feat(ciphers): add Josse - #5
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…abet
Josse (Maj. H. D. Josse, French Army, ~1889) is absent from the ACA registry and
from every hobbyist tool surveyed, so it was a genuine gap. Spec recovered by
Géraud-Stewart & Naccache (Cryptologia 45(4), 2021); broken ciphertext-only from
~75 letters by Lasry (Cryptologia 47(1), 2023).
C_1 = let(-num(P_1)); C_i = let(num(P_i) + num(C_{i-1}))
with num()/let() the position map of a keyword-derived mixed alphabet (written
row-wise into a table, read back column-wise). Decryption is a first difference.
Adds:
* encode/decode for the 26-letter generalisation (default) and the historical
25-letter form (drop="W")
* keyless two-stage crack: at the correct numbering the difference sequence
D_i = num(C_i) - num(C_{i-1}) IS a monoalphabetic image of the plaintext, so
stage 2 reduces to a simple-substitution solve
* Josse.digraph_ioc — a mono-invariant statistic that identifies a candidate
numbering (~150-letter sample: true 4.12 vs random 1.08 +- 0.23, z +13.3)
* full spec and computationally verified test vectors in docs/cipher-specs.json
* 6 tests, including the mono-invariance the attack depends on
Measured caveat, recorded in the module and the spec: free 26!-alphabet search is
overfit-undecidable at short lengths. Maximising unigram IoC overfits (a wrong
numbering reaches 2.32 vs a true 1.89); sorted-profile matching is too coarse; and
even digraph IoC — an excellent TEST — is beaten by free search (9.42 vs 4.12).
The practical attack sweeps the KEYWORD space, which is dictionary-sized rather
than 26!, and recovers planted keys exactly.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UrpA9d1jGn7fzbB1aV61JK
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