feat(ciphers): add ADFGVX cipher with CLI

Author: Saiprashanth Pulisetti

ciphers/adfgvx.py

@@ -0,0 +1,138 @@
+#!/usr/bin/env python3
+"""ADFGVX cipher implementation with CLI encode/decode.
+
+Usage examples:
+  python -m ciphers.adfgvx encode --key squarekeyword --transposition SECRET --text "attack at once"
+  python -m ciphers.adfgvx decode --key squarekeyword --transposition SECRET --text <ciphertext>
+
+The ADFGVX cipher uses a 6x6 Polybius square (A-Z + 0-9) and a columnar
+transposition with a keyword.
+"""
+from __future__ import annotations
+
+import argparse
+import math
+import sys
+from typing import List
+
+ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+HEADERS = "ADFGVX"
+
+
+def build_square(keyword: str) -> str:
+    key = "".join(ch for ch in keyword.upper() if ch.isalnum())
+    seen = set()
+    square = []
+    for ch in key + ALPHABET:
+        if ch not in seen:
+            seen.add(ch)
+            square.append(ch)
+    return "".join(square[:36])
+
+
+def to_pairs(text: str, square: str) -> str:
+    out = []
+    for ch in text.upper():
+        if not ch.isalnum():
+            continue
+        idx = square.find(ch)
+        if idx == -1:
+            continue
+        r, c = divmod(idx, 6)
+        out.append(HEADERS[r])
+        out.append(HEADERS[c])
+    return "".join(out)
+
+
+def from_pairs(pairs: str, square: str) -> str:
+    assert len(pairs) % 2 == 0
+    out = []
+    for i in range(0, len(pairs), 2):
+        r = HEADERS.index(pairs[i])
+        c = HEADERS.index(pairs[i + 1])
+        out.append(square[r * 6 + c])
+    return "".join(out)
+
+
+def columnar_transpose_encrypt(text: str, key: str) -> str:
+    key = "".join(ch for ch in key.upper() if ch.isalnum())
+    order = sorted(range(len(key)), key=lambda i: (key[i], i))
+    rows = math.ceil(len(text) / len(key))
+    grid = [list(text[i * len(key):(i + 1) * len(key)]) for i in range(rows)]
+    if grid:
+        last = grid[-1]
+        while len(last) < len(key):
+            last.append("X")
+    out = []
+    for col in order:
+        for r in range(rows):
+            if col < len(grid[r]):
+                out.append(grid[r][col])
+    return "".join(out)
+
+
+def columnar_transpose_decrypt(cipher: str, key: str) -> str:
+    key = "".join(ch for ch in key.upper() if ch.isalnum())
+    order = sorted(range(len(key)), key=lambda i: (key[i], i))
+    rows = math.ceil(len(cipher) / len(key))
+    cols = len(key)
+    grid = [[None] * cols for _ in range(rows)]
+    idx = 0
+    for col in order:
+        for r in range(rows):
+            if idx < len(cipher):
+                grid[r][col] = cipher[idx]
+                idx += 1
+    out = []
+    for r in range(rows):
+        for c in range(cols):
+            ch = grid[r][c]
+            if ch is not None:
+                out.append(ch)
+    return "".join(out)
+
+
+def encode(key: str, transposition: str, text: str) -> str:
+    square = build_square(key)
+    pairs = to_pairs(text, square)
+    return columnar_transpose_encrypt(pairs, transposition)
+
+
+def decode(key: str, transposition: str, text: str) -> str:
+    square = build_square(key)
+    pairs = columnar_transpose_decrypt(text, transposition)
+    if len(pairs) % 2 == 1:
+        pairs = pairs[:-1]
+    return from_pairs(pairs, square)
+
+
+def parse_args(argv: List[str]) -> argparse.Namespace:
+    parser = argparse.ArgumentParser(description="ADFGVX cipher encode/decode")
+    sub = parser.add_subparsers(dest="cmd", required=True)
+
+    p_enc = sub.add_parser("encode")
+    p_enc.add_argument("--key", required=True)
+    p_enc.add_argument("--transposition", required=True)
+    p_enc.add_argument("--text", required=True)
+
+    p_dec = sub.add_parser("decode")
+    p_dec.add_argument("--key", required=True)
+    p_dec.add_argument("--transposition", required=True)
+    p_dec.add_argument("--text", required=True)
+
+    return parser.parse_args(argv)
+
+
+def main(argv: List[str] | None = None) -> int:
+    args = parse_args(sys.argv[1:] if argv is None else argv)
+    if args.cmd == "encode":
+        print(encode(args.key, args.transposition, args.text))
+        return 0
+    if args.cmd == "decode":
+        print(decode(args.key, args.transposition, args.text))
+        return 0
+    return 2
+
+
+if __name__ == "__main__":  # pragma: no cover
+    raise SystemExit(main())