Add Playfair cipher codec

Co-authored-by: dhondta <9108102+dhondta@users.noreply.github.com>
Agent-Logs-Url: https://github.com/dhondta/python-codext/sessions/4b40b433-0112-42bf-ad61-b7d0d380bc9c

Author: Copilot

src/codext/crypto/__init__.py

@@ -4,6 +4,7 @@
 from .bacon import *
 from .barbie import *
 from .citrix import *
+from .playfair import *
 from .polybius import *
 from .railfence import *
 from .rot import *

src/codext/crypto/playfair.py

@@ -0,0 +1,138 @@
+# -*- coding: UTF-8 -*-
+"""Playfair Cipher Codec - playfair content encoding.
+
+The Playfair cipher is a symmetric encryption method using polygram substitution
+with bigrams (pairs of letters), invented in 1854 by Charles Wheatstone, but
+popularized by his friend Lord Playfair.
+
+This codec:
+- en/decodes strings from str to str
+- en/decodes strings from bytes to bytes
+- decodes file content to str (read)
+- encodes file content from str to bytes (write)
+
+Reference: https://www.dcode.fr/playfair-cipher
+"""
+from ..__common__ import *
+
+
+__examples__ = {
+    # Classic example from Wikipedia (key "PLAYFAIR EXAMPLE"):
+    #   the EE in "TREE" is split with an X filler during encoding, so decoding
+    #   exposes the filler: "TREESTUMP" → encoded → decoded as "TREXESTUMP"
+    'enc(playfair-playfairexample)': {'HIDETHEGOLDINTHETREESTUMP': 'BMODZBXDNABEKUDMUIXMMOUVIF'},
+    'dec(playfair-playfairexample)': {'BMODZBXDNABEKUDMUIXMMOUVIF': 'HIDETHEGOLDINTHETREXESTUMP'},
+    'enc-dec(playfair-keyword)':     ['INSTRUMENT'],
+}
+__guess__ = ["playfair"]
+
+
+# Standard 5×5 Playfair alphabet (I and J share the same cell)
+_DEFAULT_ALPHABET = "ABCDEFGHIKLMNOPQRSTUVWXYZ"
+
+
+def _build_grid(key=None):
+    """Build the 5×5 Playfair grid from an optional keyword."""
+    seen, grid = set(), []
+    if key:
+        for c in key.upper():
+            if c == 'J':
+                c = 'I'
+            if c.isalpha() and c not in seen:
+                seen.add(c)
+                grid.append(c)
+    for c in _DEFAULT_ALPHABET:
+        if c not in seen:
+            seen.add(c)
+            grid.append(c)
+    pos = {grid[i]: (i // 5, i % 5) for i in range(25)}
+    return grid, pos
+
+
+def _filler(c):
+    """Return the filler character for a given letter (X, or Q when the letter is X)."""
+    return 'Q' if c == 'X' else 'X'
+
+
+def _make_bigrams(text):
+    """Convert plaintext to bigrams, inserting fillers for repeated-letter pairs."""
+    chars = []
+    for c in ensure_str(text).upper():
+        if c == 'J':
+            chars.append('I')
+        elif c.isalpha():
+            chars.append(c)
+    bigrams = []
+    i = 0
+    while i < len(chars):
+        a = chars[i]
+        if i + 1 < len(chars):
+            b = chars[i + 1]
+            if a == b:
+                bigrams.append((a, _filler(a)))
+                i += 1
+            else:
+                bigrams.append((a, b))
+                i += 2
+        else:
+            bigrams.append((a, _filler(a)))
+            i += 1
+    return bigrams
+
+
+def _encode_bigram(grid, pos, a, b):
+    r_a, c_a = pos[a]
+    r_b, c_b = pos[b]
+    if r_a == r_b:
+        return grid[r_a * 5 + (c_a + 1) % 5], grid[r_b * 5 + (c_b + 1) % 5]
+    elif c_a == c_b:
+        return grid[((r_a + 1) % 5) * 5 + c_a], grid[((r_b + 1) % 5) * 5 + c_b]
+    else:
+        return grid[r_a * 5 + c_b], grid[r_b * 5 + c_a]
+
+
+def _decode_bigram(grid, pos, a, b):
+    r_a, c_a = pos[a]
+    r_b, c_b = pos[b]
+    if r_a == r_b:
+        return grid[r_a * 5 + (c_a - 1) % 5], grid[r_b * 5 + (c_b - 1) % 5]
+    elif c_a == c_b:
+        return grid[((r_a - 1) % 5) * 5 + c_a], grid[((r_b - 1) % 5) * 5 + c_b]
+    else:
+        return grid[r_a * 5 + c_b], grid[r_b * 5 + c_a]
+
+
+def playfair_encode(key=None):
+    grid, pos = _build_grid(key)
+    def encode(text, errors="strict"):
+        t = ensure_str(text)
+        result = []
+        for a, b in _make_bigrams(t):
+            ea, eb = _encode_bigram(grid, pos, a, b)
+            result.extend([ea, eb])
+        r = "".join(result)
+        return r, len(t)
+    return encode
+
+
+def playfair_decode(key=None):
+    grid, pos = _build_grid(key)
+    def decode(text, errors="strict"):
+        t = ensure_str(text)
+        chars = []
+        for c in t.upper():
+            if c == 'J':
+                chars.append('I')
+            elif c.isalpha():
+                chars.append(c)
+        result = []
+        for i in range(0, len(chars) - 1, 2):
+            da, db = _decode_bigram(grid, pos, chars[i], chars[i + 1])
+            result.extend([da, db])
+        r = "".join(result)
+        return r, len(t)
+    return decode
+
+
+add("playfair", playfair_encode, playfair_decode, r"^playfair(?:[-_]cipher)?(?:[-_]([a-zA-Z]+))?$",
+    printables_rate=1.)