Checklist

• No unrelated changes
• Codec is new (not already implemented)
• Tests included (if cannot be automated with tests/test_generated)
• Documentation (included in the right page in docs/pages/enc)

Description

The Phillips cipher is a polyalphabetic block cipher that enciphers bigrams using a cycle of 8 key squares. The first square is a 5×5 grid filled keyword-first (I/J combined); each subsequent square rotates every row one step left. Substitution follows Playfair rules (same row → shift right/left, same column → shift down/up, rectangle → swap columns).

Implementation highlights:

• src/codext/crypto/phillips.py — new codec:
  • Keyword-seeded 5×5 grid; 7 row-rotated variants give the 8 key squares
  • Bigrams cycle through squares 1–8 (wrapping); odd-length input padded with X
  • Non-alphabetic characters pass through in place; J → I
  • Key required — LookupError deferred to call-time (consistent with Vigenère pattern, so codec listing/lookup works without a key)
  • __examples__ with verified encode values + roundtrip tests; __guess__ for guess mode
• src/codext/crypto/__init__.py — registers the codec
• README.md — adds entry under Cryptography

import codext

codext.encode("ATTACK", "phillips-key")   # → "BSSBIC"
codext.decode("BSSBIC", "phillips-key")   # → "ATTACK"

codext.encode("HELLO WORLD", "phillips-key")  # → "DBMMM ZMTQL"  (space preserved)
codext.decode("DBMMM ZMTQL", "phillips-key")  # → "HELLO WORLD"

• Fixes Add new encoding: phillips #39

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