add readme

README.md

@@ -0,0 +1,13 @@
+# encryption method
+
+based 100% on that one Veritasium video
+
+several bugs raised + fixed by codex
+
+## what is the encryption method
+
+the enigma machine. a "rotor cipher"
+
+`App` takes several rotors, the reflector, and optionally those keyboard configs...
+
+each rotor is the pairing mechanism, and the rotating + carry/notch thing mechanism

src/whatcryption/pairs/index_mapper.py

@@ -37,7 +37,9 @@ class index_mapper:
     def verify(i: list[int]):
         if not i:
             raise ValueError("index_mapper: i must not be empty")
-        if set(i) != set(range(len(i))):  # this also clears uniqueness
+        if len(i) != len(set(i)):
+            raise ValueError("index_mapper: i must not contain duplicates")
+        if set(i) != set(range(len(i))):  # this clears range
             raise ValueError("index_mapper: i must be a permutation of range(len(i))")