salts()[0]
+
+
+def test_captcha_cmdline_filter():
+    """ Make sure that the captcha_cmdline filter gives us the
+    same challenge that get_captcha_challenge does.
+
+    Example usage:
+        <code>{{ captcha_salt | captcha_cmdline }}</code>
+    """
+    salt = captcha.captcha_salt_filter(None)
+    display1 = captcha.captcha_cmdline_filter(None, salt)
+    display2 = captcha.get_captcha_challenge(salt)
+    assert display1 == display2