Validate cell characters and digit/period sanity

Halt import on control characters or a ':'/'/' inside an issuer or
account (they collide with the OATH issuer:account separator and period/
prefix), on a composed name over 64 bytes with a clearer per-field
message, and on a digit count outside 6-8 or a non-positive period.

test_oath.py

@@ -173,6 +173,30 @@ class TestValidateInput(unittest.TestCase):
         self.assertTrue(any("sha512" in p.lower() or "algorithm" in p.lower()
                             for p in problems))
 
+    def test_control_char_in_account_reported(self):
+        problems = validate_input([cred(account="a\x01b")])
+        self.assertTrue(any("control" in p.lower() for p in problems))
+
+    def test_colon_in_issuer_reported(self):
+        problems = validate_input([cred(issuer="Goo:gle")])
+        self.assertTrue(any(":" in p and "issuer" in p.lower() for p in problems))
+
+    def test_slash_in_account_reported(self):
+        problems = validate_input([cred(account="a/b")])
+        self.assertTrue(any("/" in p and "account" in p.lower() for p in problems))
+
+    def test_digits_out_of_range_reported(self):
+        problems = validate_input([cred(digits=9)])
+        self.assertTrue(any("digit" in p.lower() for p in problems))
+
+    def test_bad_period_reported(self):
+        problems = validate_input([cred(period=0)])
+        self.assertTrue(any("period" in p.lower() for p in problems))
+
+    def test_tab_is_control_char(self):
+        problems = validate_input([cred(issuer="Goo\tgle")])
+        self.assertTrue(any("control" in p.lower() for p in problems))
+
 
 class TestLoadCredentials(unittest.TestCase):
     def _make_xlsx(self, rows):